PEE/src/gif.ts

import { Buffer } from "buffer";
import { BufferWriteStream } from "./png";

const netscape = Buffer.from("!\xFF\x0BNETSCAPE2.0", 'ascii');
const magic = Buffer.from("!\xFF\x0B" + "COOMTECH0.1", 'ascii');

const extractBuff = (gif: Buffer) => {
    const field = gif.readUInt8(10);
    const gcte = !!(field & (1 << 7));
    let end = 13;
    if (gcte) {
        end += 3 * (1 << ((field & 7) + 1));
    }
    // skip beeg blocks
    while (gif.readUInt8(end) == '!'.charCodeAt(0)) {
        if (magic.compare(gif, end, end + magic.byteLength) != 0) {
            end += 3 + gif.readUInt8(end + 2);
            // eslint-disable-next-line no-constant-condition
            while (true) { // skip sub blocks
                const v = gif.readUInt8(end++);
                if (!v)
                    break;
                end += v;
            }
        } else {
            let count = end + magic.byteLength;
            let t = 0;
            let v = 0;
            while ((v = gif.readUInt8(count)) != 0) {
                t += v;
                count += v + 1;
            }
            const buff = Buffer.alloc(t);
            count = end + magic.byteLength;
            t = 0;
            while ((v = gif.readUInt8(count)) != 0) {
                gif.copy(buff, t, count + 1, count + 1 + v);
                t += v;
                count += v + 1;
            }
            return {filename: 'embedded', data: buff};
        }
    }
    // metadata ended, nothing...
};

export const extract = extractBuff;

const write_embedding = async (writer: WritableStreamDefaultWriter<Buffer>, inj: Buffer) => {
    await writer.write(magic);
    const byte = Buffer.from([0]);
    let size = inj.byteLength;
    let ws;
    let offset = 0;
    while (size != 0) {
        ws = size >= 255 ? 255 : size;
        byte.writeUInt8(ws, 0);
        await writer.write(byte);
        await writer.write(inj.slice(offset, offset + ws));
        size -= ws;
        offset += ws;
    }
    byte.writeUInt8(0, 0);
    await writer.write(byte);
};

export const inject = async (container: File, inj: File) => {
    const [writestream, extract] = BufferWriteStream();
    const writer = writestream.getWriter();

    const contbuff = Buffer.from(await container.arrayBuffer());

    const field = contbuff.readUInt8(10);
    const gcte = !!(field & (1 << 0x7));
    let endo = 13;
    if (gcte)
        endo += 3 * (1 << ((field & 7) + 1));

    if (netscape.compare(contbuff, endo, endo + netscape.byteLength) == 0)
        endo += netscape.byteLength;
    await writer.write(contbuff.slice(0, endo));
    await write_embedding(writer, Buffer.from(await inj.arrayBuffer()));
    await writer.write(contbuff.slice(endo));
    return extract();
};

export const has_embed = (gif: Buffer) => {
    const field = gif.readUInt8(10);
    const gcte = !!(field & (1 << 7));
    let end = 13;
    if (gcte) {
        end += 3 * (1 << ((field & 7) + 1));
    }
    // skip beeg blocks
    while (end < gif.byteLength && gif.readUInt8(end) == '!'.charCodeAt(0)) {
        if (magic.compare(gif, end, end + magic.byteLength) != 0) {
            end += 3 + gif.readUInt8(end + 2);
            // eslint-disable-next-line no-constant-condition
            while (true) { // skip sub blocks
                const v = gif.readUInt8(end++);
                if (!v)
                    break;
                end += v;
            }
        } else {
            return true;
        }
    }
    if (end >= gif.byteLength)
        return; // Don't know yet, need more to decide.
    return false; // no more extension blocks, so definite no
};