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### Triggering Map Scripts With Flags
In vanilla emerald, mapscripts are conditionally triggered based on the value of a provided variable. This small change allows mapscripts to be triggered based on the value of a flag instead. This is useful if you have an event/cutscene that you need to activate only once. This also works just fine with poryscript.
To add this feature, you will modify the function `MapHeaderCheckScriptTable` in the file `src\script.c`. Simply replace that entire function with the following:
```c
u8 *MapHeaderCheckScriptTable(u8 tag)
{
u8 *ptr = MapHeaderGetScriptTable(tag);
if (!ptr)
return NULL;
while (1)
{
u16 varIndex1;
u16 varIndex2;
bool8 isFlag;
bool8 flagSet;
// Read first var (or .2byte terminal value)
varIndex1 = T1_READ_16(ptr);
if (!varIndex1)
return NULL; // Reached end of table
ptr += 2;
// Read second var
varIndex2 = T1_READ_16(ptr);
ptr += 2;
isFlag = varIndex1 < VARS_START;
flagSet = isFlag && FlagGet(varIndex1);
// Run map script if vars are equal
if (isFlag
&& ((VarGet(varIndex2) != 0 && flagSet)
|| (VarGet(varIndex2) == 0 && !flagSet))) {
return T2_READ_PTR(ptr);
}
else if (VarGet(varIndex1) == VarGet(varIndex2)) {
return T2_READ_PTR(ptr);
}
ptr += 4;
}
}
```
To make use of this, write any mapscript as you normally would but substituting a flag in place of the var. If the value provided is zero, the mapscript will trigger when the flag is **not** set. If the value is **not** zero, the mapscript will trigger if the flag **is** set.
For example, the following will trigger a script when the flag `FLAG_DO_A_THING` is set.
```
MyMap_MapScripts::
map_script MAP_SCRIPT_ON_FRAME_TABLE, NeoBay_MapScripts_MAP_SCRIPT_ON_FRAME_TABLE
.byte 0
MyMap_MapScripts_MAP_SCRIPT_ON_FRAME_TABLE:
map_script_2 FLAG_DO_A_THING, 1, MyMap_EventScript_DoSomething
.2byte 0
```
It's that easy!
## Caveats ##
Due to the values associated with them, this will NOT work for "Special" flags, i.e. those beginning at `SPECIAL_FLAGS_START`. Those overlap with the VAR indices, and thus will be read as though they are VARs instead of flags.
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