merkle_path_selector.tcc

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// DISCLAIMER:
// Content taken and adapted from:
// https://github.com/HarryR/ethsnarks/blob/master/src/gadgets/merkle_tree.hpp
// https://github.com/HarryR/ethsnarks/blob/master/src/gadgets/merkle_tree.cpp
 
#ifndef __ZETH_CIRCUITS_MERKLE_PATH_SELECTOR_TCC__
#define __ZETH_CIRCUITS_MERKLE_PATH_SELECTOR_TCC__
 
namespace libzeth
{
 
template<typename FieldT>
merkle_path_selector<FieldT>::merkle_path_selector(
    libsnark::protoboard<FieldT> &pb,
    const libsnark::pb_variable<FieldT> &input,
    const libsnark::pb_variable<FieldT> &pathvar,
    const libsnark::pb_variable<FieldT> &is_right,
    const std::string &annotation_prefix)
    : libsnark::gadget<FieldT>(pb, annotation_prefix)
    , input(input)
    , pathvar(pathvar)
    , is_right(is_right)
{
    // We allocate the selector's outputs left and right
    left.allocate(pb, FMT(this->annotation_prefix, " left"));
    right.allocate(pb, FMT(this->annotation_prefix, " right"));
};
 
template<typename FieldT>
void merkle_path_selector<FieldT>::generate_r1cs_constraints()
{
    // Constrain is_right to be boolean
    this->pb.add_r1cs_constraint(
        libsnark::r1cs_constraint<FieldT>(is_right, 1 - is_right, 0),
        FMT(this->annotation_prefix, " is_right"));
 
    // We then constrain left to be the authentication node if is_right = 1,
    // input otherwise
    this->pb.add_r1cs_constraint(
        libsnark::r1cs_constraint<FieldT>(
            is_right, pathvar - input, left - input),
        FMT(this->annotation_prefix,
            " is_right*pathvar+(1-is_right)*input=left"));
 
    // Inversely, we constrain right to be the input if is_right = 1, the
    // authentication node otherwise
    this->pb.add_r1cs_constraint(
        libsnark::r1cs_constraint<FieldT>(
            is_right, input - pathvar, right - pathvar),
        FMT(this->annotation_prefix,
            " is_right*input+(1-is_right)*pathvar=right"));
};
 
template<typename FieldT>
void merkle_path_selector<FieldT>::generate_r1cs_witness()
{
    // We compute left and right according to the constrains
    this->pb.val(left) =
        this->pb.val(input) +
        this->pb.val(is_right) * (this->pb.val(pathvar) - this->pb.val(input));
    this->pb.val(right) =
        this->pb.val(pathvar) +
        this->pb.val(is_right) * (this->pb.val(input) - this->pb.val(pathvar));
};
 
template<typename FieldT>
const libsnark::pb_variable<FieldT> &merkle_path_selector<FieldT>::get_left()
{
    return left;
};
 
template<typename FieldT>
const libsnark::pb_variable<FieldT> &merkle_path_selector<FieldT>::get_right()
{
    return right;
};
 
} // namespace libzeth
 
#endif // __ZETH_CIRCUITS_MERKLE_PATH_SELECTOR_TCC__