Add 'cache' option for caching parsed output

README.md

@@ -137,3 +137,30 @@ function compile(src, file) {
     return src;
 }
 ```
+
+### cache
+An object that will be used as a cache for parsed output of import files, indexed by absolute file path.
+This option may be useful when integrating rework with a watcher by avoiding unnecessary parsing of unchanged files.
+
+Example:
+
+```js
+var cache = {};
+
+function buildCss() {
+    // When './abc' is resolved, the cache will be checked
+    // first. If not cached, the parsed output will be added
+    // to the cache.
+    rework('@import "./abc";', { source: 'index.css' })
+        .use(reworkNPM({ cache: cache }))
+        .toString();
+}
+
+// Triggered by a watcher, perhaps
+function onCssFileChanged(file) {
+    // A css file has changed, so remove it from the cache
+    // and run rework again.
+    delete cache[path.resolve(file)];
+    buildCss();
+}
+```

index.js

@@ -18,6 +18,7 @@ function reworkNPM(opts) {
     var prefilter = opts.prefilter || identity;
     var shim = opts.shim || {};
     var alias = opts.alias || {};
+    var cache = opts.cache || null;
 
     function inline(scope, style) {
         style.rules = concatMap(style.rules, function(rule) {
@@ -41,10 +42,19 @@ function reworkNPM(opts) {
         }
         scope.push(file);
 
+        if (cache && cache.hasOwnProperty(file)) {
+            return cache[file];
+        }
+
         var contents = fs.readFileSync(file, 'utf8');
         contents = prefilter(contents, file);
         contents = parse(contents, { source: path.relative(root, file) });
-        return inline(scope, contents.stylesheet).rules;
+
+        var rules = inline(scope, contents.stylesheet).rules;
+        if (cache) {
+            cache[file] = rules;
+        }
+        return rules;
     }
 
     function resolveImport(rule) {

test.js

@@ -3,6 +3,7 @@ var reworkNPM = require('./');
 var fs = require('fs');
 var path = require('path');
 var rework = require('rework');
+var parse = require('css').parse;
 var sass = require('node-sass');
 var convertSourceMap = require('convert-source-map');
 var SourceMapConsumer = require('source-map').SourceMapConsumer;
@@ -254,3 +255,34 @@ test('Provide filename as second arg to prefilter', function(t) {
             : code;
     }
 });
+
+test('Use cached rules if cache provided', function(t) {
+    var cachedContents = '.test {\n  content: "Cached package";\n}';
+
+    var cache = {};
+    cache[path.resolve('test/node_modules/test/index.css')] = parse(cachedContents, { source: 'test/index.css' }).stylesheet.rules;
+
+    var input = '@import "test";';
+    var output = rework(input, { source: 'test/index.css' })
+            .use(reworkNPM({ cache: cache }))
+            .toString();
+    t.equal(output, cachedContents);
+
+    t.end();
+});
+
+test('Caches parsed output if cache provided', function(t) {
+    var cache = {};
+    var input = '@import "test";';
+    var output = rework(input, { source: 'test/index.css' })
+            .use(reworkNPM({ cache: cache }))
+            .toString();
+
+    t.equal(output, '.test {\n  content: "Test package";\n}');
+
+    var resolvedPath = path.resolve('test/node_modules/test/index.css');
+    t.ok(cache[resolvedPath]);
+    t.type(cache[resolvedPath], "object");
+
+    t.end();
+});