A wrapper over the external runewidth package.

cell/runewidth/runewidth.go

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+// runewidth is a wrapper over github.com/mattn/go-runewidth which gives
+// different treatment to certain runes with ambiguous width.
+package runewidth
+
+import runewidth "github.com/mattn/go-runewidth"
+
+// RuneWidth returns the number of cells needed to draw r.
+// Background in http://www.unicode.org/reports/tr11/.
+//
+// Treats runes used internally by termdash as single-cell (half-width) runes
+// regardless of the locale. I.e. runes that are used to draw lines, boxes,
+// indicate resize or text trimming was needed and runes used by the braille
+// canvas.
+//
+// This should be safe, since even in locales where these runes have ambiguous
+// width, we still place all the character content around them so they should
+// have be half-width.
+func RuneWidth(r rune) int {
+	return runewidth.RuneWidth(r)
+}
+
+// StringWidth is like RuneWidth, but returns the number of cells occupied by
+// all the runes in the string.
+func StringWidth(s string) int {
+	var width int
+	for _, r := range []rune(s) {
+		width += RuneWidth(r)
+	}
+	return width
+}

cell/runewidth/runewidth_test.go

@@ -0,0 +1,97 @@
+package runewidth
+
+import (
+	"testing"
+
+	runewidth "github.com/mattn/go-runewidth"
+)
+
+func TestRuneWidth(t *testing.T) {
+	tests := []struct {
+		desc      string
+		runes     []rune
+		eastAsian bool
+		want      int
+	}{
+		{
+			desc:  "ascii characters",
+			runes: []rune{'a', 'f', '#'},
+			want:  1,
+		},
+		{
+			desc:  "non-printable characters from mattn/runewidth/runewidth_test",
+			runes: []rune{'\x00', '\x01', '\u0300', '\u2028', '\u2029'},
+			want:  0,
+		},
+		{
+			desc:  "half-width runes from mattn/runewidth/runewidth_test",
+			runes: []rune{'ｾ', 'ｶ', 'ｲ', '☆'},
+			want:  1,
+		},
+		{
+			desc:  "full-width runes from mattn/runewidth/runewidth_test",
+			runes: []rune{'世', '界'},
+			want:  2,
+		},
+		{
+			desc:      "ambiguous so double-width in eastAsian from mattn/runewidth/runewidth_test",
+			runes:     []rune{'☆'},
+			eastAsian: true,
+			want:      2,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.desc, func(t *testing.T) {
+			runewidth.DefaultCondition.EastAsianWidth = tc.eastAsian
+			defer func() {
+				runewidth.DefaultCondition.EastAsianWidth = false
+			}()
+
+			for _, r := range tc.runes {
+				if got := RuneWidth(r); got != tc.want {
+					t.Errorf("RuneWidth(%v) => %v, want %v", r, got, tc.want)
+				}
+			}
+		})
+	}
+}
+
+func TestStringWidth(t *testing.T) {
+	tests := []struct {
+		desc      string
+		str       string
+		eastAsian bool
+		want      int
+	}{
+		{
+			desc: "ascii characters",
+			str:  "hello",
+			want: 5,
+		},
+		{
+			desc: "string from mattn/runewidth/runewidth_test",
+			str:  "■㈱の世界①",
+			want: 10,
+		},
+		{
+			desc:      "string in eastAsian from mattn/runewidth/runewidth_test",
+			str:       "■㈱の世界①",
+			eastAsian: true,
+			want:      12,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.desc, func(t *testing.T) {
+			runewidth.DefaultCondition.EastAsianWidth = tc.eastAsian
+			defer func() {
+				runewidth.DefaultCondition.EastAsianWidth = false
+			}()
+
+			if got := StringWidth(tc.str); got != tc.want {
+				t.Errorf("StringWidth(%q) => %v, want %v", tc.str, got, tc.want)
+			}
+		})
+	}
+}