Improve search performance by limiting the search scope

Find the last occurrence of the last character in the pattern and
perform the search algorithm only up to that point.

The effectiveness of this mechanism depends a lot on the shape of the
input and the pattern.

src/algo/algo.go

@@ -352,30 +352,45 @@ func isAscii(runes []rune) bool {
 	return true
 }
 
-func asciiFuzzyIndex(input *util.Chars, pattern []rune, caseSensitive bool) int {
+func asciiFuzzyIndex(input *util.Chars, pattern []rune, caseSensitive bool) (int, int) {
 	// Can't determine
 	if !input.IsBytes() {
-		return 0
+		return 0, input.Length()
 	}
 
 	// Not possible
 	if !isAscii(pattern) {
-		return -1
+		return -1, -1
 	}
 
-	firstIdx, idx := 0, 0
+	firstIdx, idx, lastIdx := 0, 0, 0
+	var b byte
 	for pidx := 0; pidx < len(pattern); pidx++ {
-		idx = trySkip(input, caseSensitive, byte(pattern[pidx]), idx)
+		b = byte(pattern[pidx])
+		idx = trySkip(input, caseSensitive, b, idx)
 		if idx < 0 {
-			return -1
+			return -1, -1
 		}
 		if pidx == 0 && idx > 0 {
 			// Step back to find the right bonus point
 			firstIdx = idx - 1
 		}
+		lastIdx = idx
 		idx++
 	}
-	return firstIdx
+
+	// Find the last appearance of the last character of the pattern to limit the search scope
+	bu := b
+	if !caseSensitive && b >= 'a' && b <= 'z' {
+		bu = b - 32
+	}
+	scope := input.Bytes()[lastIdx:]
+	for offset := len(scope) - 1; offset > 0; offset-- {
+		if scope[offset] == b || scope[offset] == bu {
+			return firstIdx, lastIdx + offset + 1
+		}
+	}
+	return firstIdx, lastIdx + 1
 }
 
 func debugV2(T []rune, pattern []rune, F []int32, lastIdx int, H []int16, C []int16) {
@@ -424,6 +439,9 @@ func FuzzyMatchV2(caseSensitive bool, normalize bool, forward bool, input *util.
 		return Result{0, 0, 0}, posArray(withPos, M)
 	}
 	N := input.Length()
+	if M > N {
+		return Result{-1, -1, 0}, nil
+	}
 
 	// Since O(nm) algorithm can be prohibitively expensive for large input,
 	// we fall back to the greedy algorithm.
@@ -432,10 +450,12 @@ func FuzzyMatchV2(caseSensitive bool, normalize bool, forward bool, input *util.
 	}
 
 	// Phase 1. Optimized search for ASCII string
-	idx := asciiFuzzyIndex(input, pattern, caseSensitive)
-	if idx < 0 {
+	minIdx, maxIdx := asciiFuzzyIndex(input, pattern, caseSensitive)
+	if minIdx < 0 {
 		return Result{-1, -1, 0}, nil
 	}
+	// fmt.Println(N, maxIdx, idx, maxIdx-idx, input.ToString())
+	N = maxIdx - minIdx
 
 	// Reuse pre-allocated integer slice to avoid unnecessary sweeping of garbages
 	offset16 := 0
@@ -448,21 +468,19 @@ func FuzzyMatchV2(caseSensitive bool, normalize bool, forward bool, input *util.
 	offset32, F := alloc32(offset32, slab, M)
 	// Rune array
 	_, T := alloc32(offset32, slab, N)
-	input.CopyRunes(T)
+	input.CopyRunes(T, minIdx)
 
 	// Phase 2. Calculate bonus for each point
 	maxScore, maxScorePos := int16(0), 0
 	pidx, lastIdx := 0, 0
 	pchar0, pchar, prevH0, prevClass, inGap := pattern[0], pattern[0], int16(0), initialCharClass, false
-	Tsub := T[idx:]
-	H0sub, C0sub, Bsub := H0[idx:][:len(Tsub)], C0[idx:][:len(Tsub)], B[idx:][:len(Tsub)]
-	for off, char := range Tsub {
+	for off, char := range T {
 		var class charClass
 		if char <= unicode.MaxASCII {
 			class = asciiCharClasses[char]
 			if !caseSensitive && class == charUpper {
 				char += 32
-				Tsub[off] = char
+				T[off] = char
 			}
 		} else {
 			class = charClassOfNonAscii(char)
@@ -472,28 +490,28 @@ func FuzzyMatchV2(caseSensitive bool, normalize bool, forward bool, input *util.
 			if normalize {
 				char = normalizeRune(char)
 			}
-			Tsub[off] = char
+			T[off] = char
 		}
 
 		bonus := bonusMatrix[prevClass][class]
-		Bsub[off] = bonus
+		B[off] = bonus
 		prevClass = class
 
 		if char == pchar {
 			if pidx < M {
-				F[pidx] = int32(idx + off)
+				F[pidx] = int32(off)
 				pidx++
 				pchar = pattern[util.Min(pidx, M-1)]
 			}
-			lastIdx = idx + off
+			lastIdx = off
 		}
 
 		if char == pchar0 {
 			score := scoreMatch + bonus*bonusFirstCharMultiplier
-			H0sub[off] = score
-			C0sub[off] = 1
+			H0[off] = score
+			C0[off] = 1
 			if M == 1 && (forward && score > maxScore || !forward && score >= maxScore) {
-				maxScore, maxScorePos = score, idx+off
+				maxScore, maxScorePos = score, off
 				if forward && bonus >= bonusBoundary {
 					break
 				}
@@ -501,24 +519,24 @@ func FuzzyMatchV2(caseSensitive bool, normalize bool, forward bool, input *util.
 			inGap = false
 		} else {
 			if inGap {
-				H0sub[off] = util.Max16(prevH0+scoreGapExtension, 0)
+				H0[off] = util.Max16(prevH0+scoreGapExtension, 0)
 			} else {
-				H0sub[off] = util.Max16(prevH0+scoreGapStart, 0)
+				H0[off] = util.Max16(prevH0+scoreGapStart, 0)
 			}
-			C0sub[off] = 0
+			C0[off] = 0
 			inGap = true
 		}
-		prevH0 = H0sub[off]
+		prevH0 = H0[off]
 	}
 	if pidx != M {
 		return Result{-1, -1, 0}, nil
 	}
 	if M == 1 {
-		result := Result{maxScorePos, maxScorePos + 1, int(maxScore)}
+		result := Result{minIdx + maxScorePos, minIdx + maxScorePos + 1, int(maxScore)}
 		if !withPos {
 			return result, nil
 		}
-		pos := []int{maxScorePos}
+		pos := []int{minIdx + maxScorePos}
 		return result, &pos
 	}
 
@@ -615,7 +633,7 @@ func FuzzyMatchV2(caseSensitive bool, normalize bool, forward bool, input *util.
 			}
 
 			if s > s1 && (s > s2 || s == s2 && preferMatch) {
-				*pos = append(*pos, j)
+				*pos = append(*pos, j+minIdx)
 				if i == 0 {
 					break
 				}
@@ -628,7 +646,7 @@ func FuzzyMatchV2(caseSensitive bool, normalize bool, forward bool, input *util.
 	// Start offset we return here is only relevant when begin tiebreak is used.
 	// However finding the accurate offset requires backtracking, and we don't
 	// want to pay extra cost for the option that has lost its importance.
-	return Result{j, maxScorePos + 1, int(maxScore)}, pos
+	return Result{minIdx + j, minIdx + maxScorePos + 1, int(maxScore)}, pos
 }
 
 // Implement the same sorting criteria as V2
@@ -696,7 +714,8 @@ func FuzzyMatchV1(caseSensitive bool, normalize bool, forward bool, text *util.C
 	if len(pattern) == 0 {
 		return Result{0, 0, 0}, nil
 	}
-	if asciiFuzzyIndex(text, pattern, caseSensitive) < 0 {
+	idx, _ := asciiFuzzyIndex(text, pattern, caseSensitive)
+	if idx < 0 {
 		return Result{-1, -1, 0}, nil
 	}
 
@@ -790,7 +809,8 @@ func ExactMatchNaive(caseSensitive bool, normalize bool, forward bool, text *uti
 		return Result{-1, -1, 0}, nil
 	}
 
-	if asciiFuzzyIndex(text, pattern, caseSensitive) < 0 {
+	idx, _ := asciiFuzzyIndex(text, pattern, caseSensitive)
+	if idx < 0 {
 		return Result{-1, -1, 0}, nil
 	}