I'm trying to get the remainder of a large number, for example:
1551690021432628813 % 64
But I find that the it's a couple of digits too long for JavaScript. ie it's getting rounded to zero.
Is there a way around this other than using a 26kb library like BigInteger.js ?
You could break the number into chunks of 10 digits (from the right) and do the modular arithmetic on the chunks, combining the result at the end:
1551690021432628813 = 155169002 * 10**10 + 1432628813
Hence
1551690021432628813 % 64 = (155169002 % 64 * (10**10) % 64 + 1432628813 % 64) % 64
(Which equals 13
).
You could write a recursive function that implements this idea. The following is in Python (which I am more fluent in) but should be easily translated into JavaScript:
def remainder(s,m):
#computes int(s) % m, while just using small numbers
#s is a string and m is an integer
n = len(s)
if n <= 10:
return int(s) % m
else:
first = s[:n-10] #first n-10 digits in s
second = s[-10:] #last 10 digits
return (remainder(first,m) * ((10**10) % m) + int(second) % m) % m
For the special case that the modulus is 64
, there is an exceptionally easy approach: 64 divides 10**6
so, absolutely always
n % 64 == (last 6 digits of n) % 64
For example,
1551690021432628813 % 64 = 628813 % 64 = 13
Similar remarks hold whenever the modulus is a power of 2.
thank you John Coleman
javascript version:
function calculateMod(str, mod) {
var n = str.length;
if (n <= 10) {
return parseInt(str) % mod;
}
else {
var first = str.substring(0, n - 10)
var second = str.substring(n - 10)
return (calculateMod(first, mod) * (Math.pow(10, 10) % mod) + parseInt(second) % mod) % mod;
}
}
You'd have to use a library (like that one you found). JavaScript's numbers ( IEEE-754 double-precision binary floating point) are just not accurate at that scale, and those are the only kind of number JavaScript has*. Once you get past Number.MAX_SAFE_INTEGER + 1
(9,007,199,254,740,992), JavaScript's numbers cannot represent every integer anymore (for instance, can't represent 9,007,199,254,740,993).
* Other than the element type of a typed array, but those don't help you with this for two reasons: 1. There's no typed array for Uint64
, the biggest integer is Uint32
, and 2. Once you're performing math operations on the entry, you're converting it to standard number.
// you can use the built-in BigInt object console.log(Number(1551690021432628813n % 64n));
There are some workarounds, but generally it is easiest to use a library Read more
BigInt(1551690021432628813) % 64
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