git-style folder-separation in cache dir

lib/snapshot.js

@@ -3,6 +3,7 @@ var spawn = require('child_process').spawn
 var fs = require('fs')
 var path = require('path')
 var EventEmitter = require('events').EventEmitter
+var crypto = require('crypto')
 
 var app = module.exports = express()
 
@@ -48,20 +49,28 @@ app.get('/:url/:resolution/:wait', function (req, res, next) {
     })
   }
 
-  var hash = new Buffer(req.params['url'] + req.params['resolution']).toString('base64')
-  var filename = cacheDir + hash
+  var hash = crypto
+    .createHash('sha1')
+    .update(req.params['url'])
+    .update(req.params['resolution'])
+    .digest('hex')
+
+  var dir = hash.slice(0, 2)
+  var filename = cacheDir + dir + '/' + hash
 
   function createCash () {
     caching[hash] = true
     snapshot(req.params, function (err, rasterized) {
       if (err) return caching[hash] = false, next(err)
 
-      fs.writeFile(filename, rasterized, function (err) {
-        caching[hash] = false
-        if (err) return next(err)
-
-        res.set('Content-Type', 'image/png').end(rasterized)
-        caches.emit(hash)
+      fs.mkdir(cacheDir + dir, function () {
+        fs.writeFile(filename, rasterized, function (err) {
+          caching[hash] = false
+          if (err) return next(err)
+
+          res.set('Content-Type', 'image/png').end(rasterized)
+          caches.emit(hash)
+        })
       })
     })
   }