abstract historyKeeper's batched reads implementation into a library

lib/batch-read.js

@@ -0,0 +1,58 @@
+/*
+
+## Purpose
+
+To avoid running expensive IO or computation concurrently.
+
+If the result of IO or computation is requested while an identical request
+is already in progress, wait until the first one completes and provide its
+result to every routine that requested it.
+
+## Usage
+
+Provide:
+
+1. a named key for the computation or resource,
+2. a callback to handle the result
+3. an implementation which calls back with the result
+
+```
+var batch = Batch();
+
+var read = function (path, cb) {
+    batch(path, cb, function (done) {
+        console.log("reading %s", path);
+        fs.readFile(path, 'utf8', done);
+    });
+};
+
+read('./pewpew.txt', function (err, data) {
+    if (err) { return void console.error(err); }
+    console.log(data);
+});
+
+read('./pewpew.txt', function (err, data) {
+    if (err) { return void console.error(err); }
+    console.log(data);
+});
+```
+
+*/
+
+module.exports = function () {
+    var map = {};
+    return function (id, cb, impl) {
+        if (typeof(cb) !== 'function' || typeof(impl) !== 'function') {
+            throw new Error("expected callback and implementation");
+        }
+        if (map[id]) { return void map[id].push(cb); }
+        map[id] = [cb];
+        impl(function () {
+            var args = Array.prototype.slice.call(arguments);
+            map[id].forEach(function (h) {
+                h.apply(null, args);
+            });
+            delete map[id];
+        });
+    };
+};