import { EventEmitter } from 'node:events'

let s = 0
const STATE_DICT = {
  PARSER_UNINITIALIZED: s++,
  START: s++,
  START_BOUNDARY: s++,
  HEADER_FIELD_START: s++,
  HEADER_FIELD: s++,
  HEADER_VALUE_START: s++,
  HEADER_VALUE: s++,
  HEADER_VALUE_ALMOST_DONE: s++,
  HEADERS_ALMOST_DONE: s++,
  PART_DATA_START: s++,
  PART_DATA: s++,
  PART_END: s++,
  END: s++
}
let f = 1
const FLAG_DICT = {
  PART_BOUNDARY: f,
  LAST_BOUNDARY: (f *= 2)
}

const LF = 10
const CR = 13
const SPACE = 32
const HYPHEN = 45
const COLON = 58
const LETTER_A = 97
const LETTER_Z = 122

function lower(c) {
  return c | 0x20
}

function stateToString(value) {
  for (let key in STATE_DICT) {
    let number = STATE_DICT[key]
    if (number === value) {
      return key
    }
  }
}

export class MultipartParser {
  boundary = null
  boundaryChars = null
  lookbehind = null
  state = STATE_DICT.PARSER_UNINITIALIZED

  index = null
  flags = 0

  constructor(str) {
    this.boundary = Buffer.alloc(str.length + 4)
    this.boundary.write('\r\n--', 0)
    this.boundary.write(str, 4)
    this.lookbehind = Buffer.alloc(this.boundary.length + 8)
    this.state = STATE_DICT.START

    this.boundaryChars = {}
    for (let i = 0; i < this.boundary.length; i++) {
      this.boundaryChars[this.boundary[i]] = true
    }
  }

  #mark(k, v) {
    this[k + 'Mark'] = v
  }

  #emit(name, buff, idx, cleanup) {
    let mark = name + 'Mark'
    if (this[mark] !== void 0) {
      let start = this[mark]
      let end = buff.length

      if (cleanup) {
        end = idx
        delete this[mark]
      } else {
        this[mark] = 0
      }

      if (start === end) {
        return
      }
      this['$' + name](buff.slice(start, end))
    }
  }

  write(buffer) {
    let idx = 0,
      len = buffer.length,
      prevIndex = this.index,
      index = this.index,
      state = this.state,
      flags = this.flags,
      lookbehind = this.lookbehind,
      boundary = this.boundary,
      boundaryChars = this.boundaryChars,
      boundaryLength = this.boundary.length,
      boundaryEnd = boundaryLength - 1,
      bufferLength = buffer.length,
      c,
      cl

    for (idx = 0; idx < len; idx++) {
      c = buffer[idx]

      switch (state) {
        case STATE_DICT.PARSER_UNINITIALIZED:
          return

        case STATE_DICT.START:
          index = 0
          state = STATE_DICT.START_BOUNDARY

        case STATE_DICT.START_BOUNDARY:
          if (index == boundary.length - 2) {
            if (c == HYPHEN) {
              flags |= FLAG_DICT.LAST_BOUNDARY
            } else if (c != CR) {
              return
            }
            index++
            break
          } else if (index - 1 == boundary.length - 2) {
            if (flags & FLAG_DICT.LAST_BOUNDARY && c == HYPHEN) {
              this.$end()
              state = STATE_DICT.END
              flags = 0
            } else if (!(flags & FLAG_DICT.LAST_BOUNDARY) && c == LF) {
              index = 0
              this.$partBegin()
              state = STATE_DICT.HEADER_FIELD_START
            } else {
              return
            }
            break
          }

          if (c != boundary[index + 2]) {
            index = -2
          }
          if (c == boundary[index + 2]) {
            index++
          }
          break

        case STATE_DICT.HEADER_FIELD_START:
          state = STATE_DICT.HEADER_FIELD
          this.#mark('headerField', idx)
          index = 0

        case STATE_DICT.HEADER_FIELD:
          if (c == CR) {
            delete this.headerFieldMark
            state = STATE_DICT.HEADERS_ALMOST_DONE
            break
          }

          index++
          if (c == HYPHEN) {
            break
          }

          if (c == COLON) {
            if (index == 1) {
              // empty header field
              return
            }
            this.#emit('headerField', buffer, idx, true)
            state = STATE_DICT.HEADER_VALUE_START
            break
          }

          cl = lower(c)
          if (cl < LETTER_A || cl > LETTER_Z) {
            return
          }
          break

        case STATE_DICT.HEADER_VALUE_START:
          if (c == SPACE) {
            break
          }

          this.#mark('headerValue', idx)
          state = STATE_DICT.HEADER_VALUE

        case STATE_DICT.HEADER_VALUE:
          if (c == CR) {
            this.#emit('headerValue', buffer, idx, true)
            this.$headerEnd()
            state = STATE_DICT.HEADER_VALUE_ALMOST_DONE
          }
          break

        case STATE_DICT.HEADER_VALUE_ALMOST_DONE:
          if (c != LF) {
            return
          }
          state = STATE_DICT.HEADER_FIELD_START
          break

        case STATE_DICT.HEADERS_ALMOST_DONE:
          if (c != LF) {
            return
          }

          this.$headersEnd()
          state = STATE_DICT.PART_DATA_START
          break

        case STATE_DICT.PART_DATA_START:
          state = STATE_DICT.PART_DATA
          this.#mark('partData', idx)

        case STATE_DICT.PART_DATA:
          prevIndex = index

          if (index === 0) {
            // boyer-moore derrived algorithm to safely skip non-boundary data
            idx += boundaryEnd
            while (idx < bufferLength && !(buffer[idx] in boundaryChars)) {
              idx += boundaryLength
            }
            idx -= boundaryEnd
            c = buffer[idx]
          }

          if (index < boundary.length) {
            if (boundary[index] == c) {
              if (index === 0) {
                this.#emit('partData', buffer, idx, true)
              }
              index++
            } else {
              index = 0
            }
          } else if (index == boundary.length) {
            index++
            if (c == CR) {
              // CR = part boundary
              flags |= FLAG_DICT.PART_BOUNDARY
            } else if (c == HYPHEN) {
              // HYPHEN = end boundary
              flags |= FLAG_DICT.LAST_BOUNDARY
            } else {
              index = 0
            }
          } else if (index - 1 == boundary.length) {
            if (flags & FLAG_DICT.PART_BOUNDARY) {
              index = 0
              if (c == LF) {
                // unset the PART_BOUNDARY flag
                flags &= ~FLAG_DICT.PART_BOUNDARY
                this.$partEnd()
                this.$partBegin()
                state = STATE_DICT.HEADER_FIELD_START
                break
              }
            } else if (flags & FLAG_DICT.LAST_BOUNDARY) {
              if (c == HYPHEN) {
                this.$partEnd()
                this.$end()
                state = STATE_DICT.END
                flags = 0
              } else {
                index = 0
              }
            } else {
              index = 0
            }
          }

          if (index > 0) {
            // when matching a possible boundary, keep a lookbehind reference
            // in case it turns out to be a false lead
            lookbehind[index - 1] = c
          } else if (prevIndex > 0) {
            // if our boundary turned out to be rubbish, the captured lookbehind
            // belongs to partData

            this.$partData(lookbehind.slice(0, prevIndex))
            prevIndex = 0
            this.#mark('partData', idx)

            // reconsider the current character even so it interrupted the sequence
            // it could be the beginning of a new sequence
            idx--
          }

          break

        case STATE_DICT.END:
          break

        default:
          return
      }
    }

    this.#emit('headerField', buffer, idx)
    this.#emit('headerValue', buffer, idx)
    this.#emit('partData', buffer, idx)

    this.index = index
    this.state = state
    this.flags = flags
  }

  end() {
    if (
      (this.state === STATE_DICT.HEADER_FIELD_START && this.index === 0) ||
      (this.state === STATE_DICT.PART_DATA &&
        this.index == this.boundary.length)
    ) {
      this.$end()
    } else if (this.state !== STATE_DICT.END) {
      return new Error(
        'MultipartParser.end(): stream ended unexpectedly: ' + this.explain()
      )
    }
  }

  explain() {
    return 'state = ' + stateToString(this.state)
  }
}