/**
 * @copyright Shachaf Ben-Kiki and the Converse.js contributors
 * @description
 *  Started as a fork of Shachaf Ben-Kiki's jsgif library
 *  https://github.com/shachaf/jsgif
 * @license MIT License
 */

function bitsToNum (ba) {
    return ba.reduce(function (s, n) {
        return s * 2 + n;
    }, 0);
}

function byteToBitArr (bite) {
    const a = [];
    for (let i = 7; i >= 0; i--) {
        a.push( !! (bite & (1 << i)));
    }
    return a;
}

function lzwDecode (minCodeSize, data) {
    // TODO: Now that the GIF parser is a bit different, maybe this should get an array of bytes instead of a String?
    let pos = 0; // Maybe this streaming thing should be merged with the Stream?
    function readCode (size) {
        let code = 0;
        for (let i = 0; i < size; i++) {
            if (data.charCodeAt(pos >> 3) & (1 << (pos & 7))) {
                code |= 1 << i;
            }
            pos++;
        }
        return code;
    }

    const output = [];
    const clearCode = 1 << minCodeSize;
    const eoiCode = clearCode + 1;

    let codeSize = minCodeSize + 1;
    let dict = [];

    const clear = function () {
        dict = [];
        codeSize = minCodeSize + 1;
        for (let i = 0; i < clearCode; i++) {
            dict[i] = [i];
        }
        dict[clearCode] = [];
        dict[eoiCode] = null;
    };

    let code = clearCode;
    let last;
    clear();

    while (true) { // eslint-disable-line no-constant-condition
        last = code;
        code = readCode(codeSize);

        if (code === clearCode) {
            clear();
            continue;
        }
        if (code === eoiCode) break;

        if (code < dict.length) {
            if (last !== clearCode) {
                dict.push(dict[last].concat(dict[code][0]));
            }
        }
        else {
            if (code !== dict.length) throw new Error('Invalid LZW code.');
            dict.push(dict[last].concat(dict[last][0]));
        }
        output.push.apply(output, dict[code]);

        if (dict.length === (1 << codeSize) && codeSize < 12) {
            // If we're at the last code and codeSize is 12, the next code will be a clearCode, and it'll be 12 bits long.
            codeSize++;
        }
    }
    // I don't know if this is technically an error, but some GIFs do it.
    //if (Math.ceil(pos / 8) !== data.length) throw new Error('Extraneous LZW bytes.');
    return output;
}


function readSubBlocks (st) {
    let size, data;
    data = '';
    do {
        size = st.readByte();
        data += st.read(size);
    } while (size !== 0);
    return data;
}

/**
 * Parses GIF image color table information
 * @param { Stream } st
 * @param { Number } entries
 */
function parseCT (st, entries) { // Each entry is 3 bytes, for RGB.
    const ct = [];
    for (let i = 0; i < entries; i++) {
        ct.push(st.readBytes(3));
    }
    return ct;
}

/**
 * Parses GIF image information
 * @param { Stream } st
 * @param { ByteStream } img
 * @param { Function } [callback]
 */
function parseImg (st, img, callback) {
    function deinterlace (pixels, width) {
        // Of course this defeats the purpose of interlacing. And it's *probably*
        // the least efficient way it's ever been implemented. But nevertheless...
        const newPixels = new Array(pixels.length);
        const rows = pixels.length / width;
        function cpRow (toRow, fromRow) {
            const fromPixels = pixels.slice(fromRow * width, (fromRow + 1) * width);
            newPixels.splice.apply(newPixels, [toRow * width, width].concat(fromPixels));
        }

        // See appendix E.
        const offsets = [0, 4, 2, 1];
        const steps = [8, 8, 4, 2];
        let fromRow = 0;
        for (let pass = 0; pass < 4; pass++) {
            for (let toRow = offsets[pass]; toRow < rows; toRow += steps[pass]) {
                cpRow(toRow, fromRow)
                fromRow++;
            }
        }
        return newPixels;
    }

    img.leftPos = st.readUnsigned();
    img.topPos = st.readUnsigned();
    img.width = st.readUnsigned();
    img.height = st.readUnsigned();

    const bits = byteToBitArr(st.readByte());
    img.lctFlag = bits.shift();
    img.interlaced = bits.shift();
    img.sorted = bits.shift();
    img.reserved = bits.splice(0, 2);
    img.lctSize = bitsToNum(bits.splice(0, 3));

    if (img.lctFlag) {
        img.lct = parseCT(st, 1 << (img.lctSize + 1));
    }
    img.lzwMinCodeSize = st.readByte();

    const lzwData = readSubBlocks(st);
    img.pixels = lzwDecode(img.lzwMinCodeSize, lzwData);

    if (img.interlaced) { // Move
        img.pixels = deinterlace(img.pixels, img.width);
    }
    callback?.(img);
}

/**
 * Parses GIF header information
 * @param { Stream } st
 * @param { Function } [callback]
 */
function parseHeader (st, callback) {
    const hdr = {};
    hdr.sig = st.read(3);
    hdr.ver = st.read(3);
    if (hdr.sig !== 'GIF') {
        throw new Error('Not a GIF file.');
    }
    hdr.width = st.readUnsigned();
    hdr.height = st.readUnsigned();

    const bits = byteToBitArr(st.readByte());
    hdr.gctFlag = bits.shift();
    hdr.colorRes = bitsToNum(bits.splice(0, 3));
    hdr.sorted = bits.shift();
    hdr.gctSize = bitsToNum(bits.splice(0, 3));

    hdr.bgColor = st.readByte();
    hdr.pixelAspectRatio = st.readByte(); // if not 0, aspectRatio = (pixelAspectRatio + 15) / 64
    if (hdr.gctFlag) {
        hdr.gct = parseCT(st, 1 << (hdr.gctSize + 1));
    }
    callback?.(hdr);
}

function parseExt (st, block, handler) {

    function parseGCExt (block) {
        st.readByte(); // blocksize, always 4
        const bits = byteToBitArr(st.readByte());
        block.reserved = bits.splice(0, 3); // Reserved; should be 000.
        block.disposalMethod = bitsToNum(bits.splice(0, 3));
        block.userInput = bits.shift();
        block.transparencyGiven = bits.shift();
        block.delayTime = st.readUnsigned();
        block.transparencyIndex = st.readByte();
        block.terminator = st.readByte();
        handler?.gce(block);
    }

    function parseComExt (block) {
        block.comment = readSubBlocks(st);
        handler.com && handler.com(block);
    }

    function parsePTExt (block) {
        // No one *ever* uses this. If you use it, deal with parsing it yourself.
        st.readByte(); // blocksize, always 12
        block.ptHeader = st.readBytes(12);
        block.ptData = readSubBlocks(st);
        handler.pte && handler.pte(block);
    }

    function parseAppExt (block) {
        function parseNetscapeExt (block) {
            st.readByte(); // blocksize, always 3
            block.unknown = st.readByte(); // ??? Always 1? What is this?
            block.iterations = st.readUnsigned();
            block.terminator = st.readByte();
            handler.app && handler.app.NETSCAPE && handler.app.NETSCAPE(block);
        }

        function parseUnknownAppExt (block) {
            block.appData = readSubBlocks(st);
            // FIXME: This won't work if a handler wants to match on any identifier.
            handler.app && handler.app[block.identifier] && handler.app[block.identifier](block);
        }

        st.readByte(); // blocksize, always 11
        block.identifier = st.read(8);
        block.authCode = st.read(3);
        switch (block.identifier) {
            case 'NETSCAPE':
                parseNetscapeExt(block);
                break;
            default:
                parseUnknownAppExt(block);
                break;
        }
    }

    function parseUnknownExt (block) {
        block.data = readSubBlocks(st);
        handler.unknown && handler.unknown(block);
    }

    block.label = st.readByte();
    switch (block.label) {
        case 0xF9:
            block.extType = 'gce';
            parseGCExt(block);
            break;
        case 0xFE:
            block.extType = 'com';
            parseComExt(block);
            break;
        case 0x01:
            block.extType = 'pte';
            parsePTExt(block);
            break;
        case 0xFF:
            block.extType = 'app';
            parseAppExt(block);
            break;
        default:
            block.extType = 'unknown';
            parseUnknownExt(block);
            break;
    }
}

/**
 * @param { Stream } st
 * @param { GIFParserHandlers } handler
 */
function parseBlock (st, handler) {
    const block = {}
    block.sentinel = st.readByte();
    switch (String.fromCharCode(block.sentinel)) { // For ease of matching
        case '!':
            block.type = 'ext';
            parseExt(st, block, handler);
            break;
        case ',':
            block.type = 'img';
            parseImg(st, block, handler?.img);
            break;
        case ';':
            block.type = 'eof';
            handler?.eof(block);
            break;
        default:
            throw new Error('Unknown block: 0x' + block.sentinel.toString(16)); // TODO: Pad this with a 0.
    }
    if (block.type !== 'eof') setTimeout(() => parseBlock(st, handler), 0);
}

/**
 * Takes a Stream and parses it for GIF data, calling the relevant handler
 * methods on the passed in `handler` object.
 * @param { Stream } st
 * @param { GIFParserHandlers } handler
 */
export function parseGIF (st, handler={}) {
    parseHeader(st, handler?.hdr);
    setTimeout(() => parseBlock(st, handler), 0);
}