_INLINE_ void find_err2()

in src/decode/decode.c [180:217]

• 24 lines of code
• 4 McCabe index (conditional complexity)

_INLINE_ void find_err2(OUT e_t *e,
                        IN e_t * pos_e,
                        IN const syndrome_t *          syndrome,
                        IN const compressed_idx_d_ar_t wlist,
                        IN const uint8_t               threshold,
                        IN const decode_ctx *ctx)
{
  DEFER_CLEANUP(syndrome_t rotated_syndrome = {0}, syndrome_cleanup);
  DEFER_CLEANUP(upc_t upc, upc_cleanup);

  for(uint32_t i = 0; i < N0; i++) {
    // UPC must start from zero at every iteration
    bike_memset(&upc, 0, sizeof(upc));

    // 1) Right-rotate the syndrome, for every index of a set bit in the secret
    // key. Then slice-add it to the UPC array.
    for(size_t j = 0; j < D; j++) {
      ctx->rotate_right(&rotated_syndrome, syndrome, wlist[i].val[j]);
      ctx->bit_sliced_adder(&upc, &rotated_syndrome, LOG2_MSB(j + 1));
    }

    // 2) Subtract the threshold from the UPC counters
    ctx->bit_slice_full_subtract(&upc, threshold);

    // 3) Update the errors vector.
    //    The last slice of the UPC array holds the MSB of the accumulated values
    //    minus the threshold. Every zero bit indicates a potential error bit.
    const r_t *last_slice = &(upc.slice[SLICES - 1].u.r.val);
    for(size_t j = 0; j < R_BYTES; j++) {
      const uint8_t sum_msb = (~last_slice->raw[j]);
      e->val[i].raw[j] ^= (pos_e->val[i].raw[j] & sum_msb);
    }

    // Ensure that the padding bits (upper bits of the last byte) are zero, so
    // they are not included in the multiplication, and in the hash function.
    e->val[i].raw[R_BYTES - 1] &= LAST_R_BYTE_MASK;
  }
}