/*
 * extmem.c
 * Zhaonian Zou
 * Harbin Institute of Technology
 * Jun 22, 2011
 */

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "extmem.h"

// 初始化缓冲区 返回指向该缓冲区的地址
Buffer *initBuffer(size_t bufSize, size_t blkSize, Buffer *buf)
{
    int i;

    buf->numIO = 0;
    buf->bufSize = bufSize;
    buf->blkSize = blkSize;
    buf->numAllBlk = bufSize / (blkSize + 1);
    buf->numFreeBlk = buf->numAllBlk;
    buf->data = (unsigned char*)malloc(bufSize * sizeof(unsigned char));

    if (!buf->data)
    {
        perror("Buffer Initialization Failed!\n");
        return NULL;
    }

    memset(buf->data, 0, bufSize * sizeof(unsigned char));
    return buf;
}

// 释放缓冲区buf占用的内存空间
void freeBuffer(Buffer *buf)
{
    free(buf->data);
}

// 在缓冲区 buf 中申请一个新的块。若申请成功，则返回该块的起始地址
unsigned char *getNewBlockInBuffer(Buffer *buf)
{
    unsigned char *blkPtr;

    if (buf->numFreeBlk == 0)
    {
        perror("Buffer is full!\n");
        return NULL;
    }

    blkPtr = buf->data;

    // 遍历缓冲区寻找下一个可用块
    while (blkPtr < buf->data + (buf->blkSize + 1) * buf->numAllBlk)
    {
        if (*blkPtr == BLOCK_AVAILABLE)
            break;
        else
            blkPtr += buf->blkSize + 1;
    }

    *blkPtr = BLOCK_UNAVAILABLE;
    buf->numFreeBlk--;
    return blkPtr + 1;
}

// 解除块blk对缓冲区内存的占用，即将 blk 占据的内存区域标记为可用
void freeBlockInBuffer(unsigned char *blk, Buffer *buf)
{
    *(blk - 1) = BLOCK_AVAILABLE;
    buf->numFreeBlk++;
}


// 从磁盘上删除地址为 addr 的磁盘块内的数据
int dropBlockOnDisk(unsigned int addr)
{
    char filename[40];

    sprintf(filename, "data/%d.blk", addr);

    // 删除这个文件
    if (remove(filename) == -1)
    {
        perror("Dropping Block Fails!\n");
        return -1;
    }

    return 0;
}

// 将磁盘上地址为 addr 的磁盘块读入缓冲区 buf。若读取成功，则返回缓冲区内该块的地址
unsigned char *readBlockFromDisk(unsigned int addr, Buffer *buf)
{
    char filename[40];
    unsigned char *blkPtr, *bytePtr;
    char ch;

    if (buf->numFreeBlk == 0)
    {
        perror("Buffer Overflows!\n");
        return NULL;
    }

    blkPtr = buf->data;

    // 遍历buf找到一个空闲块
    while (blkPtr < buf->data + (buf->blkSize + 1) * buf->numAllBlk)
    {
        if (*blkPtr == BLOCK_AVAILABLE)
            break;
        else
            blkPtr += buf->blkSize + 1;
    }

    // 打开文件
    sprintf(filename, "data/%d.blk", addr);
    FILE *fp = fopen(filename, "r");

    if (!fp)
    {
        printf("addr: %d\n", addr);
        perror("Reading Block Failed!\n");
        return NULL;
    }

    *blkPtr = BLOCK_UNAVAILABLE;
    blkPtr++;
    bytePtr = blkPtr;

    // 将对应的内容读入这个缓存块
    while (bytePtr < blkPtr + buf->blkSize)
    {
        ch = fgetc(fp);
        *bytePtr = ch;
        bytePtr++;
    }

    fclose(fp);
    buf->numFreeBlk--;
    buf->numIO++;
    return blkPtr;
}

// 将缓冲区buf内的块blk写入磁盘上地址为addr的磁盘块
int writeBlockToDisk(unsigned char *blkPtr, unsigned int addr, Buffer *buf)
{
    char filename[40];
    unsigned char *bytePtr;

    // 打开这个文件
    sprintf(filename, "data/%d.blk", addr);
    // 已存在的文件将内容清空，只允许写
    FILE *fp = fopen(filename, "w");

    if (!fp)
    {
        perror("Writing Block Failed!\n");
        return -1;
    }
    
    // 将信息写入
    for (bytePtr = blkPtr; bytePtr < blkPtr + buf->blkSize; bytePtr++)
        fputc((int)(*bytePtr), fp);

    fclose(fp);
    *(blkPtr - 1) = BLOCK_AVAILABLE;//重新将块置为可用，特别要注意这里
    buf->numFreeBlk++;
    buf->numIO++;
    return 0;
}