simplify serve_data code

split response function into standard express middleware stack - each
middleware is now responsible for a single task only

src/serve_data.js

@@ -18,6 +18,24 @@ try {
 
 var utils = require('./utils');
 
+function isGzipped(data) {
+  return data[0] === 0x1f && data[1] === 0x8b;
+}
+
+function unzipTile(tile) {
+  if (tile.isGzipped) {
+    tile.data = zlib.gunzipSync(tile.data);
+    tile.isGzipped = false;
+  }
+}
+
+function zipTile(tile) {
+  if (!tile.isGzipped) {
+    tile.data = zlib.gzipSync(tile.data);
+    tile.isGzipped = true;
+  }
+}
+
 module.exports = function(options, repo, params, id, styles) {
   var app = express().disable('x-powered-by');
 
@@ -28,10 +46,29 @@ module.exports = function(options, repo, params, id, styles) {
 
   var shrinkers = {};
 
+  function lookupShrinker(style) {
+    if (shrinkers[style]) {
+      return shrinkers[style];
+    }
+    var styleJSON = styles[style];
+    if (!styleJSON) {
+      return;
+    }
+    var sourceName = null;
+    for (var sourceName_ in styleJSON.sources) {
+      var source = styleJSON.sources[sourceName_];
+      if (source && source.type == 'vector' && source.url.endsWith('/' + id + '.json')) {
+        sourceName = sourceName_;
+      }
+    }
+    shrinkers[style] = tileshrinkGl.createPBFShrinker(styleJSON, sourceName);
+    return shrinkers[style];
+  }
+
   repo[id] = tileJSON;
 
   var mbtilesFileStats = fs.statSync(mbtilesFile);
-  if (!mbtilesFileStats.isFile() || mbtilesFileStats.size == 0) {
+  if (!mbtilesFileStats.isFile() || mbtilesFileStats.size === 0) {
     throw Error('Not valid MBTiles file: ' + mbtilesFile);
   }
   var source = new mbtiles(mbtilesFile, function(err) {
@@ -53,7 +90,7 @@ module.exports = function(options, repo, params, id, styles) {
 
   var tilePattern = '/' + id + '/:z(\\d+)/:x(\\d+)/:y(\\d+).:format([\\w.]+)';
 
-  app.get(tilePattern, function(req, res, next) {
+  function checkParams(req, res, next) {
     var z = req.params.z | 0,
         x = req.params.x | 0,
         y = req.params.y | 0;
@@ -70,87 +107,108 @@ module.exports = function(options, repo, params, id, styles) {
         x >= Math.pow(2, z) || y >= Math.pow(2, z)) {
       return res.status(404).send('Out of bounds');
     }
-    source.getTile(z, x, y, function(err, data, headers) {
+    req.params.format = format;
+    req.params.z = z;
+    req.params.x = x;
+    req.params.y = y;
+    next();
+  }
+
+  function getTile(req, res, next) {
+    var p = req.params;
+    source.getTile(p.z, p.x, p.y, function(err, data, headers) {
       if (err) {
-        if (/does not exist/.test(err.message)) {
+        var status = /does not exist/.test(err.message) ? 404 : 500;
-          return res.status(404).send(err.message);
+        return res.status(status).send(err.message);
-        } else {
-          return res.status(500).send(err.message);
-        }
-      } else {
-        if (data == null) {
-          return res.status(404).send('Not found');
-        } else {
-          if (tileJSON['format'] == 'pbf') {
-            var isGzipped = data.slice(0,2).indexOf(
-                new Buffer([0x1f, 0x8b])) === 0;
-            var style = req.query.style;
-            if (style && tileshrinkGl) {
-              if (!shrinkers[style]) {
-                var styleJSON = styles[style];
-                if (styleJSON) {
-                  var sourceName = null;
-                  for (var sourceName_ in styleJSON.sources) {
-                    var source = styleJSON.sources[sourceName_];
-                    if (source &&
-                        source.type == 'vector' &&
-                        source.url.endsWith('/' + id + '.json')) {
-                      sourceName = sourceName_;
-                    }
-                  }
-                  shrinkers[style] = tileshrinkGl.createPBFShrinker(styleJSON, sourceName);
-                }
-              }
-              if (shrinkers[style]) {
-                if (isGzipped) {
-                  data = zlib.unzipSync(data);
-                  isGzipped = false;
-                }
-                data = shrinkers[style](data, z, tileJSON.maxzoom);
-                //console.log(shrinkers[style].getStats());
-              }
-            }
-          }
-          if (format == 'pbf') {
-            headers['Content-Type'] = 'application/x-protobuf';
-          } else if (format == 'geojson') {
-            headers['Content-Type'] = 'application/json';
-
-            if (isGzipped) {
-              data = zlib.unzipSync(data);
-              isGzipped = false;
-            }
-
-            var tile = new VectorTile(new pbf(data));
-            var geojson = {
-              "type": "FeatureCollection",
-              "features": []
-            };
-            for (var layerName in tile.layers) {
-              var layer = tile.layers[layerName];
-              for (var i = 0; i < layer.length; i++) {
-                var feature = layer.feature(i);
-                var featureGeoJSON = feature.toGeoJSON(x, y, z);
-                featureGeoJSON.properties.layer = layerName;
-                geojson.features.push(featureGeoJSON);
-              }
-            }
-            data = JSON.stringify(geojson);
-          }
-          delete headers['ETag']; // do not trust the tile ETag -- regenerate
-          headers['Content-Encoding'] = 'gzip';
-          res.set(headers);
-
-          if (!isGzipped) {
-            data = zlib.gzipSync(data);
-            isGzipped = true;
-          }
-
-          return res.status(200).send(data);
-        }
       }
+      if (data == null) {
+        return res.status(404).send('Not found');
+      }
+      req.tile = {
+        data: data,
+        headers: headers,
+        contentType: 'application/x-protobuf',
+        isGzipped: isGzipped(data)
+      };
+      next();
     });
-  });
+  }
+
+  function shrinkTile(req, res, next) {
+    if (!tileshrinkGl) {
+      return next();
+    }
+    if (tileJSON['format'] !== 'pbf') {
+      return next();
+    }
+    var style = req.query.style;
+    if (!style) {
+      return next();
+    }
+    var tile = req.tile;
+    var shrinker = lookupShrinker(style);
+    if (shrinker) {
+      unzipTile(tile);
+      tile.data = shrinker(tile.data, req.params.z, tileJSON.maxzoom);
+      //console.log(shrinker.getStats());
+    }
+    next();
+  }
+
+  function formatTile(req, res, next) {
+    var format = req.params.format;
+    if (format !== 'geojson') {
+      return next();
+    }
+    var tile = req.tile;
+    var x = req.params.x,
+      y = req.params.y,
+      z = req.params.z;
+
+    tile.contentType = 'application/json';
+    unzipTile(tile);
+
+    var vectorTile = new VectorTile(new pbf(tile.data));
+    var geojson = {
+      "type": "FeatureCollection",
+      "features": []
+    };
+
+    for (var layerName in vectorTile.layers) {
+      var layer = vectorTile.layers[layerName];
+      for (var i = 0; i < layer.length; i++) {
+        var feature = layer.feature(i);
+        var featureGeoJSON = feature.toGeoJSON(x, y, z);
+        featureGeoJSON.properties.layer = layerName;
+        geojson.features.push(featureGeoJSON);
+      }
+    }
+
+    tile.data = JSON.stringify(geojson);
+
+    next();
+  }
+
+  function sendTile(req, res) {
+    var headers = req.tile.headers;
+
+    delete headers['ETag']; // do not trust the tile ETag -- regenerate
+    headers['Content-Type'] = req.tile.contentType;
+    headers['Content-Encoding'] = 'gzip';
+    res.set(headers);
+
+    zipTile(req.tile);
+
+    return res.status(200).send(req.tile.data);
+  }
+
+  app.get(tilePattern,
+    checkParams,
+    getTile,
+    shrinkTile,
+    formatTile,
+    sendTile
+  );
 
   app.get('/' + id + '.json', function(req, res, next) {
     var info = clone(tileJSON);