SkTHash: remove()

src/core/SkTHash.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkTHash_DEFINED
 #define SkTHash_DEFINED
 
 #include "SkChecksum.h"
 #include "SkTypes.h"
 #include "SkTemplates.h"
 
 // Before trying to use SkTHashTable, look below to see if SkTHashMap or SkTHashSet works for you.
 // They're easier to use, usually perform the same, and have fewer sharp edges.
 
 // T and K are treated as ordinary copyable C++ types.
 // Traits must have:
 //   - static K GetKey(T)
 //   - static uint32_t Hash(K)
 // If the key is large and stored inside T, you may want to make K a const&.
 // Similarly, if T is large you might want it to be a pointer.
 template <typename T, typename K, typename Traits = T>
 class SkTHashTable : SkNoncopyable {
 public:
-    SkTHashTable() : fCount(0), fCapacity(0) {}
+    SkTHashTable() : fCount(0), fRemoved(0), fCapacity(0) {}
 
     // Clear the table.
     void reset() {
         this->~SkTHashTable();
         SkNEW_PLACEMENT(this, SkTHashTable);
     }
 
     // How many entries are in the table?
     int count() const { return fCount; }
 
     // !!!!!!!!!!!!!!!!!                 CAUTION                   !!!!!!!!!!!!!!!!!
     // set(), find() and foreach() all allow mutable access to table entries.
     // If you change an entry so that it no longer has the same key, all hell
     // will break loose.  Do not do that!
     //
     // Please prefer to use SkTHashMap or SkTHashSet, which do not have this danger.
 
     // The pointers returned by set() and find() are valid only until the next call to set().
     // The pointers you receive in foreach() are only valid for its duration.
 
     // Copy val into the hash table, returning a pointer to the copy now in the table.
     // If there already is an entry in the table with the same key, we overwrite it.
     T* set(const T& val) {
-        if (4 * fCount >= 3 * fCapacity) {
+        if (4 * (fCount+fRemoved) >= 3 * fCapacity) {
             this->resize(fCapacity > 0 ? fCapacity * 2 : 4);
         }
         return this->uncheckedSet(val);
     }
 
     // If there is an entry in the table with this key, return a pointer to it.  If not, NULL.
     T* find(const K& key) const {
         uint32_t hash = Hash(key);
         int index = hash & (fCapacity-1);
         for (int n = 0; n < fCapacity; n++) {
             Slot& s = fSlots[index];
             if (s.empty()) {
                 return NULL;
             }
-            if (hash == s.hash && key == Traits::GetKey(s.val)) {
+            if (!s.removed() && hash == s.hash && key == Traits::GetKey(s.val)) {
                 return &s.val;
             }
             index = this->next(index, n);
         }
         SkASSERT(fCapacity == 0);
         return NULL;
     }
 
+    // Remove the value with this key from the hash table.
+    void remove(const K& key) {
+        SkASSERT(this->find(key));
+
+        uint32_t hash = Hash(key);
+        int index = hash & (fCapacity-1);
+        for (int n = 0; n < fCapacity; n++) {
+            Slot& s = fSlots[index];
+            SkASSERT(!s.empty());
+            if (!s.removed() && hash == s.hash && key == Traits::GetKey(s.val)) {
+                fRemoved++;
+                fCount--;
+                s.markRemoved();
+                return;
+            }
+            index = this->next(index, n);
+        }
+        SkASSERT(fCapacity == 0);
+    }
+
     // Call fn on every entry in the table.  You may mutate the entries, but be very careful.
     template <typename Fn>  // f(T*)
     void foreach(Fn&& fn) {
         for (int i = 0; i < fCapacity; i++) {
-            if (!fSlots[i].empty()) {
+            if (!fSlots[i].empty() && !fSlots[i].removed()) {
                 fn(&fSlots[i].val);
             }
         }
     }
 
     // Call fn on every entry in the table.  You may not mutate anything.
     template <typename Fn>  // f(T) or f(const T&)
     void foreach(Fn&& fn) const {
         for (int i = 0; i < fCapacity; i++) {
-            if (!fSlots[i].empty()) {
+            if (!fSlots[i].empty() && !fSlots[i].removed()) {
                 fn(fSlots[i].val);
             }
         }
     }
 
 private:
     T* uncheckedSet(const T& val) {
         const K& key = Traits::GetKey(val);
         uint32_t hash = Hash(key);
         int index = hash & (fCapacity-1);
         for (int n = 0; n < fCapacity; n++) {
             Slot& s = fSlots[index];
-            if (s.empty()) {
+            if (s.empty() || s.removed()) {
                 // New entry.
+                if (s.removed()) {
+                    fRemoved--;
+                }
                 s.val  = val;
                 s.hash = hash;
                 fCount++;
                 return &s.val;
             }
             if (hash == s.hash && key == Traits::GetKey(s.val)) {
                 // Overwrite previous entry.
                 // Note: this triggers extra copies when adding the same value repeatedly.
                 s.val = val;
                 return &s.val;
             }
             index = this->next(index, n);
         }
         SkASSERT(false);
         return NULL;
     }
 
     void resize(int capacity) {
         int oldCapacity = fCapacity;
         SkDEBUGCODE(int oldCount = fCount);
 
-        fCount = 0;
+        fCount = fRemoved = 0;
         fCapacity = capacity;
         SkAutoTArray<Slot> oldSlots(capacity);
         oldSlots.swap(fSlots);
 
         for (int i = 0; i < oldCapacity; i++) {
             const Slot& s = oldSlots[i];
-            if (!s.empty()) {
+            if (!s.empty() && !s.removed()) {
                 this->uncheckedSet(s.val);
             }
         }
         SkASSERT(fCount == oldCount);
     }
 
     int next(int index, int n) const {
         // A valid strategy explores all slots in [0, fCapacity) as n walks from 0 to fCapacity-1.
         // Both of these strategies are valid:
         //return (index + 0 + 1) & (fCapacity-1);      // Linear probing.
         return (index + n + 1) & (fCapacity-1);        // Quadratic probing.
     }
 
     static uint32_t Hash(const K& key) {
         uint32_t hash = Traits::Hash(key);
-        return hash == 0 ? 1 : hash;  // We reserve hash == 0 to mark empty slots.
+        return hash < 2 ? hash+2 : hash;  // We reserve hash 0 and 1 to mark empty or removed slots.
     }
 
     struct Slot {
         Slot() : hash(0) {}
-        bool empty() const { return hash == 0; }
+        bool   empty() const { return this->hash == 0; }
+        bool removed() const { return this->hash == 1; }
+
+        void markRemoved() { this->hash = 1; }
 
         T val;
         uint32_t hash;
     };
 
-    int fCount, fCapacity;
+    int fCount, fRemoved, fCapacity;
     SkAutoTArray<Slot> fSlots;
 };
 
 // Maps K->V.  A more user-friendly wrapper around SkTHashTable, suitable for most use cases.
 // K and V are treated as ordinary copyable C++ types, with no assumed relationship between the two.
 template <typename K, typename V, uint32_t(*HashK)(const K&) = &SkGoodHash>
 class SkTHashMap : SkNoncopyable {
 public:
     SkTHashMap() {}
 
@@ skipping 15 matching lines @@
 
     // If there is key/value entry in the table with this key, return a pointer to the value.
     // If not, return NULL.
     V* find(const K& key) const {
         if (Pair* p = fTable.find(key)) {
             return &p->val;
         }
         return NULL;
     }
 
+    // Remove the key/value entry in the table with this key.
+    void remove(const K& key) {
+        SkASSERT(this->find(key));
+        fTable.remove(key);
+    }
+
     // Call fn on every key/value pair in the table.  You may mutate the value but not the key.
     template <typename Fn>  // f(K, V*) or f(const K&, V*)
     void foreach(Fn&& fn) {
         fTable.foreach([&fn](Pair* p){ fn(p->key, &p->val); });
     }
 
     // Call fn on every key/value pair in the table.  You may not mutate anything.
     template <typename Fn>  // f(K, V), f(const K&, V), f(K, const V&) or f(const K&, const V&).
     void foreach(Fn&& fn) const {
         fTable.foreach([&fn](const Pair& p){ fn(p.key, p.val); });
@@ skipping 25 matching lines @@
     // Copy an item into the set.
     void add(const T& item) { fTable.set(item); }
 
     // Is this item in the set?
     bool contains(const T& item) const { return SkToBool(this->find(item)); }
 
     // If an item equal to this is in the set, return a pointer to it, otherwise null.
     // This pointer remains valid until the next call to add().
     const T* find(const T& item) const { return fTable.find(item); }
 
+    // Remove the item in the set equal to this.
+    void remove(const T& item) {
+        SkASSERT(this->contains(item));
+        fTable.remove(item);
+    }
+
     // Call fn on every item in the set.  You may not mutate anything.
     template <typename Fn>  // f(T), f(const T&)
     void foreach (Fn&& fn) const {
-        fTable.foreach (fn);
+        fTable.foreach(fn);
     }
 
 private:
     struct Traits {
         static const T& GetKey(const T& item) { return item; }
         static uint32_t Hash(const T& item) { return HashT(item); }
     };
     SkTHashTable<T, T, Traits> fTable;
 };
 
 #endif//SkTHash_DEFINED