Porting duff's device from C to JavaScript

Question

I have this kind of Duff's device in C and it works fine (format text as money):

#include <stdio.h>
#include <string.h>

char *money(const char *src, char *dst)
{
    const char *p = src;
    char *q = dst;
    size_t len;

    len = strlen(src);
    switch (len % 3) {
        do {
            *q++ = ',';
            case 0: *q++ = *p++;
            case 2: *q++ = *p++;
            case 1: *q++ = *p++;
        } while (*p);
    }
    *q++ = 0;
    return dst;
}

int main(void)
{
    char str[] = "1234567890123";
    char res[32];

    printf("%s\n", money(str, res));
    return 0;
}

Output:

1,234,567,890,123

But I have problems trying to implement the same in Javascript:

function money(src, dst) {
    var len = src.length;
    var i = 0;

    switch (len % 3) {
        do {
            dst += ',';
            case 0: dst += src[i++];
            case 2: dst += src[i++];
            case 1: dst += src[i++];
        } while (src[i]);
    }
    return dst;
}

var str = "1234567890123";
var res = "";

console.log(money(str, res));

nodejs returns this error:

        do {
        ^^
SyntaxError: Unexpected token do

My question is: Does javascript supports computed GOTO statements?

P.D: I don't want an alternative, I just want to know why is not working.

Related question: Does Duff's Device work in other languages?

score 7 · Accepted Answer · answered May 03 '16 at 23:17

My question is: Does javascript supports computed GOTO statements?

Not really.

P.D: I don't want an alternative, I just want to know why is not working.

It's not working because in JavaScript, switch statements must only contain case blocks.

And although you might not be looking for a workaround, someone else might find this question in search of one, so I'll provide one regardless.
Credit for this one goes to mnieper who suggested it at asm.js.

The basic idea is to have a while(true) loop at the top level, and a switch statement inside it.

goto:
while(true)
{
    switch(where)
    {
        // no breaks
        case 0:
            // some code...
        case 1:
            // some other code...
        // ...
    }
}

In order to simulate a goto, one would use

where = 1;
continue goto;

In order to simulate duff's device though, one would only have to make the loop the outer structure, and use a variable in the switch statement that after the first iteration is set to a value that will trigger the switch statement to start at its very first case.

So in your case, that would mean exchanging switch and do...while() and adding a default case and a control variable:

var where = len % 3;
do {
    switch (where) {
        default: dst += ',';
        case 0:  dst += src[i++];
        case 2:  dst += src[i++];
        case 1:  dst += src[i++];
    }
    where = -1;
} while (src[i]);

The one huge drawback of this method in general, is of course that it will not work across callbacks, which are used just about everywhere in JavaScript.
As long as it is used in a single, sequential context though, it should work.

For more information, see the ticket posted on the asm.js repo.

Excuse me but I don't understand: `it will not work across callbacks`, what does it means? — David Ranieri, May 03 '16 at 23:33
@AlterMann Basically it won't work across functions. In JavaScript it's common to pass function objects around as a means of reacting to future values or events, e.g. waiting for 100ms: `window.setTimeout(function(){ /* */ }, 100);`. If such a function is *declared* inside a loop, it might give the impression to *run* inside the loop (and thus be able to use `break`, `continue`, etc), but that is not the case if executed asynchronously. Just google "javascript return not working" and you'll find boat loads of newbie questions asking about exactly that problem, just with return values instead. — Siguza, May 04 '16 at 21:39

Mulan · Answer 2 · 2016-05-03T22:57:00.183

JavaScript switch statements don't work like that.

switch (expr) {
  case expr:
    statements;
    break;
  case expr:
    statements;
    break;
  default:
    statements;
    break;
}

But JavaScript does offer labels to control your loops

Example from MDN

var itemsPassed = 0;
var i, j;

top:
for (i = 0; i < items.length; i++){
  for (j = 0; j < tests.length; j++) {
    if (!tests[j].pass(items[i])) {
      continue top;
    }
  }

  itemsPassed++;
}

Another example from MDN

var allPass = true;
var i, j;

top:
for (i = 0; items.length; i++)
  for (j = 0; j < tests.length; i++)
    if (!tests[j].pass(items[i])){
      allPass = false;
      break top;
    }

Porting duff's device from C to JavaScript

2 Answers2

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