// Terminal key encoding: control characters, meta prefixes, and cursor escape
// sequences. Table-driven and easy to get subtly wrong, so it lives apart from
// the DOM and is tested directly.

export function toCtrl(ch: string): string {
  if (ch.length !== 1) return ch;
  const upper = ch.toUpperCase().codePointAt(0) ?? 0;
  if (upper >= 0x40 && upper <= 0x5f) return String.fromCodePoint(upper & 0x1f);
  if (ch === " ") return "\x00";
  if (ch === "?") return "\x7f";
  return ch;
}

/** Apply pending modifiers to a single typed character: Ctrl → control code,
 *  Alt → ESC prefix (xterm meta). Both compose (Ctrl+Alt = ESC + control). */
export function applyModsToChar(ch: string, ctrl: boolean, alt: boolean): string {
  let out = ctrl ? toCtrl(ch) : ch;
  if (alt) out = "\x1b" + out;
  return out;
}

/** Cursor/nav-key sequence, promoted to the modified CSI form when Ctrl/Alt are
 *  pending — `\x1b[1;<n><final>`, n = 1 + alt(2) + ctrl(4) (xterm convention). */
export function cursorSeq(final: string, ctrl: boolean, alt: boolean): string {
  if (!ctrl && !alt) return `\x1b[${final}`;
  return `\x1b[1;${modParam(ctrl, alt)}${final}`;
}

/** Modifier parameter of the xterm CSI forms: n = 1 + alt(2) + ctrl(4). */
function modParam(ctrl: boolean, alt: boolean): number {
  return 1 + (alt ? 2 : 0) + (ctrl ? 4 : 0);
}

/** Sequence for the `~`-terminated navigation keys (PgUp `5`, PgDn `6`), which
 *  carry modifiers as `\x1b[<code>;<n>~` rather than the cursor keys' `1;<n>`
 *  form. */
export function tildeSeq(code: number, ctrl: boolean, alt: boolean): string {
  if (!ctrl && !alt) return `\x1b[${code}~`;
  return `\x1b[${code};${modParam(ctrl, alt)}~`;
}