Merge branch 'feature-settings-tab' into develop

3 files changed, 974 insertions, 305 deletions

diff --git a/src/org/unitConverter/converterGUI/DefaultPrefixRepetitionRule.java b/src/org/unitConverter/converterGUI/DefaultPrefixRepetitionRule.java
new file mode 100644
index 0000000..bdc3a2e
--- /dev/null
+++ b/src/org/unitConverter/converterGUI/DefaultPrefixRepetitionRule.java
@@ -0,0 +1,95 @@
+/**
+ * @since 2020-08-26
+ */
+package org.unitConverter.converterGUI;
+
+import java.util.List;
+import java.util.function.Predicate;
+
+import org.unitConverter.unit.SI;
+import org.unitConverter.unit.UnitPrefix;
+
+/**
+ * A rule that specifies whether prefix repetition is allowed
+ *
+ * @since 2020-08-26
+ */
+enum DefaultPrefixRepetitionRule implements Predicate<List<UnitPrefix>> {
+	NO_REPETITION {
+		@Override
+		public boolean test(List<UnitPrefix> prefixes) {
+			return prefixes.size() <= 1;
+		}
+	},
+	NO_RESTRICTION {
+		@Override
+		public boolean test(List<UnitPrefix> prefixes) {
+			return true;
+		}
+	},
+	/**
+	 * You are allowed to have any number of Yotta/Yocto followed by possibly one
+	 * Kilo-Zetta/Milli-Zepto followed by possibly one Deca/Hecto. Same for
+	 * reducing prefixes, don't mix magnifying and reducing. Non-metric
+	 * (including binary) prefixes can't be repeated.
+	 */
+	COMPLEX_REPETITION {
+		@Override
+		public boolean test(List<UnitPrefix> prefixes) {
+			// determine whether we are magnifying or reducing
+			final boolean magnifying;
+			if (prefixes.isEmpty())
+				return true;
+			else if (prefixes.get(0).getMultiplier() > 1) {
+				magnifying = true;
+			} else {
+				magnifying = false;
+			}
+			
+			// if the first prefix is non-metric (including binary prefixes),
+			// assume we are using non-metric prefixes
+			// non-metric prefixes are allowed, but can't be repeated.
+			if (!SI.DECIMAL_PREFIXES.contains(prefixes.get(0)))
+				return NO_REPETITION.test(prefixes);
+			
+			int part = 0; // 0=yotta/yoctos, 1=kilo-zetta/milli-zepto,
+								// 2=deka,hecto,deci,centi
+			
+			for (final UnitPrefix prefix : prefixes) {
+				// check that the current prefix is metric and appropriately
+				// magnifying/reducing
+				if (!SI.DECIMAL_PREFIXES.contains(prefix))
+					return false;
+				if (magnifying != prefix.getMultiplier() > 1)
+					return false;
+					
+				// check if the current prefix is correct
+				// since part is set *after* this check, part designates the state
+				// of the *previous* prefix
+				switch (part) {
+				case 0:
+					// do nothing, any prefix is valid after a yotta
+					break;
+				case 1:
+					// after a kilo-zetta, only deka/hecto are valid
+					if (SI.THOUSAND_PREFIXES.contains(prefix))
+						return false;
+					break;
+				case 2:
+					// deka/hecto must be the last prefix, so this is always invalid
+					return false;
+				}
+				
+				// set part
+				if (SI.YOTTA.equals(prefix) || SI.YOCTO.equals(prefix)) {
+					part = 0;
+				} else if (SI.THOUSAND_PREFIXES.contains(prefix)) {
+					part = 1;
+				} else {
+					part = 2;
+				}
+			}
+			return true;
+		}
+	};
+}
diff --git a/src/org/unitConverter/converterGUI/SearchBoxList.java b/src/org/unitConverter/converterGUI/SearchBoxList.java
index 1995466..10ef589 100644
--- a/src/org/unitConverter/converterGUI/SearchBoxList.java
+++ b/src/org/unitConverter/converterGUI/SearchBoxList.java
@@ -36,13 +36,13 @@ import javax.swing.JTextField;
  * @since v0.2.0
  */
 final class SearchBoxList extends JPanel {
-
+	
 	/**
 	 * @since 2019-04-13
 	 * @since v0.2.0
 	 */
 	private static final long serialVersionUID = 6226930279415983433L;
-
+	
 	/**
 	 * The text to place in an empty search box.
 	 * 
@@ -50,7 +50,7 @@ final class SearchBoxList extends JPanel {
 	 * @since v0.2.0
 	 */
 	private static final String EMPTY_TEXT = "Search...";
-
+	
 	/**
 	 * The color to use for an empty foreground.
 	 * 
@@ -58,94 +58,92 @@ final class SearchBoxList extends JPanel {
 	 * @since v0.2.0
 	 */
 	private static final Color EMPTY_FOREGROUND = new Color(192, 192, 192);
-
+	
 	// the components
 	private final Collection<String> itemsToFilter;
 	private final DelegateListModel<String> listModel;
 	private final JTextField searchBox;
 	private final JList<String> searchItems;
-
+	
 	private boolean searchBoxEmpty = true;
-
-	// I need to do this because, for some reason, Swing is auto-focusing my search box without triggering a focus
+	
+	// I need to do this because, for some reason, Swing is auto-focusing my
+	// search box without triggering a focus
 	// event.
 	private boolean searchBoxFocused = false;
-
+	
 	private Predicate<String> customSearchFilter = o -> true;
 	private final Comparator<String> defaultOrdering;
 	private final boolean caseSensitive;
-
+	
 	/**
 	 * Creates the {@code SearchBoxList}.
 	 * 
-	 * @param itemsToFilter
-	 *            items to put in the list
+	 * @param itemsToFilter items to put in the list
 	 * @since 2019-04-14
 	 */
 	public SearchBoxList(final Collection<String> itemsToFilter) {
 		this(itemsToFilter, null, false);
 	}
-
+	
 	/**
 	 * Creates the {@code SearchBoxList}.
 	 * 
-	 * @param itemsToFilter
-	 *            items to put in the list
-	 * @param defaultOrdering
-	 *            default ordering of items after filtration (null=Comparable)
-	 * @param caseSensitive
-	 *            whether or not the filtration is case-sensitive
+	 * @param itemsToFilter   items to put in the list
+	 * @param defaultOrdering default ordering of items after filtration
+	 *                        (null=Comparable)
+	 * @param caseSensitive   whether or not the filtration is case-sensitive
 	 * 
 	 * @since 2019-04-13
 	 * @since v0.2.0
 	 */
-	public SearchBoxList(final Collection<String> itemsToFilter, final Comparator<String> defaultOrdering,
+	public SearchBoxList(final Collection<String> itemsToFilter,
+			final Comparator<String> defaultOrdering,
 			final boolean caseSensitive) {
 		super(new BorderLayout(), true);
 		this.itemsToFilter = itemsToFilter;
 		this.defaultOrdering = defaultOrdering;
 		this.caseSensitive = caseSensitive;
-
+		
 		// create the components
 		this.listModel = new DelegateListModel<>(new ArrayList<>(itemsToFilter));
 		this.searchItems = new JList<>(this.listModel);
-
+		
 		this.searchBox = new JTextField(EMPTY_TEXT);
 		this.searchBox.setForeground(EMPTY_FOREGROUND);
-
+		
 		// add them to the panel
 		this.add(this.searchBox, BorderLayout.PAGE_START);
 		this.add(new JScrollPane(this.searchItems), BorderLayout.CENTER);
-
+		
 		// set up the search box
 		this.searchBox.addFocusListener(new FocusListener() {
 			@Override
 			public void focusGained(final FocusEvent e) {
 				SearchBoxList.this.searchBoxFocusGained(e);
 			}
-
+			
 			@Override
 			public void focusLost(final FocusEvent e) {
 				SearchBoxList.this.searchBoxFocusLost(e);
 			}
 		});
-
+		
 		this.searchBox.addCaretListener(e -> this.searchBoxTextChanged());
 		this.searchBoxEmpty = true;
 	}
-
+	
 	/**
 	 * Adds an additional filter for searching.
 	 * 
-	 * @param filter
-	 *            filter to add.
+	 * @param filter filter to add.
 	 * @since 2019-04-13
 	 * @since v0.2.0
 	 */
 	public void addSearchFilter(final Predicate<String> filter) {
 		this.customSearchFilter = this.customSearchFilter.and(filter);
 	}
-
+	
 	/**
 	 * Resets the search filter.
 	 * 
@@ -155,7 +153,7 @@ final class SearchBoxList extends JPanel {
 	public void clearSearchFilters() {
 		this.customSearchFilter = o -> true;
 	}
-
+	
 	/**
 	 * @return this component's search box component
 	 * @since 2019-04-14
@@ -164,11 +162,11 @@ final class SearchBoxList extends JPanel {
 	public final JTextField getSearchBox() {
 		return this.searchBox;
 	}
-
+	
 	/**
-	 * @param searchText
-	 *            text to search for
-	 * @return a filter that filters out that text, based on this list's case sensitive setting
+	 * @param searchText text to search for
+	 * @return a filter that filters out that text, based on this list's case
+	 *         sensitive setting
 	 * @since 2019-04-14
 	 * @since v0.2.0
 	 */
@@ -176,9 +174,10 @@ final class SearchBoxList extends JPanel {
 		if (this.caseSensitive)
 			return string -> string.contains(searchText);
 		else
-			return string -> string.toLowerCase().contains(searchText.toLowerCase());
+			return string -> string.toLowerCase()
+					.contains(searchText.toLowerCase());
 	}
-
+	
 	/**
 	 * @return this component's list component
 	 * @since 2019-04-14
@@ -187,7 +186,7 @@ final class SearchBoxList extends JPanel {
 	public final JList<String> getSearchList() {
 		return this.searchItems;
 	}
-
+	
 	/**
 	 * @return index selected in item list
 	 * @since 2019-04-14
@@ -196,7 +195,7 @@ final class SearchBoxList extends JPanel {
 	public int getSelectedIndex() {
 		return this.searchItems.getSelectedIndex();
 	}
-
+	
 	/**
 	 * @return value selected in item list
 	 * @since 2019-04-13
@@ -205,7 +204,7 @@ final class SearchBoxList extends JPanel {
 	public String getSelectedValue() {
 		return this.searchItems.getSelectedValue();
 	}
-
+	
 	/**
 	 * Re-applies the filters.
 	 * 
@@ -213,29 +212,30 @@ final class SearchBoxList extends JPanel {
 	 * @since v0.2.0
 	 */
 	public void reapplyFilter() {
-		final String searchText = this.searchBoxEmpty ? "" : this.searchBox.getText();
-		final FilterComparator comparator = new FilterComparator(searchText, this.defaultOrdering, this.caseSensitive);
+		final String searchText = this.searchBoxEmpty ? ""
+				: this.searchBox.getText();
+		final FilterComparator comparator = new FilterComparator(searchText,
+				this.defaultOrdering, this.caseSensitive);
 		final Predicate<String> searchFilter = this.getSearchFilter(searchText);
-
+		
 		this.listModel.clear();
 		this.itemsToFilter.forEach(string -> {
 			if (searchFilter.test(string)) {
 				this.listModel.add(string);
 			}
 		});
-
+		
 		// applies the custom filters
 		this.listModel.removeIf(this.customSearchFilter.negate());
-
+		
 		// sorts the remaining items
 		this.listModel.sort(comparator);
 	}
-
+	
 	/**
 	 * Runs whenever the search box gains focus.
 	 * 
-	 * @param e
-	 *            focus event
+	 * @param e focus event
 	 * @since 2019-04-13
 	 * @since v0.2.0
 	 */
@@ -246,12 +246,11 @@ final class SearchBoxList extends JPanel {
 			this.searchBox.setForeground(Color.BLACK);
 		}
 	}
-
+	
 	/**
 	 * Runs whenever the search box loses focus.
 	 * 
-	 * @param e
-	 *            focus event
+	 * @param e focus event
 	 * @since 2019-04-13
 	 * @since v0.2.0
 	 */
@@ -262,7 +261,7 @@ final class SearchBoxList extends JPanel {
 			this.searchBox.setForeground(EMPTY_FOREGROUND);
 		}
 	}
-
+	
 	/**
 	 * Runs whenever the text in the search box is changed.
 	 * <p>
@@ -276,10 +275,12 @@ final class SearchBoxList extends JPanel {
 		if (this.searchBoxFocused) {
 			this.searchBoxEmpty = this.searchBox.getText().equals("");
 		}
-		final String searchText = this.searchBoxEmpty ? "" : this.searchBox.getText();
-		final FilterComparator comparator = new FilterComparator(searchText, this.defaultOrdering, this.caseSensitive);
+		final String searchText = this.searchBoxEmpty ? ""
+				: this.searchBox.getText();
+		final FilterComparator comparator = new FilterComparator(searchText,
+				this.defaultOrdering, this.caseSensitive);
 		final Predicate<String> searchFilter = this.getSearchFilter(searchText);
-
+		
 		// initialize list with items that match the filter then sort
 		this.listModel.clear();
 		this.itemsToFilter.forEach(string -> {
@@ -287,11 +288,20 @@ final class SearchBoxList extends JPanel {
 				this.listModel.add(string);
 			}
 		});
-
+		
 		// applies the custom filters
 		this.listModel.removeIf(this.customSearchFilter.negate());
-
+		
 		// sorts the remaining items
 		this.listModel.sort(comparator);
 	}
+	
+	/**
+	 * Manually updates the search box's item list.
+	 * 
+	 * @since 2020-08-27
+	 */
+	public void updateList() {
+		this.searchBoxTextChanged();
+	}
 }
diff --git a/src/org/unitConverter/converterGUI/UnitConverterGUI.java b/src/org/unitConverter/converterGUI/UnitConverterGUI.java
index 0230728..6ddc4a0 100644
--- a/src/org/unitConverter/converterGUI/UnitConverterGUI.java
+++ b/src/org/unitConverter/converterGUI/UnitConverterGUI.java
@@ -1,5 +1,5 @@
 /**
- * Copyright (C) 2018 Adrien Hopkins
+ * Copyright (C) 2018-2021 Adrien Hopkins
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU Affero General Public License as published by
@@ -17,41 +17,63 @@
 package org.unitConverter.converterGUI;
 
 import java.awt.BorderLayout;
+import java.awt.GridBagConstraints;
+import java.awt.GridBagLayout;
 import java.awt.GridLayout;
 import java.awt.event.KeyEvent;
-import java.io.File;
+import java.io.BufferedWriter;
+import java.io.IOException;
 import java.math.BigDecimal;
 import java.math.MathContext;
+import java.math.RoundingMode;
+import java.nio.file.Files;
+import java.nio.file.Path;
 import java.text.DecimalFormat;
+import java.text.NumberFormat;
+import java.text.ParseException;
 import java.util.ArrayList;
 import java.util.Comparator;
 import java.util.HashSet;
 import java.util.List;
+import java.util.NoSuchElementException;
 import java.util.Set;
 import java.util.function.Predicate;
+import java.util.stream.Collectors;
 
 import javax.swing.BorderFactory;
+import javax.swing.BoxLayout;
+import javax.swing.ButtonGroup;
 import javax.swing.JButton;
+import javax.swing.JCheckBox;
 import javax.swing.JComboBox;
 import javax.swing.JFormattedTextField;
 import javax.swing.JFrame;
 import javax.swing.JLabel;
 import javax.swing.JOptionPane;
 import javax.swing.JPanel;
+import javax.swing.JRadioButton;
+import javax.swing.JScrollPane;
 import javax.swing.JSlider;
 import javax.swing.JTabbedPane;
 import javax.swing.JTextArea;
 import javax.swing.JTextField;
+import javax.swing.UIManager;
+import javax.swing.UnsupportedLookAndFeelException;
 import javax.swing.WindowConstants;
+import javax.swing.border.TitledBorder;
 
+import org.unitConverter.math.ConditionalExistenceCollections;
 import org.unitConverter.math.ObjectProduct;
 import org.unitConverter.unit.BaseDimension;
 import org.unitConverter.unit.BritishImperial;
 import org.unitConverter.unit.LinearUnit;
+import org.unitConverter.unit.LinearUnitValue;
+import org.unitConverter.unit.NameSymbol;
 import org.unitConverter.unit.SI;
 import org.unitConverter.unit.Unit;
 import org.unitConverter.unit.UnitDatabase;
 import org.unitConverter.unit.UnitPrefix;
+import org.unitConverter.unit.UnitValue;
 
 /**
  * @author Adrien Hopkins
@@ -63,10 +85,22 @@ final class UnitConverterGUI {
 	 * A tab in the View.
 	 */
 	private enum Pane {
-		UNIT_CONVERTER, EXPRESSION_CONVERTER, UNIT_VIEWER, PREFIX_VIEWER;
+		UNIT_CONVERTER, EXPRESSION_CONVERTER, UNIT_VIEWER, PREFIX_VIEWER, ABOUT,
+		SETTINGS;
 	}
-
+	
 	private static class Presenter {
+		/** The default place where settings are stored. */
+		private static final String DEFAULT_SETTINGS_FILEPATH = "settings.txt";
+		/** The default place where units are stored. */
+		private static final Path DEFAULT_UNITS_FILE = Path.of("unitsfile.txt");
+		/** The default place where dimensions are stored. */
+		private static final Path DEFAULT_DIMENSION_FILE = Path
+				.of("dimensionfile.txt");
+		/** The default place where exceptions are stored. */
+		private static final Path DEFAULT_EXCEPTIONS_FILE = Path
+				.of("metric_exceptions.txt");
+		
 		/**
 		 * Adds default units and dimensions to a database.
 		 * 
@@ -88,33 +122,65 @@ final class UnitConverterGUI {
 			// nonlinear units - must be loaded manually
 			database.addUnit("tempCelsius", SI.CELSIUS);
 			database.addUnit("tempFahrenheit", BritishImperial.FAHRENHEIT);
-
+			
 			// load initial dimensions
 			database.addDimension("LENGTH", SI.Dimensions.LENGTH);
 			database.addDimension("MASS", SI.Dimensions.MASS);
 			database.addDimension("TIME", SI.Dimensions.TIME);
 			database.addDimension("TEMPERATURE", SI.Dimensions.TEMPERATURE);
 		}
-
+		
+		/**
+		 * @return {@code line} with any comments removed.
+		 * @since 2021-03-13
+		 */
+		private static final String withoutComments(String line) {
+			final int index = line.indexOf('#');
+			return index == -1 ? line : line.substring(index);
+		}
+		
 		/** The presenter's associated view. */
 		private final View view;
-
+		
 		/** The units known by the program. */
 		private final UnitDatabase database;
-
+		
 		/** The names of all of the units */
 		private final List<String> unitNames;
-
+		
 		/** The names of all of the prefixes */
 		private final List<String> prefixNames;
-
+		
 		/** The names of all of the dimensions */
 		private final List<String> dimensionNames;
-
+		
+		/** Unit names that are ignored by the metric-only/imperial-only filter */
+		private final Set<String> metricExceptions;
+		
 		private final Comparator<String> prefixNameComparator;
-
-		private int significantFigures = 6;
-
+		
+		/** A boolean remembering whether or not one-way conversion is on */
+		private boolean oneWay = true;
+		
+		/** The prefix rule */
+		private DefaultPrefixRepetitionRule prefixRule = null;
+		// conditions for existence of From and To entries
+		// used for one-way conversion
+		private final MutablePredicate<String> fromExistenceCondition = new MutablePredicate<>(
+				s -> true);
+		
+		private final MutablePredicate<String> toExistenceCondition = new MutablePredicate<>(
+				s -> true);
+		
+		/*
+		 * Rounding-related settings. I am using my own system, and not
+		 * MathContext, because MathContext does not support decimal place based
+		 * or scientific rounding, only significant digit based rounding.
+		 */
+		private int precision = 6;
+		
+		private RoundingType roundingType = RoundingType.SIGNIFICANT_DIGITS;
+		
 		/**
 		 * Creates the presenter.
 		 * 
@@ -124,14 +190,28 @@ final class UnitConverterGUI {
 		 */
 		Presenter(final View view) {
 			this.view = view;
-
+			
 			// load initial units
-			this.database = new UnitDatabase();
+			this.database = new UnitDatabase(
+					DefaultPrefixRepetitionRule.NO_RESTRICTION);
 			Presenter.addDefaults(this.database);
-
-			this.database.loadUnitsFile(new File("unitsfile.txt"));
-			this.database.loadDimensionFile(new File("dimensionfile.txt"));
-
+			
+			this.database.loadUnitsFile(DEFAULT_UNITS_FILE);
+			this.database.loadDimensionFile(DEFAULT_DIMENSION_FILE);
+			
+			// load metric exceptions
+			try {
+				this.metricExceptions = Files.readAllLines(DEFAULT_EXCEPTIONS_FILE)
+						.stream().map(Presenter::withoutComments)
+						.filter(s -> !s.isBlank()).collect(Collectors.toSet());
+			} catch (final IOException e) {
+				throw new AssertionError("Loading of metric_exceptions.txt failed.",
+						e);
+			}
+			
+			// load settings - requires database to exist
+			this.loadSettings();
+			
 			// a comparator that can be used to compare prefix names
 			// any name that does not exist is less than a name that does.
 			// otherwise, they are compared by value
@@ -140,37 +220,43 @@ final class UnitConverterGUI {
 					return -1;
 				else if (!Presenter.this.database.containsPrefixName(o2))
 					return 1;
-
+				
 				final UnitPrefix p1 = Presenter.this.database.getPrefix(o1);
 				final UnitPrefix p2 = Presenter.this.database.getPrefix(o2);
-
+				
 				if (p1.getMultiplier() < p2.getMultiplier())
 					return -1;
 				else if (p1.getMultiplier() > p2.getMultiplier())
 					return 1;
-
+				
 				return o1.compareTo(o2);
 			};
-
-			this.unitNames = new ArrayList<>(this.database.unitMapPrefixless().keySet());
+			
+			this.unitNames = new ArrayList<>(
+					this.database.unitMapPrefixless().keySet());
 			this.unitNames.sort(null); // sorts it using Comparable
-
+			
 			this.prefixNames = new ArrayList<>(this.database.prefixMap().keySet());
-			this.prefixNames.sort(this.prefixNameComparator); // sorts it using my comparator
-
-			this.dimensionNames = new DelegateListModel<>(new ArrayList<>(this.database.dimensionMap().keySet()));
+			this.prefixNames.sort(this.prefixNameComparator); // sorts it using my
+																				// comparator
+			
+			this.dimensionNames = new DelegateListModel<>(
+					new ArrayList<>(this.database.dimensionMap().keySet()));
 			this.dimensionNames.sort(null); // sorts it using Comparable
-
+			
 			// a Predicate that returns true iff the argument is a full base unit
-			final Predicate<Unit> isFullBase = unit -> unit instanceof LinearUnit && ((LinearUnit) unit).isBase();
-
+			final Predicate<Unit> isFullBase = unit -> unit instanceof LinearUnit
+					&& ((LinearUnit) unit).isBase();
+			
 			// print out unit counts
-			System.out.printf("Successfully loaded %d units with %d unit names (%d base units).%n",
+			System.out.printf(
+					"Successfully loaded %d units with %d unit names (%d base units).%n",
 					new HashSet<>(this.database.unitMapPrefixless().values()).size(),
 					this.database.unitMapPrefixless().size(),
-					new HashSet<>(this.database.unitMapPrefixless().values()).stream().filter(isFullBase).count());
+					new HashSet<>(this.database.unitMapPrefixless().values())
+							.stream().filter(isFullBase).count());
 		}
-
+		
 		/**
 		 * Converts in the dimension-based converter
 		 * 
@@ -180,7 +266,8 @@ final class UnitConverterGUI {
 		public final void convertDimensionBased() {
 			final String fromSelection = this.view.getFromSelection();
 			if (fromSelection == null) {
-				this.view.showErrorDialog("Error", "No unit selected in From field");
+				this.view.showErrorDialog("Error",
+						"No unit selected in From field");
 				return;
 			}
 			final String toSelection = this.view.getToSelection();
@@ -188,30 +275,35 @@ final class UnitConverterGUI {
 				this.view.showErrorDialog("Error", "No unit selected in To field");
 				return;
 			}
-
+			
 			final Unit from = this.database.getUnit(fromSelection);
-			final Unit to = this.database.getUnit(toSelection);
-
-			final String input = this.view.getDimensionConverterInput();
-			if (input.equals("")) {
-				this.view.showErrorDialog("Error", "No value to convert entered.");
+			final Unit to = this.database.getUnit(toSelection)
+					.withName(NameSymbol.ofName(toSelection));
+			
+			final UnitValue beforeValue;
+			try {
+				beforeValue = UnitValue.of(from,
+						this.view.getDimensionConverterInput());
+			} catch (final ParseException e) {
+				this.view.showErrorDialog("Error",
+						"Error in parsing: " + e.getMessage());
 				return;
 			}
-			final double beforeValue = Double.parseDouble(input);
-			final double value = from.convertTo(to, beforeValue);
-
+			final UnitValue value = beforeValue.convertTo(to);
+			
 			final String output = this.getRoundedString(value);
-
+			
 			this.view.setDimensionConverterOutputText(
-					String.format("%s %s = %s %s", input, fromSelection, output, toSelection));
+					String.format("%s = %s", beforeValue, output));
 		}
-
+		
 		/**
 		 * Runs whenever the convert button is pressed.
 		 * 
 		 * <p>
-		 * Reads and parses a unit expression from the from and to boxes, then converts
-		 * {@code from} to {@code to}. Any errors are shown in JOptionPanes.
+		 * Reads and parses a unit expression from the from and to boxes, then
+		 * converts {@code from} to {@code to}. Any errors are shown in
+		 * JOptionPanes.
 		 * </p>
 		 * 
 		 * @since 2019-01-26
@@ -220,64 +312,79 @@ final class UnitConverterGUI {
 		public final void convertExpressions() {
 			final String fromUnitString = this.view.getFromText();
 			final String toUnitString = this.view.getToText();
-
+			
 			if (fromUnitString.isEmpty()) {
-				this.view.showErrorDialog("Parse Error", "Please enter a unit expression in the From: box.");
+				this.view.showErrorDialog("Parse Error",
+						"Please enter a unit expression in the From: box.");
 				return;
 			}
 			if (toUnitString.isEmpty()) {
-				this.view.showErrorDialog("Parse Error", "Please enter a unit expression in the To: box.");
+				this.view.showErrorDialog("Parse Error",
+						"Please enter a unit expression in the To: box.");
 				return;
 			}
-
-			// try to parse from
-			final Unit from;
+			
+			final LinearUnitValue from;
+			final Unit to;
 			try {
-				from = this.database.getUnitFromExpression(fromUnitString);
-			} catch (final IllegalArgumentException e) {
-				this.view.showErrorDialog("Parse Error", "Could not recognize text in From entry: " + e.getMessage());
+				from = this.database.evaluateUnitExpression(fromUnitString);
+			} catch (final IllegalArgumentException | NoSuchElementException e) {
+				this.view.showErrorDialog("Parse Error",
+						"Could not recognize text in From entry: " + e.getMessage());
 				return;
 			}
-
-			final double value;
-			// try to parse to
-			final Unit to;
 			try {
-				if (this.database.containsUnitName(toUnitString)) {
-					// if it's a unit, convert to that
-					to = this.database.getUnit(toUnitString);
+				to = this.database.getUnitFromExpression(toUnitString);
+			} catch (final IllegalArgumentException | NoSuchElementException e) {
+				this.view.showErrorDialog("Parse Error",
+						"Could not recognize text in To entry: " + e.getMessage());
+				return;
+			}
+			
+			if (to instanceof LinearUnit) {
+				// convert to LinearUnitValue
+				final LinearUnitValue from2;
+				final LinearUnit to2 = ((LinearUnit) to)
+						.withName(NameSymbol.ofName(toUnitString));
+				final boolean useSlash;
+				
+				if (from.canConvertTo(to2)) {
+					from2 = from;
+					useSlash = false;
+				} else if (LinearUnitValue.ONE.dividedBy(from).canConvertTo(to2)) {
+					from2 = LinearUnitValue.ONE.dividedBy(from);
+					useSlash = true;
 				} else {
-					to = this.database.getUnitFromExpression(toUnitString);
+					// if I can't convert, leave
+					this.view.showErrorDialog("Conversion Error",
+							String.format("Cannot convert between %s and %s",
+									fromUnitString, toUnitString));
+					return;
 				}
-			} catch (final IllegalArgumentException e) {
-				this.view.showErrorDialog("Parse Error", "Could not recognize text in To entry: " + e.getMessage());
+				
+				final LinearUnitValue converted = from2.convertTo(to2);
+				this.view.setExpressionConverterOutputText((useSlash ? "1 / " : "")
+						+ String.format("%s = %s", fromUnitString,
+								this.getRoundedString(converted, false)));
 				return;
-			}
-
-			if (from.canConvertTo(to)) {
-				value = from.convertTo(to, 1);
-
-				// round value
-				final String output = this.getRoundedString(value);
-
-				this.view.setExpressionConverterOutputText(
-						String.format("%s = %s %s", fromUnitString, output, toUnitString));
-			} else if (from instanceof LinearUnit && SI.ONE.dividedBy((LinearUnit) from).canConvertTo(to)) {
-				// reciprocal conversion (like seconds to hertz)
-				value = SI.ONE.dividedBy((LinearUnit) from).convertTo(to, 1);
-
-				// round value
-				final String output = this.getRoundedString(value);
-
-				this.view.setExpressionConverterOutputText(
-						String.format("1 / %s = %s %s", fromUnitString, output, toUnitString));
 			} else {
-				// if I can't convert, leave
-				this.view.showErrorDialog("Conversion Error",
-						String.format("Cannot convert between %s and %s", fromUnitString, toUnitString));
+				// convert to UnitValue
+				final UnitValue from2 = from.asUnitValue();
+				if (from2.canConvertTo(to)) {
+					final UnitValue converted = from2.convertTo(to);
+					
+					this.view
+							.setExpressionConverterOutputText(String.format("%s = %s",
+									fromUnitString, this.getRoundedString(converted)));
+				} else {
+					// if I can't convert, leave
+					this.view.showErrorDialog("Conversion Error",
+							String.format("Cannot convert between %s and %s",
+									fromUnitString, toUnitString));
+				}
 			}
 		}
-
+		
 		/**
 		 * @return a list of all of the unit dimensions
 		 * @since 2019-04-13
@@ -286,7 +393,17 @@ final class UnitConverterGUI {
 		public final List<String> dimensionNameList() {
 			return this.dimensionNames;
 		}
-
+		
+		/**
+		 * @return a list of all the entries in the dimension-based converter's
+		 *         From box
+		 * @since 2020-08-27
+		 */
+		public final Set<String> fromEntries() {
+			return ConditionalExistenceCollections.conditionalExistenceSet(
+					this.unitNameSet(), this.fromExistenceCondition);
+		}
+		
 		/**
 		 * @return a comparator to compare prefix names
 		 * @since 2019-04-14
@@ -295,19 +412,54 @@ final class UnitConverterGUI {
 		public final Comparator<String> getPrefixNameComparator() {
 			return this.prefixNameComparator;
 		}
-
+		
 		/**
-		 * @param value value to round
-		 * @return string of that value rounded to {@code significantDigits} significant
-		 *         digits.
-		 * @since 2019-04-14
-		 * @since v0.2.0
+		 * Like {@link LinearUnitValue#toString(boolean)}, but obeys this unit
+		 * converter's rounding settings.
+		 * 
+		 * @since 2020-08-04
 		 */
-		private final String getRoundedString(final double value) {
-			// round value
-			final BigDecimal bigValue = new BigDecimal(value).round(new MathContext(this.significantFigures));
-			String output = bigValue.toString();
-
+		private final String getRoundedString(final LinearUnitValue value,
+				boolean showUncertainty) {
+			switch (this.roundingType) {
+			case DECIMAL_PLACES:
+			case SIGNIFICANT_DIGITS:
+				return this.getRoundedString(value.asUnitValue());
+			case SCIENTIFIC:
+				return value.toString(showUncertainty);
+			default:
+				throw new AssertionError("Invalid switch condition.");
+			}
+		}
+		
+		/**
+		 * Like {@link UnitValue#toString()}, but obeys this unit converter's
+		 * rounding settings.
+		 * 
+		 * @since 2020-08-04
+		 */
+		private final String getRoundedString(final UnitValue value) {
+			final BigDecimal unrounded = new BigDecimal(value.getValue());
+			final BigDecimal rounded;
+			int precision = this.precision;
+			
+			switch (this.roundingType) {
+			case DECIMAL_PLACES:
+				rounded = unrounded.setScale(precision, RoundingMode.HALF_EVEN);
+				break;
+			case SCIENTIFIC:
+				precision = 12;
+				//$FALL-THROUGH$
+			case SIGNIFICANT_DIGITS:
+				rounded = unrounded
+						.round(new MathContext(precision, RoundingMode.HALF_EVEN));
+				break;
+			default:
+				throw new AssertionError("Invalid switch condition.");
+			}
+			
+			String output = rounded.toString();
+			
 			// remove trailing zeroes
 			if (output.contains(".")) {
 				while (output.endsWith("0")) {
@@ -317,10 +469,74 @@ final class UnitConverterGUI {
 					output = output.substring(0, output.length() - 1);
 				}
 			}
-
-			return output;
+			
+			return output + " " + value.getUnit().getPrimaryName().get();
 		}
-
+		
+		/**
+		 * @return The file where settings are stored;
+		 * @since 2020-12-11
+		 */
+		private final Path getSettingsFile() {
+			return Path.of(DEFAULT_SETTINGS_FILEPATH);
+		}
+		
+		/**
+		 * Loads settings from the settings file.
+		 * 
+		 * @since 2021-02-17
+		 */
+		public final void loadSettings() {
+			try {
+				// read file line by line
+				final int lineNum = 0;
+				for (final String line : Files
+						.readAllLines(this.getSettingsFile())) {
+					final int equalsIndex = line.indexOf('=');
+					if (equalsIndex == -1)
+						throw new IllegalStateException(
+								"Settings file is malformed at line " + lineNum);
+					
+					final String param = line.substring(0, equalsIndex);
+					final String value = line.substring(equalsIndex + 1);
+					
+					switch (param) {
+					// set manually to avoid the unnecessary saving of the non-manual
+					// methods
+					case "precision":
+						this.precision = Integer.valueOf(value);
+						break;
+					case "rounding_type":
+						this.roundingType = RoundingType.valueOf(value);
+						break;
+					case "prefix_rule":
+						this.prefixRule = DefaultPrefixRepetitionRule.valueOf(value);
+						this.database.setPrefixRepetitionRule(this.prefixRule);
+						break;
+					case "one_way":
+						this.oneWay = Boolean.valueOf(value);
+						if (this.oneWay) {
+							this.fromExistenceCondition.setPredicate(
+									unitName -> this.metricExceptions.contains(unitName)
+											|| !this.database.getUnit(unitName)
+													.isMetric());
+							this.toExistenceCondition.setPredicate(
+									unitName -> this.metricExceptions.contains(unitName)
+											|| this.database.getUnit(unitName).isMetric());
+						} else {
+							this.fromExistenceCondition.setPredicate(unitName -> true);
+							this.toExistenceCondition.setPredicate(unitName -> true);
+						}
+						break;
+					default:
+						System.err.printf("Warning: unrecognized setting \"%s\".",
+								param);
+						break;
+					}
+				}
+			} catch (final IOException e) {}
+		}
+		
 		/**
 		 * @return a set of all prefix names in the database
 		 * @since 2019-04-14
@@ -329,7 +545,7 @@ final class UnitConverterGUI {
 		public final Set<String> prefixNameSet() {
 			return this.database.prefixMap().keySet();
 		}
-
+		
 		/**
 		 * Runs whenever a prefix is selected in the viewer.
 		 * <p>
@@ -345,23 +561,107 @@ final class UnitConverterGUI {
 				return;
 			else {
 				final UnitPrefix prefix = this.database.getPrefix(prefixName);
-
-				this.view.setPrefixTextBoxText(String.format("%s%nMultiplier: %s", prefixName, prefix.getMultiplier()));
+				
+				this.view.setPrefixTextBoxText(String.format("%s%nMultiplier: %s",
+						prefixName, prefix.getMultiplier()));
 			}
 		}
-
+		
+		/**
+		 * Saves the settings to the settings file.
+		 *
+		 * @since 2021-02-17
+		 */
+		public final void saveSettings() {
+			try (BufferedWriter writer = Files
+					.newBufferedWriter(this.getSettingsFile())) {
+				writer.write(String.format("precision=%d\n", this.precision));
+				writer.write(
+						String.format("rounding_type=%s\n", this.roundingType));
+				writer.write(String.format("prefix_rule=%s\n", this.prefixRule));
+				writer.write(String.format("one_way=%s\n", this.oneWay));
+			} catch (final IOException e) {
+				e.printStackTrace();
+				this.view.showErrorDialog("I/O Error",
+						"Error occurred while saving settings: "
+								+ e.getLocalizedMessage());
+			}
+		}
+		
+		/**
+		 * Enables or disables one-way conversion.
+		 * 
+		 * @param oneWay whether one-way conversion should be on (true) or off
+		 *               (false)
+		 * @since 2020-08-27
+		 */
+		public final void setOneWay(boolean oneWay) {
+			this.oneWay = oneWay;
+			if (oneWay) {
+				this.fromExistenceCondition.setPredicate(
+						unitName -> this.metricExceptions.contains(unitName)
+								|| !this.database.getUnit(unitName).isMetric());
+				this.toExistenceCondition.setPredicate(
+						unitName -> this.metricExceptions.contains(unitName)
+								|| this.database.getUnit(unitName).isMetric());
+			} else {
+				this.fromExistenceCondition.setPredicate(unitName -> true);
+				this.toExistenceCondition.setPredicate(unitName -> true);
+			}
+			
+			this.saveSettings();
+		}
+		
 		/**
-		 * @param significantFigures new value of significantFigures
+		 * @param precision new value of precision
 		 * @since 2019-01-15
 		 * @since v0.1.0
 		 */
-		public final void setSignificantFigures(final int significantFigures) {
-			this.significantFigures = significantFigures;
+		public final void setPrecision(final int precision) {
+			this.precision = precision;
+			
+			this.saveSettings();
 		}
-
+		
 		/**
-		 * Returns true if and only if the unit represented by {@code unitName} has the
-		 * dimension represented by {@code dimensionName}.
+		 * @param prefixRepetitionRule the prefixRepetitionRule to set
+		 * @since 2020-08-26
+		 */
+		public void setPrefixRepetitionRule(
+				Predicate<List<UnitPrefix>> prefixRepetitionRule) {
+			if (prefixRepetitionRule instanceof DefaultPrefixRepetitionRule) {
+				this.prefixRule = (DefaultPrefixRepetitionRule) prefixRepetitionRule;
+			} else {
+				this.prefixRule = null;
+			}
+			this.database.setPrefixRepetitionRule(prefixRepetitionRule);
+			
+			this.saveSettings();
+		}
+		
+		/**
+		 * @param roundingType the roundingType to set
+		 * @since 2020-07-16
+		 */
+		public final void setRoundingType(RoundingType roundingType) {
+			this.roundingType = roundingType;
+			
+			this.saveSettings();
+		}
+		
+		/**
+		 * @return a list of all the entries in the dimension-based converter's To
+		 *         box
+		 * @since 2020-08-27
+		 */
+		public final Set<String> toEntries() {
+			return ConditionalExistenceCollections.conditionalExistenceSet(
+					this.unitNameSet(), this.toExistenceCondition);
+		}
+		
+		/**
+		 * Returns true if and only if the unit represented by {@code unitName}
+		 * has the dimension represented by {@code dimensionName}.
 		 * 
 		 * @param unitName      name of unit to test
 		 * @param dimensionName name of dimension to test
@@ -369,12 +669,14 @@ final class UnitConverterGUI {
 		 * @since 2019-04-13
 		 * @since v0.2.0
 		 */
-		public final boolean unitMatchesDimension(final String unitName, final String dimensionName) {
+		public final boolean unitMatchesDimension(final String unitName,
+				final String dimensionName) {
 			final Unit unit = this.database.getUnit(unitName);
-			final ObjectProduct<BaseDimension> dimension = this.database.getDimension(dimensionName);
+			final ObjectProduct<BaseDimension> dimension = this.database
+					.getDimension(dimensionName);
 			return unit.getDimension().equals(dimension);
 		}
-
+		
 		/**
 		 * Runs whenever a unit is selected in the viewer.
 		 * <p>
@@ -390,29 +692,44 @@ final class UnitConverterGUI {
 				return;
 			else {
 				final Unit unit = this.database.getUnit(unitName);
-
+				
 				this.view.setUnitTextBoxText(unit.toString());
 			}
 		}
-
+		
 		/**
 		 * @return a set of all of the unit names
 		 * @since 2019-04-14
 		 * @since v0.2.0
 		 */
-		public final Set<String> unitNameSet() {
+		private final Set<String> unitNameSet() {
 			return this.database.unitMapPrefixless().keySet();
 		}
 	}
-
+	
+	/**
+	 * Different types of rounding.
+	 * 
+	 * Significant digits: Rounds to a number of digits. i.e. with precision 5,
+	 * 12345.6789 rounds to 12346. Decimal places: Rounds to a number of digits
+	 * after the decimal point, i.e. with precision 5, 12345.6789 rounds to
+	 * 12345.67890. Scientific: Rounds based on the number of digits and
+	 * operations, following standard scientific rounding.
+	 */
+	private static enum RoundingType {
+		SIGNIFICANT_DIGITS, DECIMAL_PLACES, SCIENTIFIC;
+	}
+	
 	private static class View {
+		private static final NumberFormat NUMBER_FORMATTER = new DecimalFormat();
+		
 		/** The view's frame. */
 		private final JFrame frame;
 		/** The view's associated presenter. */
 		private final Presenter presenter;
 		/** The master pane containing all of the tabs. */
 		private final JTabbedPane masterPane;
-
+		
 		// DIMENSION-BASED CONVERTER
 		/** The panel for inputting values in the dimension-based converter */
 		private final JTextField valueInput;
@@ -422,7 +739,7 @@ final class UnitConverterGUI {
 		private final SearchBoxList toSearch;
 		/** The output area in the dimension-based converter */
 		private final JTextArea dimensionBasedOutput;
-
+		
 		// EXPRESSION-BASED CONVERTER
 		/** The "From" entry in the conversion panel */
 		private final JTextField fromEntry;
@@ -430,7 +747,7 @@ final class UnitConverterGUI {
 		private final JTextField toEntry;
 		/** The output area in the conversion panel */
 		private final JTextArea output;
-
+		
 		// UNIT AND PREFIX VIEWERS
 		/** The searchable list of unit names in the unit viewer */
 		private final SearchBoxList unitNameList;
@@ -440,7 +757,7 @@ final class UnitConverterGUI {
 		private final JTextArea unitTextBox;
 		/** The text box for prefix data in the prefix viewer */
 		private final JTextArea prefixTextBox;
-
+		
 		/**
 		 * Creates the {@code View}.
 		 * 
@@ -451,28 +768,38 @@ final class UnitConverterGUI {
 			this.presenter = new Presenter(this);
 			this.frame = new JFrame("Unit Converter");
 			this.frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
-			this.masterPane = new JTabbedPane();
-
+			
+			// enable system look and feel
+			try {
+				UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
+			} catch (ClassNotFoundException | InstantiationException
+					| IllegalAccessException | UnsupportedLookAndFeelException e) {
+				// oh well, just use default theme
+				System.err.println("Failed to enable system look-and-feel.");
+				e.printStackTrace();
+			}
+			
 			// create the components
+			this.masterPane = new JTabbedPane();
 			this.unitNameList = new SearchBoxList(this.presenter.unitNameSet());
 			this.prefixNameList = new SearchBoxList(this.presenter.prefixNameSet(),
 					this.presenter.getPrefixNameComparator(), true);
 			this.unitTextBox = new JTextArea();
 			this.prefixTextBox = new JTextArea();
-			this.fromSearch = new SearchBoxList(this.presenter.unitNameSet());
-			this.toSearch = new SearchBoxList(this.presenter.unitNameSet());
-			this.valueInput = new JFormattedTextField(new DecimalFormat("###############0.################"));
+			this.fromSearch = new SearchBoxList(this.presenter.fromEntries());
+			this.toSearch = new SearchBoxList(this.presenter.toEntries());
+			this.valueInput = new JFormattedTextField(NUMBER_FORMATTER);
 			this.dimensionBasedOutput = new JTextArea(2, 32);
 			this.fromEntry = new JTextField();
 			this.toEntry = new JTextField();
 			this.output = new JTextArea(2, 32);
-
+			
 			// create more components
 			this.initComponents();
-
+			
 			this.frame.pack();
 		}
-
+		
 		/**
 		 * @return the currently selected pane.
 		 * @throws AssertionError if no pane (or an invalid pane) is selected
@@ -487,20 +814,30 @@ final class UnitConverterGUI {
 				return Pane.UNIT_VIEWER;
 			case 3:
 				return Pane.PREFIX_VIEWER;
+			case 4:
+				return Pane.ABOUT;
+			case 5:
+				return Pane.SETTINGS;
 			default:
 				throw new AssertionError("No selected pane, or invalid pane.");
 			}
 		}
-
+		
 		/**
 		 * @return value in dimension-based converter
-		 * @since 2019-04-13
-		 * @since v0.2.0
+		 * @throws ParseException
+		 * @since 2020-07-07
 		 */
-		public String getDimensionConverterInput() {
-			return this.valueInput.getText();
+		public double getDimensionConverterInput() throws ParseException {
+			final Number value = NUMBER_FORMATTER.parse(this.valueInput.getText());
+			if (value instanceof Double)
+				return (double) value;
+			else if (value instanceof Long)
+				return ((Long) value).longValue();
+			else
+				throw new AssertionError();
 		}
-
+		
 		/**
 		 * @return selection in "From" selector in dimension-based converter
 		 * @since 2019-04-13
@@ -509,7 +846,7 @@ final class UnitConverterGUI {
 		public String getFromSelection() {
 			return this.fromSearch.getSelectedValue();
 		}
-
+		
 		/**
 		 * @return text in "From" box in converter panel
 		 * @since 2019-01-15
@@ -518,7 +855,7 @@ final class UnitConverterGUI {
 		public String getFromText() {
 			return this.fromEntry.getText();
 		}
-
+		
 		/**
 		 * @return index of selected prefix in prefix viewer
 		 * @since 2019-01-15
@@ -527,7 +864,7 @@ final class UnitConverterGUI {
 		public String getPrefixViewerSelection() {
 			return this.prefixNameList.getSelectedValue();
 		}
-
+		
 		/**
 		 * @return selection in "To" selector in dimension-based converter
 		 * @since 2019-04-13
@@ -536,7 +873,7 @@ final class UnitConverterGUI {
 		public String getToSelection() {
 			return this.toSearch.getSelectedValue();
 		}
-
+		
 		/**
 		 * @return text in "To" box in converter panel
 		 * @since 2019-01-26
@@ -545,7 +882,7 @@ final class UnitConverterGUI {
 		public String getToText() {
 			return this.toEntry.getText();
 		}
-
+		
 		/**
 		 * @return index of selected unit in unit viewer
 		 * @since 2019-01-15
@@ -554,7 +891,7 @@ final class UnitConverterGUI {
 		public String getUnitViewerSelection() {
 			return this.unitNameList.getSelectedValue();
 		}
-
+		
 		/**
 		 * Starts up the application.
 		 * 
@@ -564,7 +901,7 @@ final class UnitConverterGUI {
 		public final void init() {
 			this.frame.setVisible(true);
 		}
-
+		
 		/**
 		 * Initializes the view's components.
 		 * 
@@ -574,229 +911,443 @@ final class UnitConverterGUI {
 		private final void initComponents() {
 			final JPanel masterPanel = new JPanel();
 			this.frame.add(masterPanel);
-
+			
 			masterPanel.setLayout(new BorderLayout());
-
+			
 			{ // pane with all of the tabs
 				masterPanel.add(this.masterPane, BorderLayout.CENTER);
-
+				
+				// update stuff
+				this.masterPane.addChangeListener(e -> this.update());
+				
 				{ // a panel for unit conversion using a selector
 					final JPanel convertUnitPanel = new JPanel();
 					this.masterPane.addTab("Convert Units", convertUnitPanel);
 					this.masterPane.setMnemonicAt(0, KeyEvent.VK_U);
-
+					
 					convertUnitPanel.setLayout(new BorderLayout());
-
+					
 					{ // panel for input part
 						final JPanel inputPanel = new JPanel();
 						convertUnitPanel.add(inputPanel, BorderLayout.CENTER);
-
+						
 						inputPanel.setLayout(new GridLayout(1, 3));
-
+						
 						final JComboBox<String> dimensionSelector = new JComboBox<>(
-								this.presenter.dimensionNameList().toArray(new String[0]));
+								this.presenter.dimensionNameList()
+										.toArray(new String[0]));
 						dimensionSelector.setSelectedItem("LENGTH");
-
+						
 						// handle dimension filter
-						final MutablePredicate<String> dimensionFilter = new MutablePredicate<>(s -> true);
-
+						final MutablePredicate<String> dimensionFilter = new MutablePredicate<>(
+								s -> true);
+						
 						// panel for From things
 						inputPanel.add(this.fromSearch);
-
+						
 						this.fromSearch.addSearchFilter(dimensionFilter);
-
-						{ // for dimension selector and arrow that represents conversion
+						
+						{ // for dimension selector and arrow that represents
+							// conversion
 							final JPanel inBetweenPanel = new JPanel();
 							inputPanel.add(inBetweenPanel);
-
+							
 							inBetweenPanel.setLayout(new BorderLayout());
-
+							
 							{ // dimension selector
-								inBetweenPanel.add(dimensionSelector, BorderLayout.PAGE_START);
+								inBetweenPanel.add(dimensionSelector,
+										BorderLayout.PAGE_START);
 							}
-
+							
 							{ // the arrow in the middle
 								final JLabel arrowLabel = new JLabel("->");
 								inBetweenPanel.add(arrowLabel, BorderLayout.CENTER);
 							}
 						}
-
+						
 						// panel for To things
-
+						
 						inputPanel.add(this.toSearch);
-
+						
 						this.toSearch.addSearchFilter(dimensionFilter);
-
+						
 						// code for dimension filter
 						dimensionSelector.addItemListener(e -> {
-							dimensionFilter.setPredicate(string -> View.this.presenter.unitMatchesDimension(string,
-									(String) dimensionSelector.getSelectedItem()));
+							dimensionFilter.setPredicate(string -> View.this.presenter
+									.unitMatchesDimension(string,
+											(String) dimensionSelector.getSelectedItem()));
 							this.fromSearch.reapplyFilter();
 							this.toSearch.reapplyFilter();
 						});
-
-						// apply the item listener once because I have a default selection
-						dimensionFilter.setPredicate(string -> View.this.presenter.unitMatchesDimension(string,
-								(String) dimensionSelector.getSelectedItem()));
+						
+						// apply the item listener once because I have a default
+						// selection
+						dimensionFilter.setPredicate(string -> View.this.presenter
+								.unitMatchesDimension(string,
+										(String) dimensionSelector.getSelectedItem()));
 						this.fromSearch.reapplyFilter();
 						this.toSearch.reapplyFilter();
 					}
-
+					
 					{ // panel for submit and output, and also value entry
 						final JPanel outputPanel = new JPanel();
 						convertUnitPanel.add(outputPanel, BorderLayout.PAGE_END);
-
+						
 						outputPanel.setLayout(new GridLayout(3, 1));
-
+						
 						{ // unit input
 							final JPanel valueInputPanel = new JPanel();
 							outputPanel.add(valueInputPanel);
-
+							
 							valueInputPanel.setLayout(new BorderLayout());
-
+							
 							{ // prompt
-								final JLabel valuePrompt = new JLabel("Value to convert: ");
-								valueInputPanel.add(valuePrompt, BorderLayout.LINE_START);
+								final JLabel valuePrompt = new JLabel(
+										"Value to convert: ");
+								valueInputPanel.add(valuePrompt,
+										BorderLayout.LINE_START);
 							}
-
+							
 							{ // value to convert
-								valueInputPanel.add(this.valueInput, BorderLayout.CENTER);
+								valueInputPanel.add(this.valueInput,
+										BorderLayout.CENTER);
 							}
 						}
-
+						
 						{ // button to convert
 							final JButton convertButton = new JButton("Convert");
 							outputPanel.add(convertButton);
-
-							convertButton.addActionListener(e -> this.presenter.convertDimensionBased());
+							
+							convertButton.addActionListener(
+									e -> this.presenter.convertDimensionBased());
 							convertButton.setMnemonic(KeyEvent.VK_ENTER);
 						}
-
+						
 						{ // output of conversion
 							outputPanel.add(this.dimensionBasedOutput);
 							this.dimensionBasedOutput.setEditable(false);
 						}
 					}
 				}
-
+				
 				{ // panel for unit conversion using expressions
 					final JPanel convertExpressionPanel = new JPanel();
-					this.masterPane.addTab("Convert Unit Expressions", convertExpressionPanel);
+					this.masterPane.addTab("Convert Unit Expressions",
+							convertExpressionPanel);
 					this.masterPane.setMnemonicAt(1, KeyEvent.VK_E);
-
-					convertExpressionPanel.setLayout(new GridLayout(5, 1));
-
+					
+					convertExpressionPanel.setLayout(new GridLayout(4, 1));
+					
 					{ // panel for units to convert from
 						final JPanel fromPanel = new JPanel();
 						convertExpressionPanel.add(fromPanel);
-
+						
 						fromPanel.setBorder(BorderFactory.createTitledBorder("From"));
 						fromPanel.setLayout(new GridLayout(1, 1));
-
+						
 						{ // entry for units
 							fromPanel.add(this.fromEntry);
 						}
 					}
-
+					
 					{ // panel for units to convert to
 						final JPanel toPanel = new JPanel();
 						convertExpressionPanel.add(toPanel);
-
+						
 						toPanel.setBorder(BorderFactory.createTitledBorder("To"));
 						toPanel.setLayout(new GridLayout(1, 1));
-
+						
 						{ // entry for units
 							toPanel.add(this.toEntry);
 						}
 					}
-
+					
 					{ // button to convert
 						final JButton convertButton = new JButton("Convert");
 						convertExpressionPanel.add(convertButton);
-
-						convertButton.addActionListener(e -> this.presenter.convertExpressions());
+						
+						convertButton.addActionListener(
+								e -> this.presenter.convertExpressions());
 						convertButton.setMnemonic(KeyEvent.VK_ENTER);
 					}
-
+					
 					{ // output of conversion
 						final JPanel outputPanel = new JPanel();
 						convertExpressionPanel.add(outputPanel);
-
-						outputPanel.setBorder(BorderFactory.createTitledBorder("Output"));
+						
+						outputPanel
+								.setBorder(BorderFactory.createTitledBorder("Output"));
 						outputPanel.setLayout(new GridLayout(1, 1));
-
+						
 						{ // output
 							outputPanel.add(this.output);
 							this.output.setEditable(false);
 						}
 					}
-
-					{ // panel for specifying precision
-						final JPanel sigDigPanel = new JPanel();
-						convertExpressionPanel.add(sigDigPanel);
-
-						sigDigPanel.setBorder(BorderFactory.createTitledBorder("Significant Digits"));
-
-						{ // slider
-							final JSlider sigDigSlider = new JSlider(0, 12);
-							sigDigPanel.add(sigDigSlider);
-
-							sigDigSlider.setMajorTickSpacing(4);
-							sigDigSlider.setMinorTickSpacing(1);
-							sigDigSlider.setSnapToTicks(true);
-							sigDigSlider.setPaintTicks(true);
-							sigDigSlider.setPaintLabels(true);
-
-							sigDigSlider.addChangeListener(
-									e -> this.presenter.setSignificantFigures(sigDigSlider.getValue()));
-						}
-					}
 				}
-
+				
 				{ // panel to look up units
 					final JPanel unitLookupPanel = new JPanel();
 					this.masterPane.addTab("Unit Viewer", unitLookupPanel);
 					this.masterPane.setMnemonicAt(2, KeyEvent.VK_V);
-
+					
 					unitLookupPanel.setLayout(new GridLayout());
-
+					
 					{ // search panel
 						unitLookupPanel.add(this.unitNameList);
-
-						this.unitNameList.getSearchList()
-								.addListSelectionListener(e -> this.presenter.unitNameSelected());
+						
+						this.unitNameList.getSearchList().addListSelectionListener(
+								e -> this.presenter.unitNameSelected());
 					}
-
+					
 					{ // the text box for unit's toString
 						unitLookupPanel.add(this.unitTextBox);
 						this.unitTextBox.setEditable(false);
 						this.unitTextBox.setLineWrap(true);
 					}
 				}
-
+				
 				{ // panel to look up prefixes
 					final JPanel prefixLookupPanel = new JPanel();
 					this.masterPane.addTab("Prefix Viewer", prefixLookupPanel);
 					this.masterPane.setMnemonicAt(3, KeyEvent.VK_P);
-
+					
 					prefixLookupPanel.setLayout(new GridLayout(1, 2));
-
+					
 					{ // panel for listing and seaching
 						prefixLookupPanel.add(this.prefixNameList);
-
-						this.prefixNameList.getSearchList()
-								.addListSelectionListener(e -> this.presenter.prefixSelected());
+						
+						this.prefixNameList.getSearchList().addListSelectionListener(
+								e -> this.presenter.prefixSelected());
 					}
-
+					
 					{ // the text box for prefix's toString
 						prefixLookupPanel.add(this.prefixTextBox);
 						this.prefixTextBox.setEditable(false);
 						this.prefixTextBox.setLineWrap(true);
 					}
 				}
+				
+				{ // Info panel
+					final JPanel infoPanel = new JPanel();
+					this.masterPane.addTab("\uD83D\uDEC8", // info (i) character
+							new JScrollPane(infoPanel));
+					
+					final JTextArea infoTextArea = new JTextArea();
+					infoTextArea.setEditable(false);
+					infoTextArea.setOpaque(false);
+					infoPanel.add(infoTextArea);
+					
+					// get info text
+					final String infoText;
+					try {
+						final Path aboutFile = Path.of("src", "about.txt");
+						infoText = Files.readAllLines(aboutFile).stream()
+								.map(Presenter::withoutComments)
+								.collect(Collectors.joining("\n"));
+					} catch (final IOException e) {
+						throw new AssertionError("I/O exception loading about.txt");
+					}
+					infoTextArea.setText(infoText);
+				}
+				
+				{ // Settings panel
+					final JPanel settingsPanel = new JPanel();
+					this.masterPane.addTab("\u2699", new JScrollPane(settingsPanel));
+					this.masterPane.setMnemonicAt(5, KeyEvent.VK_S);
+					
+					settingsPanel.setLayout(
+							new BoxLayout(settingsPanel, BoxLayout.PAGE_AXIS));
+					
+					{ // rounding settings
+						final JPanel roundingPanel = new JPanel();
+						settingsPanel.add(roundingPanel);
+						roundingPanel
+								.setBorder(new TitledBorder("Rounding Settings"));
+						roundingPanel.setLayout(new GridBagLayout());
+						
+						// rounding rule selection
+						final ButtonGroup roundingRuleButtons = new ButtonGroup();
+						
+						final JLabel roundingRuleLabel = new JLabel("Rounding Rule:");
+						roundingPanel.add(roundingRuleLabel, new GridBagBuilder(0, 0)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JRadioButton fixedPrecision = new JRadioButton(
+								"Fixed Precision");
+						if (this.presenter.roundingType == RoundingType.SIGNIFICANT_DIGITS) {
+							fixedPrecision.setSelected(true);
+						}
+						fixedPrecision.addActionListener(e -> this.presenter
+								.setRoundingType(RoundingType.SIGNIFICANT_DIGITS));
+						roundingRuleButtons.add(fixedPrecision);
+						roundingPanel.add(fixedPrecision, new GridBagBuilder(0, 1)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JRadioButton fixedDecimals = new JRadioButton(
+								"Fixed Decimal Places");
+						if (this.presenter.roundingType == RoundingType.DECIMAL_PLACES) {
+							fixedDecimals.setSelected(true);
+						}
+						fixedDecimals.addActionListener(e -> this.presenter
+								.setRoundingType(RoundingType.DECIMAL_PLACES));
+						roundingRuleButtons.add(fixedDecimals);
+						roundingPanel.add(fixedDecimals, new GridBagBuilder(0, 2)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JRadioButton relativePrecision = new JRadioButton(
+								"Scientific Precision");
+						if (this.presenter.roundingType == RoundingType.SCIENTIFIC) {
+							relativePrecision.setSelected(true);
+						}
+						relativePrecision.addActionListener(e -> this.presenter
+								.setRoundingType(RoundingType.SCIENTIFIC));
+						roundingRuleButtons.add(relativePrecision);
+						roundingPanel.add(relativePrecision, new GridBagBuilder(0, 3)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JLabel sliderLabel = new JLabel("Precision:");
+						roundingPanel.add(sliderLabel, new GridBagBuilder(0, 4)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JSlider sigDigSlider = new JSlider(0, 12);
+						roundingPanel.add(sigDigSlider, new GridBagBuilder(0, 5)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						sigDigSlider.setMajorTickSpacing(4);
+						sigDigSlider.setMinorTickSpacing(1);
+						sigDigSlider.setSnapToTicks(true);
+						sigDigSlider.setPaintTicks(true);
+						sigDigSlider.setPaintLabels(true);
+						sigDigSlider.setValue(this.presenter.precision);
+						
+						sigDigSlider.addChangeListener(e -> this.presenter
+								.setPrecision(sigDigSlider.getValue()));
+					}
+					
+					{ // prefix repetition settings
+						final JPanel prefixRepetitionPanel = new JPanel();
+						settingsPanel.add(prefixRepetitionPanel);
+						prefixRepetitionPanel.setBorder(
+								new TitledBorder("Prefix Repetition Settings"));
+						prefixRepetitionPanel.setLayout(new GridBagLayout());
+						
+						// prefix rules
+						final ButtonGroup prefixRuleButtons = new ButtonGroup();
+						
+						final JRadioButton noRepetition = new JRadioButton(
+								"No Repetition");
+						if (this.presenter.prefixRule == DefaultPrefixRepetitionRule.NO_REPETITION) {
+							noRepetition.setSelected(true);
+						}
+						noRepetition.addActionListener(
+								e -> this.presenter.setPrefixRepetitionRule(
+										DefaultPrefixRepetitionRule.NO_REPETITION));
+						prefixRuleButtons.add(noRepetition);
+						prefixRepetitionPanel.add(noRepetition,
+								new GridBagBuilder(0, 0)
+										.setAnchor(GridBagConstraints.LINE_START)
+										.build());
+						
+						final JRadioButton noRestriction = new JRadioButton(
+								"No Restriction");
+						if (this.presenter.prefixRule == DefaultPrefixRepetitionRule.NO_RESTRICTION) {
+							noRestriction.setSelected(true);
+						}
+						noRestriction.addActionListener(
+								e -> this.presenter.setPrefixRepetitionRule(
+										DefaultPrefixRepetitionRule.NO_RESTRICTION));
+						prefixRuleButtons.add(noRestriction);
+						prefixRepetitionPanel.add(noRestriction,
+								new GridBagBuilder(0, 1)
+										.setAnchor(GridBagConstraints.LINE_START)
+										.build());
+						
+						final JRadioButton customRepetition = new JRadioButton(
+								"Complex Repetition");
+						if (this.presenter.prefixRule == DefaultPrefixRepetitionRule.COMPLEX_REPETITION) {
+							customRepetition.setSelected(true);
+						}
+						customRepetition.addActionListener(
+								e -> this.presenter.setPrefixRepetitionRule(
+										DefaultPrefixRepetitionRule.COMPLEX_REPETITION));
+						prefixRuleButtons.add(customRepetition);
+						prefixRepetitionPanel.add(customRepetition,
+								new GridBagBuilder(0, 2)
+										.setAnchor(GridBagConstraints.LINE_START)
+										.build());
+					}
+					
+					{ // search settings
+						final JPanel searchingPanel = new JPanel();
+						settingsPanel.add(searchingPanel);
+						searchingPanel.setBorder(new TitledBorder("Search Settings"));
+						searchingPanel.setLayout(new GridBagLayout());
+						
+						// searching rules
+						final ButtonGroup searchRuleButtons = new ButtonGroup();
+						
+						final JRadioButton noPrefixes = new JRadioButton(
+								"Never Include Prefixed Units");
+						noPrefixes.setEnabled(false);
+						searchRuleButtons.add(noPrefixes);
+						searchingPanel.add(noPrefixes, new GridBagBuilder(0, 0)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JRadioButton fixedPrefixes = new JRadioButton(
+								"Include Some Prefixes");
+						fixedPrefixes.setEnabled(false);
+						searchRuleButtons.add(fixedPrefixes);
+						searchingPanel.add(fixedPrefixes, new GridBagBuilder(0, 1)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JRadioButton explicitPrefixes = new JRadioButton(
+								"Include Explicit Prefixes");
+						explicitPrefixes.setEnabled(false);
+						searchRuleButtons.add(explicitPrefixes);
+						searchingPanel.add(explicitPrefixes, new GridBagBuilder(0, 2)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JRadioButton alwaysInclude = new JRadioButton(
+								"Include All Single Prefixes");
+						alwaysInclude.setEnabled(false);
+						searchRuleButtons.add(alwaysInclude);
+						searchingPanel.add(alwaysInclude, new GridBagBuilder(0, 3)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+					}
+					
+					{ // miscellaneous settings
+						final JPanel miscPanel = new JPanel();
+						settingsPanel.add(miscPanel);
+						miscPanel
+								.setBorder(new TitledBorder("Miscellaneous Settings"));
+						miscPanel.setLayout(new GridBagLayout());
+						
+						final JCheckBox oneWay = new JCheckBox(
+								"Convert One Way Only");
+						oneWay.setSelected(this.presenter.oneWay);
+						oneWay.addItemListener(
+								e -> this.presenter.setOneWay(e.getStateChange() == 1));
+						miscPanel.add(oneWay, new GridBagBuilder(0, 0)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JCheckBox showAllVariations = new JCheckBox(
+								"Show Symbols in \"Convert Units\"");
+						showAllVariations.setSelected(true);
+						showAllVariations.setEnabled(false);
+						miscPanel.add(showAllVariations, new GridBagBuilder(0, 1)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JButton unitFileButton = new JButton(
+								"Manage Unit Data Files");
+						unitFileButton.setEnabled(false);
+						miscPanel.add(unitFileButton, new GridBagBuilder(0, 2)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+					}
+				}
 			}
 		}
-
+		
 		/**
 		 * Sets the text in the output of the dimension-based converter.
 		 * 
@@ -807,7 +1358,7 @@ final class UnitConverterGUI {
 		public void setDimensionConverterOutputText(final String text) {
 			this.dimensionBasedOutput.setText(text);
 		}
-
+		
 		/**
 		 * Sets the text in the output of the conversion panel.
 		 * 
@@ -818,7 +1369,7 @@ final class UnitConverterGUI {
 		public void setExpressionConverterOutputText(final String text) {
 			this.output.setText(text);
 		}
-
+		
 		/**
 		 * Sets the text of the prefix text box in the prefix viewer.
 		 * 
@@ -829,7 +1380,7 @@ final class UnitConverterGUI {
 		public void setPrefixTextBoxText(final String text) {
 			this.prefixTextBox.setText(text);
 		}
-
+		
 		/**
 		 * Sets the text of the unit text box in the unit viewer.
 		 * 
@@ -840,7 +1391,7 @@ final class UnitConverterGUI {
 		public void setUnitTextBoxText(final String text) {
 			this.unitTextBox.setText(text);
 		}
-
+		
 		/**
 		 * Shows an error dialog.
 		 * 
@@ -850,10 +1401,23 @@ final class UnitConverterGUI {
 		 * @since v0.1.0
 		 */
 		public void showErrorDialog(final String title, final String message) {
-			JOptionPane.showMessageDialog(this.frame, message, title, JOptionPane.ERROR_MESSAGE);
+			JOptionPane.showMessageDialog(this.frame, message, title,
+					JOptionPane.ERROR_MESSAGE);
+		}
+		
+		public void update() {
+			switch (this.getActivePane()) {
+			case UNIT_CONVERTER:
+				this.fromSearch.updateList();
+				this.toSearch.updateList();
+				break;
+			default:
+				// do nothing, for now
+				break;
+			}
 		}
 	}
-
+	
 	public static void main(final String[] args) {
 		new View().init();
 	}