Add a small LRU cache in front of calls to normalize

We are almost always normalizing the same strings over and over again. Think
about when iterating over mappings: we are reconstructing each mapping's source,
but the mappings are in sorted order, and so will likely have the same source
every time.

This makes us take .09x the time that we used to take on the
"iterate.already.parsed" benchmark!!

Without the LRU cache:

Samples     Total (ms)  Mean (ms)   Standard Deviation (ms)
50          257604.64   5152.09     221.19

With the new LRU cache:

Samples     Total (ms)  Mean (ms)   Standard Deviation (ms)
50          23301.74    466.03      56.14

Author: fitzgen

dist/source-map.js

@@ -145,6 +145,43 @@ function urlGenerate(aParsedUrl) {
 }
 exports.urlGenerate = urlGenerate;
 
+const MAX_CACHED_INPUTS = 32;
+
+/**
+ * Takes some function `f(input) -> result` and returns a memoized version of
+ * `f`.
+ *
+ * We keep at most `MAX_CACHED_INPUTS` memoized results of `f` alive. The
+ * memoization is a dumb-simple, linear least-recently-used cache.
+ */
+function lruMemoize(f) {
+  const cache = [];
+
+  return function (input) {
+    for (var i = 0; i < cache.length; i++) {
+      if (cache[i].input === input) {
+        var temp = cache[0];
+        cache[0] = cache[i];
+        cache[i] = temp;
+        return cache[0].result;
+      }
+    }
+
+    var result = f(input);
+
+    cache.unshift({
+      input,
+      result,
+    });
+
+    if (cache.length > MAX_CACHED_INPUTS) {
+      cache.pop();
+    }
+
+    return result;
+  };
+}
+
 /**
  * Normalizes a path, or the path portion of a URL:
  *
@@ -156,7 +193,7 @@ exports.urlGenerate = urlGenerate;
  *
  * @param aPath The path or url to normalize.
  */
-function normalize(aPath) {
+var normalize = lruMemoize(function normalize(aPath) {
   var path = aPath;
   var url = urlParse(aPath);
   if (url) {
@@ -216,7 +253,7 @@ function normalize(aPath) {
     return urlGenerate(url);
   }
   return path;
-}
+});
 exports.normalize = normalize;
 
 /**

lib/util.js

@@ -66,6 +66,43 @@ function urlGenerate(aParsedUrl) {
 }
 exports.urlGenerate = urlGenerate;
 
+const MAX_CACHED_INPUTS = 32;
+
+/**
+ * Takes some function `f(input) -> result` and returns a memoized version of
+ * `f`.
+ *
+ * We keep at most `MAX_CACHED_INPUTS` memoized results of `f` alive. The
+ * memoization is a dumb-simple, linear least-recently-used cache.
+ */
+function lruMemoize(f) {
+  const cache = [];
+
+  return function (input) {
+    for (var i = 0; i < cache.length; i++) {
+      if (cache[i].input === input) {
+        var temp = cache[0];
+        cache[0] = cache[i];
+        cache[i] = temp;
+        return cache[0].result;
+      }
+    }
+
+    var result = f(input);
+
+    cache.unshift({
+      input,
+      result,
+    });
+
+    if (cache.length > MAX_CACHED_INPUTS) {
+      cache.pop();
+    }
+
+    return result;
+  };
+}
+
 /**
  * Normalizes a path, or the path portion of a URL:
  *
@@ -77,7 +114,7 @@ exports.urlGenerate = urlGenerate;
  *
  * @param aPath The path or url to normalize.
  */
-function normalize(aPath) {
+var normalize = lruMemoize(function normalize(aPath) {
   var path = aPath;
   var url = urlParse(aPath);
   if (url) {
@@ -137,7 +174,7 @@ function normalize(aPath) {
     return urlGenerate(url);
   }
   return path;
-}
+});
 exports.normalize = normalize;
 
 /**