Initial import with skill sheet working

node_modules/classic-level/deps/leveldb/leveldb-1.20/util/cache.cc

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+// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file. See the AUTHORS file for names of contributors.
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "leveldb/cache.h"
+#include "port/port.h"
+#include "util/hash.h"
+#include "util/mutexlock.h"
+
+namespace leveldb {
+
+Cache::~Cache() {
+}
+
+namespace {
+
+// LRU cache implementation
+//
+// Cache entries have an "in_cache" boolean indicating whether the cache has a
+// reference on the entry.  The only ways that this can become false without the
+// entry being passed to its "deleter" are via Erase(), via Insert() when
+// an element with a duplicate key is inserted, or on destruction of the cache.
+//
+// The cache keeps two linked lists of items in the cache.  All items in the
+// cache are in one list or the other, and never both.  Items still referenced
+// by clients but erased from the cache are in neither list.  The lists are:
+// - in-use:  contains the items currently referenced by clients, in no
+//   particular order.  (This list is used for invariant checking.  If we
+//   removed the check, elements that would otherwise be on this list could be
+//   left as disconnected singleton lists.)
+// - LRU:  contains the items not currently referenced by clients, in LRU order
+// Elements are moved between these lists by the Ref() and Unref() methods,
+// when they detect an element in the cache acquiring or losing its only
+// external reference.
+
+// An entry is a variable length heap-allocated structure.  Entries
+// are kept in a circular doubly linked list ordered by access time.
+struct LRUHandle {
+  void* value;
+  void (*deleter)(const Slice&, void* value);
+  LRUHandle* next_hash;
+  LRUHandle* next;
+  LRUHandle* prev;
+  size_t charge;      // TODO(opt): Only allow uint32_t?
+  size_t key_length;
+  bool in_cache;      // Whether entry is in the cache.
+  uint32_t refs;      // References, including cache reference, if present.
+  uint32_t hash;      // Hash of key(); used for fast sharding and comparisons
+  char key_data[1];   // Beginning of key
+
+  Slice key() const {
+    // For cheaper lookups, we allow a temporary Handle object
+    // to store a pointer to a key in "value".
+    if (next == this) {
+      return *(reinterpret_cast<Slice*>(value));
+    } else {
+      return Slice(key_data, key_length);
+    }
+  }
+};
+
+// We provide our own simple hash table since it removes a whole bunch
+// of porting hacks and is also faster than some of the built-in hash
+// table implementations in some of the compiler/runtime combinations
+// we have tested.  E.g., readrandom speeds up by ~5% over the g++
+// 4.4.3's builtin hashtable.
+class HandleTable {
+ public:
+  HandleTable() : length_(0), elems_(0), list_(NULL) { Resize(); }
+  ~HandleTable() { delete[] list_; }
+
+  LRUHandle* Lookup(const Slice& key, uint32_t hash) {
+    return *FindPointer(key, hash);
+  }
+
+  LRUHandle* Insert(LRUHandle* h) {
+    LRUHandle** ptr = FindPointer(h->key(), h->hash);
+    LRUHandle* old = *ptr;
+    h->next_hash = (old == NULL ? NULL : old->next_hash);
+    *ptr = h;
+    if (old == NULL) {
+      ++elems_;
+      if (elems_ > length_) {
+        // Since each cache entry is fairly large, we aim for a small
+        // average linked list length (<= 1).
+        Resize();
+      }
+    }
+    return old;
+  }
+
+  LRUHandle* Remove(const Slice& key, uint32_t hash) {
+    LRUHandle** ptr = FindPointer(key, hash);
+    LRUHandle* result = *ptr;
+    if (result != NULL) {
+      *ptr = result->next_hash;
+      --elems_;
+    }
+    return result;
+  }
+
+ private:
+  // The table consists of an array of buckets where each bucket is
+  // a linked list of cache entries that hash into the bucket.
+  uint32_t length_;
+  uint32_t elems_;
+  LRUHandle** list_;
+
+  // Return a pointer to slot that points to a cache entry that
+  // matches key/hash.  If there is no such cache entry, return a
+  // pointer to the trailing slot in the corresponding linked list.
+  LRUHandle** FindPointer(const Slice& key, uint32_t hash) {
+    LRUHandle** ptr = &list_[hash & (length_ - 1)];
+    while (*ptr != NULL &&
+           ((*ptr)->hash != hash || key != (*ptr)->key())) {
+      ptr = &(*ptr)->next_hash;
+    }
+    return ptr;
+  }
+
+  void Resize() {
+    uint32_t new_length = 4;
+    while (new_length < elems_) {
+      new_length *= 2;
+    }
+    LRUHandle** new_list = new LRUHandle*[new_length];
+    memset(new_list, 0, sizeof(new_list[0]) * new_length);
+    uint32_t count = 0;
+    for (uint32_t i = 0; i < length_; i++) {
+      LRUHandle* h = list_[i];
+      while (h != NULL) {
+        LRUHandle* next = h->next_hash;
+        uint32_t hash = h->hash;
+        LRUHandle** ptr = &new_list[hash & (new_length - 1)];
+        h->next_hash = *ptr;
+        *ptr = h;
+        h = next;
+        count++;
+      }
+    }
+    assert(elems_ == count);
+    delete[] list_;
+    list_ = new_list;
+    length_ = new_length;
+  }
+};
+
+// A single shard of sharded cache.
+class LRUCache {
+ public:
+  LRUCache();
+  ~LRUCache();
+
+  // Separate from constructor so caller can easily make an array of LRUCache
+  void SetCapacity(size_t capacity) { capacity_ = capacity; }
+
+  // Like Cache methods, but with an extra "hash" parameter.
+  Cache::Handle* Insert(const Slice& key, uint32_t hash,
+                        void* value, size_t charge,
+                        void (*deleter)(const Slice& key, void* value));
+  Cache::Handle* Lookup(const Slice& key, uint32_t hash);
+  void Release(Cache::Handle* handle);
+  void Erase(const Slice& key, uint32_t hash);
+  void Prune();
+  size_t TotalCharge() const {
+    MutexLock l(&mutex_);
+    return usage_;
+  }
+
+ private:
+  void LRU_Remove(LRUHandle* e);
+  void LRU_Append(LRUHandle*list, LRUHandle* e);
+  void Ref(LRUHandle* e);
+  void Unref(LRUHandle* e);
+  bool FinishErase(LRUHandle* e);
+
+  // Initialized before use.
+  size_t capacity_;
+
+  // mutex_ protects the following state.
+  mutable port::Mutex mutex_;
+  size_t usage_;
+
+  // Dummy head of LRU list.
+  // lru.prev is newest entry, lru.next is oldest entry.
+  // Entries have refs==1 and in_cache==true.
+  LRUHandle lru_;
+
+  // Dummy head of in-use list.
+  // Entries are in use by clients, and have refs >= 2 and in_cache==true.
+  LRUHandle in_use_;
+
+  HandleTable table_;
+};
+
+LRUCache::LRUCache()
+    : usage_(0) {
+  // Make empty circular linked lists.
+  lru_.next = &lru_;
+  lru_.prev = &lru_;
+  in_use_.next = &in_use_;
+  in_use_.prev = &in_use_;
+}
+
+LRUCache::~LRUCache() {
+  assert(in_use_.next == &in_use_);  // Error if caller has an unreleased handle
+  for (LRUHandle* e = lru_.next; e != &lru_; ) {
+    LRUHandle* next = e->next;
+    assert(e->in_cache);
+    e->in_cache = false;
+    assert(e->refs == 1);  // Invariant of lru_ list.
+    Unref(e);
+    e = next;
+  }
+}
+
+void LRUCache::Ref(LRUHandle* e) {
+  if (e->refs == 1 && e->in_cache) {  // If on lru_ list, move to in_use_ list.
+    LRU_Remove(e);
+    LRU_Append(&in_use_, e);
+  }
+  e->refs++;
+}
+
+void LRUCache::Unref(LRUHandle* e) {
+  assert(e->refs > 0);
+  e->refs--;
+  if (e->refs == 0) { // Deallocate.
+    assert(!e->in_cache);
+    (*e->deleter)(e->key(), e->value);
+    free(e);
+  } else if (e->in_cache && e->refs == 1) {  // No longer in use; move to lru_ list.
+    LRU_Remove(e);
+    LRU_Append(&lru_, e);
+  }
+}
+
+void LRUCache::LRU_Remove(LRUHandle* e) {
+  e->next->prev = e->prev;
+  e->prev->next = e->next;
+}
+
+void LRUCache::LRU_Append(LRUHandle* list, LRUHandle* e) {
+  // Make "e" newest entry by inserting just before *list
+  e->next = list;
+  e->prev = list->prev;
+  e->prev->next = e;
+  e->next->prev = e;
+}
+
+Cache::Handle* LRUCache::Lookup(const Slice& key, uint32_t hash) {
+  MutexLock l(&mutex_);
+  LRUHandle* e = table_.Lookup(key, hash);
+  if (e != NULL) {
+    Ref(e);
+  }
+  return reinterpret_cast<Cache::Handle*>(e);
+}
+
+void LRUCache::Release(Cache::Handle* handle) {
+  MutexLock l(&mutex_);
+  Unref(reinterpret_cast<LRUHandle*>(handle));
+}
+
+Cache::Handle* LRUCache::Insert(
+    const Slice& key, uint32_t hash, void* value, size_t charge,
+    void (*deleter)(const Slice& key, void* value)) {
+  MutexLock l(&mutex_);
+
+  LRUHandle* e = reinterpret_cast<LRUHandle*>(
+      malloc(sizeof(LRUHandle)-1 + key.size()));
+  e->value = value;
+  e->deleter = deleter;
+  e->charge = charge;
+  e->key_length = key.size();
+  e->hash = hash;
+  e->in_cache = false;
+  e->refs = 1;  // for the returned handle.
+  memcpy(e->key_data, key.data(), key.size());
+
+  if (capacity_ > 0) {
+    e->refs++;  // for the cache's reference.
+    e->in_cache = true;
+    LRU_Append(&in_use_, e);
+    usage_ += charge;
+    FinishErase(table_.Insert(e));
+  } // else don't cache.  (Tests use capacity_==0 to turn off caching.)
+
+  while (usage_ > capacity_ && lru_.next != &lru_) {
+    LRUHandle* old = lru_.next;
+    assert(old->refs == 1);
+    bool erased = FinishErase(table_.Remove(old->key(), old->hash));
+    if (!erased) {  // to avoid unused variable when compiled NDEBUG
+      assert(erased);
+    }
+  }
+
+  return reinterpret_cast<Cache::Handle*>(e);
+}
+
+// If e != NULL, finish removing *e from the cache; it has already been removed
+// from the hash table.  Return whether e != NULL.  Requires mutex_ held.
+bool LRUCache::FinishErase(LRUHandle* e) {
+  if (e != NULL) {
+    assert(e->in_cache);
+    LRU_Remove(e);
+    e->in_cache = false;
+    usage_ -= e->charge;
+    Unref(e);
+  }
+  return e != NULL;
+}
+
+void LRUCache::Erase(const Slice& key, uint32_t hash) {
+  MutexLock l(&mutex_);
+  FinishErase(table_.Remove(key, hash));
+}
+
+void LRUCache::Prune() {
+  MutexLock l(&mutex_);
+  while (lru_.next != &lru_) {
+    LRUHandle* e = lru_.next;
+    assert(e->refs == 1);
+    bool erased = FinishErase(table_.Remove(e->key(), e->hash));
+    if (!erased) {  // to avoid unused variable when compiled NDEBUG
+      assert(erased);
+    }
+  }
+}
+
+static const int kNumShardBits = 4;
+static const int kNumShards = 1 << kNumShardBits;
+
+class ShardedLRUCache : public Cache {
+ private:
+  LRUCache shard_[kNumShards];
+  port::Mutex id_mutex_;
+  uint64_t last_id_;
+
+  static inline uint32_t HashSlice(const Slice& s) {
+    return Hash(s.data(), s.size(), 0);
+  }
+
+  static uint32_t Shard(uint32_t hash) {
+    return hash >> (32 - kNumShardBits);
+  }
+
+ public:
+  explicit ShardedLRUCache(size_t capacity)
+      : last_id_(0) {
+    const size_t per_shard = (capacity + (kNumShards - 1)) / kNumShards;
+    for (int s = 0; s < kNumShards; s++) {
+      shard_[s].SetCapacity(per_shard);
+    }
+  }
+  virtual ~ShardedLRUCache() { }
+  virtual Handle* Insert(const Slice& key, void* value, size_t charge,
+                         void (*deleter)(const Slice& key, void* value)) {
+    const uint32_t hash = HashSlice(key);
+    return shard_[Shard(hash)].Insert(key, hash, value, charge, deleter);
+  }
+  virtual Handle* Lookup(const Slice& key) {
+    const uint32_t hash = HashSlice(key);
+    return shard_[Shard(hash)].Lookup(key, hash);
+  }
+  virtual void Release(Handle* handle) {
+    LRUHandle* h = reinterpret_cast<LRUHandle*>(handle);
+    shard_[Shard(h->hash)].Release(handle);
+  }
+  virtual void Erase(const Slice& key) {
+    const uint32_t hash = HashSlice(key);
+    shard_[Shard(hash)].Erase(key, hash);
+  }
+  virtual void* Value(Handle* handle) {
+    return reinterpret_cast<LRUHandle*>(handle)->value;
+  }
+  virtual uint64_t NewId() {
+    MutexLock l(&id_mutex_);
+    return ++(last_id_);
+  }
+  virtual void Prune() {
+    for (int s = 0; s < kNumShards; s++) {
+      shard_[s].Prune();
+    }
+  }
+  virtual size_t TotalCharge() const {
+    size_t total = 0;
+    for (int s = 0; s < kNumShards; s++) {
+      total += shard_[s].TotalCharge();
+    }
+    return total;
+  }
+};
+
+}  // end anonymous namespace
+
+Cache* NewLRUCache(size_t capacity) {
+  return new ShardedLRUCache(capacity);
+}
+
+}  // namespace leveldb