few useful function added to Ord class as well

l1cache · jbgi · commit dc0f405dc9ab · 2018-03-20T14:05:09.000+01:00
diff --git a/core/src/main/java/fj/Ord.java b/core/src/main/java/fj/Ord.java
@@ -287,6 +287,39 @@ public final Ord<A> reverse() {
     return ordDef(def.dual());
   }
 
+  /**
+   * Constructs an ord instance, which compares using self and if objects are equal compares using <code>ord</code>
+   *
+   * @param ord Ord for subsequent comparison
+   * @return A new equal instance
+   */
+  public final Ord<A> andThen(final Ord<A> ord) {
+    return ordDef((a1, a2) -> {
+      final Ordering compareResult = compare(a1, a2);
+      if(compareResult == Ordering.EQ)
+        return ord.compare(a1, a2);
+      return compareResult;
+    });
+  }
+
+  /**
+   * Constructs an ord instance, which compares using self and if objects are equal compares the mapped objects
+   *
+   * @param f The function to map the original object
+   * @param ord Ord for the mapped object
+   * @return A new equal instance
+   */
+  public final <B> Ord<A> andThen(final F<A, B> f, final Ord<B> ord) {
+    return andThen(ord.contramap(f));
+  }
+
+  /**
+   * Static version of {@link #contramap(F)}
+   */
+  public static <A, B> Ord<A> contramap(final F<A, B> f, final Ord<B> ord) {
+    return ord.contramap(f);
+  }
+
   /**
    * Returns an order instance that uses the given equality test and ordering function.
    *
diff --git a/core/src/test/java/fj/OrdTest.java b/core/src/test/java/fj/OrdTest.java
@@ -24,4 +24,22 @@ public void isLessThan() {
     assertThat(pred.f(1L), is(false));
     assertThat(pred.f(2L), is(false));
   }
+
+  @Test
+  public void contramapShouldWork() {
+    Ord<String> lengthOrd = Ord.contramap(String::length, Ord.intOrd);
+
+    assertThat(lengthOrd.compare("str", "rts"), is(Ordering.EQ));
+    assertThat(lengthOrd.compare("strlong", "str"), is(Ordering.GT));
+  }
+
+  @Test
+  public void andThenShouldWork() {
+    Ord<String> lengthThenLastDigitOrd = Ord.contramap(String::length, Ord.intOrd)
+                                            .andThen(s -> s.charAt(s.length() - 1), Ord.charOrd);
+
+    assertThat(lengthThenLastDigitOrd.compare("str", "dyr"), is(Ordering.EQ));
+    assertThat(lengthThenLastDigitOrd.compare("stt", "str"), is(Ordering.GT));
+    assertThat(lengthThenLastDigitOrd.compare("str", "strr"), is(Ordering.LT));
+  }
 }
