Longest way to implement extended hamming code error detection. It doesn't even have a correction capability.

crap.js

function isEven(n) {
    return n % 2 == 0;
}

function getColumnSum(column, block, skipFirst) {
    return (skipFirst ? 0 : block[column + 0]) +
           block[column + 4] +
           block[column + 8] +
           block[column + 12];
}

function getRowSum(row, block, skipFirst) {
    return (skipFirst ? 0 : block[4 * row + 0]) +
           block[4 * row + 1] +
           block[4 * row + 2] +
           block[4 * row + 3];
}

function check1stParity(block) {
    var a = getColumnSum(1, block, true);
    var b = getColumnSum(3, block, false);
    var parityValue = block[1];
    if (parityValue)
        return !isEven(a + b);
    else
        return isEven(a + b);
}

function check2ndParity(block) {
    var a = getColumnSum(2, block, true);
    var b = getColumnSum(3, block, false);
    var parityValue = block[2];
    if (parityValue)
        return !isEven(a + b);
    else
        return isEven(a + b);
}

function check3rdParity(block) {
    var a = getRowSum(1, block, true);
    var b = getRowSum(3, block, false);
    var parityValue = block[4];
    if (parityValue)
        return !isEven(a + b);
    else
        return isEven(a + b);
}

function check4thParity(block) {
    var a = getRowSum(2, block, true);
    var b = getRowSum(3, block, false);
    var parityValue = block[8];
    if (parityValue)
        return !isEven(a + b);
    else
        return isEven(a + b);
}

function firstIndexParity(block) {
    var sum = 0;
    for(var i = 0;i < block.length; ++i) {
        sum += block[i];
    }
    var parityValue = block[0];
    if (parityValue)
        return !isEven(sum);
    else
        return isEven(sum);
}

function fifteenElevenHamming(block) {
    var a = check1stParity(block);
    var b = check2ndParity(block);
    var c = check3rdParity(block);
    var d = check4thParity(block);
    return a && b && c && d;
}

function extendedHammingCode(block) {
    var a = firstIndexParity(block);
    var b = fifteenElevenHamming(block);
    return a && b;
}

function main() {
    var correctBlock = [
        0, 0, 1, 0,
        1, 0, 1, 1,
        1, 0, 0, 0,
        1, 1, 1, 0
    ];

    var wrongBlock = [
        0, 0, 1, 0,
        1, 0, 1, 1,
        1, 0, 1, 0,
        1, 1, 1, 0
    ];

    var a = extendedHammingCode(correctBlock);
    var b = extendedHammingCode(wrongBlock);
    console.log("Correct block: " + a);
    console.log("Wrong block: " + b);
}

main();