Sonic 1 + Sonic & Knuckles/Blue Spheres - Code Validator, Javascript

Sonic 1 + Sonic & Knuckles/Blue Spheres - Code Validator

//Sonic 1 + Sonic & Knuckles/Blue Spheres - Code Validator

print(codtolev(readline()));

// helper function
function div(a, b) {
	return Math.floor(a / b);
}

// this function returns the level number (1 - 134217728) for a given level password, or "Invalid code" if the password is invalid
function codtolev(cod) {
    // cod is a password string in the format "1234 1234 1234"
    var split = parseInt(cod.substring(0, 4) + cod.substring(5, 9) + cod.substring(10));
    var cx = new Array(39);
    var cz = new Array(28);

// create array of cod's binary digits
    for (i = 0; i < 39; i++) {
        cx[i] = split % 2;
        split = div(split, 2);
    }

// invert the bits at odd indices
    for (i = 1; i < 39; i += 2) {
        cx[i] = 1 - cx[i];
    }
    
    // create array from the bits of cx in a certain order
    cz[27] = 0;
    for (i = 0; i < 6; i++) {
        cz[i] = cx[i + 26];
    }
    for (i = 6; i < 27; i++) {
        cz[i] = cx[i - 6];
    }
    
    // convert the binary number represented by cz to decimal
    var sum = 0;
    for (i = 26; i > -1; i--) {
        sum += (cz[i] * Math.pow(2, i));
    }
    
    // subtract a constant, then fiddle with the number a bit
    sum -= 19088742;
    if (sum < 1 && sum > -19088743) {
    	sum = 134217728 + sum;
    }

// sum should now be the level number; now we need to make sure the rest of the bit pattern is valid

var tmp = sum - 1;

// is this bit is 0, the code was generated with the "not sonic 1 cart" flag, so to validate we carry out the extra operation that entails
    if (cx[38] == 0) {
        tmp += 0x07654321;
        tmp &= 0x07FFFFFF;
    }
    
    // create array of tmp's binary digits
    for (i = 0; i <= 27; i++) {
        cz[i] = tmp % 2;
        tmp = div(tmp, 2);
    }
    
    // validate the bit pattern
    if (cx[37] == (1 + cz[6] + cz[23]) % 2 && cx[36] == (1 + cz[5] + cz[22] + cz[17] + cz[0]) % 2 && cx[35] == (1 + cz[4] + cz[21] + cz[16]) % 2 && cx[34] == (1 + cz[3] + cz[20]) % 2 && cx[33] == (1 + cz[2] + cz[19]) % 2 && cx[32] == (1 + cz[1] + cz[18]) % 2 && cx[25] == (0 + cz[11] + cz[16]) % 2 && cx[24] == (0 + cz[10] ) % 2 && cx[23] == (0 + cz[9] + cz[26]) % 2 && cx[22] == (0 + cz[8] + cz[25]) % 2 && cx[21] == (0 + cz[7] + cz[24]) % 2) {
        // in game, the "sonic 1 cart" flag should be set here according to the value of cx[38]
        return sum;
    }
    else {
        return "Invalid code";
    }
}


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