From 4e042bd0eb0859175d3215fd4907176d57555b6c Mon Sep 17 00:00:00 2001
From: Danila Fedorin <danila.fedorin@gmail.com>
Date: Wed, 9 Aug 2017 12:34:02 -0700
Subject: [PATCH] Implement two combinatorics operators.

---
 .../nwapw/abacus/plugin/StandardPlugin.java   | 44 +++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/src/main/java/org/nwapw/abacus/plugin/StandardPlugin.java b/src/main/java/org/nwapw/abacus/plugin/StandardPlugin.java
index fdd3674..7d4fa3b 100755
--- a/src/main/java/org/nwapw/abacus/plugin/StandardPlugin.java
+++ b/src/main/java/org/nwapw/abacus/plugin/StandardPlugin.java
@@ -136,6 +136,47 @@ public class StandardPlugin extends Plugin {
                 }*/
         }
     });
+    /**
+     * The permutation operator.
+     */
+    public static final Operator OP_NPR = new Operator(OperatorAssociativity.RIGHT, OperatorType.BINARY_INFIX, 0, new Function() {
+        @Override
+        protected boolean matchesParams(NumberInterface[] params) {
+            return params.length == 2 && params[0].fractionalPart().signum() == 0
+                    && params[1].fractionalPart().signum() == 0;
+        }
+
+        @Override
+        protected NumberInterface applyInternal(NumberInterface[] params) {
+            if(params[0].compareTo(params[1]) < 0 ||
+                    params[0].signum() < 0 ||
+                    (params[0].signum() == 0 && params[1].signum() != 0)) return fromInt(params[0].getClass(), 0);
+            NumberInterface total = fromInt(params[0].getClass(), 1);
+            NumberInterface multiplyBy = params[0];
+            NumberInterface remainingMultiplications = params[1];
+            while(remainingMultiplications.signum() > 0){
+                total = total.multiply(multiplyBy);
+                remainingMultiplications = remainingMultiplications.subtract(fromInt(params[0].getClass(), 1));
+                multiplyBy = multiplyBy.subtract(fromInt(params[0].getClass(), 1));
+            }
+            return total;
+        }
+    });
+    /**
+     * The combination operator.
+     */
+    public static final Operator OP_NCR = new Operator(OperatorAssociativity.RIGHT, OperatorType.BINARY_INFIX, 0, new Function() {
+        @Override
+        protected boolean matchesParams(NumberInterface[] params) {
+            return params.length == 2 && params[0].fractionalPart().signum() == 0
+                    && params[1].fractionalPart().signum() == 0;
+        }
+
+        @Override
+        protected NumberInterface applyInternal(NumberInterface[] params) {
+            return OP_NPR.getFunction().apply(params).divide(OP_FACTORIAL.getFunction().apply(params[1]));
+        }
+    });
     /**
      * The absolute value function, abs(-3) = 3
      */
@@ -575,6 +616,9 @@ public class StandardPlugin extends Plugin {
         registerOperator("^", OP_CARET);
         registerOperator("!", OP_FACTORIAL);
 
+        registerOperator("npr", OP_NPR);
+        registerOperator("ncr", OP_NCR);
+
         registerFunction("abs", FUNCTION_ABS);
         registerFunction("exp", FUNCTION_EXP);
         registerFunction("ln", FUNCTION_LN);