revert alphabet.Index to binary search instead of map

alphabet.go

@@ -14,7 +14,6 @@ const (
 
 type alphabet struct {
 	chars       []rune
-	indexMap    map[rune]int
 	len         int64
 	encLen      int64
 	singleBytes bool
@@ -35,13 +34,12 @@ func newAlphabet(s string) alphabet {
 		len:         int64(len(abc)),
 		encLen:      int64(math.Ceil(128 / math.Log2(float64(len(abc))))),
 		singleBytes: true,
-		indexMap:    make(map[rune]int, len(abc)),
 	}
-	for i, c := range a.chars {
+	for _, c := range a.chars {
 		if c > rune1Max {
 			a.singleBytes = false
+			break
 		}
-		a.indexMap[c] = i
 	}
 
 	return a
@@ -54,8 +52,17 @@ func (a *alphabet) Length() int64 {
 // Index returns the index of the first instance of t in the alphabet, or an
 // error if t is not present.
 func (a *alphabet) Index(t rune) (int64, error) {
-	if i, ok := a.indexMap[t]; ok {
-		return int64(i), nil
+	i, j := 0, int(a.len)
+	for i < j {
+		h := int(uint(i+j) >> 1)
+		if a.chars[h] < t {
+			i = h + 1
+		} else {
+			j = h
+		}
+	}
+	if i >= int(a.len) || a.chars[i] != t {
+		return 0, fmt.Errorf("element '%v' is not part of the alphabet", t)
 	}
-	return 0, fmt.Errorf("element '%v' is not part of the alphabet", t)
+	return int64(i), nil
 }