circomlib/circuits/compconstant.circom

/*
    Copyright 2018 0KIMS association.

    This file is part of circom (Zero Knowledge Circuit Compiler).

    circom is a free software: you can redistribute it and/or modify it
    under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    circom is distributed in the hope that it will be useful, but WITHOUT
    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
    or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public
    License for more details.

    You should have received a copy of the GNU General Public License
    along with circom. If not, see <https://www.gnu.org/licenses/>.
*/

include "bitify.circom";

// Returns 1 if in (in binary) > ct

template CompConstant(ct) {
    signal input in[254];
    signal output out;

    signal parts[127];
    signal sout;

    var clsb;
    var cmsb;
    var slsb;
    var smsb;

    var sum=0;

    var b = (1 << 128) -1;
    var a = 1;
    var e = 1;
    var i;

    for (i=0;i<127; i++) {
        clsb = (ct >> (i*2)) & 1;
        cmsb = (ct >> (i*2+1)) & 1;
        slsb = in[i*2];
        smsb = in[i*2+1];


        if ((cmsb==0)&(clsb==0)) {
            parts[i] <== -b*smsb*slsb + b*smsb + b*slsb;
        } else if ((cmsb==0)&(clsb==1)) {
            parts[i] <== a*smsb*slsb - a*slsb + b*smsb - a*smsb + a;
        } else if ((cmsb==1)&(clsb==0)) {
            parts[i] <== b*smsb*slsb - a*smsb + a;
        } else {
            parts[i] <== -a*smsb*slsb + a;
        }

        sum = sum + parts[i];

        b = b -e;
        a = a +e;
        e = e*2;
    }

    sout <== sum;

    component num2bits = Num2Bits(135);

    num2bits.in <== sout;

    out <== num2bits.out[127];
}