eighty's blog

the remainder of a big integer
By eighty, history, 6 years ago, In English

Hi Everyone.

Now I'm wondering about how to count (a*b) % c? 1 <= a,b,c <=10^18.

Any advice?

Thank you.

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6 years ago, # |
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  1. You can check if your judge allows int128

  2. You can write your own int128 class (it's a lot easier than general bignum). It's harder than 3. but faster.

  3. You can use algorithm similar to fast exponentiation. Instead of a*a*a*...*a (b times) you calculate a+a+a+...+a (b times) so just change * to + in your favorite quick power implementation.

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6 years ago, # |
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"Binary multiply" O(log(min(a, b))) :

code

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