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)
```

1 files changed, 239 insertions, 0 deletions

diff --git a/src/ansi_test.go b/src/ansi_test.go
index 497eca44..36ed0c7d 100644
--- a/src/ansi_test.go
+++ b/src/ansi_test.go
@@ -2,12 +2,190 @@ package fzf
 
 import (
 	"fmt"
+	"math/rand"
+	"regexp"
 	"strings"
 	"testing"
+	"unicode/utf8"
 
 	"github.com/junegunn/fzf/src/tui"
 )
 
+// The following regular expression will include not all but most of the
+// frequently used ANSI sequences. This regex is used as a reference for
+// testing nextAnsiEscapeSequence().
+//
+// 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
+var ansiRegexRefence = regexp.MustCompile("(?:\x1b[\\[()][0-9;]*[a-zA-Z@]|\x1b][0-9];[[:print:]]+(?:\x1b\\\\|\x07)|\x1b.|[\x0e\x0f]|.\x08)")
+
+func testParserReference(t testing.TB, str string) {
+	t.Helper()
+
+	toSlice := func(start, end int) []int {
+		if start == -1 {
+			return nil
+		}
+		return []int{start, end}
+	}
+
+	s := str
+	for i := 0; ; i++ {
+		got := toSlice(nextAnsiEscapeSequence(s))
+		exp := ansiRegexRefence.FindStringIndex(s)
+
+		equal := len(got) == len(exp)
+		if equal {
+			for i := 0; i < len(got); i++ {
+				if got[i] != exp[i] {
+					equal = false
+					break
+				}
+			}
+		}
+		if !equal {
+			var exps, gots []rune
+			if len(got) == 2 {
+				gots = []rune(s[got[0]:got[1]])
+			}
+			if len(exp) == 2 {
+				exps = []rune(s[exp[0]:exp[1]])
+			}
+			t.Errorf("%d: %q: got: %v (%q) want: %v (%q)", i, s, got, gots, exp, exps)
+			return
+		}
+		if len(exp) == 0 {
+			return
+		}
+		s = s[exp[1]:]
+	}
+}
+
+func TestNextAnsiEscapeSequence(t *testing.T) {
+	testStrs := []string{
+		"\x1b[0mhello world",
+		"\x1b[1mhello world",
+		"椙\x1b[1m椙",
+		"椙\x1b[1椙m椙",
+		"\x1b[1mhello \x1b[mw\x1b7o\x1b8r\x1b(Bl\x1b[2@d",
+		"\x1b[1mhello \x1b[Kworld",
+		"hello \x1b[34;45;1mworld",
+		"hello \x1b[34;45;1mwor\x1b[34;45;1mld",
+		"hello \x1b[34;45;1mwor\x1b[0mld",
+		"hello \x1b[34;48;5;233;1mwo\x1b[38;5;161mr\x1b[0ml\x1b[38;5;161md",
+		"hello \x1b[38;5;38;48;5;48;1mwor\x1b[38;5;48;48;5;38ml\x1b[0md",
+		"hello \x1b[32;1mworld",
+		"hello world",
+		"hello \x1b[0;38;5;200;48;5;100mworld",
+		"\x1b椙",
+		"椙\x08",
+		"\n\x08",
+		"X\x08",
+		"",
+		"\x1b]4;3;rgb:aa/bb/cc\x07 ",
+		"\x1b]4;3;rgb:aa/bb/cc\x1b\\ ",
+		ansiBenchmarkString,
+	}
+
+	for _, s := range testStrs {
+		testParserReference(t, s)
+	}
+}
+
+func TestNextAnsiEscapeSequence_Fuzz_Modified(t *testing.T) {
+	t.Parallel()
+	if testing.Short() {
+		t.Skip("short test")
+	}
+
+	testStrs := []string{
+		"\x1b[0mhello world",
+		"\x1b[1mhello world",
+		"椙\x1b[1m椙",
+		"椙\x1b[1椙m椙",
+		"\x1b[1mhello \x1b[mw\x1b7o\x1b8r\x1b(Bl\x1b[2@d",
+		"\x1b[1mhello \x1b[Kworld",
+		"hello \x1b[34;45;1mworld",
+		"hello \x1b[34;45;1mwor\x1b[34;45;1mld",
+		"hello \x1b[34;45;1mwor\x1b[0mld",
+		"hello \x1b[34;48;5;233;1mwo\x1b[38;5;161mr\x1b[0ml\x1b[38;5;161md",
+		"hello \x1b[38;5;38;48;5;48;1mwor\x1b[38;5;48;48;5;38ml\x1b[0md",
+		"hello \x1b[32;1mworld",
+		"hello world",
+		"hello \x1b[0;38;5;200;48;5;100mworld",
+		ansiBenchmarkString,
+	}
+
+	replacementBytes := [...]rune{'\x0e', '\x0f', '\x1b', '\x08'}
+
+	modifyString := func(s string, rr *rand.Rand) string {
+		n := rr.Intn(len(s))
+		b := []rune(s)
+		for ; n >= 0 && len(b) != 0; n-- {
+			i := rr.Intn(len(b))
+			switch x := rr.Intn(4); x {
+			case 0:
+				b = append(b[:i], b[i+1:]...)
+			case 1:
+				j := rr.Intn(len(replacementBytes) - 1)
+				b[i] = replacementBytes[j]
+			case 2:
+				x := rune(rr.Intn(utf8.MaxRune))
+				for !utf8.ValidRune(x) {
+					x = rune(rr.Intn(utf8.MaxRune))
+				}
+				b[i] = x
+			case 3:
+				b[i] = rune(rr.Intn(utf8.MaxRune)) // potentially invalid
+			default:
+				t.Fatalf("unsupported value: %d", x)
+			}
+		}
+		return string(b)
+	}
+
+	rr := rand.New(rand.NewSource(1))
+	for _, s := range testStrs {
+		for i := 1_000; i >= 0; i-- {
+			testParserReference(t, modifyString(s, rr))
+		}
+	}
+}
+
+func TestNextAnsiEscapeSequence_Fuzz_Random(t *testing.T) {
+	t.Parallel()
+
+	if testing.Short() {
+		t.Skip("short test")
+	}
+
+	randomString := func(rr *rand.Rand) string {
+		numChars := rand.Intn(50)
+		codePoints := make([]rune, numChars)
+		for i := 0; i < len(codePoints); i++ {
+			var r rune
+			for n := 0; n < 1000; n++ {
+				r = rune(rr.Intn(utf8.MaxRune))
+				// Allow 10% of runes to be invalid
+				if utf8.ValidRune(r) || rr.Float64() < 0.10 {
+					break
+				}
+			}
+			codePoints[i] = r
+		}
+		return string(codePoints)
+	}
+
+	rr := rand.New(rand.NewSource(1))
+	for i := 0; i < 100_000; i++ {
+		testParserReference(t, randomString(rr))
+	}
+}
+
 func TestExtractColor(t *testing.T) {
 	assert := func(offset ansiOffset, b int32, e int32, fg tui.Color, bg tui.Color, bold bool) {
 		var attr tui.Attr
@@ -185,3 +363,64 @@ func TestAnsiCodeStringConversion(t *testing.T) {
 		&ansiState{attr: tui.Dim | tui.Italic, fg: 1, bg: 1},
 		"\x1b[2;3;7;38;2;10;20;30;48;5;100m")
 }
+
+func TestParseAnsiCode(t *testing.T) {
+	tests := []struct {
+		In, Exp string
+		N       int
+	}{
+		{"123", "", 123},
+		{"1a", "", -1},
+		{"1a;12", "12", -1},
+		{"12;a", "a", 12},
+		{"-2", "", -1},
+	}
+	for _, x := range tests {
+		n, s := parseAnsiCode(x.In)
+		if n != x.N || s != x.Exp {
+			t.Fatalf("%q: got: (%d %q) want: (%d %q)", x.In, n, s, x.N, x.Exp)
+		}
+	}
+}
+
+// kernel/bpf/preload/iterators/README
+const ansiBenchmarkString = "\x1b[38;5;81m\x1b[01;31m\x1b[Kkernel/\x1b[0m\x1b[38;5;81mbpf/" +
+	"\x1b[0m\x1b[38;5;81mpreload/\x1b[0m\x1b[38;5;81miterators/" +
+	"\x1b[0m\x1b[38;5;149mMakefile\x1b[m\x1b[K\x1b[0m"
+
+func BenchmarkNextAnsiEscapeSequence(b *testing.B) {
+	b.SetBytes(int64(len(ansiBenchmarkString)))
+	for i := 0; i < b.N; i++ {
+		s := ansiBenchmarkString
+		for {
+			_, o := nextAnsiEscapeSequence(s)
+			if o == -1 {
+				break
+			}
+			s = s[o:]
+		}
+	}
+}
+
+// Baseline test to compare the speed of nextAnsiEscapeSequence() to the
+// previously used regex based implementation.
+func BenchmarkNextAnsiEscapeSequence_Regex(b *testing.B) {
+	b.SetBytes(int64(len(ansiBenchmarkString)))
+	for i := 0; i < b.N; i++ {
+		s := ansiBenchmarkString
+		for {
+			a := ansiRegexRefence.FindStringIndex(s)
+			if len(a) == 0 {
+				break
+			}
+			s = s[a[1]:]
+		}
+	}
+}
+
+func BenchmarkExtractColor(b *testing.B) {
+	b.SetBytes(int64(len(ansiBenchmarkString)))
+	for i := 0; i < b.N; i++ {
+		extractColor(ansiBenchmarkString, nil, nil)
+	}
+}