termdash/cell/runewidth/runewidth.go

// Package 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 {
	if inTable(r, exceptions) {
		return 1
	}
	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
}

// inTable determines if the rune falls within the table.
// Copied from github.com/mattn/go-runewidth/blob/master/runewidth.go.
func inTable(r rune, t table) bool {
	// func (t table) IncludesRune(r rune) bool {
	if r < t[0].first {
		return false
	}

	bot := 0
	top := len(t) - 1
	for top >= bot {
		mid := (bot + top) >> 1

		switch {
		case t[mid].last < r:
			bot = mid + 1
		case t[mid].first > r:
			top = mid - 1
		default:
			return true
		}
	}

	return false
}

type interval struct {
	first rune
	last  rune
}

type table []interval

// exceptions runes defined here are always considered to be half-width even if
// they might be ambiguous in some contexts.
var exceptions = table{
	// Characters used by termdash to indicate text trim or scroll.
	{0x2026, 0x2026},
	{0x21c4, 0x21c4},
	{0x21e7, 0x21e7},
	{0x21e9, 0x21e9},

	// Box drawing, used as line-styles.
	// https://en.wikipedia.org/wiki/Box-drawing_character
	{0x2500, 0x257F},

	// Block elements used as sparks.
	// https://en.wikipedia.org/wiki/Box-drawing_character
	{0x2580, 0x258F},
}