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fixing template ModSubThree #34

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fixing template ModSubThree #34

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16 changes: 7 additions & 9 deletions circuits/bigint.circom
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Original file line number Diff line number Diff line change
Expand Up @@ -35,22 +35,20 @@ template ModSub(n) {
}

// a - b - c
// assume a - b - c + 2**n >= 0
// checks a - b - c + 2**n >= 0
template ModSubThree(n) {
assert(n + 2 <= 253);
signal input a;
signal input b;
signal input c;
assert(a - b - c + (1 << n) >= 0);
signal output out;
signal output borrow;
signal b_plus_c;
b_plus_c <== b + c;
component lt = LessThan(n + 1);
lt.in[0] <== a;
lt.in[1] <== b_plus_c;
borrow <== lt.out;
out <== borrow * (1 << n) + a - b_plus_c;

component n2b = Num2Bits(n+2);
n2b.in <== (1 << (n + 1)) + a - b - c;
n2b.out[n+1] + n2b.out[n] === 1;
borrow <== 1 - n2b.out[n+1];
out <== borrow * (1 << n) + a - b - c;
}

template ModSumThree(n) {
Expand Down