static void rq_decoderounded()

in sntrup4591761.c [959:1021]

• 33 lines of code
• 2 McCabe index (conditional complexity)

static void rq_decoderounded(modq *f,const unsigned char *c)
{
  crypto_uint32 c0, c1, c2, c3;
  crypto_uint32 f0, f1, f2;
  int i;

  for (i = 0;i < p/3;++i) {
    c0 = *c++;
    c1 = *c++;
    c2 = *c++;
    c3 = *c++;

    /* f0 + f1*1536 + f2*1536^2 */
    /* = c0 + c1*256 + c2*256^2 + c3*256^3 */
    /* with each f between 0 and 1530 */

    /* f2 = (64/9)c3 + (1/36)c2 + (1/9216)c1 + (1/2359296)c0 - [0,0.99675] */
    /* claim: 2^21 f2 < x < 2^21(f2+1) */
    /* where x = 14913081*c3 + 58254*c2 + 228*(c1+2) */
    /* proof: x - 2^21 f2 = 456 - (8/9)c0 + (4/9)c1 - (2/9)c2 + (1/9)c3 + 2^21 [0,0.99675] */
    /* at least 456 - (8/9)255 - (2/9)255 > 0 */
    /* at most 456 + (4/9)255 + (1/9)255 + 2^21 0.99675 < 2^21 */
    f2 = (14913081*c3 + 58254*c2 + 228*(c1+2)) >> 21;

    c2 += c3 << 8;
    c2 -= (f2 * 9) << 2;
    /* f0 + f1*1536 */
    /* = c0 + c1*256 + c2*256^2 */
    /* c2 <= 35 = floor((1530+1530*1536)/256^2) */
    /* f1 = (128/3)c2 + (1/6)c1 + (1/1536)c0 - (1/1536)f0 */
    /* claim: 2^21 f1 < x < 2^21(f1+1) */
    /* where x = 89478485*c2 + 349525*c1 + 1365*(c0+1) */
    /* proof: x - 2^21 f1 = 1365 - (1/3)c2 - (1/3)c1 - (1/3)c0 + (4096/3)f0 */
    /* at least 1365 - (1/3)35 - (1/3)255 - (1/3)255 > 0 */
    /* at most 1365 + (4096/3)1530 < 2^21 */
    f1 = (89478485*c2 + 349525*c1 + 1365*(c0+1)) >> 21;

    c1 += c2 << 8;
    c1 -= (f1 * 3) << 1;

    c0 += c1 << 8;
    f0 = c0;

    *f++ = modq_freeze(f0 * 3 + q - qshift);
    *f++ = modq_freeze(f1 * 3 + q - qshift);
    *f++ = modq_freeze(f2 * 3 + q - qshift);
  }

  c0 = *c++;
  c1 = *c++;
  c2 = *c++;

  f1 = (89478485*c2 + 349525*c1 + 1365*(c0+1)) >> 21;

  c1 += c2 << 8;
  c1 -= (f1 * 3) << 1;

  c0 += c1 << 8;
  f0 = c0;

  *f++ = modq_freeze(f0 * 3 + q - qshift);
  *f++ = modq_freeze(f1 * 3 + q - qshift);
}