Enhanced ranking algorithm

Based on the patch by Matt Westcott (@mjwestcott).
But with a more conservative approach:
- Does not use linearly increasing penalties; It is agreed upon that we
  should prefer matching characters at the beginnings of the words, but
  it's not always clear that the relevance is inversely proportional to
  the distance from the beginning.
- The approach here is more conservative in that the bonus is never
  large enough to override the matchlen, so it can be thought of as the
  first implicit tiebreak criterion.
- One may argue the change breaks the contract of --tiebreak, but the
  judgement depends on the definition of "tie".

src/algo/algo.go

@@ -22,10 +22,30 @@ func runeAt(runes []rune, index int, max int, forward bool) rune {
 	return runes[max-index-1]
 }
 
+// Result conatins the results of running a match function.
+type Result struct {
+	Start int32
+	End   int32
+
+	// Items are basically sorted by the lengths of matched substrings.
+	// But we slightly adjust the score with bonus for better results.
+	Bonus int32
+}
+
+type charClass int
+
+const (
+	charNonWord charClass = iota
+	charLower
+	charUpper
+	charLetter
+	charNumber
+)
+
 // FuzzyMatch performs fuzzy-match
-func FuzzyMatch(caseSensitive bool, forward bool, runes []rune, pattern []rune) (int, int) {
+func FuzzyMatch(caseSensitive bool, forward bool, runes []rune, pattern []rune) Result {
 	if len(pattern) == 0 {
-		return 0, 0
+		return Result{0, 0, 0}
 	}
 
 	// 0. (FIXME) How to find the shortest match?
@@ -90,12 +110,76 @@ func FuzzyMatch(caseSensitive bool, forward bool, runes []rune, pattern []rune)
 				}
 			}
 		}
-		if forward {
-			return sidx, eidx
+
+		// Calculate the bonus. This can't be done at the same time as the
+		// pattern scan above because 'forward' may be false.
+		if !forward {
+			sidx, eidx = lenRunes-eidx, lenRunes-sidx
 		}
-		return lenRunes - eidx, lenRunes - sidx
+
+		var bonus int32
+		pidx := 0
+		consecutive := false
+		prevClass := charNonWord
+		for index := 0; index < eidx; index++ {
+			char := runes[index]
+			var class charClass
+			if unicode.IsLower(char) {
+				class = charLower
+			} else if unicode.IsUpper(char) {
+				class = charUpper
+			} else if unicode.IsLetter(char) {
+				class = charLetter
+			} else if unicode.IsNumber(char) {
+				class = charNumber
+			} else {
+				class = charNonWord
+			}
+
+			var point int32
+			if prevClass == charNonWord && class != charNonWord {
+				// Word boundary
+				point = 2
+			} else if prevClass == charLower && class == charUpper ||
+				prevClass != charNumber && class == charNumber {
+				// camelCase letter123
+				point = 1
+			}
+			prevClass = class
+
+			if index >= sidx {
+				if !caseSensitive {
+					if char >= 'A' && char <= 'Z' {
+						char += 32
+					} else if char > unicode.MaxASCII {
+						char = unicode.To(unicode.LowerCase, char)
+					}
+				}
+				pchar := pattern[pidx]
+				if pchar == char {
+					// Boost bonus for the first character in the pattern
+					if pidx == 0 {
+						point *= 2
+					}
+					// Bonus to consecutive matching chars
+					if consecutive {
+						point++
+					}
+					bonus += point
+
+					if pidx++; pidx == lenPattern {
+						break
+					}
+					consecutive = true
+				} else {
+					consecutive = false
+				}
+			}
+		}
+
+		return Result{int32(sidx), int32(eidx), bonus}
 	}
-	return -1, -1
+	return Result{-1, -1, 0}
 }
 
 // ExactMatchNaive is a basic string searching algorithm that handles case
@@ -105,16 +189,17 @@ func FuzzyMatch(caseSensitive bool, forward bool, runes []rune, pattern []rune)
 //
 // We might try to implement better algorithms in the future:
 // http://en.wikipedia.org/wiki/String_searching_algorithm
-func ExactMatchNaive(caseSensitive bool, forward bool, runes []rune, pattern []rune) (int, int) {
+func ExactMatchNaive(caseSensitive bool, forward bool, runes []rune, pattern []rune) Result {
+	// Note: ExactMatchNaive always return a zero bonus.
 	if len(pattern) == 0 {
-		return 0, 0
+		return Result{0, 0, 0}
 	}
 
 	lenRunes := len(runes)
 	lenPattern := len(pattern)
 
 	if lenRunes < lenPattern {
-		return -1, -1
+		return Result{-1, -1, 0}
 	}
 
 	pidx := 0
@@ -132,22 +217,23 @@ func ExactMatchNaive(caseSensitive bool, forward bool, runes []rune, pattern []r
 			pidx++
 			if pidx == lenPattern {
 				if forward {
-					return index - lenPattern + 1, index + 1
+					return Result{int32(index - lenPattern + 1), int32(index + 1), 0}
 				}
-				return lenRunes - (index + 1), lenRunes - (index - lenPattern + 1)
+				return Result{int32(lenRunes - (index + 1)), int32(lenRunes - (index - lenPattern + 1)), 0}
 			}
 		} else {
 			index -= pidx
 			pidx = 0
 		}
 	}
-	return -1, -1
+	return Result{-1, -1, 0}
 }
 
 // PrefixMatch performs prefix-match
-func PrefixMatch(caseSensitive bool, forward bool, runes []rune, pattern []rune) (int, int) {
+func PrefixMatch(caseSensitive bool, forward bool, runes []rune, pattern []rune) Result {
+	// Note: PrefixMatch always return a zero bonus.
 	if len(runes) < len(pattern) {
-		return -1, -1
+		return Result{-1, -1, 0}
 	}
 
 	for index, r := range pattern {
@@ -156,19 +242,20 @@ func PrefixMatch(caseSensitive bool, forward bool, runes []rune, pattern []rune)
 			char = unicode.ToLower(char)
 		}
 		if char != r {
-			return -1, -1
+			return Result{-1, -1, 0}
 		}
 	}
-	return 0, len(pattern)
+	return Result{0, int32(len(pattern)), 0}
 }
 
 // SuffixMatch performs suffix-match
-func SuffixMatch(caseSensitive bool, forward bool, input []rune, pattern []rune) (int, int) {
+func SuffixMatch(caseSensitive bool, forward bool, input []rune, pattern []rune) Result {
+	// Note: SuffixMatch always return a zero bonus.
 	runes := util.TrimRight(input)
 	trimmedLen := len(runes)
 	diff := trimmedLen - len(pattern)
 	if diff < 0 {
-		return -1, -1
+		return Result{-1, -1, 0}
 	}
 
 	for index, r := range pattern {
@@ -177,23 +264,24 @@ func SuffixMatch(caseSensitive bool, forward bool, input []rune, pattern []rune)
 			char = unicode.ToLower(char)
 		}
 		if char != r {
-			return -1, -1
+			return Result{-1, -1, 0}
 		}
 	}
-	return trimmedLen - len(pattern), trimmedLen
+	return Result{int32(trimmedLen - len(pattern)), int32(trimmedLen), 0}
 }
 
 // EqualMatch performs equal-match
-func EqualMatch(caseSensitive bool, forward bool, runes []rune, pattern []rune) (int, int) {
+func EqualMatch(caseSensitive bool, forward bool, runes []rune, pattern []rune) Result {
+	// Note: EqualMatch always return a zero bonus.
 	if len(runes) != len(pattern) {
-		return -1, -1
+		return Result{-1, -1, 0}
 	}
 	runesStr := string(runes)
 	if !caseSensitive {
 		runesStr = strings.ToLower(runesStr)
 	}
 	if runesStr == string(pattern) {
-		return 0, len(pattern)
+		return Result{0, int32(len(pattern)), 0}
 	}
-	return -1, -1
+	return Result{-1, -1, 0}
 }