Recover performance of ParticleFilter in SMC.

src/inference/particlefilter2.js

@@ -4,6 +4,7 @@ var _ = require('underscore');
 var util = require('../util.js');
 var erp = require('../erp.js');
 var Trace = require('../trace');
+var Particle = require('../particle');
 var assert = require('assert');
 
 module.exports = function(env) {
@@ -22,10 +23,9 @@ module.exports = function(env) {
       return wpplFn(s, env.exit, a);
     };
 
-    // Create initial particles.
-    // Particles are partial traces.
+    // Create initial particles/traces.
     for (var i = 0; i < this.numParticles; i++) {
-      var p = new Trace();
+      var p = new (this.rejuvSteps === 0 ? Particle : Trace)();
       p.saveContinuation(exitK, _.clone(s));
       this.particles.push(p);
     }

src/particle.js

@@ -0,0 +1,40 @@
+'use strict';
+
+var _ = require('underscore');
+var assert = require('assert');
+
+// Minimal Trace-like structure used to avoid unnecessary overhead in SMC
+// without rejuvenation.
+
+var Particle = function() {
+  this.score = 0;
+  this.logWeight = 0; // Importance weight.
+};
+
+Particle.prototype.saveContinuation = function(continuation, store) {
+  this.k = continuation;
+  this.store = store;
+};
+
+Particle.prototype.addChoice = function(erp, params, val, address, store, continuation) {
+  this.score += erp.score(params, val);
+};
+
+Particle.prototype.complete = function(value) {
+  // Called at coroutine exit.
+  assert(this.value === undefined);
+  this.value = value;
+  // Ensure any attempt to continue a completed Particle fails in an obvious way.
+  this.k = this.store = undefined;
+};
+
+Particle.prototype.copy = function() {
+  var t = new Particle();
+  t.score = this.score;
+  t.k = this.k;
+  t.store = _.clone(this.store);
+  t.value = this.value;
+  return t;
+};
+
+module.exports = Particle;