Re: bug in table.sort (lua 5.1.3)

[Daniel Stephens <daniel@...>]

Actually, the problem is that your sort function is invalid, it doesn't 
follow the correct contract:

"Sorts table elements in a given order, /in-place/, from |table[1]| to 
|table[n]|, where |n| is the length of the table. If |comp| is given, 
then it must be a function that receives two table elements, and returns 
true when the first is less than the second (so that |not 
comp(a[i+1],a[i])| will be true after the sort). If |comp| is not given, 
then the standard Lua operator |<| is used instead."

func(a,b) implies not func(b, a);

With your function, for any pair of entries X and Y with the same value, 
you're saying X should come before Y AND that Y should come before X.

Change == to < in your function, and your problem will go away.

Daniel.

Tommy Pettersson wrote:
> Hi,
>
> I'm not a list subscriber. Well, I am now, but I don't think
> I'll have time to read it.
>
> I just wanted to submit a bug fix. It is very minor, and only
> concerns the error reporting from Lua. But it sent me on a long
> bug hunt that could have been much shorter.
>
> Here's how to reproduce the bug:
>
>   local t = { 1 }
>   table.sort( { t, t, t, t }, function (a,b) return a[1] == b[1] end )
>
> gives:
>
>   lua: stdin:2: attempt to index local 'a' (a nil value)
>
> but should give:
>
>   lua: stdin:2: invalid order function for sorting
>
>
> It's auxsort that calls lua_rawgeti with an "out of bound" index
> and thus passes a nil value to the comparison function.
>
> The fix:
>
> diff -rN -u old-lua-5.1.3/src/ltablib.c new-lua-5.1.3/src/ltablib.c
> --- old-lua-5.1.3/src/ltablib.c	2008-04-04 00:51:38.375673575 +0200
> +++ new-lua-5.1.3/src/ltablib.c	2008-04-04 00:51:38.407673452 +0200
> @@ -216,12 +216,12 @@
>      for (;;) {  /* invariant: a[l..i] <= P <= a[j..u] */
>        /* repeat ++i until a[i] >= P */
>        while (lua_rawgeti(L, 1, ++i), sort_comp(L, -1, -2)) {
> -        if (i>u) luaL_error(L, "invalid order function for sorting");
> +        if (i>=u) luaL_error(L, "invalid order function for sorting");
>          lua_pop(L, 1);  /* remove a[i] */
>        }
>        /* repeat --j until a[j] <= P */
>        while (lua_rawgeti(L, 1, --j), sort_comp(L, -3, -1)) {
> -        if (j<l) luaL_error(L, "invalid order function for sorting");
> +        if (j<=l) luaL_error(L, "invalid order function for sorting");
>          lua_pop(L, 1);  /* remove a[j] */
>        }
>        if (j<i) {
>
>
>