From 279d6e0e656705a35337b9a58c2e9aa1ddefac42 Mon Sep 17 00:00:00 2001
From: eldritchreality <eldritchreality@gmail.com>
Date: Fri, 29 Apr 2016 15:36:51 +0000
Subject: [PATCH] propagators no longer run unless all their input cells have
 values

---
 propagator.js           | 13 ++++++++++---
 test/test-propagator.js | 19 +++++++++++++++++++
 2 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/propagator.js b/propagator.js
index cf33ca4..c8570bc 100644
--- a/propagator.js
+++ b/propagator.js
@@ -19,16 +19,23 @@ function Propagator(func,input,output) {
       this.output = output;
     
       function applyFuncToInputs() {
-          var inputValues = self.input.map((cell) => cell.getContents())
+          var inputs = self.input.map((cell) => cell.getContents())
+          var output = {"value" : undefined, "usable" : false}
           
-          return func.apply(null,inputValues);
+          inputs.fullyPopulated = inputs.every((value) => typeof value !== "undefined")
+         
+          if(inputs.fullyPopulated) {
+              output.value = func.apply(null,inputs);
+              output.usable = true;
+          }
           
+          return output;
       }
    
       this.propagate = function propagate() {
           
           var outputValues = applyFuncToInputs();
-          self.output.update(outputValues, self)
+          if (outputValues.usable) self.output.update(outputValues.value, self)
       }
     
       this.input.forEach( (cell) => cell.addListener(self.propagate) ) 
diff --git a/test/test-propagator.js b/test/test-propagator.js
index 513a13c..2a541d1 100644
--- a/test/test-propagator.js
+++ b/test/test-propagator.js
@@ -48,6 +48,25 @@ describe("The propagator constructor",function() {
         expect(mockOutputCell.getContents()).to.equal(addValuesTogether(10,1,1))
     }) 
      
+    it("should not update an output cell unless all of its input cells have values",function(){
+        var mockInputCell1 = Propagator.makeCell()
+        var mockInputCell2 = Propagator.makeCell()
+        var mockInputCell3 = Propagator.makeCell()
+        var mockOutputCell = Propagator.makeCell()
+        var addValuesTogether = ((x,y,z) => x+y+z)
+        
+        var testPropagator = new Propagator(addValuesTogether,[mockInputCell1,mockInputCell2,mockInputCell3],mockOutputCell);
+        
+        mockInputCell1.update(1)
+        expect(mockOutputCell.getContents()).to.be.undefined;
+        
+        mockInputCell2.update(10)
+        expect(mockOutputCell.getContents()).to.be.undefined;
+        
+        mockInputCell3.update(100)
+        expect(mockOutputCell.getContents()).to.equal(addValuesTogether(1,10,100))
+    })
+     
     it("should throw an error when the constructor is called with no arguments",function(){
         
         expect( () => new Propagator() ).to.throw(Error)