formats.exposed/public/gif/parseGif.js

// @ts-check

import { GifNodeType } from "./constants.js";
/** @typedef {import("../../types/gif.d.ts").GifNode} GifNode */

/**
 * @typedef {object} LogicalScreenDescriptorPackedField
 * @prop {boolean} globalColorTableFlag
 * @prop {number} colorResolution
 * @prop {boolean} sortFlag
 * @prop {number} sizeOfGlobalColorTable
 */

/**
 * @param {number} byte
 * @returns {LogicalScreenDescriptorPackedField}
 */
export const parseLogicalScreenDescriptorPackedField = (byte) => ({
  globalColorTableFlag: (byte & 0b10000000) !== 0,
  colorResolution: ((byte & 0b01110000) >> 4) + 1,
  sortFlag: (byte & 0b00001000) !== 0,
  sizeOfGlobalColorTable: 2 ** ((byte & 0b00000111) + 1),
});

/**
 * @param {Uint8Array} bytes
 * @returns {null | GifNode} The root node of the GIF tree, or null if the GIF is invalid.
 */
export default (bytes) => {
  const widthBytes = bytes.subarray(6, 6 + 2);
  const heightBytes = bytes.subarray(8, 8 + 2);
  const packedFieldBytes = bytes.subarray(10, 10 + 1);
  const packedFieldByte = packedFieldBytes[0];

  /** @type {GifNode[]} */
  const children = [
    {
      type: GifNodeType.header,
      bytes: bytes.subarray(0, 6),
      children: [
        {
          type: GifNodeType.headerSignature,
          bytes: bytes.subarray(0, 3),
        },
        {
          type: GifNodeType.headerVersion,
          bytes: bytes.subarray(3, 3 + 3),
        },
      ],
    },
    {
      type: GifNodeType.logicalScreenDescriptor,
      bytes: bytes.subarray(6, 6 + 6),
      children: [
        {
          type: GifNodeType.logicalScreenWidth,
          bytes: widthBytes,
        },
        {
          type: GifNodeType.logicalScreenHeight,
          bytes: heightBytes,
        },
        {
          type: GifNodeType.logicalScreenDescriptorPackedFields,
          bytes: packedFieldBytes,
        },
        {
          type: GifNodeType.logicalScreenBackgroundColorIndex,
          bytes: bytes.subarray(11, 11 + 1),
        },
        {
          type: GifNodeType.logicalScreenPixelAspectRatio,
          bytes: bytes.subarray(12, 12 + 1),
        },
      ],
    },
  ];

  return { type: GifNodeType.root, bytes, children };
};