void rdc_mont()

in src/P503/generic/fp_generic.c [204:256]
47 lines of code
9 McCabe index (conditional complexity)

void rdc_mont(digit_t* ma, digit_t* mc)
{ // Efficient Montgomery reduction using comba and exploiting the special form of the prime p503.
  // mc = ma*R^-1 mod p503x2, where R = 2^512.
  // If ma < 2^512*p503, the output mc is in the range [0, 2*p503-1].
  // ma is assumed to be in Montgomery representation.
    unsigned int i, j, carry, count = p503_ZERO_WORDS;
    digit_t UV[2], t = 0, u = 0, v = 0;

    for (i = 0; i < NWORDS_FIELD; i++) {
        mc[i] = 0;
    }

    for (i = 0; i < NWORDS_FIELD; i++) {
        for (j = 0; j < i; j++) {
            if (j < (i-p503_ZERO_WORDS+1)) { 
                MUL(mc[j], ((digit_t*)p503p1)[i-j], UV+1, UV[0]);
                ADDC(0, UV[0], v, carry, v); 
                ADDC(carry, UV[1], u, carry, u); 
                t += carry; 
            }
        }
        ADDC(0, v, ma[i], carry, v); 
        ADDC(carry, u, 0, carry, u); 
        t += carry; 
        mc[i] = v;
        v = u;
        u = t;
        t = 0;
    }    

    for (i = NWORDS_FIELD; i < 2*NWORDS_FIELD-1; i++) {
        if (count > 0) {
            count -= 1;
        }
        for (j = i-NWORDS_FIELD+1; j < NWORDS_FIELD; j++) {
            if (j < (NWORDS_FIELD-count)) { 
                MUL(mc[j], ((digit_t*)p503p1)[i-j], UV+1, UV[0]);
                ADDC(0, UV[0], v, carry, v); 
                ADDC(carry, UV[1], u, carry, u); 
                t += carry;
            }
        }
        ADDC(0, v, ma[i], carry, v); 
        ADDC(carry, u, 0, carry, u); 
        t += carry; 
        mc[i-NWORDS_FIELD] = v;
        v = u;
        u = t;
        t = 0;
    }
    ADDC(0, v, ma[2*NWORDS_FIELD-1], carry, v); 
    mc[NWORDS_FIELD-1] = v;
}