Add editor packages

internal/editor/buffer/glyph/glyph.go

@@ -0,0 +1,15 @@
+package glyph
+
+import "github.com/mattn/go-runewidth"
+
+// Width returns the terminal column width of a rune.
+// Tabs use a fixed width of 8 columns.
+// TODO: handle Unicode marks.
+func Width(r rune) int {
+	switch r {
+	case '\t':
+		return 8
+	default:
+		return runewidth.RuneWidth(r)
+	}
+}