block.h

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/* block.h */
/*
        Template classes Block and DBlock
        Implement adding and deleting items of the same type in blocks.

        If there there are many items then using Block or DBlock
        is more efficient than using 'new' and 'delete' both in terms
        of memory and time since
        (1) On some systems there is some minimum amount of memory
            that 'new' can allocate (e.g., 64), so if items are
            small that a lot of memory is wasted.
        (2) 'new' and 'delete' are designed for items of varying size.
            If all items has the same size, then an algorithm for
            adding and deleting can be made more efficient.
        (3) All Block and DBlock functions are inline, so there are
            no extra function calls.

        Differences between Block and DBlock:
        (1) DBlock allows both adding and deleting items,
            whereas Block allows only adding items.
        (2) Block has an additional operation of scanning
            items added so far (in the order in which they were added).
        (3) Block allows to allocate several consecutive
            items at a time, whereas DBlock can add only a single item.

        Note that no constructors or destructors are called for items.

        Example usage for items of type 'MyType':

        #include "block.h"
        #define BLOCK_SIZE 1024
        typedef struct { int a, b; } MyType;
        MyType *ptr, *array[10000];

        ...

        Block<MyType> *block = new Block<MyType>(BLOCK_SIZE);

        // adding items
        for (int i=0; i<sizeof(array); i++)
        {
                ptr = block -> New();
                ptr -> a = ptr -> b = rand();
        }

        // reading items
        for (ptr=block->ScanFirst(); ptr; ptr=block->ScanNext())
        {
                printf("%d %d\n", ptr->a, ptr->b);
        }

        delete block;

        ...

        DBlock<MyType> *dblock = new DBlock<MyType>(BLOCK_SIZE);
        
        // adding items
        for (int i=0; i<sizeof(array); i++)
        {
                array[i] = dblock -> New();
        }

        // deleting items
        for (int i=0; i<sizeof(array); i+=2)
        {
                dblock -> Delete(array[i]);
        }

        // adding items
        for (int i=0; i<sizeof(array); i++)
        {
                array[i] = dblock -> New();
        }

        delete dblock;


        Note that DBlock deletes items by marking them as
        empty (i.e., by adding them to the list of free items),
        so that this memory could be used for subsequently
        added items. Thus, at each moment the memory allocated
        is determined by the maximum number of items allocated
        simultaneously at earlier moments. All memory is
        deallocated only when the destructor is called.
*/

#ifndef __BLOCK_H__
#define __BLOCK_H__

#include <stdlib.h>

/***********************************************************************/
/***********************************************************************/
/***********************************************************************/

template <class Type> class Block
{
public:
        /* Constructor. Arguments are the block size and
           (optionally) the pointer to the function which
           will be called if allocation failed; the message
           passed to this function is "Not enough memory!" */
        Block(int size, void (*err_function)(char *) = NULL) { first = last = NULL; block_size = size; error_function = err_function; }

        /* Destructor. Deallocates all items added so far */
        ~Block() { while (first) { block *next = first -> next; delete first; first = next; } }

        /* Allocates 'num' consecutive items; returns pointer
           to the first item. 'num' cannot be greater than the
           block size since items must fit in one block */
        Type *New(int num = 1)
        {
                Type *t;

                if (!last || last->current + num > last->last)
                {
                        if (last && last->next) last = last -> next;
                        else
                        {
                                block *next = (block *) new char [sizeof(block) + (block_size-1)*sizeof(Type)];
                                if (!next) { if (error_function) (*error_function)("Not enough memory!"); exit(1); }
                                if (last) last -> next = next;
                                else first = next;
                                last = next;
                                last -> current = & ( last -> data[0] );
                                last -> last = last -> current + block_size;
                                last -> next = NULL;
                        }
                }

                t = last -> current;
                last -> current += num;
                return t;
        }

        /* Returns the first item (or NULL, if no items were added) */
        Type *ScanFirst()
        {
                for (scan_current_block=first; scan_current_block; scan_current_block = scan_current_block->next)
                {
                        scan_current_data = & ( scan_current_block -> data[0] );
                        if (scan_current_data < scan_current_block -> current) return scan_current_data ++;
                }
                return NULL;
        }

        /* Returns the next item (or NULL, if all items have been read)
           Can be called only if previous ScanFirst() or ScanNext()
           call returned not NULL. */
        Type *ScanNext()
        {
                while (scan_current_data >= scan_current_block -> current)
                {
                        scan_current_block = scan_current_block -> next;
                        if (!scan_current_block) return NULL;
                        scan_current_data = & ( scan_current_block -> data[0] );
                }
                return scan_current_data ++;
        }

        /* Marks all elements as empty */
        void Reset()
        {
                block *b;
                if (!first) return;
                for (b=first; ; b=b->next)
                {
                        b -> current = & ( b -> data[0] );
                        if (b == last) break;
                }
                last = first;
        }

/***********************************************************************/

private:

        typedef struct block_st
        {
                Type                                    *current, *last;
                struct block_st                 *next;
                Type                                    data[1];
        } block;

        int             block_size;
        block   *first;
        block   *last;

        block   *scan_current_block;
        Type    *scan_current_data;

        void    (*error_function)(char *);
};

/***********************************************************************/
/***********************************************************************/
/***********************************************************************/

template <class Type> class DBlock
{
public:
        /* Constructor. Arguments are the block size and
           (optionally) the pointer to the function which
           will be called if allocation failed; the message
           passed to this function is "Not enough memory!" */
        DBlock(int size, void (*err_function)(char *) = NULL) { first = NULL; first_free = NULL; block_size = size; error_function = err_function; }

        /* Destructor. Deallocates all items added so far */
        ~DBlock() { while (first) { block *next = first -> next; delete first; first = next; } }

        /* Allocates one item */
        Type *New()
        {
                block_item *item;

                if (!first_free)
                {
                        block *next = first;
                        first = (block *) new char [sizeof(block) + (block_size-1)*sizeof(block_item)];
                        if (!first) { if (error_function) (*error_function)("Not enough memory!"); exit(1); }
                        first_free = & (first -> data[0] );
                        for (item=first_free; item<first_free+block_size-1; item++)
                                item -> next_free = item + 1;
                        item -> next_free = NULL;
                        first -> next = next;
                }

                item = first_free;
                first_free = item -> next_free;
                return (Type *) item;
        }

        /* Deletes an item allocated previously */
        void Delete(Type *t)
        {
                ((block_item *) t) -> next_free = first_free;
                first_free = (block_item *) t;
        }

/***********************************************************************/

private:

        typedef union block_item_st
        {
                Type                    t;
                block_item_st   *next_free;
        } block_item;

        typedef struct block_st
        {
                struct block_st                 *next;
                block_item                              data[1];
        } block;

        int                     block_size;
        block           *first;
        block_item      *first_free;

        void    (*error_function)(char *);
};


#endif