Fox at Dusk

This program implements a two-player strategic board game called “Fox at Dusk” on an 8×8 checkerboard. The human controls a set of hounds (3–8, user-selectable) while the computer controls a fox; pieces move diagonally and the fox can leap over hounds. A difficulty toggle hides the fox except when it leaps or collides with a hound. The board is drawn using ZX81 block graphics characters, with each square rendered as a 3×3 character cell. The fox’s AI scores candidate moves using a weighted random formula that accounts for board depth, proximity to row 1, and whether a leap over a hound is possible.

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Program Analysis

Program Structure

The program is organised into a main flow followed by a set of subroutines accessed via GOSUB. The top-level sections are:

1. Lines 10–90: Initialisation, title screen, difficulty prompt.
2. Lines 100–140: Hound-count input and validation.
3. Lines 160–480: Board array setup and hound placement.
4. Lines 490–560: Fox placement and first display.
5. Lines 560–1160: Main game loop — human hound move, then fox AI move.
6. Lines 1170–1240: Single-digit input subroutine (returns value in Z).
7. Lines 1250–1360: Board rendering subroutines for hound, fox, and blank square.
8. Lines 1370–1420: Fox-leap logic.
9. Lines 1430–1490: Win/loss messages.
10. Lines 1510–1570: Fox flash animation subroutine.
11. Lines 1580–1600: SAVE and RUN for auto-restart.

Board Representation

The game state is held in A$(8,8), an 8×8 string array. Each cell contains one character: "B" for a black square (off-limits), "H" for a hound, "F" for the fox, or " " (space) for an empty playable square. The checkerboard pattern is set at lines 320–360 by testing whether (A+B) is odd. The fox’s current column and row are tracked separately in scalar variables E and F.

Screen Layout and Block Graphics

Each board square occupies a 3×3 character cell on screen. The column-to-screen mapping is 3*(X-1) and the row mapping inverts the board with 3*(8-Y), placing row 8 at the top and row 1 at the bottom. ZX81 block graphics characters are used to draw checkerboard shading (lines 220–290) and to render the hound (GOSUB 1250), fox (GOSUB 1300), and erased square (GOSUB 1340) as distinct 3-row sprites.

The title area in the right margin (columns 25–31) uses inverse-video characters for labels and move prompts, rendered with PRINT AT statements that leave the board area untouched.

Input Subroutine

The subroutine at line 1170 implements a robust single-keystroke digit reader. Line 1180 first flushes any held key with a busy-wait loop, lines 1190–1200 wait for a new keypress, and lines 1210–1230 reject characters outside "1"–"8" (looping back to flush again). The accepted character is echoed with PRINT C$;, converted to a column/row integer via CODE C$-28 (since CODE "1" = 29 on the ZX81), and returned in Z. The human enters the column then the row on two separate calls (lines 580–600 and 630–650).

Move Validation

A compact string expression is stored at line 170:

LET B$="A>8 OR A<1 OR B>8 OR B<1"

This string is later evaluated with VAL B$ at line 880 and line 1400 to perform bounds-checking on candidate positions — a classic ZX81 memory-saving idiom that avoids repeating the condition in code. Human hound moves are validated at line 710, which checks that the source cell contains "H", the destination is empty " ", and both column and row distances are exactly 1.

Fox AI

The fox AI (lines 820–1010) iterates over the four diagonal neighbours of the fox’s current position (E,F) using nested FOR loops over W=F-1 TO F+1 STEP 2 and V=E-1 TO E+1 STEP 2. For each candidate square it computes a score in T:

• 3*RND: random noise (0–3) to vary play.
• (B<F): +1 if the move advances toward row 1 (the fox’s goal).
• (B=1): +1 bonus for landing on the winning row immediately.

The best-scoring move is stored in (X,Y) and M tracks the maximum score. If a candidate square is occupied by a hound, the leap subroutine at line 1370 is called, which projects the fox two squares in that direction and retries with a reduced random weight (T=RND). If M remains 0 after all candidates are examined, the fox is trapped and the player wins (line 1030).

Visibility (“Dusk”) Mechanic

Variable I is set to 1 (easy mode) or 0 (hard mode) at line 80 based on the player’s "Y"/"N" answer. The fox sprite is drawn at line 1090 whenever G is true (a leap occurred), and at line 1150 when G OR I — meaning in easy mode it is shown every move, while in hard mode it is only shown after a leap. The flash animation subroutine at line 1510 toggles the fox sprite on and off four times to draw attention to leaps.

Notable Techniques and Idioms

• POKE 16418,2 at line 30 sets the system variable controlling the display file, and POKE 16418,0 at line 200 restores it — used here to manipulate the display during setup under FAST mode.
• LET H=CODE H$-28 (line 130) converts the ASCII digit character to a numeric count using the ZX81 character code offset, consistent with the same technique in the input subroutine.
• The hound-placement loop (lines 380–480) fills the bottom two rows of playable squares left-to-right until H hounds have been placed, decrementing H each time and stopping when NOT H is true.
• SAVE "1025" at line 1590 (where is an inverse digit) saves the program with the auto-run flag set, so reloading the tape starts the game immediately.

Bugs and Anomalies

• Line 1260 stores a value into Z (LET Z=3*(X-1)) but this is the hound-draw subroutine entered at line 1250 — line 1250 itself does not exist in the listing; the subroutine effectively starts at 1260. The GOSUB 1250 calls target this gap, which on the ZX81 causes execution to continue from the next higher line (1260), so it functions correctly.
• Similarly, GOSUB 1290 (called for the win/loss display at lines 1430 and 1470) targets a non-existent line; execution falls through to line 1300, which draws the fox. This means the fox is always revealed on game-end regardless of the visibility mode — likely intentional.
• The instruction text at line 50 contains the typo “COLUME” (for “COLUMN”) and the run-together word “THENTHE”.
• Lines 20 and 550 are referenced by GOTO (lines 1460 and 1160 respectively) but do not appear in the listing. GOTO 20 restarts near the beginning (falling through to line 30), and GOTO 550 re-enters the human move prompt section (falling through to line 560) — both are deliberate restart targets.