Speed up ANSI code processing (#2368)

This commit speeds up the parsing/processing of ANSI escape codes by
roughly 7.5x. The speedup is mostly accomplished by replacing the regex
with dedicated parsing logic (nextAnsiEscapeSequence()) and reducing the
number of allocations in extractColor().

#### Benchmarks
```
name             old time/op    new time/op     delta
ExtractColor-16    4.89µs ± 5%     0.64µs ± 2%   -86.87%  (p=0.000 n=9+9)

name             old speed      new speed       delta
ExtractColor-16  25.6MB/s ± 5%  194.6MB/s ± 2%  +661.43%  (p=0.000 n=9+9)

name             old alloc/op   new alloc/op    delta
ExtractColor-16    1.37kB ± 0%     0.31kB ± 0%   -77.31%  (p=0.000 n=10+10)

name             old allocs/op  new allocs/op   delta
ExtractColor-16      48.0 ± 0%        4.0 ± 0%   -91.67%  (p=0.000 n=10+10)
```

src/ansi.go

@@ -1,8 +1,6 @@
 package fzf
 
 import (
-	"bytes"
-	"regexp"
 	"strconv"
 	"strings"
 	"unicode/utf8"
@@ -82,73 +80,154 @@ func toAnsiString(color tui.Color, offset int) string {
 	return ret + ";"
 }
 
-var ansiRegex *regexp.Regexp
-
-func init() {
-	/*
-		References:
-		- https://github.com/gnachman/iTerm2
-		- http://ascii-table.com/ansi-escape-sequences.php
-		- http://ascii-table.com/ansi-escape-sequences-vt-100.php
-		- http://tldp.org/HOWTO/Bash-Prompt-HOWTO/x405.html
-		- https://invisible-island.net/xterm/ctlseqs/ctlseqs.html
-	*/
-	// The following regular expression will include not all but most of the
-	// frequently used ANSI sequences
-	ansiRegex = regexp.MustCompile("(?:\x1b[\\[()][0-9;]*[a-zA-Z@]|\x1b][0-9];[[:print:]]+(?:\x1b\\\\|\x07)|\x1b.|[\x0e\x0f]|.\x08)")
+func isPrint(c uint8) bool {
+	return '\x20' <= c && c <= '\x7e'
 }
 
-func findAnsiStart(str string) int {
-	idx := 0
-	for ; idx < len(str); idx++ {
-		b := str[idx]
-		if b == 0x1b || b == 0x0e || b == 0x0f {
-			return idx
+func matchOperatingSystemCommand(s string) int {
+	// `\x1b][0-9];[[:print:]]+(?:\x1b\\\\|\x07)`
+	//                        ^ match starting here
+	//
+	i := 5 // prefix matched in nextAnsiEscapeSequence()
+	for ; i < len(s) && isPrint(s[i]); i++ {
+	}
+	if i < len(s) {
+		if s[i] == '\x07' {
+			return i + 1
 		}
-		if b == 0x08 && idx > 0 {
-			return idx - 1
+		if s[i] == '\x1b' && i < len(s)-1 && s[i+1] == '\\' {
+			return i + 2
 		}
 	}
-	return idx
+	return -1
+}
+
+func matchControlSequence(s string) int {
+	// `\x1b[\\[()][0-9;]*[a-zA-Z@]`
+	//                   ^ match starting here
+	//
+	i := 2 // prefix matched in nextAnsiEscapeSequence()
+	for ; i < len(s) && (isNumeric(s[i]) || s[i] == ';'); i++ {
+	}
+	if i < len(s) {
+		c := s[i]
+		if 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' || c == '@' {
+			return i + 1
+		}
+	}
+	return -1
+}
+
+func isCtrlSeqStart(c uint8) bool {
+	return c == '\\' || c == '[' || c == '(' || c == ')'
+}
+
+// nextAnsiEscapeSequence returns the ANSI escape sequence and is equivalent to
+// calling FindStringIndex() on the below regex (which was originally used):
+//
+// "(?:\x1b[\\[()][0-9;]*[a-zA-Z@]|\x1b][0-9];[[:print:]]+(?:\x1b\\\\|\x07)|\x1b.|[\x0e\x0f]|.\x08)"
+//
+func nextAnsiEscapeSequence(s string) (int, int) {
+	// fast check for ANSI escape sequences
+	i := 0
+	for ; i < len(s); i++ {
+		switch s[i] {
+		case '\x0e', '\x0f', '\x1b', '\x08':
+			// We ignore the fact that '\x08' cannot be the first char
+			// in the string and be an escape sequence for the sake of
+			// speed and simplicity.
+			goto Loop
+		}
+	}
+	return -1, -1
+
+Loop:
+	for ; i < len(s); i++ {
+		switch s[i] {
+		case '\x08':
+			// backtrack to match: `.\x08`
+			if i > 0 && s[i-1] != '\n' {
+				if s[i-1] < utf8.RuneSelf {
+					return i - 1, i + 1
+				}
+				_, n := utf8.DecodeLastRuneInString(s[:i])
+				return i - n, i + 1
+			}
+		case '\x1b':
+			// match: `\x1b[\\[()][0-9;]*[a-zA-Z@]`
+			if i+2 < len(s) && isCtrlSeqStart(s[i+1]) {
+				if j := matchControlSequence(s[i:]); j != -1 {
+					return i, i + j
+				}
+			}
+
+			// match: `\x1b][0-9];[[:print:]]+(?:\x1b\\\\|\x07)`
+			if i+5 < len(s) && s[i+1] == ']' && isNumeric(s[i+2]) &&
+				s[i+3] == ';' && isPrint(s[i+4]) {
+
+				if j := matchOperatingSystemCommand(s[i:]); j != -1 {
+					return i, i + j
+				}
+			}
+
+			// match: `\x1b.`
+			if i+1 < len(s) && s[i+1] != '\n' {
+				if s[i+1] < utf8.RuneSelf {
+					return i, i + 2
+				}
+				_, n := utf8.DecodeRuneInString(s[i+1:])
+				return i, i + n + 1
+			}
+		case '\x0e', '\x0f':
+			// match: `[\x0e\x0f]`
+			return i, i + 1
+		}
+	}
+	return -1, -1
 }
 
 func extractColor(str string, state *ansiState, proc func(string, *ansiState) bool) (string, *[]ansiOffset, *ansiState) {
-	var offsets []ansiOffset
-	var output bytes.Buffer
+	// We append to a stack allocated variable that we'll
+	// later copy and return, to save on allocations.
+	offsets := make([]ansiOffset, 0, 32)
 
 	if state != nil {
 		offsets = append(offsets, ansiOffset{[2]int32{0, 0}, *state})
 	}
 
-	prevIdx := 0
-	runeCount := 0
+	var (
+		pstate    *ansiState // lazily allocated
+		output    strings.Builder
+		prevIdx   int
+		runeCount int
+	)
 	for idx := 0; idx < len(str); {
-		idx += findAnsiStart(str[idx:])
-		if idx == len(str) {
+		// Make sure that we found an ANSI code
+		start, end := nextAnsiEscapeSequence(str[idx:])
+		if start == -1 {
 			break
 		}
-
-		// Make sure that we found an ANSI code
-		offset := ansiRegex.FindStringIndex(str[idx:])
-		if len(offset) < 2 {
-			idx++
-			continue
-		}
-		offset[0] += idx
-		offset[1] += idx
-		idx = offset[1]
+		start += idx
+		idx += end
 
 		// Check if we should continue
-		prev := str[prevIdx:offset[0]]
+		prev := str[prevIdx:start]
 		if proc != nil && !proc(prev, state) {
 			return "", nil, nil
 		}
+		prevIdx = idx
 
-		prevIdx = offset[1]
-		runeCount += utf8.RuneCountInString(prev)
-		output.WriteString(prev)
+		if len(prev) != 0 {
+			runeCount += utf8.RuneCountInString(prev)
+			// Grow the buffer size to the maximum possible length (string length
+			// containing ansi codes) to avoid repetitive allocation
+			if output.Cap() == 0 {
+				output.Grow(len(str))
+			}
+			output.WriteString(prev)
+		}
 
-		newState := interpretCode(str[offset[0]:offset[1]], state)
+		newState := interpretCode(str[start:idx], state)
 		if !newState.equals(state) {
 			if state != nil {
 				// Update last offset
@@ -157,8 +236,15 @@ func extractColor(str string, state *ansiState, proc func(string, *ansiState) bo
 
 			if newState.colored() {
 				// Append new offset
-				state = newState
-				offsets = append(offsets, ansiOffset{[2]int32{int32(runeCount), int32(runeCount)}, *state})
+				if pstate == nil {
+					pstate = &ansiState{}
+				}
+				*pstate = newState
+				state = pstate
+				offsets = append(offsets, ansiOffset{
+					[2]int32{int32(runeCount), int32(runeCount)},
+					newState,
+				})
 			} else {
 				// Discard state
 				state = nil
@@ -168,7 +254,6 @@ func extractColor(str string, state *ansiState, proc func(string, *ansiState) bo
 
 	var rest string
 	var trimmed string
-
 	if prevIdx == 0 {
 		// No ANSI code found
 		rest = str
@@ -178,51 +263,75 @@ func extractColor(str string, state *ansiState, proc func(string, *ansiState) bo
 		output.WriteString(rest)
 		trimmed = output.String()
 	}
-	if len(rest) > 0 && state != nil {
-		// Update last offset
-		runeCount += utf8.RuneCountInString(rest)
-		(&offsets[len(offsets)-1]).offset[1] = int32(runeCount)
-	}
 	if proc != nil {
 		proc(rest, state)
 	}
-	if len(offsets) == 0 {
-		return trimmed, nil, state
+	if len(offsets) > 0 {
+		if len(rest) > 0 && state != nil {
+			// Update last offset
+			runeCount += utf8.RuneCountInString(rest)
+			(&offsets[len(offsets)-1]).offset[1] = int32(runeCount)
+		}
+		// Return a copy of the offsets slice
+		a := make([]ansiOffset, len(offsets))
+		copy(a, offsets)
+		return trimmed, &a, state
 	}
-	return trimmed, &offsets, state
+	return trimmed, nil, state
 }
 
-func interpretCode(ansiCode string, prevState *ansiState) *ansiState {
-	// State
-	var state *ansiState
+func parseAnsiCode(s string) (int, string) {
+	var remaining string
+	if i := strings.IndexByte(s, ';'); i >= 0 {
+		remaining = s[i+1:]
+		s = s[:i]
+	}
+
+	if len(s) > 0 {
+		// Inlined version of strconv.Atoi() that only handles positive
+		// integers and does not allocate on error.
+		code := 0
+		for _, ch := range []byte(s) {
+			ch -= '0'
+			if ch > 9 {
+				return -1, remaining
+			}
+			code = code*10 + int(ch)
+		}
+		return code, remaining
+	}
+
+	return -1, remaining
+}
+
+func interpretCode(ansiCode string, prevState *ansiState) ansiState {
+	var state ansiState
 	if prevState == nil {
-		state = &ansiState{-1, -1, 0, -1}
+		state = ansiState{-1, -1, 0, -1}
 	} else {
-		state = &ansiState{prevState.fg, prevState.bg, prevState.attr, prevState.lbg}
+		state = ansiState{prevState.fg, prevState.bg, prevState.attr, prevState.lbg}
 	}
 	if ansiCode[0] != '\x1b' || ansiCode[1] != '[' || ansiCode[len(ansiCode)-1] != 'm' {
-		if strings.HasSuffix(ansiCode, "0K") && prevState != nil {
+		if prevState != nil && strings.HasSuffix(ansiCode, "0K") {
 			state.lbg = prevState.bg
 		}
 		return state
 	}
 
-	ptr := &state.fg
-	state256 := 0
-
-	init := func() {
+	if len(ansiCode) <= 3 {
 		state.fg = -1
 		state.bg = -1
 		state.attr = 0
-		state256 = 0
+		return state
 	}
-
 	ansiCode = ansiCode[2 : len(ansiCode)-1]
-	if len(ansiCode) == 0 {
-		init()
-	}
-	for _, code := range strings.Split(ansiCode, ";") {
-		if num, err := strconv.Atoi(code); err == nil {
+
+	state256 := 0
+	ptr := &state.fg
+
+	for len(ansiCode) != 0 {
+		var num int
+		if num, ansiCode = parseAnsiCode(ansiCode); num != -1 {
 			switch state256 {
 			case 0:
 				switch num {
@@ -253,7 +362,10 @@ func interpretCode(ansiCode string, prevState *ansiState) *ansiState {
 				case 24: // tput rmul
 					state.attr = state.attr &^ tui.Underline
 				case 0:
-					init()
+					state.fg = -1
+					state.bg = -1
+					state.attr = 0
+					state256 = 0
 				default:
 					if num >= 30 && num <= 37 {
 						state.fg = tui.Color(num - 30)
@@ -289,6 +401,7 @@ func interpretCode(ansiCode string, prevState *ansiState) *ansiState {
 			}
 		}
 	}
+
 	if state256 > 0 {
 		*ptr = -1
 	}