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/* eslint no-bitwise: "off" */
const calculateWebpLength = (webpFile) => {
const d = new Uint8Array(webpFile);
// https://developers.google.com/speed/webp/docs/riff_container#animation
let duration = 0;
for (let i = 0; i < d.length; i++) {
// Find ANMF header (41 4E 4D 46)
if (d[i] === 0x41
&& d[i + 1] === 0x4E
&& d[i + 2] === 0x4D
&& d[i + 3] === 0x46) {
// Swap 5th and 6th bytes to get the delay per frame
const delay = (d[i + 21] << 8) | (d[i + 20] & 0xFF);
// Should be aware browsers have a minimum frame delay
// e.g. 6ms for IE, 2ms modern browsers (50fps)
duration += delay < 2 ? 10 : delay;
}
}
return duration;
};
export default calculateWebpLength;
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