Improve SelectVariant performance (#187)

* Improve SelectVariant performance

A three changes were made to improve performance:

- totalWeight was not needed, per the docs this will always be 100%.
This removes the Sum call which loops over the list of variants.
- The FirstOrDefault call was removed, this allows for us to loop over
the list of variants only once.
- We check if there are any overrides before calling Any.

* Fix failing tests

* Add back weight

src/Unleash/Variants/VariantUtils.cs

@@ -14,33 +14,35 @@ public static Variant SelectVariant(string groupId, UnleashContext context, List
         {
             var totalWeight = variantDefinitions.Sum(v => v.Weight);
 
-            if (totalWeight == 0) {
-                return null;
-            }
-
-            var variantOverride = GetOverride(variantDefinitions, context);
-            if (variantOverride != null)
+            if (totalWeight == 0)
             {
-                return variantOverride.ToVariant();
+                return null;
             }
 
-            var stickiness = variantDefinitions[0].Stickiness ?? "default";
+            var stickiness = variantDefinitions.FirstOrDefault()?.Stickiness ?? "default";
             var target = StrategyUtils.GetNormalizedNumber(GetIdentifier(context, stickiness), groupId, VARIANT_NORMALIZATION_SEED, totalWeight);
 
             var counter = 0;
+            Variant result = null;
             foreach (var variantDefinition in variantDefinitions)
             {
                 if (variantDefinition.Weight != 0)
                 {
+                    if (variantDefinition.Overrides.Count > 0 && variantDefinition.Overrides.Any(OverrideMatchesContext(context)))
+                    {
+                        result = variantDefinition.ToVariant();
+                        break;
+                    }
+
                     counter += variantDefinition.Weight;
-                    if (counter >= target)
+                    if (counter >= target && result == null)
                     {
-                        return variantDefinition.ToVariant();
+                        result = variantDefinition.ToVariant();
                     }
                 }
             }
 
-            return null;
+            return result;
         }
 
         public static Variant SelectVariant(FeatureToggle feature, UnleashContext context, Variant defaultVariant = null)
@@ -54,11 +56,6 @@ public static Variant SelectVariant(FeatureToggle feature, UnleashContext contex
 
         }
 
-        private static VariantDefinition GetOverride(List<VariantDefinition> variants, UnleashContext context)
-        {
-            return variants.FirstOrDefault(v => v.Overrides.Any(OverrideMatchesContext(context)));
-        }
-
         private static Func<VariantOverride, bool> OverrideMatchesContext(UnleashContext context)
         {
             return (variantOverride) =>