Add a simple GENSYM function

src/function/builtin/GENSYM.java

@@ -0,0 +1,36 @@
+package function.builtin;
+
+import java.math.BigInteger;
+
+import function.*;
+import sexpression.*;
+
+@FunctionNames({ "GENSYM" })
+public class GENSYM extends LispFunction {
+
+    public static final String GENSYM_PREFIX = "#G";
+    private static BigInteger counter = BigInteger.ZERO;
+
+    private static Symbol generateSymbol() {
+        incrementCounter();
+        return new Symbol(GENSYM_PREFIX + counter);
+    }
+
+    private static void incrementCounter() {
+        counter = counter.add(BigInteger.ONE);
+    }
+
+    private ArgumentValidator argumentValidator;
+
+    public GENSYM(String name) {
+        this.argumentValidator = new ArgumentValidator(name);
+        this.argumentValidator.setMaximumNumberOfArguments(0);
+    }
+
+    public SExpression call(Cons argumentList) {
+        argumentValidator.validate(argumentList);
+
+        return generateSymbol();
+    }
+
+}

src/table/FunctionTable.java

@@ -32,6 +32,7 @@ public class FunctionTable {
         allBuiltIns.add(EXIT.class);
         allBuiltIns.add(FIRST.class);
         allBuiltIns.add(FUNCALL.class);
+        allBuiltIns.add(GENSYM.class);
         allBuiltIns.add(NUMERIC_GREATER.class);
         allBuiltIns.add(IF.class);
         allBuiltIns.add(LAMBDA.class);

test/function/builtin/GENSYMTester.java

@@ -0,0 +1,41 @@
+package function.builtin;
+
+import static function.builtin.GENSYM.GENSYM_PREFIX;
+import static testutil.TestUtilities.*;
+import static testutil.TypeAssertions.assertSymbol;
+
+import org.junit.Test;
+
+import function.ArgumentValidator.TooManyArgumentsException;
+import token.TokenFactory.BadCharacterException;
+
+public class GENSYMTester {
+
+    @Test
+    public void gensymCreatesSymbol() {
+        assertSymbol(evaluateString("(gensym)"));
+    }
+
+    @Test
+    public void gensymCreatesUniqueSymbol() {
+        assertSExpressionsDoNotMatch(evaluateString("(gensym)"), evaluateString("(gensym)"));
+    }
+
+    @Test
+    public void simpleGensymUsage() {
+        String input = "(let ((x (gensym))) (set x 23) (eval x))";
+        assertSExpressionsMatch(evaluateString("23"), evaluateString(input));
+    }
+
+    @Test(expected = BadCharacterException.class)
+    public void cannotUseGensymValueManually() {
+        String variableName = GENSYM_PREFIX + 1;
+        evaluateString("(setq " + variableName + " 100)");
+    }
+
+    @Test(expected = TooManyArgumentsException.class)
+    public void gensymWithTooManyArguments() {
+        evaluateString("(gensym 1)");
+    }
+
+}

test/testutil/TestUtilities.java

@@ -1,7 +1,7 @@
 package testutil;
 
 import static function.builtin.EVAL.eval;
-import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.*;
 
 import java.io.*;
 
@@ -37,4 +37,8 @@ public final class TestUtilities {
         assertEquals(one.toString(), two.toString());
     }
 
+    public static void assertSExpressionsDoNotMatch(SExpression one, SExpression two) {
+        assertNotEquals(one.toString(), two.toString());
+    }
+
 }