feat: Allocate region file structures dynamically (#348)

Previously, there would always be memory reserved for 16 region files.
This is simultaneously a large amount of space to reserve by default, and
also a relatively low limit in case of multiple players and/or a large world.
Now, the memory is allocated as needed, with a capacity of 1024 pointers
to `BASED` items.

src/_copybooks/state/DD-REGION-FILES.cpy

@@ -9,29 +9,12 @@
 78 REGION-CHUNK-WIDTH           VALUE 32.
 
 *> The maximum number of region files that can be open at once.
-*> Should always be greater than the number of players or thrashing will occur if players are spread out.
-78 MAX-REGION-FILES             VALUE 16.
+78 MAX-REGION-FILES             VALUE 1024.
 
 *> The region file cache.
-01 REGION-FILES EXTERNAL.
-    02 REGION-FILE OCCURS MAX-REGION-FILES TIMES.
-        *> Whether the region file is currently open.
-        03 REGION-FILE-OPEN             BINARY-CHAR UNSIGNED.
-        *> When the region file was last accessed.
-        03 REGION-FILE-ACCESS-TIME      BINARY-LONG-LONG.
-        *> Region coordinates.
-        03 REGION-FILE-X                BINARY-LONG.
-        03 REGION-FILE-Z                BINARY-LONG.
-        *> The file name.
-        03 REGION-FILE-NAME             PIC X(1024).
-        *> The open file handle (input/output).
-        03 REGION-FILE-HANDLE           PIC X(4) USAGE COMP-X.
-        *> Chunk locations within the file, consisting of the offset (3 big-endian bytes) and length (1 byte).
-        *> Offset and length are measured in 4 kiB sectors.
-        03 REGION-FILE-OFFSET-BYTES     PIC X(REGION-SECTOR-BYTES).
-        03 REGION-FILE-OFFSETS          REDEFINES REGION-FILE-OFFSET-BYTES.
-            04 REGION-FILE-OFFSET OCCURS REGION-SECTOR-INTS TIMES PIC X(4).
-        *> TODO: Last-modified timestamps for each chunk
-        *> Boolean array; one entry per file sector, indicating whether the sector is used.
-        *> TODO: Make this use less memory (bitset?)
-        03 REGION-FILE-USED OCCURS REGION-SECTOR-COUNT TIMES BINARY-CHAR UNSIGNED.
+01 REGION-FILE-POINTERS EXTERNAL.
+    *> Number of loaded region files
+    02 REGION-FILE-COUNT        BINARY-LONG UNSIGNED.
+    *> Pointers to dynamically allocated structures, see DD-REGION-FILE-REF.cpy.
+    *> The items will be reordered such that all open files are at the beginning of the table.
+    02 REGION-FILE-POINTER      USAGE POINTER OCCURS MAX-REGION-FILES TIMES.

src/_copybooks/structs/DD-REGION-FILE-REF.cpy

@@ -0,0 +1,24 @@
+*> --- Copybook: reference to a loaded region file ---
+*> This is a BASED item, i.e., a view into a data structure referenced by a pointer. DD-REGION-FILES.cpy contains a
+*> table of pointers to these structures.
+
+*> The region file cache.
+01 REGION-FILE BASED.
+    *> When the region file was last accessed.
+    02 REGION-FILE-ACCESS-TIME      BINARY-LONG-LONG.
+    *> Region coordinates.
+    02 REGION-FILE-X                BINARY-LONG.
+    02 REGION-FILE-Z                BINARY-LONG.
+    *> The file name.
+    02 REGION-FILE-NAME             PIC X(1024).
+    *> The open file handle (input/output).
+    02 REGION-FILE-HANDLE           PIC X(4) USAGE COMP-X.
+    *> Chunk locations within the file, consisting of the offset (3 big-endian bytes) and length (1 byte).
+    *> Offset and length are measured in 4 kiB sectors.
+    02 REGION-FILE-OFFSET-BYTES     PIC X(REGION-SECTOR-BYTES).
+    02 REGION-FILE-OFFSETS          REDEFINES REGION-FILE-OFFSET-BYTES.
+        03 REGION-FILE-OFFSET OCCURS REGION-SECTOR-INTS TIMES PIC X(4).
+    *> TODO: Last-modified timestamps for each chunk
+    *> Boolean array; one entry per file sector, indicating whether the sector is used.
+    *> TODO: Make this use less memory (bitset?)
+    02 REGION-FILE-USED OCCURS REGION-SECTOR-COUNT TIMES BINARY-CHAR UNSIGNED.