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fix: Predicate handling in Grumpkin #18247

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fix: Predicate handling in Grumpkin #18247

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9 changes: 7 additions & 2 deletions barretenberg/cpp/src/barretenberg/dsl/acir_format/witness_constant.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -58,9 +58,14 @@ bb::stdlib::cycle_group<Builder> to_grumpkin_point(const WitnessOrConstant<typen
}

// If the predicate is a non-constant witness, conditionally replace coordinates with a valid point.
// We always replace the x and y coordinates with witnesses to avoid the edge case in which point_x =
// g1::affine_one.x, in which case the conditional assign would return point_x (as a constant) while with high
// probability point_y would return a witness
if (!predicate.is_constant()) {
point_x = field_ct::conditional_assign(predicate, point_x, field_ct(bb::grumpkin::g1::affine_one.x));
point_y = field_ct::conditional_assign(predicate, point_y, field_ct(bb::grumpkin::g1::affine_one.y));
point_x = field_ct::conditional_assign(
predicate, point_x, field_ct::from_witness(&builder, bb::grumpkin::g1::affine_one.x));
point_y = field_ct::conditional_assign(
predicate, point_y, field_ct::from_witness(&builder, bb::grumpkin::g1::affine_one.y));
infinite = bool_ct::conditional_assign(predicate, infinite, bool_ct(false));
} else {
BB_ASSERT(predicate.get_value(), "Creating Grumpkin point with a constant predicate equal to false.");
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