node.js解决获取图片真实文件类型的问题

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;

};