Fixed-size cache store for bitmap images.

Part of issue #23.

svg-to-img.js

@@ -6,19 +6,18 @@ var childProcess = require('child_process');
 var phantomScript = path.join(__dirname, 'phantomjs-svg2png.js');
 
 var imgCache = Object.create(null);
-
-// If available, use the font here.
-var fontPath = './Verbana.ttf';
-try {
-  // This happens at startup. Needn't be async.
-  var fontBase64 = fs.readFileSync(fontPath, 'base64');
-} catch(e) {}
+var imgCacheTime = Object.create(null);
+var imgCacheSize = 0;
+// The following is an arbitrary limit (~1.5MB, 1.5kB/image).
+var imgCacheMaxSize = 1000;
 
 module.exports = function (svg, format, out, cb) {
   var cacheIndex = format + svg;
   if (imgCache[cacheIndex] !== undefined) {
     // We own a cache for this svg conversion.
-    imgCache[cacheIndex].pipe(out);
+    (new DataStream(imgCache[cacheIndex])).pipe(out);
+    imgCacheTime[cacheIndex] = +(new Date());
+    return;
   }
   var tmpFile = path.join(os.tmpdir(),
       "svg2img-" + (Math.random()*2147483648|0) + "." + format);
@@ -37,21 +36,47 @@ module.exports = function (svg, format, out, cb) {
     // Remove the temporary file after use.
     inStream.on('end', function() {
       out.end();
-      imgCache[cacheIndex] = streamFromData(cached);
+      addToCache(cacheIndex, cached);
       fs.unlink(tmpFile, cb);
     });
   });
 };
 
-// Fake stream from the cache.
-var stream = require('stream');
-function streamFromData(data) {
-  var newStream = new stream.Readable();
-  newStream._read = function() {
-    for (var i = 0; i < data.length; i++) {
-      newStream.push(data[i]);
+function addToCache(cacheIndex, cached) {
+  imgCache[cacheIndex] = cached;
+  var mostAncient = +(new Date());
+  imgCacheTime[cacheIndex] = mostAncient;
+  if (imgCacheSize >= imgCacheMaxSize) {
+    // Find the most ancient image.
+    var ancientCacheIndex = cacheIndex;
+    for (var currentCacheIndex in imgCacheTime) {
+      if (mostAncient > imgCacheTime[currentCacheIndex]) {
+        mostAncient = imgCacheTime[currentCacheIndex];
+        ancientCacheIndex = currentCacheIndex;
+      }
     }
-    newStream.push(null)
-  };
-  return newStream;
+    // Delete that image.
+    delete imgCache[ancientCacheIndex];
+    delete imgCacheTime[ancientCacheIndex];
+  } else {
+    imgCacheSize++;
+  }
+}
+
+// Fake stream from the cache.
+var Readable = require('stream').Readable;
+var util = require('util');
+function DataStream(data) {
+  Readable.call(this);
+  this.data = data;
+  this.i = 0;
 }
+util.inherits(DataStream, Readable);
+DataStream.prototype._read = function() {
+  while (this.i < this.data.length) {
+    var stop = this.push(this.data[this.i]);
+    this.i++;
+    if (stop) { return; }
+  }
+  this.push(null);
+};