logos-storage/logos-storage-proofs-circuits
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ugly hack to work around the limitations of circom-witnesscalc

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Balazs Komuves 2025-03-14 14:53:22 +01:00
parent 72bc4ae612
commit 7f2d23a318
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2 changed files with 55 additions and 1 deletions
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2
circuit/codex/sample_cells.circom
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@ -112,7 +112,7 @@ log("top root check = ", mtop.recRoot == dataSetRoot);
//
// then we prove the individual sampled cells
component lg = Log2(maxDepth); // we allow at most 2^32 cells per slot
component lg = Log2_CircomWitnessCalc_Hack(maxDepth); // we allow at most 2^32 cells per slot
lg.inp <== nCellsPerSlot;
// NOTE: in general we need for the Merkle prover the binary decomposition

54
circuit/lib/log2.circom
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@ -36,6 +36,60 @@ template Log2(n) {
0 === (mask[i] - mask[i+1]) * (i + 1 - out);
}
inp === sum;
}
//------------------------------------------------------------------------------
//
// a version of `Log2(n)` which works with `circom-witnesscalc`
//
// yeah this is very ugly. Direct your complaints to the iden3 people. ¯\_(ツ)_/¯
//
template Log2_CircomWitnessCalc_Hack(n) {
signal input inp;
signal output out;
signal output mask[n+1];
// compute the logarithm
// as `circom-witnesscalc` does not support any kind of dynamic computation
// and support for functions, and other things also seems to be limited
// we have to implement this here as a static computation instead
// but even that's not enough for `circom-witnesscalc`! hence this uglyness
var x = inp;
var cnt = -1;
signal done[257]; done[0] <-- 0; // we need *signals* here because `circom-witnesscalc` doesn't work otherwise (seriously...)
signal log2[257]; log2[0] <-- -1; // but hopefully circom will optimize these away...
for(var i=0; i<256; i++) {
var not_done = (done[i] == 0);
var do_switch = (x==0) && not_done;
log2[i+1] <-- not_done ? cnt : log2[i];
done[i+1] <-- do_switch ? 1 : done[i];
cnt = cnt + 1;
x = x >> 1;
}
// mask will be a vector [1,1,1,...1,0,...,0,0]
// which can change only where index == out
for(var i=0; i<=n; i++) {
mask[i] <-- ((2**i) < inp) ? 1 : 0;
}
out <-- log2[256];
log("Log2 hack: input = ",inp," | logarithm = ",out);
mask[0] === 1;
mask[n] === 0;
var sum = 0;
for(var i=0; i<n; i++) {
sum += (2**(i+1)) * (mask[i] - mask[i+1]);
0 === (mask[i] - mask[i+1]) * (i + 1 - out);
}
inp === sum;
}