ClickHouse/dbms/src/Common/RadixSort.h

#pragma once

#include <string.h>
#if !defined(__APPLE__) && !defined(__FreeBSD__)
#include <malloc.h>
#endif
#include <cstdlib>
#include <cstdint>
#include <type_traits>

#include <ext/bit_cast.h>
#include <Core/Types.h>
#include <Core/Defines.h>


/** Radix sort, has the following functionality:
  * Can sort unsigned, signed numbers, and floats.
  * Can sort an array of fixed length elements that contain something else besides the key.
  * Customizable radix size.
  *
  * LSB, stable.
  * NOTE For some applications it makes sense to add MSB-radix-sort,
  *  as well as radix-select, radix-partial-sort, radix-get-permutation algorithms based on it.
  */


/** Used as a template parameter. See below.
  */
struct RadixSortMallocAllocator
{
    void * allocate(size_t size)
    {
        return malloc(size);
    }

    void deallocate(void * ptr, size_t /*size*/)
    {
        return free(ptr);
    }
};


/** A transformation that transforms the bit representation of a key into an unsigned integer number,
  *  that the order relation over the keys will match the order relation over the obtained unsigned numbers.
  * For floats this conversion does the following:
  *  if the signed bit is set, it flips all other bits.
  * In this case, NaN-s are bigger than all normal numbers.
  */
template <typename KeyBits>
struct RadixSortFloatTransform
{
    /// Is it worth writing the result in memory, or is it better to do calculation every time again?
    static constexpr bool transform_is_simple = false;

    static KeyBits forward(KeyBits x)
    {
        return x ^ ((-(x >> (sizeof(KeyBits) * 8 - 1))) | (KeyBits(1) << (sizeof(KeyBits) * 8 - 1)));
    }

    static KeyBits backward(KeyBits x)
    {
        return x ^ (((x >> (sizeof(KeyBits) * 8 - 1)) - 1) | (KeyBits(1) << (sizeof(KeyBits) * 8 - 1)));
    }
};


template <typename Float>
struct RadixSortFloatTraits
{
    using Element = Float;        /// The type of the element. It can be a structure with a key and some other payload. Or just a key.
    using Key = Float;            /// The key to sort.
    using CountType = uint32_t;   /// Type for calculating histograms. In the case of a known small number of elements, it can be less than size_t.

    /// The type to which the key is transformed to do bit operations. This UInt is the same size as the key.
    using KeyBits = std::conditional_t<sizeof(Float) == 8, uint64_t, uint32_t>;

    static constexpr size_t PART_SIZE_BITS = 8;    /// With what pieces of the key, in bits, to do one pass - reshuffle of the array.

    /// Converting a key into KeyBits is such that the order relation over the key corresponds to the order relation over KeyBits.
    using Transform = RadixSortFloatTransform<KeyBits>;

    /// An object with the functions allocate and deallocate.
    /// Can be used, for example, to allocate memory for a temporary array on the stack.
    /// To do this, the allocator itself is created on the stack.
    using Allocator = RadixSortMallocAllocator;

    /// The function to get the key from an array element.
    static Key & extractKey(Element & elem) { return elem; }
};


template <typename KeyBits>
struct RadixSortIdentityTransform
{
    static constexpr bool transform_is_simple = true;

    static KeyBits forward(KeyBits x)     { return x; }
    static KeyBits backward(KeyBits x)    { return x; }
};


template <typename KeyBits>
struct RadixSortSignedTransform
{
    static constexpr bool transform_is_simple = true;

    static KeyBits forward(KeyBits x)     { return x ^ (KeyBits(1) << (sizeof(KeyBits) * 8 - 1)); }
    static KeyBits backward(KeyBits x)    { return x ^ (KeyBits(1) << (sizeof(KeyBits) * 8 - 1)); }
};


template <typename UInt>
struct RadixSortUIntTraits
{
    using Element = UInt;
    using Key = UInt;
    using CountType = uint32_t;
    using KeyBits = UInt;

    static constexpr size_t PART_SIZE_BITS = 8;

    using Transform = RadixSortIdentityTransform<KeyBits>;
    using Allocator = RadixSortMallocAllocator;

    /// The function to get the key from an array element.
    static Key & extractKey(Element & elem) { return elem; }
};

template <typename Int>
struct RadixSortIntTraits
{
    using Element = Int;
    using Key = Int;
    using CountType = uint32_t;
    using KeyBits = std::make_unsigned_t<Int>;

    static constexpr size_t PART_SIZE_BITS = 8;

    using Transform = RadixSortSignedTransform<KeyBits>;
    using Allocator = RadixSortMallocAllocator;

    /// The function to get the key from an array element.
    static Key & extractKey(Element & elem) { return elem; }
};


template <typename Traits>
struct RadixSort
{
private:
    using Element     = typename Traits::Element;
    using Key         = typename Traits::Key;
    using CountType   = typename Traits::CountType;
    using KeyBits     = typename Traits::KeyBits;

    static constexpr size_t HISTOGRAM_SIZE = 1 << Traits::PART_SIZE_BITS;
    static constexpr size_t PART_BITMASK = HISTOGRAM_SIZE - 1;
    static constexpr size_t KEY_BITS = sizeof(Key) * 8;
    static constexpr size_t NUM_PASSES = (KEY_BITS + (Traits::PART_SIZE_BITS - 1)) / Traits::PART_SIZE_BITS;

    static ALWAYS_INLINE KeyBits getPart(size_t N, KeyBits x)
    {
        if (Traits::Transform::transform_is_simple)
            x = Traits::Transform::forward(x);

        return (x >> (N * Traits::PART_SIZE_BITS)) & PART_BITMASK;
    }

    static KeyBits keyToBits(Key x) { return ext::bit_cast<KeyBits>(x); }
    static Key bitsToKey(KeyBits x) { return ext::bit_cast<Key>(x); }

public:
    static void execute(Element * arr, size_t size)
    {
        /// If the array is smaller than 256, then it is better to use another algorithm.

        /// There are loops of NUM_PASSES. It is very important that they are unfolded at compile-time.

        /// For each of the NUM_PASSES bit ranges of the key, consider how many times each value of this bit range met.
        CountType histograms[HISTOGRAM_SIZE * NUM_PASSES] = {0};

        typename Traits::Allocator allocator;

        /// We will do several passes through the array. On each pass, the data is transferred to another array. Let's allocate this temporary array.
        Element * swap_buffer = reinterpret_cast<Element *>(allocator.allocate(size * sizeof(Element)));

        /// Transform the array and calculate the histogram.
        /// NOTE This is slightly suboptimal. Look at https://github.com/powturbo/TurboHist
        for (size_t i = 0; i < size; ++i)
        {
            if (!Traits::Transform::transform_is_simple)
                Traits::extractKey(arr[i]) = bitsToKey(Traits::Transform::forward(keyToBits(Traits::extractKey(arr[i]))));

            for (size_t j = 0; j < NUM_PASSES; ++j)
                ++histograms[j * HISTOGRAM_SIZE + getPart(j, keyToBits(Traits::extractKey(arr[i])))];
        }

        {
            /// Replace the histograms with the accumulated sums: the value in position i is the sum of the previous positions minus one.
            size_t sums[NUM_PASSES] = {0};

            for (size_t i = 0; i < HISTOGRAM_SIZE; ++i)
            {
                for (size_t j = 0; j < NUM_PASSES; ++j)
                {
                    size_t tmp = histograms[j * HISTOGRAM_SIZE + i] + sums[j];
                    histograms[j * HISTOGRAM_SIZE + i] = sums[j] - 1;
                    sums[j] = tmp;
                }
            }
        }

        /// Move the elements in the order starting from the least bit piece, and then do a few passes on the number of pieces.
        for (size_t j = 0; j < NUM_PASSES; ++j)
        {
            Element * writer = j % 2 ? arr : swap_buffer;
            Element * reader = j % 2 ? swap_buffer : arr;

            for (size_t i = 0; i < size; ++i)
            {
                size_t pos = getPart(j, keyToBits(Traits::extractKey(reader[i])));

                /// Place the element on the next free position.
                auto & dest = writer[++histograms[j * HISTOGRAM_SIZE + pos]];
                dest = reader[i];

                /// On the last pass, we do the reverse transformation.
                if (!Traits::Transform::transform_is_simple && j == NUM_PASSES - 1)
                    Traits::extractKey(dest) = bitsToKey(Traits::Transform::backward(keyToBits(Traits::extractKey(reader[i]))));
            }
        }

        /// If the number of passes is odd, the result array is in a temporary buffer. Copy it to the place of the original array.
        /// NOTE Sometimes it will be more optimal to provide non-destructive interface, that will not modify original array.
        if (NUM_PASSES % 2)
            memcpy(arr, swap_buffer, size * sizeof(Element));

        allocator.deallocate(swap_buffer, size * sizeof(Element));
    }
};


template <typename T>
std::enable_if_t<std::is_unsigned_v<T> && std::is_integral_v<T>, void>
radixSort(T * arr, size_t size)
{
    return RadixSort<RadixSortUIntTraits<T>>::execute(arr, size);
}

template <typename T>
std::enable_if_t<std::is_signed_v<T> && std::is_integral_v<T>, void>
radixSort(T * arr, size_t size)
{
    return RadixSort<RadixSortIntTraits<T>>::execute(arr, size);
}

template <typename T>
std::enable_if_t<std::is_floating_point_v<T>, void>
radixSort(T * arr, size_t size)
{
    return RadixSort<RadixSortFloatTraits<T>>::execute(arr, size);
}