/*

 * Copyright (c) 2021, the SerenityOS developers.

 *

 * SPDX-License-Identifier: BSD-2-Clause

 */

#include <AK/Random.h>

namespace AK {

u32 get_random_uniform(u32 max_bounds)

{

    // If we try to divide all 2**32 numbers into groups of "max_bounds" numbers, we may end up

    // with one group that is a bit too small. For this reason, the implementation

    // `arc4random() % max_bounds` would be insufficient. Here we compute the first number of

    // the first "full group" as 2**32 % max_bounds, and rely on the equivalence

    // `2**32 % x == (2**32 - x) % x` to keep calculations in u32 domain:

    u32 const threshold = static_cast<u32>(-max_bounds) % max_bounds;

    auto random_value = get_random<u32>();

    for (int i = 0; i < 20 && random_value < threshold; ++i) {

        // By chance we picked a value from the incomplete group. Note that this group has size at

        // most 2**31-1, so picking this group has a chance of less than 50%.

        // In practice, this means that for the worst possible input, there is still only a

        // once-in-a-million chance to get to iteration 20. In theory we should be able to loop

        // forever. Here we prefer marginally imperfect random numbers over weird runtime behavior.

        random_value = get_random<u32>();

    }

    return random_value % max_bounds;

}

u64 get_random_uniform_64(u64 max_bounds)

{

    // Uses the same algorithm as `get_random_uniform`,

    // by replacing u32 with u64.

    u64 const threshold = static_cast<u64>(-max_bounds) % max_bounds;

    auto random_value = get_random<u64>();

    for (int i = 0; i < 20 && random_value < threshold; ++i) {

        random_value = get_random<u64>();

    }

    return random_value % max_bounds;

}

}