From 296240b52cc91632027faddeb76fe533d8a1d9ed Mon Sep 17 00:00:00 2001
From: Carsten Otto <git@c-otto.de>
Date: Tue, 3 May 2022 18:35:16 +0200
Subject: [PATCH] reuse Coins.NONE object instance

---
 .../java/de/cotto/lndmanagej/model/Coins.java | 16 +++++++-
 .../de/cotto/lndmanagej/model/CoinsTest.java  | 39 +++++++++++++++++++
 2 files changed, 53 insertions(+), 2 deletions(-)

diff --git a/model/src/main/java/de/cotto/lndmanagej/model/Coins.java b/model/src/main/java/de/cotto/lndmanagej/model/Coins.java
index fae32442..d0a821a9 100644
--- a/model/src/main/java/de/cotto/lndmanagej/model/Coins.java
+++ b/model/src/main/java/de/cotto/lndmanagej/model/Coins.java
@@ -6,13 +6,16 @@ import java.util.Locale;
 
 public record Coins(long milliSatoshis) implements Comparable<Coins> {
     private static final int SCALE = 3;
-    public static final Coins NONE = Coins.ofSatoshis(0);
+    public static final Coins NONE = new Coins(0);
 
     public static Coins ofSatoshis(long satoshis) {
-        return new Coins(satoshis * 1_000);
+        return ofMilliSatoshis(satoshis * 1_000);
     }
 
     public static Coins ofMilliSatoshis(long milliSatoshis) {
+        if (milliSatoshis == 0) {
+            return NONE;
+        }
         return new Coins(milliSatoshis);
     }
 
@@ -24,10 +27,19 @@ public record Coins(long milliSatoshis) implements Comparable<Coins> {
     }
 
     public Coins add(Coins summand) {
+        if (summand.milliSatoshis == 0) {
+            return this;
+        }
+        if (this.milliSatoshis == 0) {
+            return summand;
+        }
         return Coins.ofMilliSatoshis(milliSatoshis + summand.milliSatoshis);
     }
 
     public Coins subtract(Coins subtrahend) {
+        if (subtrahend.milliSatoshis == 0) {
+            return this;
+        }
         return Coins.ofMilliSatoshis(milliSatoshis - subtrahend.milliSatoshis);
     }
 
diff --git a/model/src/test/java/de/cotto/lndmanagej/model/CoinsTest.java b/model/src/test/java/de/cotto/lndmanagej/model/CoinsTest.java
index 90aa77ca..bdd567e4 100644
--- a/model/src/test/java/de/cotto/lndmanagej/model/CoinsTest.java
+++ b/model/src/test/java/de/cotto/lndmanagej/model/CoinsTest.java
@@ -8,6 +8,7 @@ import java.util.Locale;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 
+@SuppressWarnings("PMD.GodClass")
 class CoinsTest {
 
     private static final Coins ONE_COIN = Coins.ofSatoshis(100_000_000);
@@ -22,6 +23,44 @@ class CoinsTest {
         assertThat(Coins.ofMilliSatoshis(400).add(Coins.ofMilliSatoshis(600))).isEqualTo(Coins.ofSatoshis(1));
     }
 
+    @Test
+    void zero_sum_is_none_object() {
+        assertThat(Coins.ofMilliSatoshis(400).add(Coins.ofMilliSatoshis(-400))).isSameAs(Coins.NONE);
+    }
+
+    @Test
+    void difference_resulting_in_zero_is_none_object() {
+        assertThat(Coins.ofMilliSatoshis(400).subtract(Coins.ofMilliSatoshis(400))).isSameAs(Coins.NONE);
+    }
+
+    @Test
+    void zero_milli_satoshis_is_non_object() {
+        assertThat(Coins.ofMilliSatoshis(0)).isSameAs(Coins.NONE);
+    }
+
+    @Test
+    void zero_satoshis_is_non_object() {
+        assertThat(Coins.ofSatoshis(0)).isSameAs(Coins.NONE);
+    }
+
+    @Test
+    void add_zero_gives_same_instance() {
+        Coins original = Coins.ofSatoshis(1);
+        assertThat(original.add(Coins.NONE)).isSameAs(original);
+    }
+
+    @Test
+    void adding_to_zero_gives_same_instance() {
+        Coins original = Coins.ofSatoshis(1);
+        assertThat(Coins.NONE.add(original)).isSameAs(original);
+    }
+
+    @Test
+    void subtract_zero_gives_same_instance() {
+        Coins original = Coins.ofSatoshis(1);
+        assertThat(original.subtract(Coins.NONE)).isSameAs(original);
+    }
+
     @Test
     void subtract() {
         assertThat(Coins.ofSatoshis(456).subtract(Coins.ofSatoshis(123))).isEqualTo(Coins.ofSatoshis(456 - 123));