/** TTY key parsing for /bin/edit: consume one logical key from a mutable byte queue. */

/**
 * @param {number} b1
 */
function bareEditUtf8TrailCount(b1) {
  if (b1 >= 0xc0 && b1 < 0xe0) return 1
  if (b1 >= 0xe0 && b1 < 0xf0) return 2
  if (b1 >= 0xf0) return 3
  return 0
}

/**
 * @param {number[]} bytes
 */
function bareEditUtf8DecodeKey(bytes) {
  try {
    const u = new Uint8Array(bytes)
    if (typeof TextDecoder !== 'undefined') {
      return new TextDecoder('utf-8', { fatal: false }).decode(u)
    }
  } catch {
    /* fall through */
  }
  let s = ''
  for (const b of bytes) s += String.fromCharCode(b)
  return s
}

/**
 * @param {string} seq CSI payload after ESC [, including final byte (e.g. "A", "1;5A", "3~")
 */
function bareEditCsiToEvent(seq) {
  if (seq === '3~') return { type: 'ctrl', code: 'delete' }
  if (seq === '5~') return { type: 'nav', key: 'pageup' }
  if (seq === '6~') return { type: 'nav', key: 'pagedown' }
  const last = seq.charAt(seq.length - 1)
  if (last === '~') {
    if (seq === '1~' || seq === '7~') return { type: 'nav', key: 'home' }
    if (seq === '4~' || seq === '8~') return { type: 'nav', key: 'end' }
    const tildeNum = /^(\d+)~$/.exec(seq)
    if (tildeNum) {
      const n = parseInt(tildeNum[1], 10)
      const fnMap = {
        11: 1,
        12: 2,
        13: 3,
        14: 4,
        15: 5,
        17: 6,
        18: 7,
        19: 8,
        20: 9,
        21: 10,
        23: 11,
        24: 12
      }
      if (fnMap[n] != null) return { type: 'fn', n: fnMap[n] }
    }
    return { type: 'unknown' }
  }
  if (last === 'Z') return { type: 'nav', key: 'stab' }
  if (last === 'A' || last === 'B' || last === 'C' || last === 'D') {
    const map = { A: 'up', B: 'down', C: 'right', D: 'left' }
    return { type: 'nav', key: map[last] }
  }
  if (last === 'H') return { type: 'nav', key: 'home' }
  if (last === 'F') return { type: 'nav', key: 'end' }
  return { type: 'unknown' }
}

/**
 * @param {number} b3 byte after ESC O
 */
function bareEditSs3ToEvent(b3) {
  if (b3 === 72) return { type: 'nav', key: 'home' }
  if (b3 === 70) return { type: 'nav', key: 'end' }
  if (b3 === 65) return { type: 'nav', key: 'up' }
  if (b3 === 66) return { type: 'nav', key: 'down' }
  if (b3 === 67) return { type: 'nav', key: 'right' }
  if (b3 === 68) return { type: 'nav', key: 'left' }
  if (b3 === 80) return { type: 'fn', n: 1 }
  if (b3 === 81) return { type: 'fn', n: 2 }
  if (b3 === 82) return { type: 'fn', n: 3 }
  if (b3 === 83) return { type: 'fn', n: 4 }
  return { type: 'unknown' }
}

/**
 * @param {number[]} q mutable queue (front = index 0)
 * @returns {Record<string, unknown> | null} null if more bytes needed
 */
function bareEditTryConsumeKey(q) {
  if (!q.length) return null
  const b1 = q[0]
  if (b1 === 3) {
    q.shift()
    return { type: 'ctrl', code: 'interrupt' }
  }
  if (b1 === 8 || b1 === 127) {
    q.shift()
    return { type: 'ctrl', code: 'backspace' }
  }
  if (b1 === 13 || b1 === 10) {
    q.shift()
    return { type: 'key', ch: '\n' }
  }
  if (b1 === 9) {
    q.shift()
    return { type: 'key', ch: '\t' }
  }
  if (b1 === 27) {
    if (q.length < 2) return null
    const b2 = q[1]
    if (b2 === 91) {
      let i = 2
      while (i < q.length) {
        const b = q[i]
        if (b >= 0x40 && b <= 0x7e) {
          const seq = String.fromCharCode.apply(null, q.slice(2, i + 1))
          q.splice(0, i + 1)
          return bareEditCsiToEvent(seq)
        }
        i++
      }
      return null
    }
    if (b2 === 79) {
      if (q.length < 3) return null
      const b3 = q[2]
      q.splice(0, 3)
      return bareEditSs3ToEvent(b3)
    }
    q.splice(0, 2)
    return { type: 'key', ch: String.fromCharCode(b2) }
  }
  if (b1 < 0x20) {
    q.shift()
    return { type: 'ctrl', code: b1 }
  }
  const need = bareEditUtf8TrailCount(b1)
  if (q.length < 1 + need) return null
  const chunk = q.splice(0, 1 + need)
  return { type: 'key', ch: bareEditUtf8DecodeKey(chunk) }
}