diff options
Diffstat (limited to 'libtommath/bn_s_mp_karatsuba_mul.c')
| -rw-r--r-- | libtommath/bn_s_mp_karatsuba_mul.c | 174 |
1 files changed, 174 insertions, 0 deletions
diff --git a/libtommath/bn_s_mp_karatsuba_mul.c b/libtommath/bn_s_mp_karatsuba_mul.c new file mode 100644 index 0000000..85899fb --- /dev/null +++ b/libtommath/bn_s_mp_karatsuba_mul.c @@ -0,0 +1,174 @@ +#include "tommath_private.h" +#ifdef BN_S_MP_KARATSUBA_MUL_C +/* LibTomMath, multiple-precision integer library -- Tom St Denis */ +/* SPDX-License-Identifier: Unlicense */ + +/* c = |a| * |b| using Karatsuba Multiplication using + * three half size multiplications + * + * Let B represent the radix [e.g. 2**MP_DIGIT_BIT] and + * let n represent half of the number of digits in + * the min(a,b) + * + * a = a1 * B**n + a0 + * b = b1 * B**n + b0 + * + * Then, a * b => + a1b1 * B**2n + ((a1 + a0)(b1 + b0) - (a0b0 + a1b1)) * B + a0b0 + * + * Note that a1b1 and a0b0 are used twice and only need to be + * computed once. So in total three half size (half # of + * digit) multiplications are performed, a0b0, a1b1 and + * (a1+b1)(a0+b0) + * + * Note that a multiplication of half the digits requires + * 1/4th the number of single precision multiplications so in + * total after one call 25% of the single precision multiplications + * are saved. Note also that the call to mp_mul can end up back + * in this function if the a0, a1, b0, or b1 are above the threshold. + * This is known as divide-and-conquer and leads to the famous + * O(N**lg(3)) or O(N**1.584) work which is asymptopically lower than + * the standard O(N**2) that the baseline/comba methods use. + * Generally though the overhead of this method doesn't pay off + * until a certain size (N ~ 80) is reached. + */ +mp_err s_mp_karatsuba_mul(const mp_int *a, const mp_int *b, mp_int *c) +{ + mp_int x0, x1, y0, y1, t1, x0y0, x1y1; + int B; + mp_err err = MP_MEM; /* default the return code to an error */ + + /* min # of digits */ + B = MP_MIN(a->used, b->used); + + /* now divide in two */ + B = B >> 1; + + /* init copy all the temps */ + if (mp_init_size(&x0, B) != MP_OKAY) { + goto LBL_ERR; + } + if (mp_init_size(&x1, a->used - B) != MP_OKAY) { + goto X0; + } + if (mp_init_size(&y0, B) != MP_OKAY) { + goto X1; + } + if (mp_init_size(&y1, b->used - B) != MP_OKAY) { + goto Y0; + } + + /* init temps */ + if (mp_init_size(&t1, B * 2) != MP_OKAY) { + goto Y1; + } + if (mp_init_size(&x0y0, B * 2) != MP_OKAY) { + goto T1; + } + if (mp_init_size(&x1y1, B * 2) != MP_OKAY) { + goto X0Y0; + } + + /* now shift the digits */ + x0.used = y0.used = B; + x1.used = a->used - B; + y1.used = b->used - B; + + { + int x; + mp_digit *tmpa, *tmpb, *tmpx, *tmpy; + + /* we copy the digits directly instead of using higher level functions + * since we also need to shift the digits + */ + tmpa = a->dp; + tmpb = b->dp; + + tmpx = x0.dp; + tmpy = y0.dp; + for (x = 0; x < B; x++) { + *tmpx++ = *tmpa++; + *tmpy++ = *tmpb++; + } + + tmpx = x1.dp; + for (x = B; x < a->used; x++) { + *tmpx++ = *tmpa++; + } + + tmpy = y1.dp; + for (x = B; x < b->used; x++) { + *tmpy++ = *tmpb++; + } + } + + /* only need to clamp the lower words since by definition the + * upper words x1/y1 must have a known number of digits + */ + mp_clamp(&x0); + mp_clamp(&y0); + + /* now calc the products x0y0 and x1y1 */ + /* after this x0 is no longer required, free temp [x0==t2]! */ + if (mp_mul(&x0, &y0, &x0y0) != MP_OKAY) { + goto X1Y1; /* x0y0 = x0*y0 */ + } + if (mp_mul(&x1, &y1, &x1y1) != MP_OKAY) { + goto X1Y1; /* x1y1 = x1*y1 */ + } + + /* now calc x1+x0 and y1+y0 */ + if (s_mp_add(&x1, &x0, &t1) != MP_OKAY) { + goto X1Y1; /* t1 = x1 - x0 */ + } + if (s_mp_add(&y1, &y0, &x0) != MP_OKAY) { + goto X1Y1; /* t2 = y1 - y0 */ + } + if (mp_mul(&t1, &x0, &t1) != MP_OKAY) { + goto X1Y1; /* t1 = (x1 + x0) * (y1 + y0) */ + } + + /* add x0y0 */ + if (mp_add(&x0y0, &x1y1, &x0) != MP_OKAY) { + goto X1Y1; /* t2 = x0y0 + x1y1 */ + } + if (s_mp_sub(&t1, &x0, &t1) != MP_OKAY) { + goto X1Y1; /* t1 = (x1+x0)*(y1+y0) - (x1y1 + x0y0) */ + } + + /* shift by B */ + if (mp_lshd(&t1, B) != MP_OKAY) { + goto X1Y1; /* t1 = (x0y0 + x1y1 - (x1-x0)*(y1-y0))<<B */ + } + if (mp_lshd(&x1y1, B * 2) != MP_OKAY) { + goto X1Y1; /* x1y1 = x1y1 << 2*B */ + } + + if (mp_add(&x0y0, &t1, &t1) != MP_OKAY) { + goto X1Y1; /* t1 = x0y0 + t1 */ + } + if (mp_add(&t1, &x1y1, c) != MP_OKAY) { + goto X1Y1; /* t1 = x0y0 + t1 + x1y1 */ + } + + /* Algorithm succeeded set the return code to MP_OKAY */ + err = MP_OKAY; + +X1Y1: + mp_clear(&x1y1); +X0Y0: + mp_clear(&x0y0); +T1: + mp_clear(&t1); +Y1: + mp_clear(&y1); +Y0: + mp_clear(&y0); +X1: + mp_clear(&x1); +X0: + mp_clear(&x0); +LBL_ERR: + return err; +} +#endif |