Created Reuse filler save space for Variables and Flags (markdown)

Reuse-filler-save-space-for-Variables-and-Flags.md

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+This is an alternative to [Extra save space with two lines of code](Extra-save-space-with-two-lines-of-code).
+The Tutorial linked above will increase the already existing SaveBlock areas, while this will instead add another one "in between" the original ones to keep compatibility to vanilla savegames.
+
+## Defining the space
+
+in `include/save.h`, add:
+
+```c
+#define N_ADDITIONAL_SAVE_PAYLOAD_SIZE_PER_SLOT 116
+#define N_ADDITIONAL_SAVE_PAYLOAD_SIZE (N_ADDITIONAL_SAVE_PAYLOAD_SIZE_PER_SLOT*NUM_SECTORS_PER_SLOT)
+extern u8 gnAdditionalSaveData[];
+```
+
+in `src/save.c`, add (before the `gSaveDataBuffer` at the start):
+```c
+#if NCHI__ADDITIONAL_SAVE_PAYLOAD
+EWRAM_DATA u8 gnAdditionalSaveData[N_ADDITIONAL_SAVE_PAYLOAD_SIZE];
+#endif
+```
+
+## Saving the data
+
+then, still in `src/save.c`, add this at the end of `HandleWriteSector` (right before the `return` line):
+```c
+for (i = 0; i < N_ADDITIONAL_SAVE_PAYLOAD_SIZE_PER_SLOT; i++)
+  gReadWriteSector->unused[i] = gnAdditionalSaveData[i + N_ADDITIONAL_SAVE_PAYLOAD_SIZE_PER_SLOT*sectorId];
+```
+also add the exact same code again in `HandleReplaceSector`, right before the `EraseFlashSector(sector);` line.
+
+## Loading the data
+
+still in `src/save.c`, add this in `CopySaveSlotData`, right before the end of the `if` block (and after the end of the inner `for` loop):
+```c
+for (j = 0; j < N_ADDITIONAL_SAVE_PAYLOAD_SIZE_PER_SLOT; j++)
+  gnAdditionalSaveData[j + N_ADDITIONAL_SAVE_PAYLOAD_SIZE_PER_SLOT*id] = gReadWriteSector->unused[j];
+```
+
+# Accessing Variables and Flags
+
+in `src/event_data.c`, add this at the top to the includes:
+```c
+#include "save.h"
+```
+
+then, in the same file, in `GetVarPointer`, add this right before the last `else`:
+```c
+else if (id >= 0xF000)
+  return &gnAdditionalSaveData[(id - 0xF000)*2];
+```
+
+and in `GetFlagPointer`, again before the last `else`:
+```c
+else if (id >= 0xF000)
+  return &gnAdditionalSaveData[(id - 0xF000)/8];
+```
+
+# Defining Variables and Flags
+
+define them as usual in `include/constants/flags.h` and `include/constants/vars.h`, starting from `0xF000`:
+```c
+#define FLAG_FOR_SOMETHING_SUPER_SPECIAL 0xF000
+#define FLAG_FOR_SOMETHING_TOTALLY_BORING 0xF001
+
+#define VAR_FOR_SOMETHING_SUPER_SPECIAL 0xF001
+#define VAR_FOR_SOMETHING_TOTALLY_BORING 0xF002
+```
+
+# CAVEAT
+
+To save on space (because I don't know what your game will need), variables and flags both **share the same data space**, so make sure they don't overlap. Variables are 16bit long (so their ID gets multiplied by 2 to get the byte offset into `gnAdditionalSaveData`), and Flags are 1bit long (ID gets divided by 8).
+
+so the above examples would then internally look like this:
+```c
+struct{
+  u8 FLAG_FOR_SOMETHING_SUPER_SPECIAL : 1;  // flag with ID 0xF000 at bit 0 of byte 0
+  u8 FLAG_FOR_SOMETHING_TOTALLY_BORING : 1; // flag with ID 0xF001 at bit 1 of byte 0
+  u8 filler;                                // an unused byte because none of the IDs maps to it
+  u16 VAR_FOR_SOMETHING_SUPER_SPECIAL;      // var with ID 0xF001 at bytes 2 and 3
+  u16 VAR_FOR_SOMETHING_TOTALLY_BORING;     // var with ID 0xF002 at bytes 4 and 5
+};
+```
+
+if instead you would have more flags, or e.g. a var with ID `0xF000` (which would be at bytes 0 and 1), this would clash and corrupt your data!