Alter similarity calculation

Use a slightly more sophisticated method to determine similarity than
just trying to find duplicated lines, which falls apart fairly quickly.
Instead add value to the histogram while scanning the first file, and
subtract while scanning the second. After this, any entries with a
vvalue of 0 indicate matching lines. The magnitudes of anything elsefrom
zero are summed and used to calculate a similarity fraction.

main.go

@@ -285,23 +285,20 @@ func (dm *DocumentManager) compareFiles(f1Number, f2Number int) (float64, error)
 	}
 
 	histogram := make(map[string]int)
-	for _, lines := range [][]string{f1, f2} {
-		for _, line := range lines {
-			// Skip blank lines, which can throw off the count.
-			if line == "" {
-				continue
-			}
+	for _, line := range f1 {
 			histogram[line]++
 		}
+	for _, line := range f2 {
+		histogram[line]--
 	}
 
-	var overlap int
+	var differences float64
 	for _, v := range histogram {
-		if v == 2 {
-			overlap++
-		}
+		differences += math.Abs(float64(v))
 	}
-	return float64(overlap) / float64(len(histogram)), nil
+
+	similarity := 1 - (differences / float64(len(f1)+len(f2)))
+	return similarity, nil
 }
 
 // Document stores a document ID and a list of associated files.