Misc Cleanups (#66)

+ Nullability annotations in Gatherers4j.java
+ Minimum windowSize is 1 in a lot of places, not zero.
+ Add constructor to GroupingByGatherer and handle null mapping function to make nullability checks happier
+ Parameterize tests where multiple independent assertions were taking place

src/main/java/com/ginsberg/gatherers4j/BigDecimalMovingProductGatherer.java

@@ -34,8 +34,8 @@ public class BigDecimalMovingProductGatherer<INPUT extends @Nullable Object>
             final Function<INPUT, @Nullable BigDecimal> mappingFunction,
             final int windowSize) {
         super(mappingFunction);
-        if (windowSize <= 0) {
-            throw new IllegalArgumentException("Window size must be positive");
+        if (windowSize <= 1) {
+            throw new IllegalArgumentException("Window size must be greater than 1");
         }
         this.windowSize = windowSize;
     }

src/main/java/com/ginsberg/gatherers4j/BigDecimalMovingSumGatherer.java

@@ -34,8 +34,8 @@ public class BigDecimalMovingSumGatherer<INPUT extends @Nullable Object>
             final Function<INPUT, @Nullable BigDecimal> mappingFunction,
             final int windowSize) {
         super(mappingFunction);
-        if (windowSize <= 0) {
-            throw new IllegalArgumentException("Window size must be positive");
+        if (windowSize <= 1) {
+            throw new IllegalArgumentException("Window size must be greater than 1");
         }
         this.windowSize = windowSize;
     }

src/main/java/com/ginsberg/gatherers4j/BigDecimalSimpleMovingAverageGatherer.java

@@ -34,8 +34,8 @@ public final class BigDecimalSimpleMovingAverageGatherer<INPUT extends @Nullable
             final int windowSize
     ) {
         super(mappingFunction);
-        if (windowSize <= 0) {
-            throw new IllegalArgumentException("Window size must be positive");
+        if (windowSize <= 1) {
+            throw new IllegalArgumentException("Window size must be greater than 1");
         }
         this.windowSize = windowSize;
     }

src/main/java/com/ginsberg/gatherers4j/GroupingByGatherer.java

@@ -31,9 +31,13 @@
 public class GroupingByGatherer<INPUT extends @Nullable Object> implements
         Gatherer<INPUT, GroupingByGatherer.State<INPUT>, List<INPUT>> {
 
-    private final Function<INPUT, Object> mappingFunction;
+    private final @Nullable Function<INPUT, Object> mappingFunction;
 
-    GroupingByGatherer(Function<INPUT, @Nullable Object> mappingFunction) {
+    GroupingByGatherer() {
+        mappingFunction = null;
+    }
+
+    GroupingByGatherer(Function<@Nullable INPUT, @Nullable Object> mappingFunction) {
         mustNotBeNull(mappingFunction, "Mapping function must not be null");
         this.mappingFunction = mappingFunction;
     }
@@ -46,7 +50,7 @@ public Supplier<State<INPUT>> initializer() {
     @Override
     public Integrator<State<INPUT>, INPUT, List<INPUT>> integrator() {
         return Integrator.ofGreedy((state, element, downstream) -> {
-            final Object thisMatch = mappingFunction.apply(element);
+            final Object thisMatch = mappingFunction == null ? element: mappingFunction.apply(element);
             if (state.working == null) {
                 state.working = new ArrayList<>();
                 state.working.add(element);

src/test/java/com/ginsberg/gatherers4j/BigDecimalMovingProductGathererTest.java

@@ -17,6 +17,8 @@
 package com.ginsberg.gatherers4j;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 
 import java.math.BigDecimal;
 import java.util.List;
@@ -172,14 +174,11 @@ void treatNullAsOne() {
                 );
     }
 
-    @Test
-    void windowSizeMustBePositive() {
-        assertThatThrownBy(() ->
-                Stream.of(BigDecimal.ONE).gather(Gatherers4j.movingProduct(0))
-        ).isExactlyInstanceOf(IllegalArgumentException.class);
-
+    @ParameterizedTest(name = "windowSize of {0}")
+    @ValueSource(ints = {-1, 0, 1})
+    void windowSizeMustBeGreaterThanOne(final int windowSize) {
         assertThatThrownBy(() ->
-                Stream.of(BigDecimal.ONE).gather(Gatherers4j.movingProduct(-1))
+                Stream.of(BigDecimal.ONE).gather(Gatherers4j.movingProduct(windowSize))
         ).isExactlyInstanceOf(IllegalArgumentException.class);
     }
 }

src/test/java/com/ginsberg/gatherers4j/BigDecimalMovingSumGathererTest.java

@@ -17,6 +17,8 @@
 package com.ginsberg.gatherers4j;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 
 import java.math.BigDecimal;
 import java.util.List;
@@ -172,14 +174,13 @@ void treatNullAsOne() {
                 );
     }
 
-    @Test
-    void windowSizeMustBePositive() {
-        assertThatThrownBy(() ->
-                Stream.of(BigDecimal.ONE).gather(Gatherers4j.movingSum(0))
-        ).isExactlyInstanceOf(IllegalArgumentException.class);
 
+    @ParameterizedTest(name = "windowSize of {0}")
+    @ValueSource(ints = {-1, 0, 1})
+    void windowSizeMustBeGreaterThanOne(final int windowSize) {
         assertThatThrownBy(() ->
-                Stream.of(BigDecimal.ONE).gather(Gatherers4j.movingSum(-1))
+                Stream.of(BigDecimal.ONE).gather(Gatherers4j.movingSum(windowSize))
         ).isExactlyInstanceOf(IllegalArgumentException.class);
     }
+
 }

src/test/java/com/ginsberg/gatherers4j/BigDecimalSimpleMovingAverageGathererTest.java

@@ -17,6 +17,8 @@
 package com.ginsberg.gatherers4j;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 
 import java.math.BigDecimal;
 import java.math.MathContext;
@@ -187,14 +189,11 @@ void treatNullAsZero() {
                 );
     }
 
-    @Test
-    void windowSizeMustBePositive() {
-        assertThatThrownBy(() ->
-                Stream.of(BigDecimal.ONE).gather(Gatherers4j.simpleMovingAverage(0))
-        ).isExactlyInstanceOf(IllegalArgumentException.class);
-
+    @ParameterizedTest(name = "windowSize of {0}")
+    @ValueSource(ints = {-1, 0, 1})
+    void windowSizeMustBeGreaterThanOne(final int windowSize) {
         assertThatThrownBy(() ->
-                Stream.of(BigDecimal.ONE).gather(Gatherers4j.simpleMovingAverage(-1))
+                Stream.of(BigDecimal.ONE).gather(Gatherers4j.simpleMovingAverage(windowSize))
         ).isExactlyInstanceOf(IllegalArgumentException.class);
     }