Heap.h

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

// -*- c++ -*-

/*
 * C++ support for heaps. The set of functions is tailored for
 * efficient similarity search.
 *
 * There is no specific object for a heap, and the functions that
 * operate on a signle heap are inlined, because heaps are often
 * small. More complex functions are implemented in Heaps.cpp
 *
 */


#ifndef FAISS_Heap_h
#define FAISS_Heap_h

#include <climits>
#include <cstring>
#include <cmath>

#include <cassert>
#include <cstdio>

#include <limits>


namespace faiss {

/*******************************************************************
 * C object: uniform handling of min and max heap
 *******************************************************************/

/** The C object gives the type T of the values in the heap, the type
 *  of the keys, TI and the comparison that is done: > for the minheap
 *  and < for the maxheap. The neutral value will always be dropped in
 *  favor of any other value in the heap.
 */

template <typename T_, typename TI_>
struct CMax;

// traits of minheaps = heaps where the minimum value is stored on top
// useful to find the *max* values of an array
template <typename T_, typename TI_>
struct CMin {
    typedef T_ T;
    typedef TI_ TI;
    typedef CMax<T_, TI_> Crev;
    inline static bool cmp (T a, T b) {
        return a < b;
    }
    // value that will be popped first -> must be smaller than all others
    // for int types this is not strictly the smallest val (-max - 1)
    inline static T neutral () {
        return -std::numeric_limits<T>::max();
    }
};


template <typename T_, typename TI_>
struct CMax {
    typedef T_ T;
    typedef TI_ TI;
    typedef CMin<T_, TI_> Crev;
    inline static bool cmp (T a, T b) {
        return a > b;
    }
    inline static T neutral () {
        return std::numeric_limits<T>::max();
    }
};


/*******************************************************************
 * Basic heap ops: push and pop
 *******************************************************************/

/** Pops the top element from the heap defined by bh_val[0..k-1] and
 * bh_ids[0..k-1].  on output the element at k-1 is undefined.
 */
template <class C> inline
void heap_pop (size_t k, typename C::T * bh_val, typename C::TI * bh_ids)
{
    bh_val--; /* Use 1-based indexing for easier node->child translation */
    bh_ids--;
    typename C::T val = bh_val[k];
    size_t i = 1, i1, i2;
    while (1) {
        i1 = i << 1;
        i2 = i1 + 1;
        if (i1 > k)
            break;
        if (i2 == k + 1 || C::cmp(bh_val[i1], bh_val[i2])) {
            if (C::cmp(val, bh_val[i1]))
                break;
            bh_val[i] = bh_val[i1];
            bh_ids[i] = bh_ids[i1];
            i = i1;
        }
        else {
            if (C::cmp(val, bh_val[i2]))
                break;
            bh_val[i] = bh_val[i2];
            bh_ids[i] = bh_ids[i2];
            i = i2;
        }
    }
    bh_val[i] = bh_val[k];
    bh_ids[i] = bh_ids[k];
}



/** Pushes the element (val, ids) into the heap bh_val[0..k-2] and
 * bh_ids[0..k-2].  on output the element at k-1 is defined.
 */
template <class C> inline
void heap_push (size_t k,
                typename C::T * bh_val, typename C::TI * bh_ids,
                typename C::T val, typename C::TI ids)
{
    bh_val--; /* Use 1-based indexing for easier node->child translation */
    bh_ids--;
    size_t i = k, i_father;
    while (i > 1) {
        i_father = i >> 1;
        if (!C::cmp (val, bh_val[i_father]))  /* the heap structure is ok */
            break;
        bh_val[i] = bh_val[i_father];
        bh_ids[i] = bh_ids[i_father];
        i = i_father;
    }
    bh_val[i] = val;
    bh_ids[i] = ids;
}



/* Partial instanciation for heaps with TI = long */

template <typename T> inline
void minheap_pop (size_t k, T * bh_val, long * bh_ids)
{
    heap_pop<CMin<T, long> > (k, bh_val, bh_ids);
}


template <typename T> inline
void minheap_push (size_t k, T * bh_val, long * bh_ids, T val, long ids)
{
    heap_push<CMin<T, long> > (k, bh_val, bh_ids, val, ids);
}


template <typename T> inline
void maxheap_pop (size_t k, T * bh_val, long * bh_ids)
{
    heap_pop<CMax<T, long> > (k, bh_val, bh_ids);
}


template <typename T> inline
void maxheap_push (size_t k, T * bh_val, long * bh_ids, T val, long ids)
{
    heap_push<CMax<T, long> > (k, bh_val, bh_ids, val, ids);
}



/*******************************************************************
 * Heap initialization
 *******************************************************************/

/* Initialization phase for the heap (with unconditionnal pushes).
 * Store k0 elements in a heap containing up to k values. Note that
 * (bh_val, bh_ids) can be the same as (x, ids) */
template <class C> inline
void heap_heapify (
        size_t k,
        typename C::T *  bh_val,
        typename C::TI *  bh_ids,
        const typename C::T * x = nullptr,
        const typename C::TI * ids = nullptr,
        size_t k0 = 0)
{
   if (k0 > 0) assert (x);

   if (ids) {
       for (size_t i = 0; i < k0; i++)
           heap_push<C> (i+1, bh_val, bh_ids, x[i], ids[i]);
   } else {
       for (size_t i = 0; i < k0; i++)
           heap_push<C> (i+1, bh_val, bh_ids, x[i], i);
   }

   for (size_t i = k0; i < k; i++) {
       bh_val[i] = C::neutral();
       bh_ids[i] = -1;
   }

}

template <typename T> inline
void minheap_heapify (
        size_t k, T *  bh_val,
        long * bh_ids,
        const T * x = nullptr,
        const long * ids = nullptr,
        size_t k0 = 0)
{
    heap_heapify< CMin<T, long> > (k, bh_val, bh_ids, x, ids, k0);
}


template <typename T> inline
void maxheap_heapify (
        size_t k,
        T * bh_val,
        long * bh_ids,
         const T * x = nullptr,
         const long * ids = nullptr,
         size_t k0 = 0)
{
    heap_heapify< CMax<T, long> > (k, bh_val, bh_ids, x, ids, k0);
}



/*******************************************************************
 * Add n elements to the heap
 *******************************************************************/


/* Add some elements to the heap  */
template <class C> inline
void heap_addn (size_t k,
                typename C::T * bh_val, typename C::TI * bh_ids,
                const typename C::T * x,
                const typename C::TI * ids,
                size_t n)
{
    size_t i;
    if (ids)
        for (i = 0; i < n; i++) {
            if (C::cmp (bh_val[0], x[i])) {
                heap_pop<C> (k, bh_val, bh_ids);
                heap_push<C> (k, bh_val, bh_ids, x[i], ids[i]);
            }
        }
    else
        for (i = 0; i < n; i++) {
            if (C::cmp (bh_val[0], x[i])) {
                heap_pop<C> (k, bh_val, bh_ids);
                heap_push<C> (k, bh_val, bh_ids, x[i], i);
            }
        }
}


/* Partial instanciation for heaps with TI = long */

template <typename T> inline
void minheap_addn (size_t k, T * bh_val, long * bh_ids,
                   const T * x, const long * ids, size_t n)
{
    heap_addn<CMin<T, long> > (k, bh_val, bh_ids, x, ids, n);
}

template <typename T> inline
void maxheap_addn (size_t k, T * bh_val, long * bh_ids,
                   const T * x, const long * ids, size_t n)
{
    heap_addn<CMax<T, long> > (k, bh_val, bh_ids, x, ids, n);
}






/*******************************************************************
 * Heap finalization (reorder elements)
 *******************************************************************/


/* This function maps a binary heap into an sorted structure.
   It returns the number  */
template <typename C> inline
size_t heap_reorder (size_t k, typename C::T * bh_val, typename C::TI * bh_ids)
{
    size_t i, ii;

    for (i = 0, ii = 0; i < k; i++) {
        /* top element should be put at the end of the list */
        typename C::T val = bh_val[0];
        typename C::TI id = bh_ids[0];

        /* boundary case: we will over-ride this value if not a true element */
        heap_pop<C> (k-i, bh_val, bh_ids);
        bh_val[k-ii-1] = val;
        bh_ids[k-ii-1] = id;
        if (id != -1) ii++;
    }
    /* Count the number of elements which are effectively returned */
    size_t nel = ii;

    memmove (bh_val, bh_val+k-ii, ii * sizeof(*bh_val));
    memmove (bh_ids, bh_ids+k-ii, ii * sizeof(*bh_ids));

    for (; ii < k; ii++) {
        bh_val[ii] = C::neutral();
        bh_ids[ii] = -1;
    }
    return nel;
}

template <typename T> inline
size_t minheap_reorder (size_t k, T * bh_val, long * bh_ids)
{
    return heap_reorder< CMin<T, long> > (k, bh_val, bh_ids);
}

template <typename T> inline
size_t maxheap_reorder (size_t k, T * bh_val, long * bh_ids)
{
    return heap_reorder< CMax<T, long> > (k, bh_val, bh_ids);
}





/*******************************************************************
 * Operations on heap arrays
 *******************************************************************/

/** a template structure for a set of [min|max]-heaps it is tailored
 * so that the actual data of the heaps can just live in compact
 * arrays.
 */
template <typename C>
struct HeapArray {
    typedef typename C::TI TI;
    typedef typename C::T T;

    size_t nh;    ///< number of heaps
    size_t k;     ///< allocated size per heap
    TI * ids;     ///< identifiers (size nh * k)
    T * val;      ///< values (distances or similarities), size nh * k

    /// Return the list of values for a heap
    T * get_val (size_t key) { return val + key * k; }

    /// Correspponding identifiers
    TI * get_ids (size_t key) { return ids + key * k; }

    /// prepare all the heaps before adding
    void heapify ();

    /** add nj elements to heaps i0:i0+ni, with sequential ids
     *
     * @param nj    nb of elements to add to each heap
     * @param vin   elements to add, size ni * nj
     * @param j0    add this to the ids that are added
     * @param i0    first heap to update
     * @param ni    nb of elements to update (-1 = use nh)
     */
    void addn (size_t nj, const T *vin, TI j0 = 0,
               size_t i0 = 0, long ni = -1);

    /** same as addn
     *
     * @param id_in     ids of the elements to add, size ni * nj
     * @param id_stride stride for id_in
     */
    void addn_with_ids (
        size_t nj, const T *vin, const TI *id_in = nullptr,
        long id_stride = 0, size_t i0 = 0, long ni = -1);

    /// reorder all the heaps
    void reorder ();

    /** this is not really a heap function. It just finds the per-line
     *   extrema of each line of array D
     * @param vals_out    extreme value of each line (size nh, or NULL)
     * @param idx_out     index of extreme value (size nh or NULL)
     */
    void per_line_extrema (T *vals_out, TI *idx_out) const;

};


/* Define useful heaps */
typedef HeapArray<CMin<float, long> > float_minheap_array_t;
typedef HeapArray<CMin<int, long> > int_minheap_array_t;

typedef HeapArray<CMax<float, long> > float_maxheap_array_t;
typedef HeapArray<CMax<int, long> > int_maxheap_array_t;

// The heap templates are instanciated explicitly in Heap.cpp



















/*********************************************************************
 * Indirect heaps: instead of having
 *
 *          node i = (bh_ids[i], bh_val[i]),
 *
 * in indirect heaps,
 *
 *          node i = (bh_ids[i], bh_val[bh_ids[i]]),
 *
 *********************************************************************/


template <class C>
inline
void indirect_heap_pop (
    size_t k,
    const typename C::T * bh_val,
    typename C::TI * bh_ids)
{
    bh_ids--; /* Use 1-based indexing for easier node->child translation */
    typename C::T val = bh_val[bh_ids[k]];
    size_t i = 1;
    while (1) {
        size_t i1 = i << 1;
        size_t i2 = i1 + 1;
        if (i1 > k)
            break;
        typename C::TI id1 = bh_ids[i1], id2 = bh_ids[i2];
        if (i2 == k + 1 || C::cmp(bh_val[id1], bh_val[id2])) {
            if (C::cmp(val, bh_val[id1]))
                break;
            bh_ids[i] = id1;
            i = i1;
        } else {
            if (C::cmp(val, bh_val[id2]))
                break;
            bh_ids[i] = id2;
            i = i2;
        }
    }
    bh_ids[i] = bh_ids[k];
}



template <class C>
inline
void indirect_heap_push (size_t k,
                         const typename C::T * bh_val, typename C::TI * bh_ids,
                         typename C::TI id)
{
    bh_ids--; /* Use 1-based indexing for easier node->child translation */
    typename C::T val = bh_val[id];
    size_t i = k;
    while (i > 1) {
        size_t i_father = i >> 1;
        if (!C::cmp (val, bh_val[bh_ids[i_father]]))
            break;
        bh_ids[i] = bh_ids[i_father];
        i = i_father;
    }
    bh_ids[i] = id;
}


} // namespace faiss

#endif  /* FAISS_Heap_h */