如何进行v8源码解析hashmap

如何进行v8源码解析hashmap，相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。

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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_


   
     
 
    
    

hashmap.cc

   
     
 
    
   
// 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(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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