Include RE2 library

third_party/re2/util/sparse_array.h

+// 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.
+
+// DESCRIPTION
+// 
+// SparseArray<T>(m) is a map from integers in [0, m) to T values.
+// It requires (sizeof(T)+sizeof(int))*m memory, but it provides
+// fast iteration through the elements in the array and fast clearing
+// of the array.  The array has a concept of certain elements being
+// uninitialized (having no value).
+// 
+// Insertion and deletion are constant time operations.
+// 
+// Allocating the array is a constant time operation 
+// when memory allocation is a constant time operation.
+// 
+// Clearing the array is a constant time operation (unusual!).
+// 
+// Iterating through the array is an O(n) operation, where n
+// is the number of items in the array (not O(m)).
+//
+// The array iterator visits entries in the order they were first 
+// inserted into the array.  It is safe to add items to the array while
+// using an iterator: the iterator will visit indices added to the array
+// during the iteration, but will not re-visit indices whose values
+// change after visiting.  Thus SparseArray can be a convenient
+// implementation of a work queue.
+// 
+// The SparseArray implementation is NOT thread-safe.  It is up to the
+// caller to make sure only one thread is accessing the array.  (Typically
+// these arrays are temporary values and used in situations where speed is
+// important.)
+// 
+// The SparseArray 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.
+// 
+// Briggs & Torczon popularized this technique, but it had been known
+// long before their paper.  They point out that Aho, Hopcroft, and
+// Ullman's 1974 Design and Analysis of Computer Algorithms and Bentley's
+// 1986 Programming Pearls both hint at the technique in exercises to the
+// reader (in Aho & Hopcroft, exercise 2.12; in Bentley, column 1
+// exercise 8).
+// 
+// Briggs & Torczon describe a sparse set implementation.  I have
+// trivially generalized it to create a sparse array (actually the original
+// target of the AHU and Bentley exercises).
+
+// IMPLEMENTATION
+//
+// SparseArray uses a vector dense_ and an array sparse_to_dense_, both of
+// size max_size_. At any point, the number of elements in the sparse array is
+// size_.
+// 
+// The vector dense_ contains the size_ elements in the sparse array (with
+// their indices),
+// in the order that the elements were first inserted.  This array is dense:
+// the size_ pairs are dense_[0] through dense_[size_-1].
+//
+// The array sparse_to_dense_ maps from indices in [0,m) to indices in
+// [0,size_).
+// For indices present in the array, dense_[sparse_to_dense_[i]].index_ == i.
+// For indices not present in the array, sparse_to_dense_ can contain 
+// any value at all, perhaps outside the range [0, size_) but perhaps not.
+// 
+// The lax requirement on sparse_to_dense_ values makes clearing
+// the array very easy: set size_ to 0.  Lookups are slightly more
+// complicated.  An index i has a value in the array if and only if:
+//   sparse_to_dense_[i] is in [0, size_) AND
+//   dense_[sparse_to_dense_[i]].index_ == i.
+// If both these properties hold, only then it is safe to refer to 
+//   dense_[sparse_to_dense_[i]].value_
+// as the value associated with index i.
+//
+// To insert a new entry, set sparse_to_dense_[i] to size_,
+// initialize dense_[size_], and then increment size_.
+//
+// Deletion of specific values from the array is implemented by
+// swapping dense_[size_-1] and the dense_ being deleted and then
+// updating the appropriate sparse_to_dense_ entries.
+// 
+// To make the sparse array as efficient as possible for non-primitive types,
+// elements may or may not be destroyed when they are deleted from the sparse
+// array through a call to erase(), erase_existing() or resize(). They
+// immediately become inaccessible, but they are only guaranteed to be
+// destroyed when the SparseArray destructor is called.
+
+#ifndef RE2_UTIL_SPARSE_ARRAY_H__
+#define RE2_UTIL_SPARSE_ARRAY_H__
+
+#include "util/util.h"
+
+namespace re2 {
+
+template<typename Value>
+class SparseArray {
+ public:
+  SparseArray();
+  SparseArray(int max_size);
+  ~SparseArray();
+
+  // IndexValue pairs: exposed in SparseArray::iterator.
+  class IndexValue;
+
+  typedef IndexValue value_type;
+  typedef typename vector<IndexValue>::iterator iterator;
+  typedef typename vector<IndexValue>::const_iterator const_iterator;
+
+  inline const IndexValue& iv(int i) const;
+
+  // Return the number of entries in the array.
+  int size() const {
+    return size_;
+  }
+
+  // Iterate over the array.
+  iterator begin() {
+    return dense_.begin();
+  }
+  iterator end() {
+    return dense_.begin() + size_;
+  }
+
+  const_iterator begin() const {
+    return dense_.begin();
+  }
+  const_iterator end() const {
+    return dense_.begin() + size_;
+  }
+
+  // Change the maximum size of the array.
+  // Invalidates all iterators.
+  void resize(int max_size);
+
+  // Return the maximum size of the array.
+  // Indices can be in the range [0, max_size).
+  int max_size() const {
+    return max_size_;
+  }
+
+  // Clear the array.
+  void clear() {
+    size_ = 0;
+  }
+
+  // Check whether index i is in the array.
+  inline bool has_index(int i) const;
+
+  // Comparison function for sorting.
+  // Can sort the sparse array so that future iterations
+  // will visit indices in increasing order using
+  // sort(arr.begin(), arr.end(), arr.less);
+  static bool less(const IndexValue& a, const IndexValue& b);
+
+ public:
+  // Set the value at index i to v.
+  inline iterator set(int i, Value v);
+
+  pair<iterator, bool> insert(const value_type& new_value);
+
+  // Returns the value at index i
+  // or defaultv if index i is not initialized in the array.
+  inline Value get(int i, Value defaultv) const;
+
+  iterator find(int i);
+
+  const_iterator find(int i) const;
+
+  // Change the value at index i to v.
+  // Fast but unsafe: only use if has_index(i) is true.
+  inline iterator set_existing(int i, Value v);
+
+  // Set the value at the new index i to v.
+  // Fast but unsafe: only use if has_index(i) is false.
+  inline iterator set_new(int i, Value v);
+
+  // Get the value at index i from the array..
+  // Fast but unsafe: only use if has_index(i) is true.
+  inline Value get_existing(int i) const;
+
+  // Erasing items from the array during iteration is in general
+  // NOT safe.  There is one special case, which is that the current
+  // index-value pair can be erased as long as the iterator is then
+  // checked for being at the end before being incremented.
+  // For example:
+  //
+  //   for (i = m.begin(); i != m.end(); ++i) {
+  //     if (ShouldErase(i->index(), i->value())) {
+  //       m.erase(i->index());
+  //       --i;
+  //     }
+  //   }
+  //
+  // Except in the specific case just described, elements must
+  // not be erased from the array (including clearing the array)
+  // while iterators are walking over the array.  Otherwise,
+  // the iterators could walk past the end of the array.
+
+  // Erases the element at index i from the array.
+  inline void erase(int i);
+
+  // Erases the element at index i from the array.
+  // Fast but unsafe: only use if has_index(i) is true.
+  inline void erase_existing(int i);
+
+ private:
+  // Add the index i to the array.
+  // Only use if has_index(i) is known to be false.
+  // Since it doesn't set the value associated with i,
+  // this function is private, only intended as a helper
+  // for other methods.
+  inline 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.
+  inline void DebugCheckInvariants() const;
+
+  int size_;
+  int max_size_;
+  int* sparse_to_dense_;
+  vector<IndexValue> dense_;
+
+  DISALLOW_EVIL_CONSTRUCTORS(SparseArray);
+};
+
+template<typename Value>
+SparseArray<Value>::SparseArray()
+    : size_(0), max_size_(0), sparse_to_dense_(NULL), dense_() {}
+
+// IndexValue pairs: exposed in SparseArray::iterator.
+template<typename Value>
+class SparseArray<Value>::IndexValue {
+  friend class SparseArray;
+ public:
+  typedef int first_type;
+  typedef Value second_type;
+
+  IndexValue() {}
+  IndexValue(int index, const Value& value) : second(value), index_(index) {}
+
+  int index() const { return index_; }
+  Value value() const { return second; }
+
+  // Provide the data in the 'second' member so that the utilities
+  // in map-util work.
+  Value second;
+
+ private:
+  int index_;
+};
+
+template<typename Value>
+const typename SparseArray<Value>::IndexValue&
+SparseArray<Value>::iv(int i) const {
+  DCHECK_GE(i, 0);
+  DCHECK_LT(i, size_);
+  return dense_[i];
+}
+
+// Change the maximum size of the array.
+// Invalidates all iterators.
+template<typename Value>
+void SparseArray<Value>::resize(int new_max_size) {
+  DebugCheckInvariants();
+  if (new_max_size > max_size_) {
+    int* a = new int[new_max_size];
+    if (sparse_to_dense_) {
+      memmove(a, sparse_to_dense_, max_size_*sizeof a[0]);
+      // Don't need to zero the memory but appease Valgrind.
+      if (RunningOnValgrind()) {
+        for (int i = max_size_; i < new_max_size; i++)
+          a[i] = 0xababababU;
+      }
+      delete[] sparse_to_dense_;
+    }
+    sparse_to_dense_ = a;
+
+    dense_.resize(new_max_size);
+  }
+  max_size_ = new_max_size;
+  if (size_ > max_size_)
+    size_ = max_size_;
+  DebugCheckInvariants();
+}
+
+// Check whether index i is in the array.
+template<typename Value>
+bool SparseArray<Value>::has_index(int i) const {
+  DCHECK_GE(i, 0);
+  DCHECK_LT(i, max_size_);
+  if (static_cast<uint>(i) >= max_size_) {
+    return false;
+  }
+  // Unsigned comparison avoids checking sparse_to_dense_[i] < 0.
+  return (uint)sparse_to_dense_[i] < (uint)size_ && 
+    dense_[sparse_to_dense_[i]].index_ == i;
+}
+
+// Set the value at index i to v.
+template<typename Value>
+typename SparseArray<Value>::iterator SparseArray<Value>::set(int i, Value v) {
+  DebugCheckInvariants();
+  if (static_cast<uint>(i) >= max_size_) {
+    // 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 (!has_index(i))
+    create_index(i);
+  return set_existing(i, v);
+}
+
+template<typename Value>
+pair<typename SparseArray<Value>::iterator, bool> SparseArray<Value>::insert(
+    const value_type& new_value) {
+  DebugCheckInvariants();
+  pair<typename SparseArray<Value>::iterator, bool> p;
+  if (has_index(new_value.index_)) {
+    p = make_pair(dense_.begin() + sparse_to_dense_[new_value.index_], false);
+  } else {
+    p = make_pair(set_new(new_value.index_, new_value.second), true);
+  }
+  DebugCheckInvariants();
+  return p;
+}
+
+template<typename Value>
+Value SparseArray<Value>::get(int i, Value defaultv) const {
+  if (!has_index(i))
+    return defaultv;
+  return get_existing(i);
+}
+
+template<typename Value>
+typename SparseArray<Value>::iterator SparseArray<Value>::find(int i) {
+  if (has_index(i))
+    return dense_.begin() + sparse_to_dense_[i];
+  return end();
+}
+
+template<typename Value>
+typename SparseArray<Value>::const_iterator
+SparseArray<Value>::find(int i) const {
+  if (has_index(i)) {
+    return dense_.begin() + sparse_to_dense_[i];
+  }
+  return end();
+}
+
+template<typename Value>
+typename SparseArray<Value>::iterator
+SparseArray<Value>::set_existing(int i, Value v) {
+  DebugCheckInvariants();
+  DCHECK(has_index(i));
+  dense_[sparse_to_dense_[i]].second = v;
+  DebugCheckInvariants();
+  return dense_.begin() + sparse_to_dense_[i];
+}
+
+template<typename Value>
+typename SparseArray<Value>::iterator
+SparseArray<Value>::set_new(int i, Value v) {
+  DebugCheckInvariants();
+  if (static_cast<uint>(i) >= max_size_) {
+    // 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();
+  }
+  DCHECK(!has_index(i));
+  create_index(i);
+  return set_existing(i, v);
+}
+
+template<typename Value>
+Value SparseArray<Value>::get_existing(int i) const {
+  DCHECK(has_index(i));
+  return dense_[sparse_to_dense_[i]].second;
+}
+
+template<typename Value>
+void SparseArray<Value>::erase(int i) {
+  DebugCheckInvariants();
+  if (has_index(i))
+    erase_existing(i);
+  DebugCheckInvariants();
+}
+
+template<typename Value>
+void SparseArray<Value>::erase_existing(int i) {
+  DebugCheckInvariants();
+  DCHECK(has_index(i));
+  int di = sparse_to_dense_[i];
+  if (di < size_ - 1) {
+    dense_[di] = dense_[size_ - 1];
+    sparse_to_dense_[dense_[di].index_] = di;
+  }
+  size_--;
+  DebugCheckInvariants();
+}
+
+template<typename Value>
+void SparseArray<Value>::create_index(int i) {
+  DCHECK(!has_index(i));
+  DCHECK_LT(size_, max_size_);
+  sparse_to_dense_[i] = size_;
+  dense_[size_].index_ = i;
+  size_++;
+}
+
+template<typename Value> SparseArray<Value>::SparseArray(int max_size) {
+  max_size_ = max_size;
+  sparse_to_dense_ = new int[max_size];
+  dense_.resize(max_size);
+  // Don't need to zero the new memory, but appease Valgrind.
+  if (RunningOnValgrind()) {
+    for (int i = 0; i < max_size; i++) {
+      sparse_to_dense_[i] = 0xababababU;
+      dense_[i].index_ = 0xababababU;
+    }
+  }
+  size_ = 0;
+  DebugCheckInvariants();
+}
+
+template<typename Value> SparseArray<Value>::~SparseArray() {
+  DebugCheckInvariants();
+  delete[] sparse_to_dense_;
+}
+
+template<typename Value> void SparseArray<Value>::DebugCheckInvariants() const {
+  DCHECK_LE(0, size_);
+  DCHECK_LE(size_, max_size_);
+  DCHECK(size_ == 0 || sparse_to_dense_ != NULL);
+}
+
+// Comparison function for sorting.
+template<typename Value> bool SparseArray<Value>::less(const IndexValue& a,
+                                                       const IndexValue& b) {
+  return a.index_ < b.index_;
+}
+
+}  // namespace re2
+
+#endif  // RE2_UTIL_SPARSE_ARRAY_H__