Guard against invalid quantity input

The user can now type anything into the quantity field and some of it
may not be valid. These safe guards ensure that the buttons still work
even if the quantity is undefined or out of range.

Angular guards against the value being out of range but that has other
side-effects. We want to be able to de-activate some of Angular's
behaviour.

app/assets/javascripts/darkswarm/controllers/shop_variant_controller.js.coffee

@@ -17,21 +17,33 @@ Darkswarm.controller "ShopVariantCtrl", ($scope, $modal, Cart) ->
       if item.max_quantity < item.quantity
         item.quantity = item.max_quantity
 
+  $scope.quantity = ->
+    $scope.variant.line_item.quantity || 0
+
+  $scope.maxQuantity = ->
+    $scope.variant.line_item.max_quantity || $scope.sanitizedQuantity()
+
+  $scope.sanitizedQuantity = ->
+    Math.max(0, Math.min($scope.quantity(), $scope.available()))
+
+  $scope.sanitizedMaxQuantity = ->
+    Math.max($scope.sanitizedQuantity(), Math.min($scope.maxQuantity(), $scope.available()))
+
   $scope.add = (quantity) ->
     item = $scope.variant.line_item
-    item.quantity += quantity
+    item.quantity = $scope.sanitizedQuantity() + quantity
 
   $scope.addMax = (quantity) ->
     item = $scope.variant.line_item
-    item.max_quantity += quantity
+    item.max_quantity = $scope.sanitizedMaxQuantity() + quantity
 
   $scope.canAdd = (quantity) ->
-    wantedQuantity = $scope.variant.line_item.quantity + quantity
+    wantedQuantity = $scope.sanitizedQuantity() + quantity
     $scope.quantityValid(wantedQuantity)
 
   $scope.canAddMax = (quantity) ->
     variant = $scope.variant
-    wantedQuantity = variant.line_item.max_quantity + quantity
+    wantedQuantity = $scope.sanitizedMaxQuantity() + quantity
     $scope.quantityValid(wantedQuantity) && variant.line_item.quantity > 0
 
   $scope.available = ->

spec/javascripts/unit/darkswarm/controllers/checkout/shop_variant_controller_spec.js.coffee

@@ -27,11 +27,29 @@ describe "ShopVariantCtrl", ->
     scope.add 5
     expect(scope.variant.line_item.quantity).toEqual 6
 
+  it "adds to an invalid quantity", ->
+    scope.$apply ->
+      scope.variant.line_item.quantity = -5
+    scope.add 1
+    expect(scope.variant.line_item.quantity).toEqual 1
+
+  it "adds to an undefined quantity", ->
+    scope.$apply ->
+      scope.variant.line_item.quantity = undefined
+    scope.add 1
+    expect(scope.variant.line_item.quantity).toEqual 1
+
   it "adds to the max quantity", ->
     scope.addMax 5
     expect(scope.variant.line_item.quantity).toEqual 0
     expect(scope.variant.line_item.max_quantity).toEqual 5
 
+  it "adds to an undefined max quantity", ->
+    scope.variant.line_item.quantity = 3
+    scope.variant.line_item.max_quantity = undefined
+    scope.addMax 1
+    expect(scope.variant.line_item.max_quantity).toEqual 4
+
   it "adds to the max quantity to be at least min quantity", ->
     scope.$apply ->
       scope.variant.line_item.max_quantity = 2
@@ -74,6 +92,42 @@ describe "ShopVariantCtrl", ->
     expect(scope.canAdd(2)).toEqual true
     expect(scope.canAdd(3)).toEqual false
 
+  it "denies adding if quantity is too high", ->
+    scope.variant.on_demand = false
+    scope.variant.on_hand = 5
+    scope.variant.line_item.quantity = 7
+    scope.variant.line_item.max_quantity = 7
+
+    expect(scope.canAdd(1)).toEqual false
+    expect(scope.canAddMax(1)).toEqual false
+
+  it "allows decrease when quantity is too high", ->
+    scope.variant.on_demand = false
+    scope.variant.on_hand = 5
+    scope.variant.line_item.quantity = 7
+    scope.variant.line_item.max_quantity = 7
+    expect(scope.canAdd(-1)).toEqual true
+    expect(scope.canAddMax(-1)).toEqual true
+
+  it "allows increase when quantity is negative", ->
+    scope.variant.on_demand = false
+    scope.variant.on_hand = 5
+    scope.variant.line_item.quantity = -3
+    scope.variant.line_item.max_quantity = -3
+    expect(scope.canAdd(1)).toEqual true
+    expect(scope.canAddMax(1)).toEqual false
+
+    scope.variant.line_item.quantity = 1
+    expect(scope.canAddMax(1)).toEqual true
+
+  it "denies decrease when quantity is negative", ->
+    scope.variant.on_demand = false
+    scope.variant.on_hand = 5
+    scope.variant.line_item.quantity = -3
+    scope.variant.line_item.max_quantity = -3
+    expect(scope.canAdd(-1)).toEqual false
+    expect(scope.canAddMax(-1)).toEqual false
+
   it "denies declaring max quantity before item is in cart", ->
     expect(scope.canAddMax(1)).toEqual false