// based on https://developers.google.com/speed/webp/docs/riff_container
import { toHexString, toUTF8String, readInt16LE, readUInt24LE } from './utils'

function calculateExtended (input) {
  return {
    height: 1 + readUInt24LE(input, 7),
    width: 1 + readUInt24LE(input, 4)
  }
}

function calculateLossless (input) {
  return {
    height:
      1 +
      (((input[4] & 0xf) << 10) | (input[3] << 2) | ((input[2] & 0xc0) >> 6)),
    width: 1 + (((input[2] & 0x3f) << 8) | input[1])
  }
}

function calculateLossy (input) {
  // `& 0x3fff` returns the last 14 bits
  // TO-DO: include webp scaling in the calculations
  return {
    height: readInt16LE(input, 8) & 0x3fff,
    width: readInt16LE(input, 6) & 0x3fff
  }
}

export const WEBP = {
  validate (input) {
    const riffHeader = 'RIFF' === toUTF8String(input, 0, 4)
    const webpHeader = 'WEBP' === toUTF8String(input, 8, 12)
    const vp8Header = 'VP8' === toUTF8String(input, 12, 15)
    return riffHeader && webpHeader && vp8Header
  },

  calculate (input) {
    const chunkHeader = toUTF8String(input, 12, 16)
    input = input.slice(20, 30)

    // Extended webp stream signature
    if (chunkHeader === 'VP8X') {
      const extendedHeader = input[0]
      const validStart = (extendedHeader & 0xc0) === 0
      const validEnd = (extendedHeader & 0x01) === 0
      if (validStart && validEnd) {
        return calculateExtended(input)
      } else {
        // TODO: breaking change
        throw new TypeError('Invalid WebP')
      }
    }

    // Lossless webp stream signature
    if (chunkHeader === 'VP8 ' && input[0] !== 0x2f) {
      return calculateLossy(input)
    }

    // Lossy webp stream signature
    const signature = toHexString(input, 3, 6)
    if (chunkHeader === 'VP8L' && signature !== '9d012a') {
      return calculateLossless(input)
    }

    throw new TypeError('Invalid WebP')
  }
}
