Minimize SkTHash object copying

src/core/SkTHash.h

 #ifndef SkTHash_DEFINED
 #define SkTHash_DEFINED
 
 #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.
@@ skipping 15 matching lines @@
     // 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(T val) {
+    T* set(const T& val) {
         if (4 * fCount >= 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(K key) const {
+    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)) {
                 return &s.val;
             }
             index = this->next(index, n);
         }
         SkASSERT(fCapacity == 0);
         return NULL;
     }
 
     // Call fn on every entry in the table.  You may mutate the entries, but be very careful.
     template <typename Arg>
     void foreach(void(*fn)(T*, Arg), Arg arg) {
         for (int i = 0; i < fCapacity; i++) {
             Slot& s = fSlots[i];
             if (!s.empty()) {
                 fn(&s.val, arg);
             }
         }
     }
 
 private:
-    T* uncheckedSet(T val) {
-        K key = Traits::GetKey(val);
+    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()) {
                 // New entry.
                 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) {
@@ skipping 14 matching lines @@
         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(K key) {
+    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.
     }
 
     struct Slot {
         Slot() : hash(0) {}
         bool empty() const { return hash == 0; }
 
         T val;
         uint32_t hash;
     };
 
     int fCount, 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)(K)>
+template <typename K, typename V, uint32_t(*HashK)(const K&)>
 class SkTHashMap : SkNoncopyable {
 public:
     SkTHashMap() {}
 
     // How many key/value pairs are in the table?
     int count() const { return fTable.count(); }
 
     // N.B. The pointers returned by set() and find() are valid only until the next call to set().
 
     // Set key to val in the table, replacing any previous value with the same key.
     // We copy both key and val, and return a pointer to the value copy now in the table.
-    V* set(K key, V val) {
+    V* set(const K& key, const V& val) {
         Pair in = { key, val };
         Pair* out = fTable.set(in);
         return &out->val;
     }
 
     // If there is key/value entry in the table with this key, return a pointer to the value.
     // If not, return NULL.
-    V* find(K key) const {
+    V* find(const K& key) const {
         if (Pair* p = fTable.find(key)) {
             return &p->val;
         }
         return NULL;
     }
 
     // Call fn on every key/value pair in the table.  You may mutate the value but not the key.
     void foreach(void(*fn)(K, V*)) { fTable.foreach(ForEach, fn); }
 
 private:
     struct Pair {
         K key;
         V val;
-        static K GetKey(Pair p) { return p.key; }
-        static uint32_t Hash(K key) { return HashK(key); }
+        static const K& GetKey(const Pair& p) { return p.key; }
+        static uint32_t Hash(const K& key) { return HashK(key); }
     };
     static void ForEach(Pair* p, void (*fn)(K, V*)) { fn(p->key, &p->val); }
 
     SkTHashTable<Pair, K> fTable;
 };
 
 // A set of T.  T is treated as an ordiary copyable C++ type.
-template <typename T, uint32_t(*HashT)(T)>
+template <typename T, uint32_t(*HashT)(const T&)>
 class SkTHashSet : SkNoncopyable {
 public:
     SkTHashSet() {}
 
     // How many items are in the set?
     int count() const { return fTable.count(); }
 
     // Copy an item into the set.
-    void add(T item) { fTable.set(item); }
+    void add(const T& item) { fTable.set(item); }
 
     // Is this item in the set?
-    bool contains(T item) const { return SkToBool(fTable.find(item)); }
+    bool contains(const T& item) const { return SkToBool(fTable.find(item)); }
 
 private:
     struct Traits {
-        static T GetKey(T item) { return item; }
-        static uint32_t Hash(T item) { return HashT(item); }
+        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