helper.h

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#pragma once
 
#include <numeric>
#include <random>
#include <vector>
 
#include "number.h"
 
namespace faker::helper
{
 
template <typename T>
concept input_range_with_faster_size_compute_than_linear_rng =
    std::ranges::input_range<T> && (std::ranges::sized_range<T> || std::ranges::forward_range<T>);
 
template <input_range_with_faster_size_compute_than_linear_rng Range>
decltype(auto) randomElement(Range&& range)
{
    if (std::ranges::empty(range))
    {
        throw std::invalid_argument{"Range [start, end) is empty."};
    }
 
    const auto size = std::ranges::distance(range);
 
    const auto index = number::integer(size - 1);
 
    return (*std::ranges::next(range.begin(), index));
}
 
template <std::ranges::input_range Range>
auto randomElement(Range&& range)
{
    auto const end = range.end();
    auto itr = range.begin();
 
    if (itr == end)
    {
        throw std::invalid_argument{"Range [start, end) is empty."};
    }
 
    std::random_device rd;
    std::mt19937 gen(rd());
 
    using RangeValue = std::ranges::range_value_t<decltype(range)>;
    auto consume_itr = [&itr]() -> decltype(auto)
    {
        using reference_type = std::ranges::range_reference_t<decltype(range)>;
        if constexpr (std::is_reference_v<reference_type>)
        {
            return std::move(*itr);
        }
        else
        {
            return *itr;
        }
    };
 
    RangeValue result = consume_itr();
 
    ++itr;
 
    std::size_t count = 1;
 
    for (; itr != end; ++itr, ++count)
    {
        std::uniform_int_distribution<size_t> distrib(0, count);
        if (distrib(gen) == 0)
        {
            result = consume_itr();
        }
    }
 
    return result;
}
 
template <class T>
struct WeightedElement
{
    unsigned weight;
    T value;
};
 
template <class T>
T weightedRandomElement(const std::vector<WeightedElement<T>>& data)
{
    if (data.empty())
    {
        throw std::invalid_argument{"Data is empty."};
    }
 
    const auto sumOfWeights =
        std::accumulate(data.begin(), data.end(), 0u,
                        [](unsigned sum, const WeightedElement<T>& element) { return sum + element.weight; });
 
    if (sumOfWeights == 0u)
    {
        throw std::invalid_argument{"Sum of weights cannot be zero."};
    }
 
    const auto targetWeightValue = number::integer(1u, sumOfWeights);
 
    auto currentSum = 0u;
 
    for (const auto& elem : data)
    {
        currentSum += elem.weight;
 
        if (currentSum > targetWeightValue)
        {
            return elem.value;
        }
    }
 
    return data.back().value;
}
 
}