问题
I coded 3 factorial algorithms:
- First, I expect to fail by Stack Overflow. No problem.
- Second, I try tail recusive call, convert previous algorithm from recursive to iterative. It doesn't work but I don't understand why.
- Third, I use
trampoline()
method and works fine as I expect.
def factorial
factorial = { BigInteger n ->
if (n == 1) return 1
n * factorial(n - 1)
}
factorial(1000) // Stack Overflow
factorial = { Integer n, BigInteger acc = 1 ->
if (n == 1) return acc
factorial(n - 1, n * acc)
}
factorial(1000) // Stack Overflow, why???
factorial = { Integer n, BigInteger acc = 1 ->
if (n == 1) return acc
factorial.trampoline(n - 1, n * acc)
}.trampoline()
factorial(1000) // It works
回答1:
Starting with version 2.3 Groovy supports tail recursion with the @TailRecursive annotation for methods: http://java.dzone.com/articles/groovy-goodness-more-efficient
回答2:
There is no tail recursion in Java, and hence there is none in Groovy either (without using something like trampoline() as you have shown)
The closest I have seen to this, is an AST transformation which cleverly wraps the return recursion into a while loop
Edit
You're right, Java (and hence Groovy) do support this sort of tail-call iteration, however, it doesn't seem to work with Closures...
This code however (using a method rather than a closure for the fact
call):
public class Test {
BigInteger fact( Integer a, BigInteger acc = 1 ) {
if( a == 1 ) return acc
fact( a - 1, a * acc )
}
static main( args ) {
def t = new Test()
println "${t.fact( 1000 )}"
}
}
When saved as Test.groovy
and executed with groovy Test.groovy
runs, and prints the answer:
402387260077093773543702433923003985719374864210714632543799910429938512398629020592044208486969404800479988610197196058631666872994808558901323829669944590997424504087073759918823627727188732519779505950995276120874975462497043601418278094646496291056393887437886487337119181045825783647849977012476632889835955735432513185323958463075557409114262417474349347553428646576611667797396668820291207379143853719588249808126867838374559731746136085379534524221586593201928090878297308431392844403281231558611036976801357304216168747609675871348312025478589320767169132448426236131412508780208000261683151027341827977704784635868170164365024153691398281264810213092761244896359928705114964975419909342221566832572080821333186116811553615836546984046708975602900950537616475847728421889679646244945160765353408198901385442487984959953319101723355556602139450399736280750137837615307127761926849034352625200015888535147331611702103968175921510907788019393178114194545257223865541461062892187960223838971476088506276862967146674697562911234082439208160153780889893964518263243671616762179168909779911903754031274622289988005195444414282012187361745992642956581746628302955570299024324153181617210465832036786906117260158783520751516284225540265170483304226143974286933061690897968482590125458327168226458066526769958652682272807075781391858178889652208164348344825993266043367660176999612831860788386150279465955131156552036093988180612138558600301435694527224206344631797460594682573103790084024432438465657245014402821885252470935190620929023136493273497565513958720559654228749774011413346962715422845862377387538230483865688976461927383814900140767310446640259899490222221765904339901886018566526485061799702356193897017860040811889729918311021171229845901641921068884387121855646124960798722908519296819372388642614839657382291123125024186649353143970137428531926649875337218940694281434118520158014123344828015051399694290153483077644569099073152433278288269864602789864321139083506217095002597389863554277196742822248757586765752344220207573630569498825087968928162753848863396909959826280956121450994871701244516461260379029309120889086942028510640182154399457156805941872748998094254742173582401063677404595741785160829230135358081840096996372524230560855903700624271243416909004153690105933983835777939410970027753472000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
As a guess, I would say that the JVM does not know how to optimise closures (like it does with methods), so this tail call does not get optimised out in the bytecode before it is executed
来源:https://stackoverflow.com/questions/7374887/tail-recursion-with-groovy