#import "/common.typ": post

#show: post.with(
    title: "mapcat",
    author: "GPT 5.5",
    date: datetime(year: 2026, month: 6, day: 15),
)

#emph[(I generated this article with AI from mapcat's source code, so this text is in the public domain.)]

mapcat is a tiny C program I wrote in 2016 for #link("https://redman.xyz/git/an_almango/")[An Almango], a Tremulous map made collaboratively with MaeJong. The problem was simple: GtkRadiant stores a whole map in one text file, which is not a great format for two people editing different rooms at the same time. So we split the map into parts and used mapcat to glue them back together before compiling.

It is not clever. It parses Quake 3 `.map` files, appends all entities and brushes, keeps the first `worldspawn`, and writes out one big `.map` file. In An Almango's Makefile it was used like this:

```
mapcat maps/p0_master.map maps/p1_bases.map ... -o maps/analmango.map
```

= Usage

The source code is available at #link("https://redman.xyz/git/mapcat/")[mapcat]. Build it with the Makefile and put the resulting binary somewhere in your `PATH`.

Each map part can define a `mapcat_prefix` key in its `worldspawn`. mapcat applies it to `target`, `targetname`, and `team` values so that two parts can both have an entity named `door_trigger` without breaking each other. Names starting with `global_` are left alone, which is how parts can intentionally talk to shared entities from the master map.

There is also a small discard mechanism. An entity with `mapcat_discard` is thrown away, and any brush or patch textured with `common/discard` is skipped. This let us put editor-only helper geometry in a part without it showing up in the final map.