问题
During recent discussions at work, someone referred to a trampoline function.
I have read the description at Wikipedia. It is enough to give a general idea of the functionality, but I would like something a bit more concrete.
Do you have a simple snippet of code that would illustrate a trampoline?
回答1:
There is also the LISP sense of 'trampoline' as described on Wikipedia:
Used in some LISP implementations, a trampoline is a loop that iteratively invokes thunk-returning functions. A single trampoline is sufficient to express all control transfers of a program; a program so expressed is trampolined or in "trampolined style"; converting a program to trampolined style is trampolining. Trampolined functions can be used to implement tail recursive function calls in stack-oriented languages
Let us say we are using Javascript and want to write the naive Fibonacci function in continuation-passing-style. The reason we would do this is not relevant - to port Scheme to JS for instance, or to play with CPS which we have to use anyway to call server-side functions.
So, the first attempt is
function fibcps(n, c) {
if (n <= 1) {
c(n);
} else {
fibcps(n - 1, function (x) {
fibcps(n - 2, function (y) {
c(x + y)
})
});
}
}
But, running this with n = 25 in Firefox gives an error 'Too much recursion!'. Now this is exactly the problem (missing tail-call optimization in Javascript) that trampolining solves. Instead of making a (recursive) call to a function, let us return an instruction (thunk) to call that function, to be interpreted in a loop.
function fibt(n, c) {
function trampoline(x) {
while (x && x.func) {
x = x.func.apply(null, x.args);
}
}
function fibtramp(n, c) {
if (n <= 1) {
return {func: c, args: [n]};
} else {
return {
func: fibtramp,
args: [n - 1,
function (x) {
return {
func: fibtramp,
args: [n - 2, function (y) {
return {func: c, args: [x + y]}
}]
}
}
]
}
}
}
trampoline({func: fibtramp, args: [n, c]});
}
回答2:
Let me add few examples for factorial function implemented with trampolines, in different languages:
Scala:
sealed trait Bounce[A]
case class Done[A](result: A) extends Bounce[A]
case class Call[A](thunk: () => Bounce[A]) extends Bounce[A]
def trampoline[A](bounce: Bounce[A]): A = bounce match {
case Call(thunk) => trampoline(thunk())
case Done(x) => x
}
def factorial(n: Int, product: BigInt): Bounce[BigInt] = {
if (n <= 2) Done(product)
else Call(() => factorial(n - 1, n * product))
}
object Factorial extends Application {
println(trampoline(factorial(100000, 1)))
}
Java:
import java.math.BigInteger;
class Trampoline<T>
{
public T get() { return null; }
public Trampoline<T> run() { return null; }
T execute() {
Trampoline<T> trampoline = this;
while (trampoline.get() == null) {
trampoline = trampoline.run();
}
return trampoline.get();
}
}
public class Factorial
{
public static Trampoline<BigInteger> factorial(final int n, final BigInteger product)
{
if(n <= 1) {
return new Trampoline<BigInteger>() { public BigInteger get() { return product; } };
}
else {
return new Trampoline<BigInteger>() {
public Trampoline<BigInteger> run() {
return factorial(n - 1, product.multiply(BigInteger.valueOf(n)));
}
};
}
}
public static void main( String [ ] args )
{
System.out.println(factorial(100000, BigInteger.ONE).execute());
}
}
C (unlucky without big numbers implementation):
#include <stdio.h>
typedef struct _trampoline_data {
void(*callback)(struct _trampoline_data*);
void* parameters;
} trampoline_data;
void trampoline(trampoline_data* data) {
while(data->callback != NULL)
data->callback(data);
}
//-----------------------------------------
typedef struct _factorialParameters {
int n;
int product;
} factorialParameters;
void factorial(trampoline_data* data) {
factorialParameters* parameters = (factorialParameters*) data->parameters;
if (parameters->n <= 1) {
data->callback = NULL;
}
else {
parameters->product *= parameters->n;
parameters->n--;
}
}
int main() {
factorialParameters params = {5, 1};
trampoline_data t = {&factorial, ¶ms};
trampoline(&t);
printf("\n%d\n", params.product);
return 0;
}
回答3:
I'll give you an example that I used in an anti-cheat patch for an online game.
I needed to be able to scan all files that were being loaded by the game for modification. So the most robust way I found to do this was to use a trampoline for CreateFileA. So when the game was launched I would find the address for CreateFileA using GetProcAddress, then I would modify the first few bytes of the function and insert assembly code that would jump to my own "trampoline" function, where I would do some things, and then I would jump back to the next location in CreateFile after my jmp code. To be able to do it reliably is a little trickier than that, but the basic concept is just to hook one function, force it to redirect to another function, and then jump back to the original function.
Edit: Microsoft has a framework for this type of thing that you can look at. Called Detours
回答4:
Here's an example of nested functions:
#include <stdlib.h>
#include <string.h>
/* sort an array, starting at address `base`,
* containing `nmemb` members, separated by `size`,
* comparing on the first `nbytes` only. */
void sort_bytes(void *base, size_t nmemb, size_t size, size_t nbytes) {
int compar(const void *a, const void *b) {
return memcmp(a, b, nbytes);
}
qsort(base, nmemb, size, compar);
}
compar can't be an external function, because it uses nbytes, which only exists during the sort_bytes call. On some architectures, a small stub function -- the trampoline -- is generated at runtime, and contains the stack location of the current invocation of sort_bytes. When called, it jumps to the compar code, passing that address.
This mess isn't required on architectures like PowerPC, where the ABI specifies that a function pointer is actually a "fat pointer", a structure containing both a pointer to the executable code and another pointer to data. However, on x86, a function pointer is just a pointer.
回答5:
I am currently experimenting with ways to implement tail call optimization for a Scheme interpreter, and so at the moment I am trying to figure out whether the trampoline would be feasible for me.
As I understand it, it is basically just a series of function calls performed by a trampoline function. Each function is called a thunk and returns the next step in the computation until the program terminates (empty continuation).
Here is the first piece of code that I wrote to improve my understanding of the trampoline:
#include <stdio.h>
typedef void *(*CONTINUATION)(int);
void trampoline(CONTINUATION cont)
{
int counter = 0;
CONTINUATION currentCont = cont;
while (currentCont != NULL) {
currentCont = (CONTINUATION) currentCont(counter);
counter++;
}
printf("got off the trampoline - happy happy joy joy !\n");
}
void *thunk3(int param)
{
printf("*boing* last thunk\n");
return NULL;
}
void *thunk2(int param)
{
printf("*boing* thunk 2\n");
return thunk3;
}
void *thunk1(int param)
{
printf("*boing* thunk 1\n");
return thunk2;
}
int main(int argc, char **argv)
{
trampoline(thunk1);
}
results in:
meincompi $ ./trampoline
*boing* thunk 1
*boing* thunk 2
*boing* last thunk
got off the trampoline - happy happy joy joy !
回答6:
For C, a trampoline would be a function pointer:
size_t (*trampoline_example)(const char *, const char *);
trampoline_example= strcspn;
size_t result_1= trampoline_example("xyzbxz", "abc");
trampoline_example= strspn;
size_t result_2= trampoline_example("xyzbxz", "abc");
Edit: More esoteric trampolines would be implicitly generated by the compiler. One such use would be a jump table. (Although there are clearly more complicated ones the farther down you start attempting to generate complicated code.)
回答7:
Now that C# has Local Functions, the Bowling Game coding kata can be elegantly solved with a trampoline:
using System.Collections.Generic;
using System.Linq;
class Game
{
internal static int RollMany(params int[] rs)
{
return Trampoline(1, 0, rs.ToList());
int Trampoline(int frame, int rsf, IEnumerable<int> rs) =>
frame == 11 ? rsf
: rs.Count() == 0 ? rsf
: rs.First() == 10 ? Trampoline(frame + 1, rsf + rs.Take(3).Sum(), rs.Skip(1))
: rs.Take(2).Sum() == 10 ? Trampoline(frame + 1, rsf + rs.Take(3).Sum(), rs.Skip(2))
: Trampoline(frame + 1, rsf + rs.Take(2).Sum(), rs.Skip(2));
}
}
The method Game.RollMany is called with a number of rolls: typically 20 rolls if there are no spares or strikes.
The first line immediately calls the trampoline function: return Trampoline(1, 0, rs.ToList());. This local function recursively traverses the rolls array. The local function (the trampoline) allows the traversal to start with two additional values: start with frame 1 and the rsf (result so far) 0.
Within the local function there is ternary operator that handles five cases:
- Game ends at frame 11: return the result so far
- Game ends if there are no more rolls: return the result so far
- Strike: calculate the frame score and continue traversal
- Spare: calculate the frame score and continue traversal
- Normal score: calculate the frame score and continue traversal
Continuing the traversal is done by calling the trampoline again, but now with updated values.
For more information, search for: "tail recursion accumulator". Keep in mind that the compiler does not optimize tail recursion. So as elegant as this solution may be, it will likely not be the fasted.
回答8:
typedef void* (*state_type)(void);
void* state1();
void* state2();
void* state1() {
return state2;
}
void* state2() {
return state1;
}
// ...
state_type state = state1;
while (1) {
state = state();
}
// ...
来源：https://stackoverflow.com/questions/189725/what-is-a-trampoline-function