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uint256_mod_inv standard function with a hint #144

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uint256_mod_inv standard function with a hint #144

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ae98b69
uint256 modular inversion cairo lib
dragan2234 Jan 26, 2023
183adbb
change variable names
dragan2234 Jan 26, 2023
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16 changes: 16 additions & 0 deletions src/starkware/cairo/common/uint256.cairo
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Original file line number Diff line number Diff line change
Expand Up @@ -408,6 +408,22 @@ func uint256_pow2{range_check_ptr}(exp: Uint256) -> (res: Uint256) {
}
}

// Computes modular inversion a^{-1} % div.
func uint256_mod_inv{range_check_ptr: felt}(a: Uint256, div: Uint256) -> (res: Uint256) {
alloc_locals;
local res: Uint256;
%{
a = (ids.a.high << 128) + ids.a.low
div = (ids.div.high << 128) + ids.div.low
res = pow(a, -1, div)
ids.res.low = res & ((1 << 128) - 1)
ids.res.high = res >> 128
%}
let (quotient_low, quotient_high, remainder) = uint256_mul_div_mod(a,res,div);
assert Uint256(low=1,high=0) = (remainder);
return (res=res);
}

// Computes the logical left shift of a uint256 integer.
func uint256_shl{range_check_ptr}(a: Uint256, b: Uint256) -> (res: Uint256) {
let (c) = uint256_pow2(b);
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