Lua - Cocoa - String Concat - Simple Table to NSArray

Question

Hi all, Mac OS X 10.5 compatibility, Lua 5.0 compatibility required (hence cannot use current batch of LuaObjc bridges.)

My lua script produces an indexed table containing tens of thousands of strings.

Basic problem: how to concat those strings with a newline separator, to one string, quickly?

Fly in ointment: even using garbage-collection friendly concat code (provided at stackoverflow) the results take far too long for this purpose. (10 seconds vs 1 minute for a brute force solution.)

Proposed solution: offload the job to Cocoa, where it can be done in a fraction of a second, using NSArray's -componentsJoinedByString method.

New fly in ointment: how to get table data from Lua to Cocoa?

The script calls a registered C function, passing it the table. The C function tries to grab the table on the stack:

// Get an NSArray of strings from the first argument on the stack (a table).
NSArray *strings = nsArrayFromIndexedTable(luaState, index_1Based);

...

// Given a simple table consisting of numbers or strings, returns an NSArray.
// Nested subtables are not followed.

NSArray * nsArrayFromIndexedTable(lua_State *L, int argIdx)
{
    // (Allegedly) stops GC.
    lua_setgcthreshold(L, INT_MAX);

    // Arg must be a table.
    luaL_checktype(L, argIdx, LUA_TTABLE);

    // Get num elements in table, build an array with that many.
    int count = luaL_getn(L, 1);

    NSMutableArray *array = [NSMutableArray arrayWithCapacity: count];

    int i;
    for (i = 1; i <= count; i++) {

        lua_rawgeti(L, argIdx, i);
        int valueType = lua_type(L, -1);
        id value = 0x00;

        if (valueType is_eq LUA_TNUMBER) {
            value = [NSNumber numberWithDouble:lua_tonumber(L, -1)];
        } else if (valueType is_eq LUA_TSTRING) {
            value = [NSString stringWithUTF8String:lua_tostring(L,  -1)];
        }

        if (value) {
            [array addObject:value];
        }
    }

    // Resume GC
    lua_setgcthreshold(L, 0);    // INTERMITTENT EXC_BAD_ACCESS CRASH HERE!!!!

    return array;
}

Problem: calling this function with a (very large) Lua table of strings (intermittently) results in a EXC_BAD_ACCESS.

Debugger results are sporadic; sometimes not providing anything useful, but I've been able to glean that:

• If those Lua GC lines included, the crash happens at lua_setgcthreshold, near the end of the function.

• But... if those Lua GC lines are commented out, the crash happens at [array addObject:value]

(NSZombieEnabled is on, but is not providing useful info.)

Any help is appreciated.

Answer 1

This:

int count = luaL_getn(L, 1);

Should be:

int count = luaL_getn(L, argIdx);    

So you may be getting an incorrect row count and scanning off the end of the table.

Answer 2

Maybe you grow your C stack too much. I am not familiar with Cocoa, but I guess that the Lua values need not be accessible all the time - the string should be copied into NSString. If it is so, try including a lua_pop(L, 1) at the end of the loop, to clean up the C stack and keep it from growing.