通过二维码传输少量文本文件

如果公司电脑所有接口都锁上了，如何能传出一些文档呢？手抄？拍照后再文字识别？都不方便，要是能将文档转成二维码，扫描的结果再转成文档就方便了些。

一 思路

二维码能传输的信息量是很少的，直接将文档内容转成二维码很不现实，所以先用WinRAR对原文件进行压缩。
然后需要将rar转换成可打印字符，一个汉字的二维码比两个ascii字符的二维码复杂，这里考虑将rar转成可打印的ascii字符。标准ascii里有95个可打印字符，那就选64个，可表示6个比特位。

编码前	编码后
6个比特位	1个字符
24个比特位/3个字节	4个字符

二 编码

将rar文件转换为可打印字符的代码：

// encode.c
// input: src.rar output: dest.txt
#include <stdio.h>
#include <string.h>

const unsigned char table[]=
{
	'0','1','2','3','4','5','6','7','8','9','<','>',
	'A','B','C','D','E','F','G','H','I','J','K','L','M',
	'N','O','P','Q','R','S','T','U','V','W','X','Y','Z',
	'a','b','c','d','e','f','g','h','i','j','k','l','m',
	'n','o','p','q','r','s','t','u','v','w','x','y','z'
};

// 当到达文件尾不足3个字符时, 末尾补0, 最多会有2个空字符, 对rar文件无影响
int read3chars(FILE *fin, unsigned char *re)
{
	int t;
	unsigned char buf[3];
	memset(buf, 0, 3*sizeof(unsigned char));

	t = fgetc(fin);
	if(t == EOF) return -1;
	buf[0] = t;
	t = fgetc(fin);
	if(t == EOF) goto last;
	buf[1] = t;
	t = fgetc(fin);
	if(t == EOF) goto last;
	buf[2] = t;

	unsigned char a,b,c,d;
last:
	a = (buf[0]&0xFC)>>2;
	b = (buf[0]&0x03)<<4 | (buf[1]&0xF0)>>4;
	c = (buf[1]&0x0F)<<2 | buf[2]>>6;
	d = buf[2]&0x3F;
	re[0] = table[a];
	re[1] = table[b];
	re[2] = table[c];
	re[3] = table[d];

	return 0;
}

int main()
{
	FILE *fin = fopen("src.rar", "rb");
	if(fin == NULL)
	{
		puts("cann't open src.rar!");
		getchar();
		return -1;
	}
	FILE *fout = fopen("dest.txt", "w");
	for(;;)
	{
		unsigned char buf[4];
		int ret = read3chars(fin, buf);
		if(ret != 0) break;
		fputc(buf[0], fout);
		fputc(buf[1], fout);
		fputc(buf[2], fout);
		fputc(buf[3], fout);
	}
	fclose(fin);
	fclose(fout);
	return 0;
}

三 解码

如果用记事本编辑文本文件，可能会出现BOM字符，这里并没有对其进行特殊处理。

// decode.cpp
// input: dest.txt output: dest.rar
#include <cstdio>
#include <map>

const char table[]=
{
	'0','1','2','3','4','5','6','7','8','9','<','>',
	'A','B','C','D','E','F','G','H','I','J','K','L','M',
	'N','O','P','Q','R','S','T','U','V','W','X','Y','Z',
	'a','b','c','d','e','f','g','h','i','j','k','l','m',
	'n','o','p','q','r','s','t','u','v','w','x','y','z'
};
std::map<unsigned char, unsigned char> retable;

// 一定是4字节的整数倍
int read4chars(FILE *fin, unsigned char *ret)
{
	unsigned char buf[4];
	int t = fgetc(fin);
	if(t == EOF) return -1;
	buf[0] = t;
	buf[1] = fgetc(fin);
	buf[2] = fgetc(fin);
	buf[3] = fgetc(fin);

	unsigned char a = retable[buf[0]];
	unsigned char b = retable[buf[1]];
	unsigned char c = retable[buf[2]];
	unsigned char d = retable[buf[3]];
	ret[0] = a<<2 | b>>4;
	ret[1] = b<<4 | c>>2;
	ret[2] = c<<6 | d;

	return 0;
}

int main()
{
	for(unsigned char i=0; i<64; ++i)
		retable[table[i]] = i;

	FILE *fin = fopen("dest.txt", "r");
	if(fin == NULL)
	{
		puts("cann't open dest.txt!");
		getchar();
		return -1;
	}
	FILE *fout = fopen("dest.rar", "wb");
	for(;;)
	{
		unsigned char buf[3];
		int res = read4chars(fin, buf);
		if(res != 0) break;
		fputc(buf[0], fout);
		fputc(buf[1], fout);
		fputc(buf[2], fout);
	}

	fclose(fin);
	fclose(fout);
	return 0;
}