cmath.c

cmath.c

/*
* Copyright (c) 2023 Calvin Rose
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to
* deal in the Software without restriction, including without limitation the
* rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
* sell copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
* IN THE SOFTWARE.
*/

#include <janet.h>
#include <math.h>

double _mod_impl(double a, double m) {
    if (m == 0) return a;

    return a - floor(a / m) * m;
}

double _mulmod_impl(double a, double b, double m) {
    double res = 0;
    a = _mod_impl(a, m);
    b = _mod_impl(b, m);
    if (a < b) {
        double t = a;
        a = b;
        b = t;
    }
    if (b < 0) b -= m;
    while (b > 0) {
        if (_mod_impl(b, 2) == 1) {
            res = _mod_impl(res + a, m);
            b -= 1;
        }
        a = _mod_impl(a * 2, m);
        b /= 2;
    }
    return res;
}

double _modinv_impl(double a, double m) {
    double x = 0;
    double u = 1;
    double n = abs(m);
    a = _mod_impl(a, n);
    while (a != 0) {
        double tmpx = x;
        x = u;
        u = tmpx - floor(n / a) * u;
        double tmpa = a;
        a = _mod_impl(n, a);
        n = tmpa;
    }
    if (n == 1) return x;
#ifdef NAN
    return NAN;
#else
    return 0.0 / 0.0;
#endif
}

JANET_FN(cfun_cmath_modinv,
        "(cmath/modinv a m)",
        "Modular multiplicative inverse of a mod m.") {
    janet_fixarity(argc, 2);
    double a = janet_getnumber(argv, 0);
    double m = janet_getnumber(argv, 1);

    return janet_wrap_number(_modinv_impl(a, m));
}

JANET_FN(cfun_cmath_mulmod,
        "(cmath/mulmod a b m)",
        "Modular multiplication of a and b mod m.") {
    janet_fixarity(argc, 3);
    double a = janet_getnumber(argv, 0);
    double b = janet_getnumber(argv, 1);
    double m = janet_getnumber(argv, 2);

    return janet_wrap_number(_mulmod_impl(a, b, m));
}

JANET_FN(cfun_cmath_powmod,
        "(cmath/powmod a b m)",
        "Modular exponentiation of a to the power of b mod m.") {
    janet_fixarity(argc, 3);
    double res = 1;
    double a = janet_getnumber(argv, 0);
    double b = janet_getnumber(argv, 1);
    double m = janet_getnumber(argv, 2);
    if (b < 0) {
        a = _modinv_impl(a, m);
        if (isnan(a)) return janet_wrap_number(a);
        b = -b;
    }
    while (b > 0) {
        if (_mod_impl(b, 2) == 1) {
            res = _mulmod_impl(res, a, m);
            b -= 1;
        }
        a = _mulmod_impl(a, a, m);
        b /= 2;
    }

    return janet_wrap_number(res);
}

JANET_MODULE_ENTRY(JanetTable *env) {
    JanetRegExt cfuns[] = {
        JANET_REG("modinv", cfun_cmath_modinv),
        JANET_REG("mulmod", cfun_cmath_mulmod),
        JANET_REG("powmod", cfun_cmath_powmod),
        JANET_REG_END
    };
    janet_cfuns_ext(env, "cmath", cfuns);
}