function crashPointFromHash(serverSeed, clientSeed) {
function divisible(hash, mod) {
// We will read in 4 hex at a time, but the first chunk might be a bit smaller
// So ABCDEFGHIJ should be chunked like AB CDEF GHIJ
var val = 0;
var o = hash.length % 4;
for (var i = o > 0 ? o - 4 : 0; i < hash.length; i += 4) {
val = ((val << 16) + parseInt(hash.substring(i, i+4), 16)) % mod;
}
return val === 0;
}
var hash = crypto.createHmac('sha256', serverSeed).update(clientSeed).digest('hex');
/* In 1 of 101 games the game crashes instantly. */
if (divisible(hash, 101))
return 0;
/* Use the most significant 52-bit from the hash
to calculate the crash point */
var h = parseInt(hash.slice(0,52/4),16);
var e = Math.pow(2,52);
return Math.floor((100 * e - h) / (e - h));
}
Or, in English:
To get from the sha256 hash to the crashpoint:
1) if the hash is exactly divisible by 101, crashpoint is 0x
2) otherwise crashpoint is
(1 + 99e/(e-h)) / 100 to 2 decimal places, where e is
2^52 and h is the first 13 characters of the hash
Intuitively:
h ranges from 0 to 2^52-1.
If h is very small, e-h is very big, 99e/(e-h) is very close to 99, and the crashpoint is 1.00x.
If h is very big, e-h is small, 99e/(e-h) is very big, and the crashpoint is huge.
