如何进行v8源码解析hashmap

hashmap是哈希表的实现。 #ifndef V8_HASHMAP_H_ #define V8_HASHMAP_H_ namespace v8 { namespace internal { // Allocator defines the memory allocator interface // used by HashMap and implements a default allocator. class Allocator BASE_EMBEDDED { public: virtual ~Allocator() {} virtual void* New(size_t size) { return Malloced::New(size); } virtual void Delete(void* p) { Malloced::Delete(p); } }; class HashMap { public: static Allocator DefaultAllocator; typedef bool (*MatchFun) (void* key1, void* key2); // Dummy constructor. This constructor doesn't set up the hash // map properly so don't use it unless you have good reason. HashMap(); // initial_capacity is the size of the initial hash map; // it must be a power of 2 (and thus must not be 0). HashMap(MatchFun match, Allocator* allocator = &DefaultAllocator, uint32_t initial_capacity = 8); ~HashMap(); // HashMap entries are (key, value, hash) tripplets. // Some clients may not need to use the value slot // (e.g. implementers of sets, where the key is the value). struct Entry { void* key; void* value; uint32_t hash; // the full hash value for key }; // If an entry with matching key is found, Lookup() // returns that entry. If no matching entry is found, // but insert is set, a new entry is inserted with // corresponding key, key hash, and NULL value. // Otherwise, NULL is returned. Entry* Lookup(void* key, uint32_t hash, bool insert); // Empties the hash map (occupancy() == 0). void Clear(); // The number of (non-empty) entries in the table. uint32_t occupancy() const { return occupancy_; } // The capacity of the table. The implementation // makes sure that occupancy is at most 80% of // the table capacity. uint32_t capacity() const { return capacity_; } // Iteration // // for (Entry* p = map.Start(); p != NULL; p = map.Next(p)) { // ... // } // // If entries are inserted during iteration, the effect of // calling Next() is undefined. Entry* Start() const; Entry* Next(Entry* p) const; private: Allocator* allocator_; MatchFun match_; Entry* map_; // 可分配的元素个数 uint32_t capacity_; // 已分配的元素个数 uint32_t occupancy_; // 数组末地址 Entry* map_end() const { return map_ + capacity_; } Entry* Probe(void* key, uint32_t hash); void Initialize(uint32_t capacity); void Resize(); }; } } // namespace v8::internal #endif // V8_HASHMAP_H_

// Copyright 2008 Google Inc. All Rights Reserved. // Redistribution and use in source and binary forms, with or without // modification, are permitted provided that the following conditions are // met: // // * Redistributions of source code must retain the above copyright // notice, this list of conditions and the following disclaimer. // * Redistributions in binary form must reproduce the above // copyright notice, this list of conditions and the following // disclaimer in the documentation and/or other materials provided // with the distribution. // * Neither the name of Google Inc. nor the names of its // contributors may be used to endorse or promote products derived // from this software without specific prior written permission. // // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. #include "v8.h" #include "hashmap.h" namespace v8 { namespace internal { /* 判断x是不是有且仅有一位是1.如果是则下面的式子成立。假设x的第n位是1，x - 1后，n的左边位都是0，右边都是1，n变成0. 00001000 => 00000111,再和x与，n以及n的右边位是肯定为0的。右边就看 n的左边的位就可以了。 */ static inline bool IsPowerOf2(uint32_t x) { ASSERT(x != 0); return (x & (x - 1)) == 0; } // 内存分配器 Allocator HashMap::DefaultAllocator; // 默认构造函数 HashMap::HashMap() { allocator_ = NULL; match_ = NULL; } // 初始化属性，分配内存 HashMap::HashMap(MatchFun match, Allocator* allocator, uint32_t initial_capacity) { allocator_ = allocator; match_ = match; Initialize(initial_capacity); } // 析构函数，释放内存 HashMap::~HashMap() { if (allocator_) { allocator_->Delete(map_); } } // 查找或插入一个元素 HashMap::Entry* HashMap::Lookup(void* key, uint32_t hash, bool insert) { // Find a matching entry. // 找到key和hash对应的索引。 Entry* p = Probe(key, hash); // 找到则返回 if (p->key != NULL) { return p; } // No entry found; insert one if necessary. // 没有找到判断是否需要插入 if (insert) { p->key = key; p->value = NULL; p->hash = hash; // 更新使用的元素个数 occupancy_++; // Grow the map if we reached >= 80% occupancy. // 分配的元素过多，重新分配内存，否则导致冲突频繁，影响效率 if (occupancy_ + occupancy_/4 >= capacity_) { Resize(); // 重新查找对应的元素 p = Probe(key, hash); } return p; } // No entry found and none inserted. return NULL; } // void HashMap::Clear() { // Mark all entries as empty. // 最后一个元素的末地址 const Entry* end = map_end(); // 遍历数组，清空key字段 for (Entry* p = map_; p < end; p++) { p->key = NULL; } // 分配出去的元素个数为0 occupancy_ = 0; } // 用于迭代 HashMap::Entry* HashMap::Start() const { // Next函数的for执行了p++，所以这里要回退一个元素，见Next函数 return Next(map_ - 1); } HashMap::Entry* HashMap::Next(Entry* p) const { // 最后一个元素的末地址 const Entry* end = map_end(); ASSERT(map_ - 1 <= p && p < end); /* 遍历数组，返回遇到的第一个key非空的节点， p++，所以初始化的时候，p指向第一个元素的第一个元素 */ for (p++; p < end; p++) { if (p->key != NULL) { return p; } } return NULL; } // 根据key和hash找到哈希表中可用的索引，hash值由调用方提供 HashMap::Entry* HashMap::Probe(void* key, uint32_t hash) { ASSERT(key != NULL); ASSERT(IsPowerOf2(capacity_)); // capacity_ - 1防止溢出，实现回环 Entry* p = map_ + (hash & (capacity_ - 1)); // 最后一个元素的末地址 const Entry* end = map_end(); ASSERT(map_ <= p && p < end); // 至少有一个非NULL，使p->key != NULL成立 ASSERT(occupancy_ < capacity_); // guarantees loop termination /* 如果key等于空说明这个项还没被使用，则返回，如果key非空，并且hash和key都匹配，则返回。 hash值不相等或者名字不match,则查找下一个可用的元素，即开放地址法 */ while (p->key != NULL && (hash != p->hash || !match_(key, p->key))) { p++; // 到底了，从头开始 if (p >= end) { p = map_; } } return p; } // 申请一个Entry* 数组 void HashMap::Initialize(uint32_t capacity) { ASSERT(IsPowerOf2(capacity)); map_ = reinterpret_cast<Entry*>(allocator_->New(capacity * sizeof(Entry))); if (map_ == NULL) V8::FatalProcessOutOfMemory("HashMap::Initialize"); capacity_ = capacity; // 初始化内存数据 Clear(); } // 扩展 void HashMap::Resize() { // 先保存旧地址的指针 Entry* map = map_; uint32_t n = occupancy_; // 重新分配一个更大的数组 // Allocate larger map. Initialize(capacity_ * 2); // Rehash all current entries. // 重新计算当前哈希表中的元素的位置，n的作用是迁移完n个可用退出循环了，不需要遍历到底 for (Entry* p = map; n > 0; p++) { if (p->key != NULL) { // 把旧的元素插入到新的数组中，因为map_更新了，里面是空的，所以会一直插入新的元素到map_ Lookup(p->key, p->hash, true)->value = p->value; n--; } } // 释放旧的地址 // Delete old map. allocator_->Delete(map); } } } // namespace v8::internal

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