factors: Remove most panics

Panics are not the best way of handling errors in Go.  I've replaced
panics with default values whenever a sensible one exists.
Factors(0) does not have a sensible default value (as every number is a
factor of zero), so it still panics.

1 files changed, 8 insertions, 4 deletions

diff --git a/factors/score.go b/factors/score.go
index b47aac4..9288e8f 100644
--- a/factors/score.go
+++ b/factors/score.go
@@ -1,5 +1,7 @@
 package factors
 
+import "math"
+
 // Score returns a "factor score" equal to the sum of the reciprocoals
 // of the number n's factors.
 // Rationale:
@@ -17,7 +19,7 @@ package factors
 // a factor's score is the probability a random number is divisible by it.
 func Score(n uint) float64 {
 	if n == 0 {
-		panic("Cannot get factor score of 0.")
+		return math.NaN()
 	}
 
 	factorSum := uint(0)
@@ -27,9 +29,11 @@ func Score(n uint) float64 {
 	return float64(factorSum) / float64(n)
 }
 
-// BasicRank returns a rank describing how well a base handles the simplest
-// fractions (1/2, 1/3, 1/4 and 1/5)
-// also known as 2345 Rank
+// BasicRank returns a rank describing how well a radix handles the simplest
+// fractions (1/2, 1/3, 1/4 and 1/5).  Zero and one are not true radices,
+// but because this rank otherwise only depends on a radix's remainder
+// mod 60, they have the same ranks as 60 and 61 (A+, F~).
+// Also known as 2345 Rank.
 func BasicRank(n uint) string {
 	var firstRank, secondRank string
 	if n%2 == 0 {