/*
 * Simple random number generator for testing.
 * Copyright © 2022-2024 Nick Bowler
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 *
 * The RNG implementation is adapted from xoshiro256+ and splitmix64
 * by David Blackman and Sebastiano Vigna, originally distributed under
 * the Creative Commons Zero public domain dedication.
 */

#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <inttypes.h>
#include <errno.h>
#include <limits.h>

#include "test.h"

#define B64(x) ((x) & 0xffffffffffffffff)

struct test_rng {
	unsigned long long state[4];
};

/* Rotate val left by n bits.  The behaviour is undefined if n is zero. */
static unsigned long long rot_left64(unsigned long long val, int n)
{
	return B64( (val << n) | (val >> (64 - n)) );
}

static unsigned long long xoshiro256p(unsigned long long *s)
{
	unsigned long long tmp, ret;

	ret = B64(s[0] + s[3]);
	tmp = B64(s[1] << 17);

	s[2] ^= s[0];
	s[3] ^= s[1];
	s[1] ^= s[2];
	s[0] ^= s[3];
	s[2] ^= tmp;
	s[3] = rot_left64(s[3], 45);

	return ret;
}

static unsigned long long splitmix64(unsigned long long *state)
{
	unsigned long long z;

	z = B64(*state += 0x9e3779b97f4a7c15);
	z = B64((z ^ (z >> 30)) * 0xbf58476d1ce4e5b9);
	z = B64((z ^ (z >> 27)) * 0x94d049bb133111eb);

	return z ^ (z >> 31);
}

struct test_rng *test_rng_alloc(const char *seed_str)
{
	unsigned long long seed;
	uintmax_t limit, seed_val;
	struct test_rng *rng;

	limit  = (uintmax_t)0xffffffff;
	limit |= (limit << 16 << 16);

	if (!test_strtoumax(&seed_val, seed_str, limit)) {
		fprintf(stderr, "%s: invalid seed\n", seed_str);
		return NULL;
	}
	seed = seed_val;

	rng = malloc_nofail(sizeof *rng);
	rng->state[0] = splitmix64(&seed);
	rng->state[1] = splitmix64(&seed);
	rng->state[2] = splitmix64(&seed);
	rng->state[3] = splitmix64(&seed);

	return rng;
}

void test_rng_free(struct test_rng *rng)
{
	free(rng);
}

/* Calculate the least power of two greater than val, minus 1. */
static unsigned rng_mask(unsigned val)
{
	val |= val >> 1;
	val |= val >> 2;
	val |= val >> 4;
	val |= val >> 8;

	if (UINT_MAX >= 65536)
		val |= val >> 16;

	return val;
}

unsigned test_rng_uniform_int(struct test_rng *rng, unsigned max)
{
	unsigned mask = rng_mask(max-1);
	unsigned long long val;

	do {
		val = ( xoshiro256p(rng->state) >> 32 ) & mask;
	} while (val >= max);

	return val;
}