`wcwidth` implementation for Go. Uses the `unicode` and `golang.org/x/text/width` packages.

width.go

package textwidth

import (
	"unicode"

	"golang.org/x/text/width"
)

// IsComb returns true if r is a Unicode combining character. Alias of:
//
//	unicode.Is(unicode.Mn, r)
//
func IsComb(r rune) bool { return unicode.Is(unicode.Mn, r) }

// RuneWidth returns fixed-width width of rune.
// https://en.wikipedia.org/wiki/Halfwidth_and_fullwidth_forms#In_Unicode
func RuneWidth(r rune) int {
	if r == '\x00' || !unicode.IsPrint(r) || IsComb(r) {
		return 0
	}
	k := width.LookupRune(r)
	switch k.Kind() {
	case width.EastAsianWide, width.EastAsianFullwidth:
		return 2
	case width.EastAsianNarrow, width.EastAsianHalfwidth, width.EastAsianAmbiguous, width.Neutral:
		return 1
	default:
		return 0
	}
}

// StringWidth returns fixed-width width of string.
// https://en.wikipedia.org/wiki/Halfwidth_and_fullwidth_forms#In_Unicode
func StringWidth(s string) (n int) {
	for _, r := range s {
		n += RuneWidth(r)
	}
	return n
}

width_test.go

package textwidth

import (
	"testing"

	"github.com/mattn/go-runewidth"
)

// test cases copied from https://github.com/mattn/go-runewidth/raw/master/runewidth_test.go

var stringwidthtests = []struct {
	in    string
	out   int
	eaout int
}{
	{"■㈱の世界①", 10, 12},
	{"スター☆", 7, 8},
	{"つのだ☆HIRO", 11, 12},
}

func BenchmarkStringWidth(b *testing.B) {
	for i := 0; i < b.N; i++ {
		StringWidth(stringwidthtests[i%len(stringwidthtests)].in)
	}
}

func BenchmarkStringWidthOriginal(b *testing.B) {
	for i := 0; i < b.N; i++ {
		runewidth.StringWidth(stringwidthtests[i%len(stringwidthtests)].in)
	}
}

func TestStringWidth(t *testing.T) {
	for _, tt := range stringwidthtests {
		if out := StringWidth(tt.in); out != tt.out {
			t.Errorf("StringWidth(%q) = %d, want %d", tt.in, out, tt.out)
		}
	}
	//c := runewidth.NewCondition()
	//c.EastAsianWidth = false
	//for _, tt := range stringwidthtests {
	//	if out := c.StringWidth(tt.in); out != tt.out {
	//		t.Errorf("StringWidth(%q) = %d, want %d", tt.in, out, tt.out)
	//	}
	//}
	//c.EastAsianWidth = true
	//for _, tt := range stringwidthtests {
	//	if out := c.StringWidth(tt.in); out != tt.eaout {
	//		t.Errorf("StringWidth(%q) = %d, want %d (EA)", tt.in, out, tt.eaout)
	//	}
	//}
}

var runewidthtests = []struct {
	in     rune
	out    int
	eaout  int
	nseout int
}{
	{'世', 2, 2, 2},
	{'界', 2, 2, 2},
	{'ｾ', 1, 1, 1},
	{'ｶ', 1, 1, 1},
	{'ｲ', 1, 1, 1},
	{'☆', 1, 2, 2}, // double width in ambiguous
	{'☺', 1, 1, 2},
	{'☻', 1, 1, 2},
	{'♥', 1, 2, 2},
	{'♦', 1, 1, 2},
	{'♣', 1, 2, 2},
	{'♠', 1, 2, 2},
	{'♂', 1, 2, 2},
	{'♀', 1, 2, 2},
	{'♪', 1, 2, 2},
	{'♫', 1, 1, 2},
	{'☼', 1, 1, 2},
	{'↕', 1, 2, 2},
	{'‼', 1, 1, 2},
	{'↔', 1, 2, 2},
	{'\x00', 0, 0, 0},
	{'\x01', 0, 0, 0},
	{'\u0300', 0, 0, 0},
	{'\u2028', 0, 0, 0},
	{'\u2029', 0, 0, 0},
	{'a', 1, 1, 1}, // ASCII classified as "na" (narrow)
	{'⟦', 1, 1, 1}, // non-ASCII classified as "na" (narrow)
	{'👁', 1, 1, 2},
}

func BenchmarkRuneWidth(b *testing.B) {
	for i := 0; i < b.N; i++ {
		RuneWidth(runewidthtests[i%len(runewidthtests)].in)
	}
}

func BenchmarkRuneWidthOriginal(b *testing.B) {
	for i := 0; i < b.N; i++ {
		runewidth.RuneWidth(runewidthtests[i%len(runewidthtests)].in)
	}
}

func TestRuneWidth(t *testing.T) {
	for i, tt := range runewidthtests {
		if out := RuneWidth(tt.in); out != tt.out {
			t.Errorf("case %d: RuneWidth(%q) = %d, want %d", i, tt.in, out, tt.out)
		}
	}
	//c := runewidth.NewCondition()
	//c.EastAsianWidth = false
	//for _, tt := range runewidthtests {
	//	if out := c.RuneWidth(tt.in); out != tt.out {
	//		t.Errorf("RuneWidth(%q) = %d, want %d (EastAsianWidth=false)", tt.in, out, tt.out)
	//	}
	//}
	//c.EastAsianWidth = true
	//for _, tt := range runewidthtests {
	//	if out := c.RuneWidth(tt.in); out != tt.eaout {
	//		t.Errorf("RuneWidth(%q) = %d, want %d (EastAsianWidth=true)", tt.in, out, tt.eaout)
	//	}
	//}
}