add options.cache to save file io

README.md

@@ -160,6 +160,13 @@ It's to optimize output and skip similar files like `normalize.css` for example.
 If this behavior is not what you want, just set this option to `false` to
 disable it.
 
+#### `cache`
+
+Type: `Object`  
+Default: `null`
+
+Cache to save expensive file io. It maps the resolved file name to the corresponding transformed file contents (`String`).
+
 #### Example with some options
 
 ```js

index.js

@@ -74,6 +74,7 @@ function AtImport(options) {
       importedFiles: {},
       ignoredAtRules: [],
       hashFiles: {},
+      cache: options.cache || {},
     }
     if (opts.from) {
       state.importedFiles[opts.from] = {
@@ -347,7 +348,8 @@ function readImportedContent(
     options.encoding,
     options.transform || function(value) {
       return value
-    }
+    },
+    state.cache
   )
 
   if (fileContent.trim() === "") {
@@ -537,8 +539,11 @@ function resolveFilename(name, root, paths, source, resolver) {
  *
  * @param {String} file
  */
-function readFile(file, encoding, transform) {
-  return transform(fs.readFileSync(file, encoding || "utf8"), file)
+function readFile(file, encoding, transform, cache) {
+  if (!cache[file]) {
+    cache[file] = transform(fs.readFileSync(file, encoding || "utf8"), file)
+  }
+  return cache[file]
 }
 
 /**

test/index.js

@@ -345,3 +345,23 @@ test("sync", function(t) {
 
   t.end()
 })
+
+test("cache option", function(t) {
+  t.plan(1)
+
+  postcss()
+    .use(atImport({
+      resolve: function(file) {
+        return path.basename(file)
+      },
+      cache: {
+        b: ".b{}",
+        c: "@import '/d';.c{}",
+        d: ".d{}",
+      },
+    }))
+    .process("@import '/c';@import '/b';.a{}")
+    .then(function(res) {
+      t.equal(res.css, ".d{}.c{}.b{}.a{}")
+    })
+})