/**
 * Copyright (C) 2018 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
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */
package org.unitConverter.unit;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.util.AbstractSet;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.Set;
import java.util.function.BiFunction;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import org.unitConverter.math.DecimalComparison;
import org.unitConverter.math.ExpressionParser;
import org.unitConverter.math.ObjectProduct;

/**
 * A database of units, prefixes and dimensions, and their names.
 * 
 * @author Adrien Hopkins
 * @since 2019-01-07
 * @since v0.1.0
 */
public final class UnitDatabase {
	/**
	 * A map for units that allows the use of prefixes.
	 * <p>
	 * As this map implementation is intended to be used as a sort of "augmented view" of a unit and prefix map, it is
	 * unmodifiable but instead reflects the changes to the maps passed into it. Do not edit this map, instead edit the
	 * maps that were passed in during construction.
	 * </p>
	 * <p>
	 * The rules for applying prefixes onto units are the following:
	 * <ul>
	 * <li>Prefixes can only be applied to linear units.</li>
	 * <li>Before attempting to search for prefixes in a unit name, this map will first search for a unit name. So, if
	 * there are two units, "B" and "AB", and a prefix "A", this map will favour the unit "AB" over the unit "B" with
	 * the prefix "A", even though they have the same string.</li>
	 * <li>Longer prefixes are preferred to shorter prefixes. So, if you have units "BC" and "C", and prefixes "AB" and
	 * "A", inputting "ABC" will return the unit "C" with the prefix "AB", not "BC" with the prefix "A".</li>
	 * </ul>
	 * </p>
	 * <p>
	 * This map is infinite in size if there is at least one unit and at least one prefix. If it is infinite, some
	 * operations that only work with finite collections, like converting name/entry sets to arrays, will throw an
	 * {@code IllegalStateException}.
	 * </p>
	 * <p>
	 * Because of ambiguities between prefixes (i.e. kilokilo = mega), {@link #containsValue} and {@link #values()}
	 * currently ignore prefixes.
	 * </p>
	 * 
	 * @author Adrien Hopkins
	 * @since 2019-04-13
	 * @since v0.2.0
	 */
	private static final class PrefixedUnitMap implements Map<String, Unit> {
		/**
		 * The class used for entry sets.
		 * 
		 * <p>
		 * If the map that created this set is infinite in size (has at least one unit and at least one prefix), this
		 * set is infinite as well. If this set is infinite in size, {@link #toArray} will fail with a
		 * {@code IllegalStateException} instead of creating an infinite-sized array.
		 * </p>
		 * 
		 * @author Adrien Hopkins
		 * @since 2019-04-13
		 * @since v0.2.0
		 */
		private static final class PrefixedUnitEntrySet extends AbstractSet<Map.Entry<String, Unit>> {
			/**
			 * The entry for this set.
			 * 
			 * @author Adrien Hopkins
			 * @since 2019-04-14
			 * @since v0.2.0
			 */
			private static final class PrefixedUnitEntry implements Entry<String, Unit> {
				private final String key;
				private final Unit value;

				/**
				 * Creates the {@code PrefixedUnitEntry}.
				 * 
				 * @param key
				 *            key
				 * @param value
				 *            value
				 * @since 2019-04-14
				 * @since v0.2.0
				 */
				public PrefixedUnitEntry(final String key, final Unit value) {
					this.key = key;
					this.value = value;
				}

				/**
				 * @since 2019-05-03
				 */
				@Override
				public boolean equals(final Object o) {
					if (!(o instanceof Map.Entry))
						return false;
					final Map.Entry<?, ?> other = (Map.Entry<?, ?>) o;
					return Objects.equals(this.getKey(), other.getKey())
							&& Objects.equals(this.getValue(), other.getValue());
				}

				@Override
				public String getKey() {
					return this.key;
				}

				@Override
				public Unit getValue() {
					return this.value;
				}

				/**
				 * @since 2019-05-03
				 */
				@Override
				public int hashCode() {
					return (this.getKey() == null ? 0 : this.getKey().hashCode())
							^ (this.getValue() == null ? 0 : this.getValue().hashCode());
				}

				@Override
				public Unit setValue(final Unit value) {
					throw new UnsupportedOperationException("Cannot set value in an immutable entry");
				}

				/**
				 * Returns a string representation of the entry. The format of the string is the string representation
				 * of the key, then the equals ({@code =}) character, then the string representation of the value.
				 * 
				 * @since 2019-05-03
				 */
				@Override
				public String toString() {
					return this.getKey() + "=" + this.getValue();
				}
			}

			/**
			 * An iterator that iterates over the units of a {@code PrefixedUnitNameSet}.
			 * 
			 * @author Adrien Hopkins
			 * @since 2019-04-14
			 * @since v0.2.0
			 */
			private static final class PrefixedUnitEntryIterator implements Iterator<Entry<String, Unit>> {
				// position in the unit list
				private int unitNamePosition = 0;
				// the indices of the prefixes attached to the current unit
				private final List<Integer> prefixCoordinates = new ArrayList<>();

				// values from the unit entry set
				private final Map<String, Unit> map;
				private transient final List<String> unitNames;
				private transient final List<String> prefixNames;

				/**
				 * Creates the {@code UnitsDatabase.PrefixedUnitMap.PrefixedUnitNameSet.PrefixedUnitNameIterator}.
				 * 
				 * @since 2019-04-14
				 * @since v0.2.0
				 */
				public PrefixedUnitEntryIterator(final PrefixedUnitMap map) {
					this.map = map;
					this.unitNames = new ArrayList<>(map.units.keySet());
					this.prefixNames = new ArrayList<>(map.prefixes.keySet());
				}

				/**
				 * @return current unit name
				 * @since 2019-04-14
				 * @since v0.2.0
				 */
				private String getCurrentUnitName() {
					final StringBuilder unitName = new StringBuilder();
					for (final int i : this.prefixCoordinates) {
						unitName.append(this.prefixNames.get(i));
					}
					unitName.append(this.unitNames.get(this.unitNamePosition));

					return unitName.toString();
				}

				@Override
				public boolean hasNext() {
					if (this.unitNames.isEmpty())
						return false;
					else {
						if (this.prefixNames.isEmpty())
							return this.unitNamePosition >= this.unitNames.size() - 1;
						else
							return true;
					}
				}

				/**
				 * Changes this iterator's position to the next available one.
				 * 
				 * @since 2019-04-14
				 * @since v0.2.0
				 */
				private void incrementPosition() {
					this.unitNamePosition++;

					if (this.unitNamePosition >= this.unitNames.size()) {
						// we have used all of our units, go to a different prefix
						this.unitNamePosition = 0;

						// if the prefix coordinates are empty, then set it to [0]
						if (this.prefixCoordinates.isEmpty()) {
							this.prefixCoordinates.add(0, 0);
						} else {
							// get the prefix coordinate to increment, then increment
							int i = this.prefixCoordinates.size() - 1;
							this.prefixCoordinates.set(i, this.prefixCoordinates.get(i) + 1);

							// fix any carrying errors
							while (i >= 0 && this.prefixCoordinates.get(i) >= this.prefixNames.size()) {
								// carry over
								this.prefixCoordinates.set(i--, 0); // null and decrement at the same time

								if (i < 0) { // we need to add a new coordinate
									this.prefixCoordinates.add(0, 0);
								} else { // increment an existing one
									this.prefixCoordinates.set(i, this.prefixCoordinates.get(i) + 1);
								}
							}
						}
					}
				}

				@Override
				public Entry<String, Unit> next() {
					// get next element
					final Entry<String, Unit> nextEntry = this.peek();

					// iterate to next position
					this.incrementPosition();

					return nextEntry;
				}

				/**
				 * @return the next element in the iterator, without iterating over it
				 * @since 2019-05-03
				 */
				private Entry<String, Unit> peek() {
					if (!this.hasNext())
						throw new NoSuchElementException("No units left!");

					// if I have prefixes, ensure I'm not using a nonlinear unit
					// since all of the unprefixed stuff is done, just remove nonlinear units
					if (!this.prefixCoordinates.isEmpty()) {
						while (this.unitNamePosition < this.unitNames.size()
								&& !(this.map.get(this.unitNames.get(this.unitNamePosition)) instanceof LinearUnit)) {
							this.unitNames.remove(this.unitNamePosition);
						}
					}

					final String nextName = this.getCurrentUnitName();

					return new PrefixedUnitEntry(nextName, this.map.get(nextName));
				}

				/**
				 * Returns a string representation of the object. The exact details of the representation are
				 * unspecified and subject to change.
				 * 
				 * @since 2019-05-03
				 */
				@Override
				public String toString() {
					return String.format("Iterator iterating over name-unit entries; next value is \"%s\"",
							this.peek());
				}
			}

			// the map that created this set
			private final PrefixedUnitMap map;

			/**
			 * Creates the {@code PrefixedUnitNameSet}.
			 * 
			 * @param map
			 *            map that created this set
			 * @since 2019-04-13
			 * @since v0.2.0
			 */
			public PrefixedUnitEntrySet(final PrefixedUnitMap map) {
				this.map = map;
			}

			@Override
			public boolean add(final Map.Entry<String, Unit> e) {
				throw new UnsupportedOperationException("Cannot add to an immutable set");
			}

			@Override
			public boolean addAll(final Collection<? extends Map.Entry<String, Unit>> c) {
				throw new UnsupportedOperationException("Cannot add to an immutable set");
			}

			@Override
			public void clear() {
				throw new UnsupportedOperationException("Cannot clear an immutable set");
			}

			@Override
			public boolean contains(final Object o) {
				// get the entry
				final Entry<String, Unit> entry;

				try {
					// This is OK because I'm in a try-catch block, catching the exact exception that would be thrown.
					@SuppressWarnings("unchecked")
					final Entry<String, Unit> tempEntry = (Entry<String, Unit>) o;
					entry = tempEntry;
				} catch (final ClassCastException e) {
					throw new IllegalArgumentException("Attempted to test for an entry using a non-entry.");
				}

				return this.map.containsKey(entry.getKey()) && this.map.get(entry.getKey()).equals(entry.getValue());
			}

			@Override
			public boolean containsAll(final Collection<?> c) {
				for (final Object o : c)
					if (!this.contains(o))
						return false;
				return true;
			}

			@Override
			public boolean isEmpty() {
				return this.map.isEmpty();
			}

			@Override
			public Iterator<Entry<String, Unit>> iterator() {
				return new PrefixedUnitEntryIterator(this.map);
			}

			@Override
			public boolean remove(final Object o) {
				throw new UnsupportedOperationException("Cannot remove from an immutable set");
			}

			@Override
			public boolean removeAll(final Collection<?> c) {
				throw new UnsupportedOperationException("Cannot remove from an immutable set");
			}

			@Override
			public boolean removeIf(final Predicate<? super Entry<String, Unit>> filter) {
				throw new UnsupportedOperationException("Cannot remove from an immutable set");
			}

			@Override
			public boolean retainAll(final Collection<?> c) {
				throw new UnsupportedOperationException("Cannot remove from an immutable set");
			}

			@Override
			public int size() {
				if (this.map.units.isEmpty())
					return 0;
				else {
					if (this.map.prefixes.isEmpty())
						return this.map.units.size();
					else
						// infinite set
						return Integer.MAX_VALUE;
				}
			}

			/**
			 * @throws IllegalStateException
			 *             if the set is infinite in size
			 */
			@Override
			public Object[] toArray() {
				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
					return super.toArray();
				else
					// infinite set
					throw new IllegalStateException("Cannot make an infinite set into an array.");
			}

			/**
			 * @throws IllegalStateException
			 *             if the set is infinite in size
			 */
			@Override
			public <T> T[] toArray(final T[] a) {
				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
					return super.toArray(a);
				else
					// infinite set
					throw new IllegalStateException("Cannot make an infinite set into an array.");
			}

			@Override
			public String toString() {
				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
					return super.toString();
				else
					return String.format("Infinite set of name-unit entries created from units %s and prefixes %s",
							this.map.units, this.map.prefixes);
			}
		}

		/**
		 * The class used for unit name sets.
		 * 
		 * <p>
		 * If the map that created this set is infinite in size (has at least one unit and at least one prefix), this
		 * set is infinite as well. If this set is infinite in size, {@link #toArray} will fail with a
		 * {@code IllegalStateException} instead of creating an infinite-sized array.
		 * </p>
		 * 
		 * @author Adrien Hopkins
		 * @since 2019-04-13
		 * @since v0.2.0
		 */
		private static final class PrefixedUnitNameSet extends AbstractSet<String> {
			/**
			 * An iterator that iterates over the units of a {@code PrefixedUnitNameSet}.
			 * 
			 * @author Adrien Hopkins
			 * @since 2019-04-14
			 * @since v0.2.0
			 */
			private static final class PrefixedUnitNameIterator implements Iterator<String> {
				// position in the unit list
				private int unitNamePosition = 0;
				// the indices of the prefixes attached to the current unit
				private final List<Integer> prefixCoordinates = new ArrayList<>();

				// values from the unit name set
				private final Map<String, Unit> map;
				private transient final List<String> unitNames;
				private transient final List<String> prefixNames;

				/**
				 * Creates the {@code UnitsDatabase.PrefixedUnitMap.PrefixedUnitNameSet.PrefixedUnitNameIterator}.
				 * 
				 * @since 2019-04-14
				 * @since v0.2.0
				 */
				public PrefixedUnitNameIterator(final PrefixedUnitMap map) {
					this.map = map;
					this.unitNames = new ArrayList<>(map.units.keySet());
					this.prefixNames = new ArrayList<>(map.prefixes.keySet());
				}

				/**
				 * @return current unit name
				 * @since 2019-04-14
				 * @since v0.2.0
				 */
				private String getCurrentUnitName() {
					final StringBuilder unitName = new StringBuilder();
					for (final int i : this.prefixCoordinates) {
						unitName.append(this.prefixNames.get(i));
					}
					unitName.append(this.unitNames.get(this.unitNamePosition));

					return unitName.toString();
				}

				@Override
				public boolean hasNext() {
					if (this.unitNames.isEmpty())
						return false;
					else {
						if (this.prefixNames.isEmpty())
							return this.unitNamePosition >= this.unitNames.size() - 1;
						else
							return true;
					}
				}

				/**
				 * Changes this iterator's position to the next available one.
				 * 
				 * @since 2019-04-14
				 * @since v0.2.0
				 */
				private void incrementPosition() {
					this.unitNamePosition++;

					if (this.unitNamePosition >= this.unitNames.size()) {
						// we have used all of our units, go to a different prefix
						this.unitNamePosition = 0;

						// if the prefix coordinates are empty, then set it to [0]
						if (this.prefixCoordinates.isEmpty()) {
							this.prefixCoordinates.add(0, 0);
						} else {
							// get the prefix coordinate to increment, then increment
							int i = this.prefixCoordinates.size() - 1;
							this.prefixCoordinates.set(i, this.prefixCoordinates.get(i) + 1);

							// fix any carrying errors
							while (i >= 0 && this.prefixCoordinates.get(i) >= this.prefixNames.size()) {
								// carry over
								this.prefixCoordinates.set(i--, 0); // null and decrement at the same time

								if (i < 0) { // we need to add a new coordinate
									this.prefixCoordinates.add(0, 0);
								} else { // increment an existing one
									this.prefixCoordinates.set(i, this.prefixCoordinates.get(i) + 1);
								}
							}
						}
					}
				}

				@Override
				public String next() {
					final String nextName = this.peek();

					this.incrementPosition();

					return nextName;
				}

				/**
				 * @return the next element in the iterator, without iterating over it
				 * @since 2019-05-03
				 */
				private String peek() {
					if (!this.hasNext())
						throw new NoSuchElementException("No units left!");
					// if I have prefixes, ensure I'm not using a nonlinear unit
					// since all of the unprefixed stuff is done, just remove nonlinear units
					if (!this.prefixCoordinates.isEmpty()) {
						while (this.unitNamePosition < this.unitNames.size()
								&& !(this.map.get(this.unitNames.get(this.unitNamePosition)) instanceof LinearUnit)) {
							this.unitNames.remove(this.unitNamePosition);
						}
					}

					return this.getCurrentUnitName();
				}

				/**
				 * Returns a string representation of the object. The exact details of the representation are
				 * unspecified and subject to change.
				 * 
				 * @since 2019-05-03
				 */
				@Override
				public String toString() {
					return String.format("Iterator iterating over unit names; next value is \"%s\"", this.peek());
				}
			}

			// the map that created this set
			private final PrefixedUnitMap map;

			/**
			 * Creates the {@code PrefixedUnitNameSet}.
			 * 
			 * @param map
			 *            map that created this set
			 * @since 2019-04-13
			 * @since v0.2.0
			 */
			public PrefixedUnitNameSet(final PrefixedUnitMap map) {
				this.map = map;
			}

			@Override
			public boolean add(final String e) {
				throw new UnsupportedOperationException("Cannot add to an immutable set");
			}

			@Override
			public boolean addAll(final Collection<? extends String> c) {
				throw new UnsupportedOperationException("Cannot add to an immutable set");
			}

			@Override
			public void clear() {
				throw new UnsupportedOperationException("Cannot clear an immutable set");
			}

			@Override
			public boolean contains(final Object o) {
				return this.map.containsKey(o);
			}

			@Override
			public boolean containsAll(final Collection<?> c) {
				for (final Object o : c)
					if (!this.contains(o))
						return false;
				return true;
			}

			@Override
			public boolean isEmpty() {
				return this.map.isEmpty();
			}

			@Override
			public Iterator<String> iterator() {
				return new PrefixedUnitNameIterator(this.map);
			}

			@Override
			public boolean remove(final Object o) {
				throw new UnsupportedOperationException("Cannot remove from an immutable set");
			}

			@Override
			public boolean removeAll(final Collection<?> c) {
				throw new UnsupportedOperationException("Cannot remove from an immutable set");
			}

			@Override
			public boolean removeIf(final Predicate<? super String> filter) {
				throw new UnsupportedOperationException("Cannot remove from an immutable set");
			}

			@Override
			public boolean retainAll(final Collection<?> c) {
				throw new UnsupportedOperationException("Cannot remove from an immutable set");
			}

			@Override
			public int size() {
				if (this.map.units.isEmpty())
					return 0;
				else {
					if (this.map.prefixes.isEmpty())
						return this.map.units.size();
					else
						// infinite set
						return Integer.MAX_VALUE;
				}
			}

			/**
			 * @throws IllegalStateException
			 *             if the set is infinite in size
			 */
			@Override
			public Object[] toArray() {
				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
					return super.toArray();
				else
					// infinite set
					throw new IllegalStateException("Cannot make an infinite set into an array.");

			}

			/**
			 * @throws IllegalStateException
			 *             if the set is infinite in size
			 */
			@Override
			public <T> T[] toArray(final T[] a) {
				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
					return super.toArray(a);
				else
					// infinite set
					throw new IllegalStateException("Cannot make an infinite set into an array.");
			}

			@Override
			public String toString() {
				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
					return super.toString();
				else
					return String.format("Infinite set of name-unit entries created from units %s and prefixes %s",
							this.map.units, this.map.prefixes);
			}
		}

		/**
		 * The units stored in this collection, without prefixes.
		 * 
		 * @since 2019-04-13
		 * @since v0.2.0
		 */
		private final Map<String, Unit> units;

		/**
		 * The available prefixes for use.
		 * 
		 * @since 2019-04-13
		 * @since v0.2.0
		 */
		private final Map<String, UnitPrefix> prefixes;

		// caches
		private transient Collection<Unit> values = null;
		private transient Set<String> keySet = null;
		private transient Set<Entry<String, Unit>> entrySet = null;

		/**
		 * Creates the {@code PrefixedUnitMap}.
		 * 
		 * @param units
		 *            map mapping unit names to units
		 * @param prefixes
		 *            map mapping prefix names to prefixes
		 * @since 2019-04-13
		 * @since v0.2.0
		 */
		public PrefixedUnitMap(final Map<String, Unit> units, final Map<String, UnitPrefix> prefixes) {
			// I am making unmodifiable maps to ensure I don't accidentally make changes.
			this.units = Collections.unmodifiableMap(units);
			this.prefixes = Collections.unmodifiableMap(prefixes);
		}

		@Override
		public void clear() {
			throw new UnsupportedOperationException("Cannot clear an immutable map");
		}

		@Override
		public Unit compute(final String key,
				final BiFunction<? super String, ? super Unit, ? extends Unit> remappingFunction) {
			throw new UnsupportedOperationException("Cannot edit an immutable map");
		}

		@Override
		public Unit computeIfAbsent(final String key, final Function<? super String, ? extends Unit> mappingFunction) {
			throw new UnsupportedOperationException("Cannot edit an immutable map");
		}

		@Override
		public Unit computeIfPresent(final String key,
				final BiFunction<? super String, ? super Unit, ? extends Unit> remappingFunction) {
			throw new UnsupportedOperationException("Cannot edit an immutable map");
		}

		@Override
		public boolean containsKey(final Object key) {
			// First, test if there is a unit with the key
			if (this.units.containsKey(key))
				return true;

			// Next, try to cast it to String
			if (!(key instanceof String))
				throw new IllegalArgumentException("Attempted to test for a unit using a non-string name.");
			final String unitName = (String) key;

			// Then, look for the longest prefix that is attached to a valid unit
			String longestPrefix = null;
			int longestLength = 0;

			for (final String prefixName : this.prefixes.keySet()) {
				// a prefix name is valid if:
				// - it is prefixed (i.e. the unit name starts with it)
				// - it is longer than the existing largest prefix (since I am looking for the longest valid prefix)
				// - the part after the prefix is a valid unit name
				// - the unit described that name is a linear unit (since only linear units can have prefixes)
				if (unitName.startsWith(prefixName) && prefixName.length() > longestLength) {
					final String rest = unitName.substring(prefixName.length());
					if (this.containsKey(rest) && this.get(rest) instanceof LinearUnit) {
						longestPrefix = prefixName;
						longestLength = prefixName.length();
					}
				}
			}

			return longestPrefix != null;
		}

		/**
		 * {@inheritDoc}
		 * 
		 * <p>
		 * Because of ambiguities between prefixes (i.e. kilokilo = mega), this method only tests for prefixless units.
		 * </p>
		 */
		@Override
		public boolean containsValue(final Object value) {
			return this.units.containsValue(value);
		}

		@Override
		public Set<Entry<String, Unit>> entrySet() {
			if (this.entrySet == null) {
				this.entrySet = new PrefixedUnitEntrySet(this);
			}
			return this.entrySet;
		}

		@Override
		public Unit get(final Object key) {
			// First, test if there is a unit with the key
			if (this.units.containsKey(key))
				return this.units.get(key);

			// Next, try to cast it to String
			if (!(key instanceof String))
				throw new IllegalArgumentException("Attempted to obtain a unit using a non-string name.");
			final String unitName = (String) key;

			// Then, look for the longest prefix that is attached to a valid unit
			String longestPrefix = null;
			int longestLength = 0;

			for (final String prefixName : this.prefixes.keySet()) {
				// a prefix name is valid if:
				// - it is prefixed (i.e. the unit name starts with it)
				// - it is longer than the existing largest prefix (since I am looking for the longest valid prefix)
				// - the part after the prefix is a valid unit name
				// - the unit described that name is a linear unit (since only linear units can have prefixes)
				if (unitName.startsWith(prefixName) && prefixName.length() > longestLength) {
					final String rest = unitName.substring(prefixName.length());
					if (this.containsKey(rest) && this.get(rest) instanceof LinearUnit) {
						longestPrefix = prefixName;
						longestLength = prefixName.length();
					}
				}
			}

			// if none found, returns null
			if (longestPrefix == null)
				return null;
			else {
				// get necessary data
				final String rest = unitName.substring(longestLength);
				// this cast will not fail because I verified that it would work before selecting this prefix
				final LinearUnit unit = (LinearUnit) this.get(rest);
				final UnitPrefix prefix = this.prefixes.get(longestPrefix);

				return unit.withPrefix(prefix);
			}
		}

		@Override
		public boolean isEmpty() {
			return this.units.isEmpty();
		}

		@Override
		public Set<String> keySet() {
			if (this.keySet == null) {
				this.keySet = new PrefixedUnitNameSet(this);
			}
			return this.keySet;
		}

		@Override
		public Unit merge(final String key, final Unit value,
				final BiFunction<? super Unit, ? super Unit, ? extends Unit> remappingFunction) {
			throw new UnsupportedOperationException("Cannot merge into an immutable map");
		}

		@Override
		public Unit put(final String key, final Unit value) {
			throw new UnsupportedOperationException("Cannot add entries to an immutable map");
		}

		@Override
		public void putAll(final Map<? extends String, ? extends Unit> m) {
			throw new UnsupportedOperationException("Cannot add entries to an immutable map");
		}

		@Override
		public Unit putIfAbsent(final String key, final Unit value) {
			throw new UnsupportedOperationException("Cannot add entries to an immutable map");
		}

		@Override
		public Unit remove(final Object key) {
			throw new UnsupportedOperationException("Cannot remove entries from an immutable map");
		}

		@Override
		public boolean remove(final Object key, final Object value) {
			throw new UnsupportedOperationException("Cannot remove entries from an immutable map");
		}

		@Override
		public Unit replace(final String key, final Unit value) {
			throw new UnsupportedOperationException("Cannot replace entries in an immutable map");
		}

		@Override
		public boolean replace(final String key, final Unit oldValue, final Unit newValue) {
			throw new UnsupportedOperationException("Cannot replace entries in an immutable map");
		}

		@Override
		public void replaceAll(final BiFunction<? super String, ? super Unit, ? extends Unit> function) {
			throw new UnsupportedOperationException("Cannot replace entries in an immutable map");
		}

		@Override
		public int size() {
			if (this.units.isEmpty())
				return 0;
			else {
				if (this.prefixes.isEmpty())
					return this.units.size();
				else
					// infinite set
					return Integer.MAX_VALUE;
			}
		}

		@Override
		public String toString() {
			if (this.units.isEmpty() || this.prefixes.isEmpty())
				return super.toString();
			else
				return String.format("Infinite map of name-unit entries created from units %s and prefixes %s",
						this.units, this.prefixes);
		}

		/**
		 * {@inheritDoc}
		 * 
		 * <p>
		 * Because of ambiguities between prefixes (i.e. kilokilo = mega), this method ignores prefixes.
		 * </p>
		 */
		@Override
		public Collection<Unit> values() {
			if (this.values == null) {
				this.values = Collections.unmodifiableCollection(this.units.values());
			}
			return this.values;
		}
	}

	/**
	 * A regular expression that separates names and expressions in unit files.
	 */
	private static final Pattern NAME_EXPRESSION = Pattern.compile("(\\S+)\\s+(\\S.*)");

	/**
	 * The exponent operator
	 * 
	 * @param base
	 *            base of exponentiation
	 * @param exponentUnit
	 *            exponent
	 * @return result
	 * @since 2019-04-10
	 * @since v0.2.0
	 */
	private static final LinearUnit exponentiateUnits(final LinearUnit base, final LinearUnit exponentUnit) {
		// exponent function - first check if o2 is a number,
		if (exponentUnit.getBase().equals(SI.ONE.getBase())) {
			// then check if it is an integer,
			final double exponent = exponentUnit.getConversionFactor();
			if (DecimalComparison.equals(exponent % 1, 0))
				// then exponentiate
				return base.toExponent((int) (exponent + 0.5));
			else
				// not an integer
				throw new UnsupportedOperationException("Decimal exponents are currently not supported.");
		} else
			// not a number
			throw new IllegalArgumentException("Exponents must be numbers.");
	}

	/**
	 * The units in this system, excluding prefixes.
	 * 
	 * @since 2019-01-07
	 * @since v0.1.0
	 */
	private final Map<String, Unit> prefixlessUnits;

	/**
	 * The unit prefixes in this system.
	 * 
	 * @since 2019-01-14
	 * @since v0.1.0
	 */
	private final Map<String, UnitPrefix> prefixes;

	/**
	 * The dimensions in this system.
	 * 
	 * @since 2019-03-14
	 * @since v0.2.0
	 */
	private final Map<String, ObjectProduct<BaseDimension>> dimensions;

	/**
	 * A map mapping strings to units (including prefixes)
	 * 
	 * @since 2019-04-13
	 * @since v0.2.0
	 */
	private final Map<String, Unit> units;

	/**
	 * A parser that can parse unit expressions.
	 * 
	 * @since 2019-03-22
	 * @since v0.2.0
	 */
	private final ExpressionParser<LinearUnit> unitExpressionParser = new ExpressionParser.Builder<>(
			this::getLinearUnit).addBinaryOperator("+", (o1, o2) -> o1.plus(o2), 0)
					.addBinaryOperator("-", (o1, o2) -> o1.minus(o2), 0)
					.addBinaryOperator("*", (o1, o2) -> o1.times(o2), 1).addSpaceFunction("*")
					.addBinaryOperator("/", (o1, o2) -> o1.dividedBy(o2), 1)
					.addBinaryOperator("^", UnitDatabase::exponentiateUnits, 2).build();

	/**
	 * A parser that can parse unit prefix expressions
	 * 
	 * @since 2019-04-13
	 * @since v0.2.0
	 */
	private final ExpressionParser<UnitPrefix> prefixExpressionParser = new ExpressionParser.Builder<>(this::getPrefix)
			.addBinaryOperator("*", (o1, o2) -> o1.times(o2), 0).addSpaceFunction("*")
			.addBinaryOperator("/", (o1, o2) -> o1.dividedBy(o2), 0)
			.addBinaryOperator("^", (o1, o2) -> o1.toExponent(o2.getMultiplier()), 1).build();

	/**
	 * A parser that can parse unit dimension expressions.
	 * 
	 * @since 2019-04-13
	 * @since v0.2.0
	 */
	private final ExpressionParser<ObjectProduct<BaseDimension>> unitDimensionParser = new ExpressionParser.Builder<>(
			this::getDimension).addBinaryOperator("*", (o1, o2) -> o1.times(o2), 0).addSpaceFunction("*")
					.addBinaryOperator("/", (o1, o2) -> o1.dividedBy(o2), 0).build();

	/**
	 * Creates the {@code UnitsDatabase}.
	 * 
	 * @since 2019-01-10
	 * @since v0.1.0
	 */
	public UnitDatabase() {
		this.prefixlessUnits = new HashMap<>();
		this.prefixes = new HashMap<>();
		this.dimensions = new HashMap<>();
		this.units = new PrefixedUnitMap(this.prefixlessUnits, this.prefixes);
	}

	/**
	 * Adds a unit dimension to the database.
	 * 
	 * @param name
	 *            dimension's name
	 * @param dimension
	 *            dimension to add
	 * @throws NullPointerException
	 *             if name or dimension is null
	 * @since 2019-03-14
	 * @since v0.2.0
	 */
	public void addDimension(final String name, final ObjectProduct<BaseDimension> dimension) {
		this.dimensions.put(Objects.requireNonNull(name, "name must not be null."),
				Objects.requireNonNull(dimension, "dimension must not be null."));
	}

	/**
	 * Adds to the list from a line in a unit dimension file.
	 * 
	 * @param line
	 *            line to look at
	 * @param lineCounter
	 *            number of line, for error messages
	 * @since 2019-04-10
	 * @since v0.2.0
	 */
	private void addDimensionFromLine(final String line, final long lineCounter) {
		// ignore lines that start with a # sign - they're comments
		if (line.isEmpty())
			return;
		if (line.contains("#")) {
			this.addDimensionFromLine(line.substring(0, line.indexOf("#")), lineCounter);
			return;
		}

		// divide line into name and expression
		final Matcher lineMatcher = NAME_EXPRESSION.matcher(line);
		if (!lineMatcher.matches())
			throw new IllegalArgumentException(String.format(
					"Error at line %d: Lines of a dimension file must consist of a dimension name, then spaces or tabs, then a dimension expression.",
					lineCounter));
		final String name = lineMatcher.group(1);
		final String expression = lineMatcher.group(2);

		if (name.endsWith(" ")) {
			System.err.printf("Warning - line %d's dimension name ends in a space", lineCounter);
		}

		// if expression is "!", search for an existing dimension
		// if no unit found, throw an error
		if (expression.equals("!")) {
			if (!this.containsDimensionName(name))
				throw new IllegalArgumentException(
						String.format("! used but no dimension found (line %d).", lineCounter));
		} else {
			// it's a unit, get the unit
			final ObjectProduct<BaseDimension> dimension;
			try {
				dimension = this.getDimensionFromExpression(expression);
			} catch (final IllegalArgumentException e) {
				System.err.printf("Parsing error on line %d:%n", lineCounter);
				throw e;
			}

			this.addDimension(name, dimension);
		}
	}

	/**
	 * Adds a unit prefix to the database.
	 * 
	 * @param name
	 *            prefix's name
	 * @param prefix
	 *            prefix to add
	 * @throws NullPointerException
	 *             if name or prefix is null
	 * @since 2019-01-14
	 * @since v0.1.0
	 */
	public void addPrefix(final String name, final UnitPrefix prefix) {
		this.prefixes.put(Objects.requireNonNull(name, "name must not be null."),
				Objects.requireNonNull(prefix, "prefix must not be null."));
	}

	/**
	 * Adds a unit to the database.
	 * 
	 * @param name
	 *            unit's name
	 * @param unit
	 *            unit to add
	 * @throws NullPointerException
	 *             if unit is null
	 * @since 2019-01-10
	 * @since v0.1.0
	 */
	public void addUnit(final String name, final Unit unit) {
		this.prefixlessUnits.put(Objects.requireNonNull(name, "name must not be null."),
				Objects.requireNonNull(unit, "unit must not be null."));
	}

	/**
	 * Adds to the list from a line in a unit file.
	 * 
	 * @param line
	 *            line to look at
	 * @param lineCounter
	 *            number of line, for error messages
	 * @since 2019-04-10
	 * @since v0.2.0
	 */
	private void addUnitOrPrefixFromLine(final String line, final long lineCounter) {
		// ignore lines that start with a # sign - they're comments
		if (line.isEmpty())
			return;
		if (line.contains("#")) {
			this.addUnitOrPrefixFromLine(line.substring(0, line.indexOf("#")), lineCounter);
			return;
		}

		// divide line into name and expression
		final Matcher lineMatcher = NAME_EXPRESSION.matcher(line);
		if (!lineMatcher.matches())
			throw new IllegalArgumentException(String.format(
					"Error at line %d: Lines of a unit file must consist of a unit name, then spaces or tabs, then a unit expression.",
					lineCounter));
		final String name = lineMatcher.group(1);
		final String expression = lineMatcher.group(2);

		if (name.endsWith(" ")) {
			System.err.printf("Warning - line %d's unit name ends in a space", lineCounter);
		}

		// if expression is "!", search for an existing unit
		// if no unit found, throw an error
		if (expression.equals("!")) {
			if (!this.containsUnitName(name))
				throw new IllegalArgumentException(String.format("! used but no unit found (line %d).", lineCounter));
		} else {
			if (name.endsWith("-")) {
				final UnitPrefix prefix;
				try {
					prefix = this.getPrefixFromExpression(expression);
				} catch (final IllegalArgumentException e) {
					System.err.printf("Parsing error on line %d:%n", lineCounter);
					throw e;
				}
				this.addPrefix(name.substring(0, name.length() - 1), prefix);
			} else {
				// it's a unit, get the unit
				final Unit unit;
				try {
					unit = this.getUnitFromExpression(expression);
				} catch (final IllegalArgumentException e) {
					System.err.printf("Parsing error on line %d:%n", lineCounter);
					throw e;
				}

				this.addUnit(name, unit);
			}
		}
	}

	/**
	 * Tests if the database has a unit dimension with this name.
	 * 
	 * @param name
	 *            name to test
	 * @return if database contains name
	 * @since 2019-03-14
	 * @since v0.2.0
	 */
	public boolean containsDimensionName(final String name) {
		return this.dimensions.containsKey(name);
	}

	/**
	 * Tests if the database has a unit prefix with this name.
	 * 
	 * @param name
	 *            name to test
	 * @return if database contains name
	 * @since 2019-01-13
	 * @since v0.1.0
	 */
	public boolean containsPrefixName(final String name) {
		return this.prefixes.containsKey(name);
	}

	/**
	 * Tests if the database has a unit with this name, taking prefixes into consideration
	 * 
	 * @param name
	 *            name to test
	 * @return if database contains name
	 * @since 2019-01-13
	 * @since v0.1.0
	 */
	public boolean containsUnitName(final String name) {
		return this.units.containsKey(name);
	}

	/**
	 * @return a map mapping dimension names to dimensions
	 * @since 2019-04-13
	 * @since v0.2.0
	 */
	public Map<String, ObjectProduct<BaseDimension>> dimensionMap() {
		return Collections.unmodifiableMap(this.dimensions);
	}

	/**
	 * Gets a unit dimension from the database using its name.
	 * 
	 * <p>
	 * This method accepts exponents, like "L^3"
	 * </p>
	 * 
	 * @param name
	 *            dimension's name
	 * @return dimension
	 * @since 2019-03-14
	 * @since v0.2.0
	 */
	public ObjectProduct<BaseDimension> getDimension(final String name) {
		Objects.requireNonNull(name, "name must not be null.");
		if (name.contains("^")) {
			final String[] baseAndExponent = name.split("\\^");

			final ObjectProduct<BaseDimension> base = this.getDimension(baseAndExponent[0]);

			final int exponent;
			try {
				exponent = Integer.parseInt(baseAndExponent[baseAndExponent.length - 1]);
			} catch (final NumberFormatException e2) {
				throw new IllegalArgumentException("Exponent must be an integer.");
			}

			return base.toExponent(exponent);
		}
		return this.dimensions.get(name);
	}

	/**
	 * Uses the database's data to parse an expression into a unit dimension
	 * <p>
	 * The expression is a series of any of the following:
	 * <ul>
	 * <li>The name of a unit dimension, which multiplies or divides the result based on preceding operators</li>
	 * <li>The operators '*' and '/', which multiply and divide (note that just putting two unit dimensions next to each
	 * other is equivalent to multiplication)</li>
	 * <li>The operator '^' which exponentiates. Exponents must be integers.</li>
	 * </ul>
	 * 
	 * @param expression
	 *            expression to parse
	 * @throws IllegalArgumentException
	 *             if the expression cannot be parsed
	 * @throws NullPointerException
	 *             if expression is null
	 * @since 2019-04-13
	 * @since v0.2.0
	 */
	public ObjectProduct<BaseDimension> getDimensionFromExpression(final String expression) {
		Objects.requireNonNull(expression, "expression must not be null.");

		// attempt to get a dimension as an alias first
		if (this.containsDimensionName(expression))
			return this.getDimension(expression);

		// force operators to have spaces
		String modifiedExpression = expression;
		modifiedExpression = modifiedExpression.replaceAll("\\*", " \\* ");
		modifiedExpression = modifiedExpression.replaceAll("/", " / ");
		modifiedExpression = modifiedExpression.replaceAll(" *\\^ *", "\\^");

		// fix broken spaces
		modifiedExpression = modifiedExpression.replaceAll(" +", " ");

		return this.unitDimensionParser.parseExpression(modifiedExpression);
	}

	/**
	 * Gets a unit. If it is linear, cast it to a LinearUnit and return it. Otherwise, throw an
	 * {@code IllegalArgumentException}.
	 * 
	 * @param name
	 *            unit's name
	 * @return unit
	 * @since 2019-03-22
	 * @since v0.2.0
	 */
	private LinearUnit getLinearUnit(final String name) {
		// see if I am using a function-unit like tempC(100)
		if (name.contains("(") && name.contains(")")) {
			// break it into function name and value
			final List<String> parts = Arrays.asList(name.split("\\("));
			if (parts.size() != 2)
				throw new IllegalArgumentException("Format nonlinear units like: unit(value).");

			// solve the function
			final Unit unit = this.getUnit(parts.get(0));
			final double value = Double.parseDouble(parts.get(1).substring(0, parts.get(1).length() - 1));
			return LinearUnit.fromUnitValue(unit, value);
		} else {
			// get a linear unit
			final Unit unit = this.getUnit(name);
			if (unit instanceof LinearUnit)
				return (LinearUnit) unit;
			else
				throw new IllegalArgumentException(String.format("%s is not a linear unit.", name));
		}
	}

	/**
	 * Gets a unit prefix from the database from its name
	 * 
	 * @param name
	 *            prefix's name
	 * @return prefix
	 * @since 2019-01-10
	 * @since v0.1.0
	 */
	public UnitPrefix getPrefix(final String name) {
		try {
			return UnitPrefix.valueOf(Double.parseDouble(name));
		} catch (final NumberFormatException e) {
			return this.prefixes.get(name);
		}
	}

	/**
	 * Gets a unit prefix from a prefix expression
	 * <p>
	 * Currently, prefix expressions are much simpler than unit expressions: They are either a number or the name of
	 * another prefix
	 * </p>
	 * 
	 * @param expression
	 *            expression to input
	 * @return prefix
	 * @throws IllegalArgumentException
	 *             if expression cannot be parsed
	 * @throws NullPointerException
	 *             if any argument is null
	 * @since 2019-01-14
	 * @since v0.1.0
	 */
	public UnitPrefix getPrefixFromExpression(final String expression) {
		Objects.requireNonNull(expression, "expression must not be null.");

		// attempt to get a unit as an alias first
		if (this.containsUnitName(expression))
			return this.getPrefix(expression);

		// force operators to have spaces
		String modifiedExpression = expression;
		modifiedExpression = modifiedExpression.replaceAll("\\*", " \\* ");
		modifiedExpression = modifiedExpression.replaceAll("/", " / ");
		modifiedExpression = modifiedExpression.replaceAll("\\^", " \\^ ");

		// fix broken spaces
		modifiedExpression = modifiedExpression.replaceAll(" +", " ");

		return this.prefixExpressionParser.parseExpression(modifiedExpression);
	}

	/**
	 * Gets a unit from the database from its name, looking for prefixes.
	 * 
	 * @param name
	 *            unit's name
	 * @return unit
	 * @since 2019-01-10
	 * @since v0.1.0
	 */
	public Unit getUnit(final String name) {
		try {
			final double value = Double.parseDouble(name);
			return SI.ONE.times(value);
		} catch (final NumberFormatException e) {
			return this.units.get(name);
		}

	}

	/**
	 * Uses the database's unit data to parse an expression into a unit
	 * <p>
	 * The expression is a series of any of the following:
	 * <ul>
	 * <li>The name of a unit, which multiplies or divides the result based on preceding operators</li>
	 * <li>The operators '*' and '/', which multiply and divide (note that just putting two units or values next to each
	 * other is equivalent to multiplication)</li>
	 * <li>The operator '^' which exponentiates. Exponents must be integers.</li>
	 * <li>A number which is multiplied or divided</li>
	 * </ul>
	 * This method only works with linear units.
	 * 
	 * @param expression
	 *            expression to parse
	 * @throws IllegalArgumentException
	 *             if the expression cannot be parsed
	 * @throws NullPointerException
	 *             if expression is null
	 * @since 2019-01-07
	 * @since v0.1.0
	 */
	public Unit getUnitFromExpression(final String expression) {
		Objects.requireNonNull(expression, "expression must not be null.");

		// attempt to get a unit as an alias first
		if (this.containsUnitName(expression))
			return this.getUnit(expression);

		// force operators to have spaces
		String modifiedExpression = expression;
		modifiedExpression = modifiedExpression.replaceAll("\\+", " \\+ ");
		modifiedExpression = modifiedExpression.replaceAll("-", " - ");
		modifiedExpression = modifiedExpression.replaceAll("\\*", " \\* ");
		modifiedExpression = modifiedExpression.replaceAll("/", " / ");
		modifiedExpression = modifiedExpression.replaceAll("\\^", " \\^ ");

		// fix broken spaces
		modifiedExpression = modifiedExpression.replaceAll(" +", " ");

		// the previous operation breaks negative numbers, fix them!
		// (i.e. -2 becomes - 2)
		for (int i = 2; i < modifiedExpression.length(); i++) {
			if (modifiedExpression.charAt(i) == '-'
					&& Arrays.asList('+', '-', '*', '/', '^').contains(modifiedExpression.charAt(i - 2))) {
				// found a broken negative number
				modifiedExpression = modifiedExpression.substring(0, i + 1) + modifiedExpression.substring(i + 2);
			}
		}

		return this.unitExpressionParser.parseExpression(modifiedExpression);
	}

	/**
	 * Adds all dimensions from a file, using data from the database to parse them.
	 * <p>
	 * Each line in the file should consist of a name and an expression (parsed by getDimensionFromExpression) separated
	 * by any number of tab characters.
	 * <p>
	 * <p>
	 * Allowed exceptions:
	 * <ul>
	 * <li>Anything after a '#' character is considered a comment and ignored.</li>
	 * <li>Blank lines are also ignored</li>
	 * <li>If an expression consists of a single exclamation point, instead of parsing it, this method will search the
	 * database for an existing unit. If no unit is found, an IllegalArgumentException is thrown. This is used to define
	 * initial units and ensure that the database contains them.</li>
	 * </ul>
	 * 
	 * @param file
	 *            file to read
	 * @throws IllegalArgumentException
	 *             if the file cannot be parsed, found or read
	 * @throws NullPointerException
	 *             if file is null
	 * @since 2019-01-13
	 * @since v0.1.0
	 */
	public void loadDimensionFile(final File file) {
		Objects.requireNonNull(file, "file must not be null.");
		try (FileReader fileReader = new FileReader(file); BufferedReader reader = new BufferedReader(fileReader)) {
			// while the reader has lines to read, read a line, then parse it, then add it
			long lineCounter = 0;
			while (reader.ready()) {
				this.addDimensionFromLine(reader.readLine(), ++lineCounter);
			}
		} catch (final FileNotFoundException e) {
			throw new IllegalArgumentException("Could not find file " + file, e);
		} catch (final IOException e) {
			throw new IllegalArgumentException("Could not read file " + file, e);
		}
	}

	/**
	 * Adds all units from a file, using data from the database to parse them.
	 * <p>
	 * Each line in the file should consist of a name and an expression (parsed by getUnitFromExpression) separated by
	 * any number of tab characters.
	 * <p>
	 * <p>
	 * Allowed exceptions:
	 * <ul>
	 * <li>Anything after a '#' character is considered a comment and ignored.</li>
	 * <li>Blank lines are also ignored</li>
	 * <li>If an expression consists of a single exclamation point, instead of parsing it, this method will search the
	 * database for an existing unit. If no unit is found, an IllegalArgumentException is thrown. This is used to define
	 * initial units and ensure that the database contains them.</li>
	 * </ul>
	 * 
	 * @param file
	 *            file to read
	 * @throws IllegalArgumentException
	 *             if the file cannot be parsed, found or read
	 * @throws NullPointerException
	 *             if file is null
	 * @since 2019-01-13
	 * @since v0.1.0
	 */
	public void loadUnitsFile(final File file) {
		Objects.requireNonNull(file, "file must not be null.");
		try (FileReader fileReader = new FileReader(file); BufferedReader reader = new BufferedReader(fileReader)) {
			// while the reader has lines to read, read a line, then parse it, then add it
			long lineCounter = 0;
			while (reader.ready()) {
				this.addUnitOrPrefixFromLine(reader.readLine(), ++lineCounter);
			}
		} catch (final FileNotFoundException e) {
			throw new IllegalArgumentException("Could not find file " + file, e);
		} catch (final IOException e) {
			throw new IllegalArgumentException("Could not read file " + file, e);
		}
	}

	/**
	 * @return a map mapping prefix names to prefixes
	 * @since 2019-04-13
	 * @since v0.2.0
	 */
	public Map<String, UnitPrefix> prefixMap() {
		return Collections.unmodifiableMap(this.prefixes);
	}

	@Override
	public String toString() {
		return String.format("Unit Database with %d units and %d unit prefixes", this.prefixlessUnits.size(),
				this.prefixes.size());
	}

	/**
	 * Returns a map mapping unit names to units, including units with prefixes.
	 * <p>
	 * The returned map is infinite in size if there is at least one unit and at least one prefix. If it is infinite,
	 * some operations that only work with finite collections, like converting name/entry sets to arrays, will throw an
	 * {@code IllegalStateException}.
	 * </p>
	 * <p>
	 * Specifically, the operations that will throw an IllegalStateException if the map is infinite in size are:
	 * <ul>
	 * <li>{@code unitMap.entrySet().toArray()} (either overloading)</li>
	 * <li>{@code unitMap.keySet().toArray()} (either overloading)</li>
	 * </ul>
	 * </p>
	 * <p>
	 * Because of ambiguities between prefixes (i.e. kilokilo = mega), the map's {@link PrefixedUnitMap#containsValue
	 * containsValue} and {@link PrefixedUnitMap#values() values()} methods currently ignore prefixes.
	 * </p>
	 * 
	 * @return a map mapping unit names to units, including prefixed names
	 * @since 2019-04-13
	 * @since v0.2.0
	 */
	public Map<String, Unit> unitMap() {
		return this.units; // PrefixedUnitMap is immutable so I don't need to make an unmodifiable map.
	}

	/**
	 * @return a map mapping unit names to units, ignoring prefixes
	 * @since 2019-04-13
	 * @since v0.2.0
	 */
	public Map<String, Unit> unitMapPrefixless() {
		return Collections.unmodifiableMap(this.prefixlessUnits);
	}
}