/usr/include/fst/heap.h is in libfst-dev 1.5.3+r3-2.
This file is owned by root:root, with mode 0o644.
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// finite-state transducer library.
//
// Implementation of a heap as in STL, but allows tracking positions
// in heap using a key. The key can be used to do an in-place update of
// values in the heap.
#ifndef FST_LIB_HEAP_H_
#define FST_LIB_HEAP_H_
#include <utility>
#include <vector>
#include <fst/compat.h>
namespace fst {
// A templated heap implementation that supports in-place update of values.
//
// The templated heap implementation is a little different from the
// STL priority_queue and the *_heap operations in STL. This heap
// supports indexing of values in the heap via an associated key.
//
// Each value is internally associated with a key which is returned
// to the calling functions on heap insert. This key can be used
// to later update the specific value in the heap.
//
// T: the element type of the hash, can be POD, Data or Ptr to Data
// Compare: comparison class for determining min-heapness.
template <class T, class Compare>
class Heap {
public:
using Value = T;
enum { kNoKey = -1 };
// Initializes with a specific comparator.
Heap(Compare comp) : comp_(comp), size_(0) {}
// Creates a heap with initial size of internal arrays of 0.
Heap() : size_(0) {}
// Inserts a value into the heap.
int Insert(const Value& val) {
if (size_ < values_.size()) {
values_[size_] = val;
pos_[key_[size_]] = size_;
} else {
values_.push_back(val);
pos_.push_back(size_);
key_.push_back(size_);
}
++size_;
return Insert(val, size_ - 1);
}
// Updates a value at position given by the key. The pos array is first
// indexed by the key. The position gives the position in the heap array.
// Once we have the position we can then use the standard heap operations
// to calculate the parent and child positions.
void Update(int key, const Value& val) {
const int i = pos_[key];
const bool is_better = comp_(val, values_[Parent(i)]);
values_[i] = val;
if (is_better) {
Insert(val, i);
} else {
Heapify(i);
}
}
// Returns the greatest (max=true) / least (max=false) value w.r.t.
// from the heap.
Value Pop() {
Value top = values_.front();
Swap(0, size_-1);
size_--;
Heapify(0);
return top;
}
// Returns the greatest (max=true) / least (max=false) value w.r.t.
// comp object from the heap.
const Value& Top() const {
return values_.front();
}
// Returns the element for the given key.
const Value& Get(int key) const {
return values_[pos_[key]];
}
// Checks if the heap is empty.
bool Empty() const {
return size_ == 0;
}
void Clear() {
size_ = 0;
}
int Size() const {
return size_;
}
void Reserve(int size) {
values_.reserve(size);
pos_.reserve(size);
key_.reserve(size);
}
private:
// The following private routines are used in a supportive role
// for managing the heap and keeping the heap properties.
// Computes left child of parent.
static int Left(int i) {
return 2 * (i + 1) - 1; // 0 -> 1, 1 -> 3
}
// Computes right child of parent.
static int Right(int i) {
return 2 * (i + 1); // 0 -> 2, 1 -> 4
}
// Given a child computes parent.
static int Parent(int i) {
return (i - 1) / 2; // 0 -> 0, 1 -> 0, 2 -> 0, 3 -> 1, 4 -> 1, ...
}
// Swaps a child, parent. Use to move element up/down tree.
// Note a little tricky here. When we swap we need to swap:
// the value
// the associated keys
// the position of the value in the heap
void Swap(int j, int k) {
const int tkey = key_[j];
pos_[key_[j] = key_[k]] = j;
pos_[key_[k] = tkey] = k;
using std::swap;
swap(values_[j], values_[k]);
}
// Heapifies subtree rooted at index i.
void Heapify(int i) {
const int l = Left(i);
const int r = Right(i);
int largest = (l < size_ && comp_(values_[l], values_[i])) ? l : i;
if (r < size_ && comp_(values_[r], values_[largest]) )
largest = r;
if (largest != i) {
Swap(i, largest);
Heapify(largest);
}
}
// Inserts (updates) element at subtree rooted at index i.
int Insert(const Value& val, int i) {
int p;
while (i > 0 && !comp_(values_[p = Parent(i)], val)) {
Swap(i, p);
i = p;
}
return key_[i];
}
private:
Compare comp_;
std::vector<int> pos_;
std::vector<int> key_;
std::vector<Value> values_;
int size_;
};
} // namespace fst
#endif // FST_LIB_HEAP_H_
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