Add editor packages

internal/editor/buffer/line/edit.go

@@ -0,0 +1,51 @@
+package line
+
+import "strings"
+
+func (l *Line) Append(s string) {
+	l.data = l.data + s
+}
+
+func (l *Line) DeleteRune(index int) (deleted rune) {
+	if index < 0 || index >= len(l.data) {
+		return
+	}
+
+	var str strings.Builder
+	for i, r := range l.data {
+		if i == index {
+			deleted = r
+			continue
+		}
+		str.WriteRune(r)
+	}
+
+	l.data = str.String()
+
+	return deleted
+}
+
+func (l *Line) Insert(index int, nr rune) {
+	if index < 0 || index > len(l.data) {
+		return
+	}
+
+	var str strings.Builder
+	switch {
+
+	// Handle empty lines and the end of the line.
+	case index == len(l.data):
+		str.WriteString(l.data)
+		str.WriteRune(nr)
+
+	default:
+		for i, r := range l.data {
+			if i == index {
+				str.WriteRune(nr)
+			}
+			str.WriteRune(r)
+		}
+	}
+
+	l.data = str.String()
+}