Brain Terrain

Brain Terrain is a text-based mystery/adventure game in which the player must identify a missing person’s home year by accessing memory zones labelled with single-letter keys (A, B, C, E, F, M, P, S, T, V). Each zone contains one or more encoded clues stored in a compressed string table; the player cycles through clues displayed with a scrolling marquee effect and ultimately guesses a four-digit year. Clue text is stored compactly in the string T$ using a delta-encoding scheme where each character’s code is offset relative to the first character of each entry, with comma (CHR$ 26 in this context) as a delimiter. A help feature, unlocked only after a failed guess, reveals the last words heard by the character Mr. Winkler.

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Program Analysis

Program Structure

The program is divided into a main loop and three subroutines:

1. Lines 5–350: Initialisation — dimensions, string constants, and decompression of the clue table from T$ into the array A$.
2. Lines 360–860: Main game loop — title display, zone selection, marquee display, guess handling, and help.
3. Lines 1000–1130: Subroutine GOSUB 1000 — word-wrapping text printer that splits R$ on CHR$ 0 boundaries.
4. Lines 2000–2020: Subroutine GOSUB 2000 — centred single-line printer using TAB.
5. Line 3000: SAVE statement (not reached during normal play).

Delta-Encoded Clue Table

All clue text is packed into the single string T$ (line 180) and decompressed at startup (lines 190–340) into the three-dimensional string array A$(11,4,22). The 11 zones have between 2 and 4 entries each, as specified by L$="42232232222". Each entry is terminated by CHR$ 26 (used here as a comma/delimiter sentinel — notably not a standard ASCII comma).

The encoding scheme (lines 280–310) is a character-level delta code:

• The first character of each raw entry sets a base value C = CODE D$(1) - 27.
• Each subsequent character is stored as N = CODE D$(Z) - C, then adjusted modulo-36 via the expression N + 36*((N<28) AND (N+C>27)) + 28*(N<0) to keep values in a printable range.
• The decoded character count is stored in byte 1 of each A$(X,Y) entry, so retrieval at line 600 uses A$(FL,M)(2 TO CODE A$(FL,M,1)).

This encoding reduces the storage footprint of the clue strings and makes casual inspection of the source listing unrevealing.

Scrolling Marquee Display

Lines 430–490 implement a simple horizontally scrolling marquee for the currently selected clue. A padding string S$ of 32 asterisks (built by doubling "*" five times at lines 120–150) and a blank string B$ of 32 null bytes (CHR$ 0, similarly constructed) are used to pad Q$ on both sides. The scroll offset NS is randomised each iteration (line 444–455): normally 1–3 characters, but after a failed guess (M=1) it ranges up to half the display width, creating a faster, more chaotic scroll. The previous line is erased by printing B$ at row 18 before the new frame.

Game Logic and Zone Keys

Ten letter keys map to memory zones via V$="ACVEFSBMPT" (line 160). Pressing one selects a random clue from that zone (M=INT(RND*VAL L$(FL)+1), line 590). Zone 11 is special: it holds the answer year in A$(11,1)(2 TO 5) and a hint phrase in A$(11,2)(2 TO 21), but has no key assigned in V$ — it is only accessed internally.

Key	V$ index	Clue count
A	1	4
C	2	2
V	3	2
E	4	3
F	5	2
S	6	2
B	7	3
M	8	2
P	9	2
T	10	2

Word-Wrap Subroutine

The subroutine at line 1000 splits long strings stored in R$ at CHR$ 0 boundaries (not spaces). It searches backward from position WL+1 for a null byte, prints the segment before it, then recurses via GOTO 1000 with the remainder. This means the author must manually insert null bytes into message strings where line breaks are desired — a deliberate authoring convention rather than automatic word wrapping.

Help Gate and Guess Flow

The help key H is only advertised and active after the player has made at least one failed guess (GW=1, set at line 720). The help screen (lines 760–790) reveals the string stored in zone 11, entry 2 — a contextual hint. After either a failed guess or viewing help, Q$ and M are reset (lines 800–810) and the game loops back to the title.

Notable Techniques and Observations

• The display width constant WL=32 (line 50) is used throughout, making the layout logic easy to retarget to a different line length.
• E$=CHR$ 11 (line 90) is used in the instructions string as an inline formatting code, suggesting the display routine may interpret it, though no explicit handler is visible — it may simply render as a control character or be incidental.
• The FAST/SLOW pair (lines 10, 350) confines the slow decompression loop to FAST mode, then returns to SLOW for display.
• The SAVE "1003%2" at line 3000 uses an inverse-2 character in the filename, which on ZX81/TS1000 acts as an auto-run flag causing the program to execute immediately on LOAD.
• The sentinel value CHR$ 26 in T$ (checked at line 250) is unusual — this is not a printable character, so it survives in the string literal without visual ambiguity but could be fragile if the listing is re-entered manually.