var assign = require('object-assign');
var vary = require('vary');

var defaults = {
origin: '*',methods: 'GET,HEAD,PUT,PATCH,POST,DELETE',preflightContinue: false,optionsSuccessStatus: 204
};

function isString(s) {
return typeof s === 'string' || s instanceof String;
}

function isOriginAllowed(origin,allowedOrigin) {
if (Array.isArray(allowedOrigin)) {
for (var i = 0; i < allowedOrigin.length; ++i) {
if (isOriginAllowed(origin,allowedOrigin[i])) {
return true;
}
}
return false;
} else if (isString(allowedOrigin)) {
return origin === allowedOrigin;
} else if (allowedOrigin instanceof RegExp) {
return allowedOrigin.test(origin);
} else {
return !!allowedOrigin;
}
}

function configureOrigin(options,req) {
var requestOrigin = req.headers.origin,headers = [],isAllowed;

if (!options.origin || options.origin === '') {
// allow any origin
headers.push([{
key: 'Access-Control-Allow-Origin',value: ''
}]);
} else if (isString(options.origin)) {
// fixed origin
headers.push([{
key: 'Access-Control-Allow-Origin',value: options.origin
}]);
headers.push([{
key: 'Vary',value: 'Origin'
}]);
} else {
isAllowed = isOriginAllowed(requestOrigin,options.origin);
// reflect origin
headers.push([{
key: 'Access-Control-Allow-Origin',value: isAllowed ? requestOrigin : false
}]);
headers.push([{
key: 'Vary',value: 'Origin'
}]);
}

return headers;
}

function configureMethods(options) {
var methods = options.methods;
if (methods.join) {
methods = options.methods.join(','); // .methods is an array,so turn it into a string
}
return {
key: 'Access-Control-Allow-Methods',value: methods
};
}

function configureCredentials(options) {
if (options.credentials === true) {
return {
key: 'Access-Control-Allow-Credentials',value: 'true'
};
}
return null;
}

function configureAllowedHeaders(options,req) {
var allowedHeaders = options.allowedHeaders || options.headers;
var headers = [];

if (!allowedHeaders) {
allowedHeaders = req.headers['access-control-request-headers']; // .headers wasn't specified,so reflect the request headers
headers.push([{
key: 'Vary',value: 'Access-Control-Request-Headers'
}]);
} else if (allowedHeaders.join) {
allowedHeaders = allowedHeaders.join(','); // .headers is an array,so turn it into a string
}
if (allowedHeaders && allowedHeaders.length) {
headers.push([{
key: 'Access-Control-Allow-Headers',value: allowedHeaders
}]);
}

return headers;
}

function configureExposedHeaders(options) {
var headers = options.exposedHeaders;
if (!headers) {
return null;
} else if (headers.join) {
headers = headers.join(',so turn it into a string
}
if (headers && headers.length) {
return {
key: 'Access-Control-Expose-Headers',value: headers
};
}
return null;
}

function configureMaxAge(options) {
var maxAge = options.maxAge && options.maxAge.toString();
if (maxAge && maxAge.length) {
return {
key: 'Access-Control-Max-Age',value: maxAge
};
}
return null;
}

function applyHeaders(headers,res) {
for (var i = 0,n = headers.length; i < n; i++) {
var header = headers[i];
if (header) {
if (Array.isArray(header)) {
applyHeaders(header,res);
} else if (header.key === 'Vary' && header.value) {
vary(res,header.value);
} else if (header.value) {
res.setHeader(header.key,header.value);
}
}
}
}

function cors(options,req,res,next) {
var headers = [],method = req.method && req.method.toUpperCase && req.method.toUpperCase();

if (method === 'OPTIONS') {
// preflight
headers.push(configureOrigin(options,req));
headers.push(configureCredentials(options,req));
headers.push(configureMethods(options,req));
headers.push(configureAllowedHeaders(options,req));
headers.push(configureMaxAge(options,req));
headers.push(configureExposedHeaders(options,req));
applyHeaders(headers,res);

if (options.preflightContinue ) {
next();
} else {
res.statusCode = options.optionsSuccessStatus || defaults.optionsSuccessStatus;
res.end();
}
} else {
// actual response
headers.push(configureOrigin(options,res);
next();
}
}

function middlewareWrapper(o) {
if (typeof o !== 'function') {
o = assign({},defaults,o);
}

// if options are static (either via defaults or custom options passed in),wrap in a function
var optionsCallback = null;
if (typeof o === 'function') {
optionsCallback = o;
} else {
optionsCallback = function (req,cb) {
cb(null,o);
};
}

return function corsMiddleware(req,next) {
optionsCallback(req,function (err,options) {
if (err) {
next(err);
} else {
var originCallback = null;
if (options.origin && typeof options.origin === 'function') {
originCallback = options.origin;
} else if (options.origin) {
originCallback = function (origin,cb) {
cb(null,options.origin);
};
}

if (originCallback) {
originCallback(req.headers.origin,function (err2,origin) {
if (err2 || !origin) {
next(err2);
} else {
var corsOptions = Object.create(options);
corsOptions.origin = origin;
cors(corsOptions,next);
}
});
} else {
next();
}
}
});
};
}

// can pass either an options hash,an options delegate,or nothing
module.exports = middlewareWrapper;

}());

app.post('a/b',function (req,next) {
let reqOrigin = req.headers.origin; // request响应头的origin属性

// 判断请求是否在域名白名单内
if(isOriginAllowed(reqOrigin,ALLOW_ORIGIN)) {
// 设置CORS为请求的Origin值
res.header("Access-Control-Allow-Origin",reqOrigin);
res.header('Access-Control-Allow-Credentials','true');

// 你的业务代码逻辑代码 ...
// ...
} else {
res.send({ code: -2,msg: '非法请求' });
}
});