const pngSignature = Buffer.from([137, 80, 78, 71, 13, 10, 26, 10]);
const hashKey = 'react-snapshot-hash';
const crcTable = [];
const initialCrc = 0xffffffff;

for (let n = 0; n < 256; n++) {
    let c = n;

    for (let k = 0; k < 8; k++) {
        if (c & 1) {
            c = 0xedb88320 ^ (c >>> 1);
        } else {
            c = c >>> 1;
        }
    }

    crcTable[n] = c;
}

function updateCrc(crc, data, length) {
    let c = crc;

    for (let n = 0; n < length; n++) {
        c = crcTable[(c ^ data[n]) & 0xff] ^ (c >>> 8);
    }

    return c;
}

function crc(data, length) {
    return (updateCrc(initialCrc, data, length) ^ initialCrc) >>> 0;
}

function fetchRequestHash(data) {
    const view = new DataView(data.buffer);

    if (pngSignature.compare(data, 0, pngSignature.length) !== 0) {
        return false;
    }

    // fast png scan
    for (let offset = pngSignature.length; offset < data.length;) {
        const len = view.getUint32(offset);
        const type = data.toString('ascii', offset + 4, offset + 8);

        // search for text chunk with `hashKey` as a key
        if (type === 'tEXt') {
            const keyEnd = data.indexOf(0, offset + 8);

            if (keyEnd !== -1) {
                const key = data.toString('ascii', offset + 8, keyEnd);

                if (key === hashKey) {
                    return data.toString('utf8', keyEnd + 1, offset + 8 + len);
                }
            }
        }

        offset += len + 12; // len + type + crc
    }

    return false;
}

function buildRequestHashChunk(hash) {
    const data = Buffer.from([
        'tEXt',  // type
        hashKey, // key
        '\0',    // key terminator
        hash     // data
    ].join(''));
    const res = Buffer.alloc(4 + data.length + 4);
    const view = new DataView(res.buffer);

    view.setUint32(0, data.length - 4);
    view.setUint32(4 + data.length, crc(data, data.length));
    data.copy(res, 4);

    return res;
}

function injectRequestHash(buffer, hash) {
    return Buffer.concat([
        buffer.slice(0, buffer.length - 12),
        buildRequestHashChunk(hash),
        buffer.slice(buffer.length - 12)
    ]);
}

module.exports = {
    crc,
    fetchRequestHash,
    injectRequestHash
};