bigint.c

#include "bigint.h"

void bigint_fprint(FILE *stream, const struct bigint *dest)
{
    size_t i, j, k = LIMB_BITS/4 - ((dest->bits + 3) % LIMB_BITS)/4;
    for (i = 1, j = bigint_limbs(dest); i <= j; i++) {
        const limb_t limb = dest->limb[j-i];
        for (k %= LIMB_BITS/4; k < LIMB_BITS/4; k++) {
            unsigned char nibble = (limb >> (LIMB_BITS - 4*(k+1))) & 0x0F;
            fputc("0123456789abcdef"[nibble], stream);
        }
    }
}

struct bigint *bigint_from_string(struct bigint *dest, const char *hex)
{
    int neg;
    size_t len, bits, i;

    /* Validate input string */
    if ((neg = (hex[0] == '-')))
        hex++;
    if (hex[0] == '0' && hex[1] == 'x')
        hex += 2;
    len = strspn(hex, "0123456789abcdefABCDEF");
    if (!len || hex[len] != '\0')
        return NULL;

    /* Determine length in bits */
    for (i = 0; i < len-1 && hex[i] == '0'; i++);
    bits = (len-i)*4;
    bits -= (hex[i] < '8') + (hex[i] < '4') /* trollface.jpg */
          + (hex[i] < '2') + (hex[i] < '1');
    if (bits > dest->bits)
        return NULL; /* need moar space */

    /* Parse into limbs */
    bigint_load_zeros(dest);
    for (i = 0; i < bits; i += 4) {
        char v, c = hex[len-1 - i/4];
        if (c >= '0' && c <= '9') {
            v = c - '0';
        } else if (c >= 'A' && c <= 'F') {
            v = 10 + (c - 'A');
        } else /*if (c >= 'a' && c <= 'f')*/ {
            v = 10 + (c - 'a');
        }
        dest->limb[i/LIMB_BITS] |= (limb_t)v << (i % LIMB_BITS);
    }

    /* Handle negative values */
    if (neg) {
        limb_t carry = 1;
        size_t j = bigint_limbs(dest);
        for (i = 0; i < j; i++) {
            dest->limb[i] = ~dest->limb[i] + carry;
            carry &= dest->limb[i] == 0;
        }
    }

    return dest;
}