Gridmonger saves are corrupted under some conditions

[Jagholin]

Gridmonger saves broken files under some conditions(probable cause: saving small levels where #rows != #columns with a specific wall/floor configuration)

This was confirmed on both 1.2.0 and a version built from the current master branch (0cde3f8)

Steps to reproduce:

1. Create a new map, set in "coordinates" tab: origin - northwest, column/row style = number (unknown if this is necessary to reproduce the bug)
2. Create a new level, # of columns = 3, # of rows = 4
3. excavate (Draw) the entire level, you will have a level that looks just like this, with one room surrounded by simple walls on all sides:

4. Save the file (ctrl s) and close the program.
5. Reopen gridmonger, it will try to open the last file and will fail:

I could finish my report here, but since I'm developing some stuff that uses gmm files, we can continue our investigation:

6. Using gmm2json, we can read the corrupted file. This means that at least RIFF structure is not affected:
   ./gmm2json broken.gmm | jq | tee broken.json

Output:

[
  {
    "chunk_type": "LIST",
    "list_type": "map ",
    "children": [
      {
        "chunk_type": "MAP_PROP",
        "version": 4,
        "title": "broken",
        "game": "",
        "author": "jagholin",
        "creation_time": "2026-03-21 12:39:51",
        "notes": ""
      },
      {
        "chunk_type": "MAP_COOR",
        "origin": 0,
        "row_style": 0,
        "column_style": 0,
        "row_start": 1,
        "column_start": 1
      }
    ]
  },
  {
    "chunk_type": "LIST",
    "list_type": "lvls",
    "children": [
      {
        "chunk_type": "LIST",
        "list_type": "lvl ",
        "children": [
          {
            "chunk_type": "LVL_PROP",
            "location_name": "broken_location",
            "level_name": "broken map",
            "elevation": 0,
            "num_rows": 4,
            "num_columns": 3,
            "override_coord_opts": 0,
            "notes": ""
          },
          {
            "chunk_type": "LVL_COOR",
            "origin": 0,
            "row_style": 0,
            "column_style": 0,
            "row_start": 1,
            "column_start": 1
          },
          {
            "chunk_type": "LVL_REGN",
            "enable_regions": 0,
            "rows_per_region": 16,
            "columns_per_region": 16,
            "per_region_coords": 1,
            "num_regions": 0,
            "records": []
          },
          {
            "chunk_type": "LVL_CELL",
            "floor": [
              1,
              1,
              1,
              0,
              1,
              1,
              1,
              0,
              1,
              1,
              1,
              0,
              1,
              1,
              1,
              0,
              0,
              0,
              0,
              0
            ],
            "floor_orientation": [
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0
            ],
            "floor_color": [
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0
            ],
            "wall_north": [
              10,
              10,
              10,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              10,
              10,
              10,
              0
            ],
            "wall_west": [
              10,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0
            ],
            "trail": [
              0,
              0,
              0,
              10,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              10,
              0,
              0,
              0,
              0,
              0,
              0,
              0,
              0
            ]
          },
          {
            "chunk_type": "LVL_ANNO",
            "num_annotations": 0,
            "records": []
          }
        ]
      }
    ]
  },
  {
    "chunk_type": "MAP_LINKS",
    "num_links": 0,
    "records": []
  },
  {
    "chunk_type": "LIST",
    "list_type": "stat",
    "children": [
      {
        "chunk_type": "TYPE_UNKNOWN"
      },
      {
        "chunk_type": "TYPE_UNKNOWN"
      },
      {
        "chunk_type": "TYPE_UNKNOWN"
      },
      {
        "chunk_type": "TYPE_UNKNOWN"
      }
    ]
  }
]

As we can see, there are indeed values 10 in the trail array. But we didn't create any trails!

Also, something weird is happening with walls arrays. We can see this clearly if we try to open the json in a secret godot project(WIP):

OOPS, most of the walls are gone!

[Jagholin]

additional files:

logs when saving:

gridmonger_save.log

logs trying to open:

gridmonger_openerr.log

Corrupted save (zipped since github doesnt allow .gmm upload):

broken.zip

[Jagholin]

After a bit more experimentation, there are actually more strangeness if you try to make small levels.

(following was tried on the build from the current master branch):

For example, if you just make 1x1, it won't save any walls (though it won't corrupt saves either).

If you create 3x3 and fill it with simple floor surrounded by walls on all sides, it will corrupt the save(in the same manner as above).