From 254276b2af0a46124dfc272a5e3904abbff9a9d1 Mon Sep 17 00:00:00 2001
From: Arthur Drobot <drobotarthur@gmail.com>
Date: Tue, 25 Jul 2017 11:09:23 -0700
Subject: [PATCH 1/2] In Number interface: remove one and zero (they can't be
 static), add compareTo and signum. Modify NaiveNumber accordingly.

---
 src/org/nwapw/abacus/number/NaiveNumber.java | 13 +++++++++----
 src/org/nwapw/abacus/number/Number.java      |  5 ++---
 2 files changed, 11 insertions(+), 7 deletions(-)
 mode change 100644 => 100755 src/org/nwapw/abacus/number/NaiveNumber.java

diff --git a/src/org/nwapw/abacus/number/NaiveNumber.java b/src/org/nwapw/abacus/number/NaiveNumber.java
old mode 100644
new mode 100755
index 484f773..51fda91
--- a/src/org/nwapw/abacus/number/NaiveNumber.java
+++ b/src/org/nwapw/abacus/number/NaiveNumber.java
@@ -8,6 +8,9 @@ public class NaiveNumber implements Number {
         this.value = value;
     }
 
+    static final NaiveNumber ZERO = new NaiveNumber(0);
+    static final NaiveNumber ONE = new NaiveNumber(1);
+
     @Override
     public int precision() {
         return 4;
@@ -39,13 +42,14 @@ public class NaiveNumber implements Number {
     }
 
     @Override
-    public Number zero() {
-        return new NaiveNumber(0);
+    public int compareTo(Number number) {
+        NaiveNumber num = (NaiveNumber) number;
+        return Double.compare(value, num.value);
     }
 
     @Override
-    public Number one() {
-        return new NaiveNumber(1);
+    public int signum() {
+        return this.compareTo(ZERO);
     }
 
     @Override
@@ -53,5 +57,6 @@ public class NaiveNumber implements Number {
         if(toClass == this.getClass()) return this;
         return null;
     }
+
 }
 
diff --git a/src/org/nwapw/abacus/number/Number.java b/src/org/nwapw/abacus/number/Number.java
index 4541287..ce42ff6 100755
--- a/src/org/nwapw/abacus/number/Number.java
+++ b/src/org/nwapw/abacus/number/Number.java
@@ -8,9 +8,8 @@ public interface Number {
     Number add(Number summand);
     Number subtract(Number subtrahend);
     Number negate();
-
-    Number zero();
-    Number one();
+    int compareTo(Number number);
+    int signum();
 
     Number promoteTo(Class<? extends Number> toClass);
 

From 67f8c648dbc050cb1cdbeeede3cd1dbde5f90389 Mon Sep 17 00:00:00 2001
From: Arthur Drobot <drobotarthur@gmail.com>
Date: Tue, 25 Jul 2017 11:12:25 -0700
Subject: [PATCH 2/2] Add factorial (external).

---
 .../nwapw/abacus/number/FunctionDatabase.java | 20 ++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/src/org/nwapw/abacus/number/FunctionDatabase.java b/src/org/nwapw/abacus/number/FunctionDatabase.java
index 891c009..909c9b7 100755
--- a/src/org/nwapw/abacus/number/FunctionDatabase.java
+++ b/src/org/nwapw/abacus/number/FunctionDatabase.java
@@ -7,7 +7,6 @@ public class FunctionDatabase {
     private HashMap<String, Function> functions;
 
     private void registerDefault(){
-
         functions.put("+", new Function() {
             @Override
             protected boolean matchesParams(Number[] params) {
@@ -63,6 +62,25 @@ public class FunctionDatabase {
                 return params[0].divide(params[1]);
             }
         });
+
+        functions.put("!", new Function() {
+            @Override
+            protected boolean matchesParams(Number[] params) {
+                return params.length == 1;
+            }
+
+            @Override
+            protected Number applyInternal(Number[] params) {
+                Number factorial = params[0];
+                Number multiplier = params[0];
+                //It is necessary to later prevent calls of factorial on anything but non-negative integers.
+                while(multiplier.signum() == 1){
+                    multiplier = multiplier.subtract(NaiveNumber.ONE.promoteTo(multiplier.getClass()));
+                    factorial = factorial.multiply(multiplier);
+                }
+                return factorial;
+            }
+        });
     }
 
     public FunctionDatabase(){