遇到一个需求：假定有一个图片文件，真实的类型为jpg，而有人偷懒把jpg直接复制一张，存为同名的png文件，这样在as3读取文件时不会遇到问题，但手机c++在读取文件时却遇到问题了 - -!
现在就需要写一个程序，遍历所有文件夹下的文件，查找文件格式“不正常”的文件。我们的资源主要是gif、png、jpg，最开始，我到网上找到一篇文章：根据二进制流及文件头获取文件类型mime-type，然后读取文件二进制的头信息，获取其真实的文件类型，对与通过后缀名获得的文件类型进行比较。
var fd = fs.openSync(new_file_path, 'r');
var buffer = new Buffer(8);
var mineType = mime.lookup(new_file_path);
var fileType = mime.extension(mineType);
fs.readSync(fd, buffer, 0, 8, 0);
var newBuf = buffer.slice(0, 4);
var head_1 = newBuf[0].toString(16);
var head_2 = newBuf[1].toString(16);
var head_3 = newBuf[2].toString(16);
var head_4 = newBuf[3].toString(16);
var head_iden = head_1 + head_2;
var tempFileType = FILE_TYPE_CONFIG[head_iden];
if (!tempFileType) {
head_iden += head_3;
tempFileType = FILE_TYPE_CONFIG[head_iden];
if (!tempFileType) {
var msg = "Unknow fileType " + new_file_path + '-' + fileType;
showLog(msg);
continue;
}
}
if (tempFileType != fileType) {
var msg = "Error fileType" + new_file_path + '-' + fileType + '|' + tempFileType + '--正确的图像文件格式';
showLog(msg);
g_errorFileTypArr.push(msg);
}
后来搜索node image相关的信息时，找到这篇文章：node.js module ranking>> (images)
然后筛选到一个模块“node-imageinfo”，写了一个例子进行测试(故意把jpg文件直接修改后缀名为png)：
它的源码，有兴趣可以研究一下：
function readUInt32(buffer, offset, bigEndian) {
if (buffer.readUInt32) {
return buffer.readUInt32(offset, bigEndian);
}
var value;
if (bigEndian) {
if (buffer.readUInt32BE) {
return buffer.readUInt32BE(offset);
}
value = (buffer[offset] << 24) + (buffer[offset+1] << 16) + (buffer[offset+2] << 8) + buffer[offset+3];
}
else {
if (buffer.readUInt32LE) {
return buffer.readUInt32LE(offset);
}
value = buffer[offset] + (buffer[offset+1] << 8) + (buffer[offset+2] << 16) + (buffer[offset+3] << 24);
}
return value;
}
function readUInt16(buffer, offset, bigEndian) {
if (buffer.readUInt16) {
return buffer.readUInt16(offset, bigEndian);
}
var value;
if (bigEndian) {
if (buffer.readUInt16BE) {
return buffer.readUInt16BE(offset);
}
value = (buffer[offset] << 8) + buffer[offset+1];
}
else {
if (buffer.readUInt16LE) {
return buffer.readUInt16LE(offset);
}
value = buffer[offset] + (buffer[offset+1] << 8);
}
return value;
}
function readBit(buffer, offset, bitOffset) {
if (bitOffset > 7) {
offset += Math.floor(bitOffset / 8);
bitOffset = bitOffset % 8;
}
var b = buffer[offset];
if (bitOffset < 7) {
b >>>= (7 - bitOffset);
}
var val = b & 0x01;
return val;
}
function readBits(buffer, offset, bitOffset, bitLen, signed) {
var val = 0;
var neg = false;
if (signed) {
if (readBit(buffer, offset, bitOffset) > 0) {
neg = true;
}
bitLen--;
bitOffset++;
}
var bytes = [];
for (var i = 0; i < bitLen; i++) {
var b = readBit(buffer, offset, bitOffset + i);
if (i>0 && (bitLen - i) % 8 == 0) {
bytes.push(val);
val = 0;
}
val <<= 1;
val |= b;
}
bytes.push(val);
val = new Buffer(bytes);
val.negative = neg?true:false;
return val;
}
function imageInfoPng(buffer) {
var imageHeader = [0x49, 0x48, 0x44, 0x52],
pos = 12;
if (!checkSig(buffer, pos, imageHeader)) {
return false;
}
pos += 4;
return {
type: 'image',
format: 'PNG',
mimeType: 'image/png',
width: readUInt32(buffer, pos, true),
height: readUInt32(buffer, pos+4, true),
};
}
function imageInfoJpg(buffer) {
var pos = 2,
len = buffer.length,
sizeSig = [0xff, [0xc0, 0xc2]];
while (pos < len) {
if (checkSig(buffer, pos, sizeSig)) {
pos += 5;
return {
type: 'image',
format: 'JPG',
mimeType: 'image/jpeg',
width: readUInt16(buffer, pos+2, true),
height: readUInt16(buffer, pos, true),
};
}
pos += 2;
var size = readUInt16(buffer, pos, true);
pos += size;
}
}
function imageInfoGif(buffer) {
var pos = 6;
return {
type: 'image',
format: 'GIF',
mimeType: 'image/gif',
width: readUInt16(buffer, pos, false),
height: readUInt16(buffer, pos+2, false),
};
}
function imageInfoSwf(buffer) {
var pos = 8,
bitPos = 0,
val;
if (buffer[0] === 0x43) {
try {
// If you have zlib available ( npm install zlib ) then we can read compressed flash files
buffer = require('zlib').inflate(buffer.slice(8, 100));
pos = 0;
}
catch (ex) {
// Can't get width/height of compressed flash files... yet (need zlib)
return {
type: 'flash',
format: 'SWF',
mimeType: 'application/x-shockwave-flash',
width: null,
height: null,
}
}
}
var numBits = readBits(buffer, pos, bitPos, 5)[0];
bitPos += 5;
val = readBits(buffer, pos, bitPos, numBits, true);
var xMin = (numBits > 9 ? readUInt16(val, 0, true) : val[0]) * (val.negative ? -1 : 1);
bitPos += numBits;
val = readBits(buffer, pos, bitPos, numBits, true);
var xMax = (numBits > 9 ? readUInt16(val, 0, true) : val[0]) * (val.negative ? -1 : 1);
bitPos += numBits;
val = readBits(buffer, pos, bitPos, numBits, true);
var yMin = (numBits > 9 ? readUInt16(val, 0, true) : val[0]) * (val.negative ? -1 : 1);
bitPos += numBits;
val = readBits(buffer, pos, bitPos, numBits, true);
var yMax = (numBits > 9 ? readUInt16(val, 0, true) : val[0]) * (val.negative ? -1 : 1);
return {
type: 'flash',
format: 'SWF',
mimeType: 'application/x-shockwave-flash',
width: Math.ceil((xMax - xMin) / 20),
height: Math.ceil((yMax - yMin) / 20),
};
}
function checkSig(buffer, offset, sig) {
var len = sig.length;
for (var i = 0; i < len; i++) {
var b = buffer[i+offset],
s = sig[i],
m = false;
if ('number' == typeof s) {
m = s === b;
}
else {
for (var k in s) {
var o = s[k];
if (o === b) {
m = true;
}
}
}
if (!m) {
return false;
}
}
return true;
}
module.exports = function imageInfo(buffer, path) {
var pngSig = [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a];
var jpgSig = [0xff, 0xd8, 0xff];
var gifSig = [0x47, 0x49, 0x46, 0x38, [0x37, 0x39], 0x61];
var swfSig = [[0x46, 0x43], 0x57, 0x53];
if (checkSig(buffer, 0, pngSig)) return imageInfoPng(buffer);
if (checkSig(buffer, 0, jpgSig)) return imageInfoJpg(buffer);
if (checkSig(buffer, 0, gifSig)) return imageInfoGif(buffer);
if (checkSig(buffer, 0, swfSig)) return imageInfoSwf(buffer);
return false;
};