Closure Trampoline Patch

closure-trampoline.diff

Index: groovy-core/src/main/groovy/lang/Closure.java
===================================================================
--- groovy-core/src/main/groovy/lang/Closure.java	(revision 21027)
+++ groovy-core/src/main/groovy/lang/Closure.java	(revision )
@@ -610,6 +610,39 @@
         return Memoize.buildSoftReferenceMemoizeFunction(protectedCacheSize, new LRUCache(maxCacheSize), this);
     }
 
+    /**
+     * Executes the current closure on a functional trampoline.
+     * To prevent stack overflow due to deep recursion, functions can instead leverage the trampoline mechanism
+     * and avoid recursive calls altogether. Under trampoline, the function is supposed to perform one step of
+     * the calculation and, instead of a recursive call to itself or another function, it return back a new closure,
+     * which will be executed by the trampoline as the next step.
+     * Once a non-closure value is returned, the trampoline stops and returns the value as the final result.
+     * @param args Parameters to the closure, so as the trampoline mechanism can call it
+     * @return The final result returned by the last step of the trampolined calculation
+     */
+    public Object trampoline(final Object... args) {
+        return this.curry(args).trampoline();
+    }
+
+    /**
+     * Executes the current closure on a functional trampoline.
+     * To prevent stack overflow due to deep recursion, functions can instead leverage the trampoline mechanism
+     * and avoid recursive calls altogether. Under trampoline, the function is supposed to perform one step of
+     * the calculation and, instead of a recursive call to itself or another function, it return back a new closure,
+     * which will be executed by the trampoline as the next step.
+     * Once a non-closure value is returned, the trampoline stops and returns the value as the final result.
+     * @return The final result returned by the last step of the trampolined calculation
+     */
+    public Object trampoline() {
+        Closure currentFunction = this;
+        for(;;) {
+            final Object result = currentFunction.call();
+            if (result instanceof Closure) {
+                currentFunction = (Closure) result;
+            } else return result;
+        }
+    }
+    
     /* (non-Javadoc)
      * @see java.lang.Object#clone()
      */
Index: groovy-core/src/test/org/codehaus/groovy/runtime/trampoline/TrampolineTest.groovy
===================================================================
--- groovy-core/src/test/org/codehaus/groovy/runtime/trampoline/TrampolineTest.groovy	(revision )
+++ groovy-core/src/test/org/codehaus/groovy/runtime/trampoline/TrampolineTest.groovy	(revision )
@@ -0,0 +1,27 @@
+package org.codehaus.groovy.runtime.trampoline
+
+/**
+ *
+ * @author Vaclav Pech
+ */
+class TrampolineTest extends GroovyTestCase {
+    public void testFactorial() {
+        def fact
+        fact = {int n, BigInteger accumulator ->
+            n > 1 ? fact.curry(n - 1, n * accumulator) : accumulator
+        }
+        def factorial = {int n -> fact(n, 1)}
+        assert 1 == factorial.trampoline(1)
+        assert 2 == factorial.trampoline(2)
+        assert 6 == factorial.trampoline(3)
+        assert 402387260077093773543702433923003985719374864210714632543799910429938512398629020592044208486969404800479988610197196058631666872994808558901323829669944590997424504087073759918823627727188732519779505950995276120874975462497043601418278094646496291056393887437886487337119181045825783647849977012476632889835955735432513185323958463075557409114262417474349347553428646576611667797396668820291207379143853719588249808126867838374559731746136085379534524221586593201928090878297308431392844403281231558611036976801357304216168747609675871348312025478589320767169132448426236131412508780208000261683151027341827977704784635868170164365024153691398281264810213092761244896359928705114964975419909342221566832572080821333186116811553615836546984046708975602900950537616475847728421889679646244945160765353408198901385442487984959953319101723355556602139450399736280750137837615307127761926849034352625200015888535147331611702103968175921510907788019393178114194545257223865541461062892187960223838971476088506276862967146674697562911234082439208160153780889893964518263243671616762179168909779911903754031274622289988005195444414282012187361745992642956581746628302955570299024324153181617210465832036786906117260158783520751516284225540265170483304226143974286933061690897968482590125458327168226458066526769958652682272807075781391858178889652208164348344825993266043367660176999612831860788386150279465955131156552036093988180612138558600301435694527224206344631797460594682573103790084024432438465657245014402821885252470935190620929023136493273497565513958720559654228749774011413346962715422845862377387538230483865688976461927383814900140767310446640259899490222221765904339901886018566526485061799702356193897017860040811889729918311021171229845901641921068884387121855646124960798722908519296819372388642614839657382291123125024186649353143970137428531926649875337218940694281434118520158014123344828015051399694290153483077644569099073152433278288269864602789864321139083506217095002597389863554277196742822248757586765752344220207573630569498825087968928162753848863396909959826280956121450994871701244516461260379029309120889086942028510640182154399457156805941872748998094254742173582401063677404595741785160829230135358081840096996372524230560855903700624271243416909004153690105933983835777939410970027753472000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 == factorial.trampoline(1000)
+    }
+
+    public void testMutualRecursion() {
+        def funB
+        def funA = {int num -> num == 0 ? 0 : funB.curry(num - 1)}
+        funB = {int num -> num == 0 ? 0 : funA.curry(num - 1)}
+        assert 0 == funA.trampoline(1000)
+        assert 0 == funA.trampoline(2000)
+    }
+}