A solidity library for performing arithmetic.

The contract source is in the repo at contracts/Arithmetic.sol. Simply copy the contract file into your source tree and import it with:

import "./path/to/Arithmetic.sol";

contract MyContract {
    function myFunction() {
        uint a, b, d;
        // your code here
        uint result = Arithmetic.overflowResistantFraction(a, b, d);
    }
}

function mul256By256(uint a, uint b)
    constant
    returns (uint ab32, uint ab1, uint ab0)

This function takes two unsigned 256-bit integers and multiplies them, returning a 512-bit result split into a high 256-bit limb, a middle 128-bit limb, and a low 128-bit limb.

function div256_128By256(uint a21, uint a0, uint b)
    constant
    returns (uint q, uint r)

This function takes a unsigned 384-bit integer and divides it by a 256-bit integer, returning a high-bits truncated 256-bit quotient and a remainder. The 384-bit dividend is represented as a high 256-bit limb and a low 128-bit limb.

function overflowResistantFraction(uint a, uint b, uint divisor)
    returns (uint)

This function returns a 256-bit truncated a * b / divisor, where the division is integer division. The overflow from a * b is handled in a 512-bit buffer, so this method calculates the expression correctly for high values of a and b.