78 lines
2.4 KiB
TypeScript
78 lines
2.4 KiB
TypeScript
const PNG_SIGNATURE = new Uint8Array([137, 80, 78, 71, 13, 10, 26, 10]);
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const ACTL_CHUNK_BYTES = new TextEncoder().encode('acTL');
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const IDAT_CHUNK_BYTES = new TextEncoder().encode('IDAT');
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const MAX_BYTES_TO_READ = 1024 * 1024;
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interface AnimatedPngData {
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numPlays: number;
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}
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/**
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* This is a naïve implementation. It only performs two checks:
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*
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* 1. Do the bytes start with the [PNG signature][0]?
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* 2. If so, does it contain the [`acTL` chunk][1] before the [`IDAT` chunk][2], in the
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* first megabyte?
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*
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* Though we _could_ only check for the precense of the `acTL` chunk anywhere, we make
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* sure it's before the `IDAT` chunk and within the first megabyte. This adds a small
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* amount of validity checking and helps us avoid problems with large PNGs.
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*
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* It doesn't make sure the PNG is valid. It doesn't verify [the CRC code][3] of each PNG
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* chunk; it doesn't verify any of the chunk's data; it doesn't verify that the chunks are
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* in the right order; etc.
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*
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* [0]: https://www.w3.org/TR/PNG/#5PNG-file-signature
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* [1]: https://wiki.mozilla.org/APNG_Specification#.60acTL.60:_The_Animation_Control_Chunk
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* [2]: https://www.w3.org/TR/PNG/#11IDAT
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* [3]: https://www.w3.org/TR/PNG/#5Chunk-layout
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*/
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export function getAnimatedPngDataIfExists(
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bytes: Uint8Array
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): null | AnimatedPngData {
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if (!hasPngSignature(bytes)) {
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return null;
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}
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let numPlays: void | number;
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const dataView = new DataView(bytes.buffer);
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let i = PNG_SIGNATURE.length;
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while (i < bytes.byteLength && i <= MAX_BYTES_TO_READ) {
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const chunkTypeBytes = bytes.slice(i + 4, i + 8);
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if (areBytesEqual(chunkTypeBytes, ACTL_CHUNK_BYTES)) {
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// 4 bytes for the length; 4 bytes for the type; 4 bytes for the number of frames.
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numPlays = dataView.getUint32(i + 12);
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if (numPlays === 0) {
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numPlays = Infinity;
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}
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return { numPlays };
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}
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if (areBytesEqual(chunkTypeBytes, IDAT_CHUNK_BYTES)) {
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return null;
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}
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// Jump over the length (4 bytes), the type (4 bytes), the data, and the CRC checksum
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// (4 bytes).
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i += 12 + dataView.getUint32(i);
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}
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return null;
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}
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function hasPngSignature(bytes: Uint8Array): boolean {
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return areBytesEqual(bytes.slice(0, 8), PNG_SIGNATURE);
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}
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function areBytesEqual(a: Uint8Array, b: Uint8Array): boolean {
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if (a.byteLength !== b.byteLength) {
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return false;
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}
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for (let i = 0; i < a.byteLength; i += 1) {
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if (a[i] !== b[i]) {
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return false;
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}
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}
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return true;
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}
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