Treasure Chase

Treasure Chase is a cat-and-mouse game in which the player moves a treasure token around a grid while a pursuing “hound” automatically closes in. The 22×32 playing field is populated procedurally: each cell has a 5% chance of becoming a wall (value 2, displayed as inverse asterisk) and a 10% chance of becoming a coin (value 1, displayed as inverse period), stored in a two-dimensional array H(22,32). The player scores 10 points for landing on a wall cell and 1 point for a coin cell, with both clearing on collection; a running high score is preserved across rounds in C1. Movement uses the classic ZX81 key layout (5/8 for left/right, 6/7 for up/down), and pressing 0 teleports the treasure to a random position, while the hound steps one cell closer each tick using simple Chebyshev-style tracking logic.

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Program Analysis

Program Structure

The program is divided into four logical phases:

1. Initialisation (lines 1–13): Resets player position, places the hound randomly, zeroes the score, and enters FAST mode.
2. Map generation (lines 20–61): Fills the 22×32 grid with walls and coins using probabilistic RND checks, storing terrain in array H and printing inverse characters.
3. Game loop (lines 65–160): Renders player and hound, updates scores, advances the hound one step toward the player, reads keys to move the treasure, clamps positions, and checks for capture.
4. End / restart (lines 200–240): Prints a capture message, prompts to play again, and either stops or jumps back to line 5 (preserving high score in C1).

Display and Character Encoding

All interactive characters are rendered as ZX81 inverse video. The player token is %0 (inverse 0), the hound is %A (inverse A), walls are %* (inverse asterisk), and coins are %. (inverse period). The status line at row 0 spells out %Y%O%U%R and %H%I%G%H% %S%C%O%R%E in inverse characters to give a header-bar effect. Erasing a character is done by printing "% " (inverse space).

Terrain Array and 1-Based Indexing

The terrain array H(22,32) uses 1-based indices in ZX81 BASIC, so screen positions (I,J) — which run 0–21 and 0–31 — are stored as H(I+1,J+1). Cell values encode terrain type:

Value	Meaning	Score	Display
0	Empty	0	inverse space
1	Coin	+1	inverse period
2	Wall	+10	inverse asterisk

After the player’s treasure lands on a cell, its value is read, the score updated, and the cell zeroed — preventing double-counting on subsequent passes.

Hound (Pursuer) AI

Lines 81–87 implement simple axis-aligned stepping: the hound’s target coordinates (X1,Y1) are incremented or decremented independently on each axis toward the treasure (A,B). This is equivalent to Chebyshev movement — the hound can close diagonally one step per frame and will inevitably catch the treasure on an open field.

Player Input and Movement

Key mapping follows the standard ZX81 numeric-key convention:

• 5 — move left (B=B-1)
• 8 — move right (B=B+1)
• 6 — move down (A=A+1)
• 7 — move up (A=A-1)
• 0 — teleport to random position

Position is clamped at lines 132–135 to keep the treasure within the visible grid. There is no boundary check on the hound’s coordinates X and Y, though in practice the hound is clamped indirectly because it always moves toward a clamped target.

High Score Persistence

The variable C1 is initialised only once at line 1 and is not reset when the player restarts at line 5. This means the high score accumulates across games within a single session. Line 72 updates C1 only when the current score C exceeds it, providing correct high-score semantics.

Notable Bugs and Anomalies

• INKEY$ read multiple times per loop: Lines 89–120 each call INKEY$ independently. On the ZX81, INKEY$ is re-evaluated on every reference, so a key held down can trigger multiple branches simultaneously (e.g., both the erase at line 89 and a movement at a later line).
• Row 0 terrain exclusion: Line 31 skips wall/coin placement when I=0 or J=0, but the condition only catches the first row and first column. This leaves row 0 clear for the score display but means column 0 is also always empty.
• PAUSE 9000: Line 80 pauses for approximately 9000 display frames before processing input. On the unexpanded ZX81 this is very long; this is the primary pacing mechanism but also means input is only sampled after the pause expires, making the game sluggish.
• Lines 220–240 (CLEAR / SAVE / RUN) are unreachable during normal gameplay and appear to be a loader/saver block executed manually or at load time.
• Hound erased by INKEY$ check: Line 89 erases the hound’s current cell (AT A,B) if any key is pressed, before movement keys are checked — this means the hound is temporarily invisible for one frame whenever a key is held.

FAST/SLOW Mode Usage

FAST is entered at line 13 before the map-drawing loop to suppress the display interrupt and speed up the 704-cell fill operation. SLOW is restored at line 61 before the game loop begins, re-enabling the display so the player can see the action.