UT_Permute.h

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/*
 * PROPRIETARY INFORMATION.  This software is proprietary to
 * Side Effects Software Inc., and is not to be reproduced,
 * transmitted, or disclosed in any way without written permission.
 *
 * NAME:        UT_Permute.h ( UT Library, C++)
 *
 * COMMENTS:    
 */

#ifndef __UT_Permute__
#define __UT_Permute__

#include "UT_API.h"
#include "UT_Assert.h"
#include "UT_MTwister.h"
#include "UT_StackBuffer.h"
#include "UT_Swap.h"
#include "UT_TBBParallelInvoke.h"

#include <SYS/SYS_TypeTraits.h>

#include <algorithm>
#include <iterator>

#include <string.h>


/// This is a custom implementation of std::partition,
/// just so that we don't keep having issues of different
/// platforms giving different results.
///
/// This implementation requires bidirectional iterators.
template<typename IT, typename PRED>
IT UTpartition(IT start, IT end, PRED isbeforesplit)
{
    while (true)
    {
        if (start == end)
            return start;

        // Skip elements at the start that belong before the split.
        while (isbeforesplit(*start))
        {
            ++start;
            if (start == end)
                return start;
        }

        // Skip elements at the end that belong after the split.
        do
        {
            --end;
            if (start == end)
                return start;
        } while(!isbeforesplit(*end));

        // start points to something that goes after the split,
        // and end points to something that goes before the split.
        UTswap(*start, *end);
        ++start;
    }
}

/// This is a custom implementation of std::nth_element,
/// just so that we don't keep having issues of different
/// platforms giving different results.
///
/// (Having a comparator that forced strict ordering wasn't
/// sufficient for std::nth_element, because that only guarantees
/// that nth is greater than everything before it and less than
/// everything after it.  We'd have to then sort the two sections
/// with the comparator have consistency.)
///
/// This implementation requires a comparator.
template<typename IT, typename COMPARE>
void UTnth_element(IT start, IT nth, IT end, COMPARE isAbeforeB)
{
    if (start == end)
        return;

    while (start+2 < end)
    {
        // We have at least 3 elements

        // We could do a more formal introselect that falls back
        // to the proper median of medians algorithm that guarantees
        // O(n) time, but for now, let's just pick a few arbitrary
        // points and pick the median as the pivot.

        auto diff = (end-start);
        auto hdiff = (diff>>1);
        const IT mid = (start+hdiff);
        const IT last = end-1;

        // This is set up so that if they're equal,
        // pivot will be mid.
        typename std::iterator_traits<IT>::value_type pivot;
        if (isAbeforeB(*last, *start))
        {
            if (isAbeforeB(*mid, *start))
            {
                if (isAbeforeB(*last, *mid))
                    pivot = *mid;    // LMS
                else
                    pivot = *last;   // MLS
            }
            else
                pivot = *start;      // LSM
        }
        else
        {
            if (isAbeforeB(*mid, *start))
                pivot = *start;      // MSL
            else
            {
                if (isAbeforeB(*last, *mid))
                    pivot = *last;   // SLM
                else
                    pivot = *mid;    // SML
            }
        }

        // This partition moves everything belonging before the pivot
        // to the beginning, i.e.:
        // [(elements < pivot) (elements >= pivot)]
        IT split = UTpartition(start, end,
            [&pivot,&isAbeforeB](const typename std::iterator_traits<IT>::value_type &v)
            { return isAbeforeB(v, pivot); }
        );
        // Since there may be many duplicates and pivot could be
        // the smallest element, we must move all elements that equal
        // pivot (i.e. (v <= pivot) && (v >= pivot), the latter of which
        // is dealt with above) to the beginning of the split.
        // If we don't do this, we may make no progress on some iteration
        // and loop forever.
        // [(elements < pivot) (elements == pivot) (elements > pivot)]
        IT split2 = UTpartition(split, end,
            [&pivot,&isAbeforeB](const typename std::iterator_traits<IT>::value_type &v)
        { return !isAbeforeB(pivot, v); } // !(pivot < v) <--> (pivot >= v) <--> (v <= pivot)
        );

        // Just "recurse" on the section containing nth.
        if (nth < split)
            end = split;
        else if (nth < split2)
            return; // Between split and split2, everything is equal to pivot.
        else
            start = split2;
    }

    if (start+1 == end)
        return;

    // 2 elements left
    if (isAbeforeB(*(end-1), *start))
        UTswap(*start, *(end-1));
}

/// This is a custom implementation of std::nth_element,
/// just so that we don't keep having issues of different
/// platforms giving different results.
/// This implementation uses std::less.
template<typename IT>
void UTnth_element(IT start, IT nth, IT end)
{
    UTnth_element(start, nth, end, std::less<typename std::iterator_traits<IT>::value_type>());
}

struct UT_VariableSizeRef
{
    UT_VariableSizeRef(void *p, size_t element_bytes)
        : myPtr(p)
        , myElementBytes(element_bytes)
    {}

    UT_VariableSizeRef &operator=(const UT_VariableSizeRef &that) = delete;

    void *ptr() const
    {
        return myPtr;
    }
    size_t elementBytes() const
    {
        return myElementBytes;
    }

    void *myPtr;
    const size_t myElementBytes;
};

static inline void UTswap(UT_VariableSizeRef a, UT_VariableSizeRef b)
{
    // Swap the actual content, not a and b themselves.
    UT_ASSERT_P(a.elementBytes() == b.elementBytes());
    size_t nbytes = a.elementBytes();
    UT_StackBuffer<char> tmp(nbytes);
    memcpy(tmp.array(), a.ptr(), nbytes);
    memcpy(a.ptr(), b.ptr(), nbytes);
    memcpy(b.ptr(), tmp.array(), nbytes);
}

struct UT_VariableSizePtr
{
    UT_VariableSizePtr(void *p, size_t element_bytes)
        : myPtr(p)
        , myElementBytes(element_bytes)
    {}

    bool operator==(const UT_VariableSizePtr &that) const
    {
        UT_ASSERT_P(myElementBytes == that.myElementBytes);
        return myPtr == that.myPtr;
    }
    bool operator!=(const UT_VariableSizePtr &that) const
    {
        UT_ASSERT_P(myElementBytes == that.myElementBytes);
        return myPtr != that.myPtr;
    }
    bool operator<(const UT_VariableSizePtr &that) const
    {
        UT_ASSERT_P(myElementBytes == that.myElementBytes);
        return myPtr < that.myPtr;
    }
    bool operator<=(const UT_VariableSizePtr &that) const
    {
        UT_ASSERT_P(myElementBytes == that.myElementBytes);
        return myPtr <= that.myPtr;
    }
    bool operator>(const UT_VariableSizePtr &that) const
    {
        UT_ASSERT_P(myElementBytes == that.myElementBytes);
        return myPtr > that.myPtr;
    }
    bool operator>=(const UT_VariableSizePtr &that) const
    {
        UT_ASSERT_P(myElementBytes == that.myElementBytes);
        return myPtr >= that.myPtr;
    }
    void operator+=(const size_t i)
    {
        ((char *&)myPtr) += i*myElementBytes;
    }
    void operator-=(const size_t i)
    {
        ((char *&)myPtr) -= i*myElementBytes;
    }
    UT_VariableSizePtr operator+(const size_t i) const
    {
        return UT_VariableSizePtr((void *)(((char *)myPtr) + i*myElementBytes), myElementBytes);
    }
    UT_VariableSizePtr operator-(const size_t i) const
    {
        return UT_VariableSizePtr((void *)(((char *)myPtr) - i*myElementBytes), myElementBytes);
    }
    UT_VariableSizePtr &operator++()
    {
        ((char *&)myPtr) += myElementBytes;
        return *this;
    }
    UT_VariableSizePtr &operator--()
    {
        ((char *&)myPtr) -= myElementBytes;
        return *this;
    }
    UT_VariableSizeRef operator*() const
    {
        return UT_VariableSizeRef(myPtr, myElementBytes);
    }
    UT_VariableSizeRef operator[](const size_t i) const
    {
        return UT_VariableSizeRef((void *)(((char *)myPtr) + i*myElementBytes), myElementBytes);
    }

    void *ptr() const
    {
        return myPtr;
    }
    size_t elementBytes() const
    {
        return myElementBytes;
    }

    void *myPtr;
    const size_t myElementBytes;
};


namespace UT_Permute {

template <typename T, typename INT>
class Partition
{
public:
    Partition(T *low, const INT *permute, INT mid)
        : myLow(low)
        , myPermute(permute)
        , myMid(mid) {}
    bool operator()(const T &rhs) const
    {
        return myPermute[&rhs-myLow] < myMid;
    }
private:
    T           *myLow;
    const INT   *myPermute;
    INT          myMid;
};

template <typename INT>
class PartitionUnknown
{
public:
    PartitionUnknown(UT_VariableSizePtr low, const INT *permute, INT mid)
        : myLow(low)
        , myPermute(permute)
        , myMid(mid)
    {}
    bool operator()(const UT_VariableSizeRef &rhs) const
    {
        return myPermute[(((char*)rhs.ptr())-((char*)myLow.ptr()))/myLow.elementBytes()] < myMid;
    }
private:
    UT_VariableSizePtr myLow;
    const INT   *myPermute;
    INT          myMid;
};

template<typename INT>
class PartitionPermute
{
public:
    PartitionPermute(INT mid) : myMid(mid) {}
    bool operator()(INT rhs) const
    {
        return rhs < myMid;
    }
private:
    INT myMid;
};

static const int theCrossover = 1024;

template <typename T, typename INT>
static inline void
permuteInPlaceRHelper(T *vals, INT *permute, INT low, INT high)
{
    INT size = high - low;
    T tmp[theCrossover];
    for (INT i = low; i < high; i++)
        tmp[permute[i] - low] = vals[i];

    // Similar to UT_Array, avoid memcpy if the type cannot be trivially
    // relocated.
    if constexpr (
            !UnsafeTrivialRelocationNoCV<T>::value
            || LegacyTrivialRelocationNoCV<T>::value)
    {
        memcpy(reinterpret_cast<void *>(vals + low), tmp, size * sizeof(T));
    }
    else
    {
        std::move(tmp, tmp + size, vals + low);
    }
}

template <typename INT>
static inline void
permuteInPlaceRHelper(void *vals, size_t element_bytes, INT *permute, INT low, INT high)
{
    INT size = high-low;
    UT_StackBuffer<char, 16*1024> tmp(element_bytes*size);
    for (INT i = low; i < high; i++)
    {
        memcpy(tmp.array() + element_bytes*(permute[i]-low), ((const char *)vals) + element_bytes*i, element_bytes);
    }
    memcpy(((char *)vals) + element_bytes*low, tmp.array(), size*element_bytes);
}

// Permute by parallel recursive partitioning
template <typename T, typename INT>
static inline void
permuteInPlaceR(T *vals, INT *permute, INT low, INT high)
{
    INT size = high-low;
    if (size < theCrossover)
    {
        permuteInPlaceRHelper(vals, permute, low, high);
        return;
    }

    INT mid = (low + high) / 2;

    UTpartition(vals + low,
                vals + high,
                Partition<T,INT>(vals + low, permute + low, mid));
    UTpartition(permute + low,
                permute + high,
                PartitionPermute<INT>(mid));

    tbb::parallel_invoke(
        [=]() { permuteInPlaceR(vals, permute, low, mid); },
        [=]() { permuteInPlaceR(vals, permute, mid, high); }
    );
}

// Permute by parallel recursive partitioning
template <typename INT>
static inline void
permuteInPlaceR(UT_VariableSizePtr vals, INT *permute, INT low, INT high)
{
    INT size = high-low;
    if (size < theCrossover)
    {
        permuteInPlaceRHelper(vals.ptr(), vals.elementBytes(), permute, low, high);
        return;
    }

    INT mid = (low + high) / 2;

    UTpartition(vals + low,
                vals + high,
                PartitionUnknown<INT>(vals + low, permute + low, mid));
    UTpartition(permute + low,
                permute + high,
                PartitionPermute<INT>(mid));

    tbb::parallel_invoke(
        [=]() { permuteInPlaceR(vals, permute, low, mid); },
        [=]() { permuteInPlaceR(vals, permute, mid, high); }
    );
}

} // UT_Permute

// This method will reorder vals using the given permutation int array
// which must contain at least size elements.  T[i] is moved to
// T[permute[i]] for all valid indices i. The permute array is modified by
// this method.
template <typename T, typename INT>
static inline void
UTpermuteInPlace(T *vals, INT *permute, INT size)
{
    UT_Permute::permuteInPlaceR(vals, permute, INT(0), size);
}

// Makes a temporary copy of permute before calling UTpermuteInPlace.
template <typename T, typename INT>
static inline void
UTpermute(T *vals, const INT *permute, INT size)
{
    UT_StackBuffer<INT> tmp_permute(size);
    memcpy(tmp_permute, permute, size*sizeof(INT));

    UT_Permute::permuteInPlaceR(vals, tmp_permute.array(), INT(0), size);
}

// This method will reorder vals using the given permutation int array
// which must contain at least size elements.  T[permute[i]] is moved to
// T[i] for all valid indices i.
template <typename T, typename INT>
static inline void
UTinversePermute(T *vals, const INT *permute, INT size)
{
    UT_StackBuffer<INT> tmp_permute(size);
    for (INT i = 0; i < size; i++)
        tmp_permute[permute[i]] = i;

    UT_Permute::permuteInPlaceR(vals, tmp_permute.array(), INT(0), size);
}

// This method will reorder vals using the given permutation int array
// which must contain at least nelements elements.  T[permute[i]] is moved to
// T[i] for all valid indices i, where each array element is element_bytes
// in size.
template <typename INT>
static inline void
UTinversePermute(void *vals, size_t element_bytes, const INT *permute, INT nelements)
{
    UT_StackBuffer<INT> tmp_permute(nelements);
    for (INT i = 0; i < nelements; i++)
        tmp_permute[permute[i]] = i;

    UT_Permute::permuteInPlaceR(UT_VariableSizePtr(vals, element_bytes), tmp_permute.array(), INT(0), nelements);
}

template <typename INT>
static inline void
UTfillRandomPermutation(UT_MersenneTwister &twist, INT *permute, INT size)
{
    for (int i = 0; i < size; i++)
        permute[i] = i;

    for (int i = size; i-- > 1; )
    {
        int k = twist.urandom() % (i+1);
        if (k != i)
            std::swap(permute[k], permute[i]);
    }
}

#endif