Spin off SkTHashTable, SkTHashMap, SkTHashSet

src/core/SkTHash.h

Index: src/core/SkTHash.h
diff --git a/src/core/SkTHash.h b/src/core/SkTHash.h
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+#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.

[reed1, 2015/02/12 20:42:56]

btw -- this form of comment will not get scooped by Doxygen, but /** ... */
will.

+// 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) {}
+
+    // 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(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 {
+        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);
+        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.
+                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;
+        fCapacity = capacity;
+        SkAutoTArray<Slot> oldSlots(capacity);
+        oldSlots.swap(fSlots);
+
+        for (int i = 0; i < oldCapacity; i++) {
+            const Slot& s = oldSlots[i];
+            if (!s.empty()) {
+                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(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)>
+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) {
+        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 {
+        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 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)>
+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); }
+
+    // Is this item in the set?
+    bool contains(T item) const { return fTable.find(item); }
+
+private:
+    struct Traits {
+        static T GetKey(T item) { return item; }
+        static uint32_t Hash(T item) { return HashT(item); }
+    };
+    SkTHashTable<T, T, Traits> fTable;
+};
+
+#endif//SkTHash_DEFINED