// Copyright 2006 The RE2 Authors.  All Rights Reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

#ifndef RE2_SPARSE_SET_H_
#define RE2_SPARSE_SET_H_

// DESCRIPTION
//
// SparseSet(m) is a set of integers in [0, m).
// It requires sizeof(int)*m memory, but it provides
// fast iteration through the elements in the set and fast clearing
// of the set.
//
// Insertion and deletion are constant time operations.
//
// Allocating the set is a constant time operation
// when memory allocation is a constant time operation.
//
// Clearing the set is a constant time operation (unusual!).
//
// Iterating through the set is an O(n) operation, where n
// is the number of items in the set (not O(m)).
//
// The set iterator visits entries in the order they were first
// inserted into the set.  It is safe to add items to the set while
// using an iterator: the iterator will visit indices added to the set
// during the iteration, but will not re-visit indices whose values
// change after visiting.  Thus SparseSet can be a convenient
// implementation of a work queue.
//
// The SparseSet implementation is NOT thread-safe.  It is up to the
// caller to make sure only one thread is accessing the set.  (Typically
// these sets are temporary values and used in situations where speed is
// important.)
//
// The SparseSet interface does not present all the usual STL bells and
// whistles.
//
// Implemented with reference to Briggs & Torczon, An Efficient
// Representation for Sparse Sets, ACM Letters on Programming Languages
// and Systems, Volume 2, Issue 1-4 (March-Dec.  1993), pp.  59-69.
//
// This is a specialization of sparse array; see sparse_array.h.

// IMPLEMENTATION
//
// See sparse_array.h for implementation details.

// Doing this simplifies the logic below.
#ifndef __has_feature
#define __has_feature(x) 0
#endif

#include <assert.h>
#include <stdint.h>
#if __has_feature(memory_sanitizer)
#include <sanitizer/msan_interface.h>
#endif
#include <algorithm>
#include <memory>
#include <utility>

#include "re2/pod_array.h"

namespace re2 {

template <typename Value>
class SparseSetT {
public:
    SparseSetT();
    explicit SparseSetT(int max_size);
    ~SparseSetT();

    typedef int *iterator;
    typedef const int *const_iterator;

    // Return the number of entries in the set.
    int size() const { return size_; }

    // Indicate whether the set is empty.
    int empty() const { return size_ == 0; }

    // Iterate over the set.
    iterator begin() { return dense_.data(); }
    iterator end() { return dense_.data() + size_; }

    const_iterator begin() const { return dense_.data(); }
    const_iterator end() const { return dense_.data() + size_; }

    // Change the maximum size of the set.
    // Invalidates all iterators.
    void resize(int new_max_size);

    // Return the maximum size of the set.
    // Indices can be in the range [0, max_size).
    int max_size() const {
        if (dense_.data() != NULL)
            return dense_.size();
        else
            return 0;
    }

    // Clear the set.
    void clear() { size_ = 0; }

    // Check whether index i is in the set.
    bool contains(int i) const;

    // Comparison function for sorting.
    // Can sort the sparse set so that future iterations
    // will visit indices in increasing order using
    // std::sort(arr.begin(), arr.end(), arr.less);
    static bool less(int a, int b);

public:
    // Insert index i into the set.
    iterator insert(int i) { return InsertInternal(true, i); }

    // Insert index i into the set.
    // Fast but unsafe: only use if contains(i) is false.
    iterator insert_new(int i) { return InsertInternal(false, i); }

private:
    iterator InsertInternal(bool allow_existing, int i) {
        DebugCheckInvariants();
        if (static_cast<uint32_t>(i) >= static_cast<uint32_t>(max_size())) {
            assert(false && "illegal index");
            // Semantically, end() would be better here, but we already know
            // the user did something stupid, so begin() insulates them from
            // dereferencing an invalid pointer.
            return begin();
        }
        if (!allow_existing) {
            assert(!contains(i));
            create_index(i);
        } else {
            if (!contains(i))
                create_index(i);
        }
        DebugCheckInvariants();
        return dense_.data() + sparse_[i];
    }

    // Add the index i to the set.
    // Only use if contains(i) is known to be false.
    // This function is private, only intended as a helper
    // for other methods.
    void create_index(int i);

    // In debug mode, verify that some invariant properties of the class
    // are being maintained. This is called at the end of the constructor
    // and at the beginning and end of all public non-const member functions.
    void DebugCheckInvariants() const;

    // Initializes memory for elements [min, max).
    void MaybeInitializeMemory(int min, int max) {
#if __has_feature(memory_sanitizer)
        __msan_unpoison(sparse_.data() + min, (max - min) * sizeof sparse_[0]);
#elif defined(RE2_ON_VALGRIND)
        for (int i = min; i < max; i++) {
            sparse_[i] = 0xababababU;
        }
#endif
    }

    int size_ = 0;
    PODArray<int> sparse_;
    PODArray<int> dense_;
};

template <typename Value>
SparseSetT<Value>::SparseSetT() = default;

// Change the maximum size of the set.
// Invalidates all iterators.
template <typename Value>
void SparseSetT<Value>::resize(int new_max_size) {
    DebugCheckInvariants();
    if (new_max_size > max_size()) {
        const int old_max_size = max_size();

        // Construct these first for exception safety.
        PODArray<int> a(new_max_size);
        PODArray<int> b(new_max_size);

        std::copy_n(sparse_.data(), old_max_size, a.data());
        std::copy_n(dense_.data(), old_max_size, b.data());

        sparse_ = std::move(a);
        dense_ = std::move(b);

        MaybeInitializeMemory(old_max_size, new_max_size);
    }
    if (size_ > new_max_size)
        size_ = new_max_size;
    DebugCheckInvariants();
}

// Check whether index i is in the set.
template <typename Value>
bool SparseSetT<Value>::contains(int i) const {
    assert(i >= 0);
    assert(i < max_size());
    if (static_cast<uint32_t>(i) >= static_cast<uint32_t>(max_size())) {
        return false;
    }
    // Unsigned comparison avoids checking sparse_[i] < 0.
    return (uint32_t)sparse_[i] < (uint32_t)size_ && dense_[sparse_[i]] == i;
}

template <typename Value>
void SparseSetT<Value>::create_index(int i) {
    assert(!contains(i));
    assert(size_ < max_size());
    sparse_[i] = size_;
    dense_[size_] = i;
    size_++;
}

template <typename Value>
SparseSetT<Value>::SparseSetT(int max_size) : sparse_(max_size), dense_(max_size) {
    MaybeInitializeMemory(size_, max_size);
    DebugCheckInvariants();
}

template <typename Value>
SparseSetT<Value>::~SparseSetT() {
    DebugCheckInvariants();
}

template <typename Value>
void SparseSetT<Value>::DebugCheckInvariants() const {
    assert(0 <= size_);
    assert(size_ <= max_size());
}

// Comparison function for sorting.
template <typename Value>
bool SparseSetT<Value>::less(int a, int b) {
    return a < b;
}

typedef SparseSetT<void> SparseSet;

} // namespace re2

#endif // RE2_SPARSE_SET_H_
