factory-hole-core/include/Util/RandomPicker.h

#pragma once

#include <memory>
#include <type_traits>

template <typename T, typename WeightType>
class RandomPickerT
{
public:
    struct Entry
    {
        Entry() = default;
        Entry(const T& val, WeightType weight) : Val{ val }, Weight{ weight } {}

        T Val{};
        const WeightType Weight{};
        WeightType AccumulatedWeight{};
        WeightType GetWeightSum() { return AccumulatedWeight + Weight; }
    };

public:
    std::unique_ptr<Entry[]> Entries{};
    WeightType TotalWeight{};
    uint32_t TotalEntries{};
    const uint32_t DataSize{};

public:
    RandomPickerT() = default;

    template <typename Container, typename WeightGetter>
    RandomPickerT(const Container& container, WeightGetter getter)
        : Entries{ std::make_unique<Entry[]>(container.size()) }
        , TotalEntries{ container.size() }
        , DataSize{ container.size() }
    {
        int counter{};
        for (const auto& entry : container)
        {
            auto weight = getter(entry);
            Entries[counter++] = Entry{entry, weight};
        }

        RecalculateWeights();
    }

    template <typename EntriesT>
    RandomPickerT(const EntriesT& entries)
        : Entries{ std::make_unique<Entry[]>(entries.size()) }
        , TotalEntries{ entries.size() }
        , DataSize{ entries.size() }
    {
        int counter{};
        for (const auto& entry : entries)
        {
            Entries[counter++] = entry;
        }

        RecalculateWeights();
    }

public:
    T GetRandom(int& index, WeightType randomVal) const
    {
        WeightType weight = GetRandomWeight(randomVal);
        index = GetIndex(weight);

        return Entries[index].Val;
    }

    int GetIndex(WeightType weight) const
    {
        if (weight < 0 || weight > TotalWeight || TotalEntries == 0) 
            return -1;
        if (TotalEntries == 1) 
            return 0;
        return BinarySearchRecursive(weight, 0, TotalEntries - 1);
    }

    void RemoveEntry(int index)
    {
        if (ValidateIndex(index)) RemoveEntryInternal(index);
    }

    T GetAndRemoveRandom(int& index, WeightType randomVal)
    {
        WeightType weight = GetRandomWeight(randomVal);
        int index = GetIndex(weight);

        auto returnVal = Entries[index].Val;
        RemoveEntryInternal(index);

        return returnVal;
    }

    T GetAndRemoveRandom(WeightType randomVal)
    {
        int index{};
        return GetAndRemoveRandom(index, randomVal);
    }

    T Peek(int index) const
    {
        _ASSERT(ValidateIndex(index));
        return Entries[index];
    }

    void Reset()
    {
        TotalEntries = DataSize;
        RecalculateWeights();
    }

    template <typename WeightGetter>
    void SetObjectAtIndex(int index, const T& val, WeightGetter getter)
    {
        if (!ValidateIndex(index)) return;

        auto entry = Entries[index];

        auto newWeight = getter(val);
        bool recalculateWeights = newWeight != entry.Weight;

        Entries[index] = Entry{ val, newWeight };

        if (recalculateWeights) RecalculateWeights();
    }

    void SetEntryAtIndex(int index, const Entry& entry)
    {
        if (!ValidateIndex(index)) return;

        bool recalculateWeights = entry.Weight != Entries[index].Weight;
        Entries[index] = entry;

        if (recalculateWeights) RecalculateWeights();
    }

private:
    void RecalculateWeights()
    {
        WeightType accumulatedWeight = 0;
        for (uint32_t i{}; i < TotalEntries; ++i)
        {
            Entries[i].AccumulatedWeight = accumulatedWeight;
            accumulatedWeight += Entries[i].Weight;
        }
        TotalWeight = accumulatedWeight;
    }

    WeightType GetRandomWeight(WeightType randomVal) const
    {
        if constexpr (std::is_integral_v<WeightType>)
        {
            return randomVal % TotalWeight;
        }
        if constexpr (std::is_floating_point_v<WeightType>)
        {
            return std::fmod(randomVal, TotalWeight);
        }
    }

    int BinarySearchRecursive(WeightType weight, int min, int max) const
    {
        int middle = (min + max) >> 1;

        auto entry = Entries[middle];
        if (weight >= entry.AccumulatedWeight && weight < entry.GetWeightSum())
            return middle;
        
        return (weight < entry.AccumulatedWeight) ? BinarySearchRecursive(weight, min, middle - 1) : BinarySearchRecursive(weight, middle + 1, max);
    }

    void RemoveEntryInternal(int index)
    {
        if (!ValidateIndex(index))
            return;
        
        std::swap(Entries[index], Entries.back());
        --TotalEntries;

        if (TotalEntries != 0) RecalculateWeights();
        else TotalWeight = 0;
    }

    bool ValidateIndex(int index) const
    {
        return index >= 0 && index < TotalEntries;
    }
};

template <typename T>
using RandomPicker32 = RandomPickerT<T, uint32_t>;
template <typename T>
using RandomPicker64 = RandomPickerT<T, uint64_t>;
template <typename T>
using RandomPickerF = RandomPickerT<T, float>;
template <typename T>
using RandomPickerD = RandomPickerT<T, double>;