1 files changed, 110 insertions, 0 deletions
diff --git a/internal/render/width.go b/internal/render/width.go
new file mode 100644
index 0000000..c5676d5
--- /dev/null
+++ b/internal/render/width.go
@@ -0,0 +1,110 @@
+package render
+
+import (
+	"strings"
+	"unicode"
+)
+
+// wideRanges are the code point ranges this program can emit that occupy two
+// terminal cells. Only the sets we actually produce are covered -- our own icon
+// glyphs, plus the CJK blocks a place name could contain -- rather than the
+// whole Unicode width table, which would be a dependency or a large generated
+// file for no gain.
+var wideRanges = [][2]rune{
+	{0x1100, 0x115F}, // Hangul Jamo
+	{0x2329, 0x232A},
+	{0x2E80, 0x303E}, // CJK radicals, Kangxi
+	{0x3041, 0x33FF}, // kana, CJK compatibility
+	{0x3400, 0x4DBF}, // CJK extension A
+	{0x4E00, 0x9FFF}, // CJK unified
+	{0xA000, 0xA4CF}, // Yi
+	{0xAC00, 0xD7A3}, // Hangul syllables
+	{0xF900, 0xFAFF}, // CJK compatibility ideographs
+	{0xFE30, 0xFE6F}, // CJK compatibility forms
+	{0xFF00, 0xFF60}, // fullwidth forms
+	{0xFFE0, 0xFFE6},
+	{0x1F300, 0x1F64F}, // emoji: weather, faces
+	{0x1F680, 0x1F6FF}, // emoji: transport and symbols
+	{0x1F900, 0x1F9FF}, // supplemental symbols
+	{0x26C4, 0x26C8},   // snowman, thundercloud
+	{0x2614, 0x2615},   // umbrella with rain, hot beverage
+}
+
+// ambiguousWide lists the individual code points we emit whose East-Asian width
+// is Ambiguous but which terminals in this estate render as two cells.
+var ambiguousWide = map[rune]bool{
+	0x26C5: true, // sun behind cloud
+	0x26C8: true, // thunder cloud and rain
+}
+
+func runeWidth(r rune) int {
+	switch {
+	case r == 0xFE0F:
+		// Variation Selector-16 requests emoji presentation. It has no width of
+		// its own; its effect is already counted on the base rune.
+		return 0
+	case r == 0xFE0E:
+		return 0
+	case unicode.Is(unicode.Mn, r) || unicode.Is(unicode.Me, r) || unicode.Is(unicode.Cf, r):
+		return 0 // combining and formatting marks occupy no cell
+	case r == '‍':
+		return 0 // zero-width joiner
+	case r < 0x20:
+		return 0
+	case ambiguousWide[r]:
+		return 2
+	}
+	for _, rng := range wideRanges {
+		if r >= rng[0] && r <= rng[1] {
+			return 2
+		}
+	}
+	return 1
+}
+
+// DisplayWidth is the number of terminal cells a string occupies.
+//
+// Neither len() nor a rune count will do: an emoji may be two cells, a
+// variation selector is zero, and combining marks are zero. Padding with the
+// wrong number shears every column to the right of it -- and only in a real
+// terminal, never when the output is piped, which is what makes it easy to miss.
+func DisplayWidth(s string) int {
+	w := 0
+	for _, r := range s {
+		w += runeWidth(r)
+	}
+	return w
+}
+
+// Pad returns s padded with spaces to at least w display cells (left aligned).
+func Pad(s string, w int) string {
+	if n := w - DisplayWidth(s); n > 0 {
+		return s + strings.Repeat(" ", n)
+	}
+	return s
+}
+
+// PadLeft returns s padded with spaces to at least w display cells (right aligned).
+func PadLeft(s string, w int) string {
+	if n := w - DisplayWidth(s); n > 0 {
+		return strings.Repeat(" ", n) + s
+	}
+	return s
+}
+
+// Truncate cuts s to at most w display cells, never splitting a rune.
+func Truncate(s string, w int) string {
+	if DisplayWidth(s) <= w {
+		return s
+	}
+	out, used := make([]rune, 0, len(s)), 0
+	for _, r := range s {
+		rw := runeWidth(r)
+		if used+rw > w {
+			break
+		}
+		out = append(out, r)
+		used += rw
+	}
+	return string(out)
+}