TinySTL/TinySTL/Alloc.h

#ifndef _ALLOC_H_
#define _ALLOC_H_

#include <cstdlib>

namespace TinySTL{

	/*
	**空间配置器，以字节数为单位分配
	**内部使用
	*/
	class alloc{
	private:
		enum EAlign{ ALIGN = 8};//小型区块的上调边界
		enum EMaxBytes{ MAXBYTES = 128};//小型区块的上限，超过的区块由malloc分配
		enum ENFreeLists{ NFREELISTS = (EMaxBytes::MAXBYTES / EAlign::ALIGN)};//free-lists的个数
		enum ENObjs{ NOBJS = 20};//每次增加的节点数
	private:
		//free-lists的节点构造
		union obj{
			union obj *next;
			char client[1];
		};

		static obj *free_list[ENFreeLists::NFREELISTS];
	private:
		static char *start_free;//内存池起始位置
		static char *end_free;//内存池结束位置
		static size_t heap_size;//
	private:
		//将bytes上调至8的倍数
		static size_t ROUND_UP(size_t bytes){
			return ((bytes + EAlign::ALIGN - 1) & ~(EAlign::ALIGN - 1));
		}
		//根据区块大小，决定使用第n号free-list，n从0开始计算
		static size_t FREELIST_INDEX(size_t bytes){
			return (((bytes)+EAlign::ALIGN - 1) / EAlign::ALIGN - 1);
		}
		//返回一个大小为n的对象，并可能加入大小为n的其他区块到free-list
		static void *refill(size_t n);
		//配置一大块空间，可容纳nobjs个大小为size的区块
		//如果配置nobjs个区块有所不便，nobjs可能会降低
		static char *chunk_alloc(size_t size, size_t& nobjs);

	public:
		static void *allocate(size_t bytes);
		static void deallocate(void *ptr, size_t bytes);
		static void *reallocate(void *ptr, size_t old_sz, size_t new_sz);
	};

	char *alloc::start_free = 0;
	char *alloc::end_free = 0;
	size_t alloc::heap_size = 0;
	alloc::obj *alloc::free_list[alloc::ENFreeLists::NFREELISTS] = {
		0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
	};

	void *alloc::allocate(size_t bytes){
		if (bytes > EMaxBytes::MAXBYTES){
			return malloc(bytes);
		}
		size_t index = FREELIST_INDEX(bytes);
		obj *list = free_list[index];
		if (list){//此list还有空间给我们
			free_list[index] = list->next;
			return list;
		}else{//此list没有足够的空间，需要从内存池里面取空间
			return refill(ROUND_UP(bytes));
		}
	}
	void alloc::deallocate(void *ptr, size_t bytes){
		if (bytes > EMaxBytes::MAXBYTES){
			free(ptr);
		}
		size_t index = FREELIST_INDEX(bytes);
		obj *node = static_cast<obj *>(ptr);
		node->next = free_list[index];
		free_list[index] = node;
	}
	void *alloc::reallocate(void *ptr, size_t old_sz, size_t new_sz){
		deallocate(ptr, old_sz);
		ptr = allocate(new_sz);

		return ptr;
	}
	//返回一个大小为n的对象，并且有时候会为适当的free list增加节点
	//假设bytes已经上调为8的倍数
	void *alloc::refill(size_t bytes){
		size_t nobjs = ENObjs::NOBJS;
		//从内存池里取
		char *chunk = chunk_alloc(bytes, nobjs);
		obj **my_free_list = 0;
		obj *result = 0;
		obj *current_obj = 0, *next_obj = 0;

		if (nobjs == 1){//取出的空间只够一个对象使用
			return chunk;
		}else{
			my_free_list = free_list + FREELIST_INDEX(bytes);
			result = (obj *)(chunk);
			*my_free_list = next_obj = (obj *)(chunk + bytes);
			//将取出的多余的空间加入到相应的free list里面去
			for (int i = 1;; ++i){
				current_obj = next_obj;
				next_obj = (obj *)((char *)next_obj + bytes);
				if (nobjs - 1 == i){
					current_obj->next = 0;
					break;
				}else{
					current_obj->next = next_obj;
				}
			}
			return result;
		}
	}
	//假设bytes已经上调为8的倍数
	char *alloc::chunk_alloc(size_t bytes, size_t& nobjs){
		char *result = 0;
		size_t total_bytes = bytes * nobjs;
		size_t bytes_left = end_free - start_free;

		if (bytes_left >= total_bytes){//内存池剩余空间完全满足需要
			result = start_free;
			start_free = start_free + total_bytes;
			return result;
		}else if(bytes_left >= bytes){//内存池剩余空间不能完全满足需要，但足够供应一个或以上的区块
			nobjs = bytes_left / bytes;
			total_bytes = nobjs * bytes;
			result = start_free;
			start_free += total_bytes;
			return result;
		}else{//内存池剩余空间连一个区块的大小都无法提供
			size_t bytes_to_get = 2 * total_bytes + ROUND_UP(heap_size >> 4);
			if(bytes_left > 0){
				obj **my_free_list = free_list + FREELIST_INDEX(bytes_left);
				((obj *)start_free)->next = *my_free_list;
				*my_free_list = (obj *)start_free;
			}
			start_free = (char *)malloc(bytes_to_get);
			if (!start_free){
				obj **my_free_list = 0, *p = 0;
				for(int i = 0; i <= EMaxBytes::MAXBYTES; i += EAlign::ALIGN){
					my_free_list = free_list + FREELIST_INDEX(i);
					p = *my_free_list;
					if(!p){
						*my_free_list = p->next;
						start_free = (char *)p;
						end_free = start_free + i;
						return chunk_alloc(bytes, nobjs);
					}
				}
				end_free = 0;
			}
			heap_size += bytes_to_get;
			end_free = start_free + bytes_to_get;
			return chunk_alloc(bytes, nobjs);
		}
	}
}

#endif