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)(const 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)(const 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)(const 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)(const char *);
};


#endif