1/*
  2    Copyright 2018 0KIMS association.
  3
  4    This file is part of circom (Zero Knowledge Circuit Compiler).
  5
  6    circom is a free software: you can redistribute it and/or modify it
  7    under the terms of the GNU General Public License as published by
  8    the Free Software Foundation, either version 3 of the License, or
  9    (at your option) any later version.
 10
 11    circom is distributed in the hope that it will be useful, but WITHOUT
 12    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 13    or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public
 14    License for more details.
 15
 16    You should have received a copy of the GNU General Public License
 17    along with circom. If not, see <https://www.gnu.org/licenses/>.
 18*/
 19
 20/*
 21
 22Binary Sum
 23==========
 24
 25This component creates a binary sum componet of ops operands and n bits each operand.
 26
 27e is Number of carries: Depends on the number of operands in the input.
 28
 29Main Constraint:
 30   in[0][0]     * 2^0  +  in[0][1]     * 2^1  + ..... + in[0][n-1]    * 2^(n-1)  +
 31 + in[1][0]     * 2^0  +  in[1][1]     * 2^1  + ..... + in[1][n-1]    * 2^(n-1)  +
 32 + ..
 33 + in[ops-1][0] * 2^0  +  in[ops-1][1] * 2^1  + ..... + in[ops-1][n-1] * 2^(n-1)  +
 34 ===
 35   out[0] * 2^0  + out[1] * 2^1 +   + out[n+e-1] *2(n+e-1)
 36
 37To waranty binary outputs:
 38
 39    out[0]     * (out[0] - 1) === 0
 40    out[1]     * (out[0] - 1) === 0
 41    .
 42    .
 43    .
 44    out[n+e-1] * (out[n+e-1] - 1) == 0
 45
 46 */
 47
 48
 49/*
 50    This function calculates the number of extra bits in the output to do the full sum.
 51 */
 52
 53function nbits(a) {
 54    var n = 1;
 55    var r = 0;
 56    while (n-1<a) {
 57        r++;
 58        n *= 2;
 59    }
 60    return r;
 61}
 62
 63
 64template BinSum(n, ops) {
 65    var nout = nbits((2**n -1)*ops);
 66    signal input in[ops][n];
 67    signal output out[nout];
 68
 69    var lin = 0;
 70    var lout = 0;
 71
 72    var k;
 73    var j;
 74
 75    var e2;
 76
 77    e2 = 1;
 78    for (k=0; k<n; k++) {
 79        for (j=0; j<ops; j++) {
 80            lin += in[j][k] * e2;
 81        }
 82        e2 = e2 + e2;
 83    }
 84
 85    e2 = 1;
 86    for (k=0; k<nout; k++) {
 87        out[k] <-- (lin >> k) & 1;
 88
 89        // Ensure out is binary
 90        out[k] * (out[k] - 1) === 0;
 91
 92        lout += out[k] * e2;
 93
 94        e2 = e2+e2;
 95    }
 96
 97    // Ensure the sum;
 98
 99    lin === lout;
100}