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faster uint256_add. No new hints needed. #176

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faster uint256_add. No new hints needed. #176

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38 changes: 29 additions & 9 deletions src/starkware/cairo/common/uint256.cairo
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Original file line number Diff line number Diff line change
Expand Up @@ -69,7 +69,6 @@ func felt_to_uint256{range_check_ptr}(value: felt) -> Uint256 {
// Adds two integers. Returns the result as a 256-bit integer and the (1-bit) carry.
func uint256_add{range_check_ptr}(a: Uint256, b: Uint256) -> (res: Uint256, carry: felt) {
alloc_locals;
local res: Uint256;
local carry_low: felt;
local carry_high: felt;
%{
Expand All @@ -79,14 +78,35 @@ func uint256_add{range_check_ptr}(a: Uint256, b: Uint256) -> (res: Uint256, carr
ids.carry_high = 1 if sum_high >= ids.SHIFT else 0
%}

assert carry_low * carry_low = carry_low;
assert carry_high * carry_high = carry_high;

assert res.low = a.low + b.low - carry_low * SHIFT;
assert res.high = a.high + b.high + carry_low - carry_high * SHIFT;
uint256_check(res);

return (res, carry_high);
if (carry_low != 0) {
if (carry_high != 0) {
tempvar range_check_ptr = range_check_ptr + 2;
tempvar res = Uint256(low=a.low + b.low - SHIFT, high=a.high + b.high + 1 - SHIFT);
assert [range_check_ptr - 2] = res.low;
assert [range_check_ptr - 1] = res.high;
return (res, 1);
} else {
tempvar range_check_ptr = range_check_ptr + 2;
tempvar res = Uint256(low=a.low + b.low - SHIFT, high=a.high + b.high + 1);
assert [range_check_ptr - 2] = res.low;
assert [range_check_ptr - 1] = res.high;
return (res, 0);
}
} else {
if (carry_high != 0) {
tempvar range_check_ptr = range_check_ptr + 2;
tempvar res = Uint256(low=a.low + b.low, high=a.high + b.high - SHIFT);
assert [range_check_ptr - 2] = res.low;
assert [range_check_ptr - 1] = res.high;
return (res, 1);
} else {
tempvar range_check_ptr = range_check_ptr + 2;
tempvar res = Uint256(low=a.low + b.low, high=a.high + b.high);
assert [range_check_ptr - 2] = res.low;
assert [range_check_ptr - 1] = res.high;
return (res, 0);
}
}
}

// Splits a field element in the range [0, 2^192) to its low 64-bit and high 128-bit parts.
Expand Down