// NOTE: we only support baseline and progressive JPGs here
// due to the structure of the loader class, we only get a buffer
// with a maximum size of 4096 bytes. so if the SOF marker is outside
// if this range we can't detect the file size correctly.
import { readUInt, readUInt16BE, toHexString } from './utils'

const EXIF_MARKER = '45786966'
const APP1_DATA_SIZE_BYTES = 2
const EXIF_HEADER_BYTES = 6
const TIFF_BYTE_ALIGN_BYTES = 2
const BIG_ENDIAN_BYTE_ALIGN = '4d4d'
const LITTLE_ENDIAN_BYTE_ALIGN = '4949'

// Each entry is exactly 12 bytes
const IDF_ENTRY_BYTES = 12
const NUM_DIRECTORY_ENTRIES_BYTES = 2

function isEXIF (input) {
  return toHexString(input, 2, 6) === EXIF_MARKER
}

function extractSize (input, index) {
  return {
    height: readUInt16BE(input, index),
    width: readUInt16BE(input, index + 2)
  }
}

function extractOrientation (exifBlock, isBigEndian) {
  // TODO: assert that this contains 0x002A
  // let STATIC_MOTOROLA_TIFF_HEADER_BYTES = 2
  // let TIFF_IMAGE_FILE_DIRECTORY_BYTES = 4

  // TODO: derive from TIFF_IMAGE_FILE_DIRECTORY_BYTES
  const idfOffset = 8

  // IDF osset works from right after the header bytes
  // (so the offset includes the tiff byte align)
  const offset = EXIF_HEADER_BYTES + idfOffset

  const idfDirectoryEntries = readUInt(exifBlock, 16, offset, isBigEndian)

  for (
    let directoryEntryNumber = 0;
    directoryEntryNumber < idfDirectoryEntries;
    directoryEntryNumber++
  ) {
    const start =
      offset +
      NUM_DIRECTORY_ENTRIES_BYTES +
      directoryEntryNumber * IDF_ENTRY_BYTES
    const end = start + IDF_ENTRY_BYTES

    // Skip on corrupt EXIF blocks
    if (start > exifBlock.length) {
      return
    }

    const block = exifBlock.slice(start, end)
    const tagNumber = readUInt(block, 16, 0, isBigEndian)

    // 0x0112 (decimal: 274) is the `orientation` tag ID
    if (tagNumber === 274) {
      const dataFormat = readUInt(block, 16, 2, isBigEndian)
      if (dataFormat !== 3) {
        return
      }

      // unsinged int has 2 bytes per component
      // if there would more than 4 bytes in total it's a pointer
      const numberOfComponents = readUInt(block, 32, 4, isBigEndian)
      if (numberOfComponents !== 1) {
        return
      }

      return readUInt(block, 16, 8, isBigEndian)
    }
  }
}

function validateExifBlock (input, index) {
  // Skip APP1 Data Size
  const exifBlock = input.slice(APP1_DATA_SIZE_BYTES, index)

  // Consider byte alignment
  const byteAlign = toHexString(
    exifBlock,
    EXIF_HEADER_BYTES,
    EXIF_HEADER_BYTES + TIFF_BYTE_ALIGN_BYTES
  )

  // Ignore Empty EXIF. Validate byte alignment
  const isBigEndian = byteAlign === BIG_ENDIAN_BYTE_ALIGN
  const isLittleEndian = byteAlign === LITTLE_ENDIAN_BYTE_ALIGN

  if (isBigEndian || isLittleEndian) {
    return extractOrientation(exifBlock, isBigEndian)
  }
}

function validateInput (input, index) {
  // index should be within buffer limits
  if (index > input.length) {
    throw new TypeError('Corrupt JPG, exceeded buffer limits')
  }
  // Every JPEG block must begin with a 0xFF
  if (input[index] !== 0xff) {
    throw new TypeError('Invalid JPG, marker table corrupted')
  }
}

export const JPG = {
  validate: (input) => toHexString(input, 0, 2) === 'ffd8',

  calculate (input) {
    // Skip 4 chars, they are for signature
    input = input.slice(4)

    let orientation
    let next
    while (input.length) {
      // read length of the next block
      const i = readUInt16BE(input, 0)

      if (isEXIF(input)) {
        orientation = validateExifBlock(input, i)
      }

      // ensure correct format
      validateInput(input, i)

      // 0xFFC0 is baseline standard(SOF)
      // 0xFFC1 is baseline optimized(SOF)
      // 0xFFC2 is progressive(SOF2)
      next = input[i + 1]
      if (next === 0xc0 || next === 0xc1 || next === 0xc2) {
        const size = extractSize(input, i + 5)

        // TODO: is orientation=0 a valid answer here?
        if (!orientation) {
          return size
        }

        return {
          height: size.height,
          orientation,
          width: size.width
        }
      }

      // move to the next block
      input = input.slice(i + 2)
    }

    throw new TypeError('Invalid JPG, no size found')
  }
}
