Spinner

This program implements a 4×4 sliding-tile puzzle called “SPINNER T.M.” where sixteen tiles are arranged in a grid and the player rotates groups of four tiles in a cycle using keyboard keys. The array A$(16,1) stores the current symbol in each of the 16 tile positions, with each cell holding one of four inverse-video characters (X, colon, O, or space) indicating the tile type. The 16 move subroutines (lines 1000–1269) each perform a four-way cyclic rotation among four specific array indices, using a temporary variable B$ for the swap. The display loop redraws the board frame using inverse-video border characters and then plots the current tile symbols into the grid at lines 500–550; PAUSE 60000 inside the loop effectively halts until a keypress is detected in the next iteration. The program auto-saves itself with an auto-run flag via the SAVE command at line 1280.

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Program Analysis

Program Structure

The program is divided into four functional regions:

1. Initialisation (lines 5–60): Populates a 16-element string array with the starting tile symbols.
2. Main game loop (lines 70–570): Scans 16 keys, redraws the board frame and tile grid, then pauses before repeating.
3. Move subroutines (lines 1000–1269): One subroutine per key, each performing a four-element cyclic rotation in the tile array.
4. Save/restart (lines 1270–1290): Clears memory, saves the program with an auto-run flag, and restarts.

Data Representation

The board is modelled as A$(16,1), a 16-row string array where each row holds exactly one character. The four tile types are initialised in lines 10–60:

Array indices	Character	Meaning
1–4	%X (inverse X)	Top row tiles
5–8	%: (inverse colon/block)	Second row tiles
9–12	%O (inverse O)	Third row tiles
13–16	% (inverse space)	Bottom row tiles

The solved state therefore has all tiles of the same type in their respective row, providing a clear visual goal.

Board Layout and Display

The frame is drawn unconditionally on every loop iteration (lines 400–430) using inverse-video border characters including %=, %S, %P etc. to form decorative text and box-drawing lines. The tile symbols are then overlaid in lines 500–550 using PRINT AT 6,(9+2*A) and similar expressions, placing tiles in every other column to give visual spacing. The PAUSE 60000 at line 560 acts as a long delay that is effectively interrupted by the INKEY$ polling on the next loop iteration — since PAUSE does not exit early on keypress in this context, the game relies on the outer FOR T=1 TO 60000 loop cycling rapidly through the INKEY$ checks before reaching the PAUSE.

Move Subroutines and the Rotation Mechanic

Each of the 16 keys (1–4, Q–R, A–F, Z–V) calls a dedicated subroutine that rotates four specific array indices in a cycle. The canonical pattern uses a temporary string B$:

1. Save the first element into B$.
2. Copy element 2 → position 1, element 3 → position 2, element 4 → position 3.
3. Restore B$ into position 4.

This is a standard four-element left-rotation. The indices chosen for each subroutine define which “spinner” on the board is activated, linking adjacent or diagonal cells in a pattern consistent with the game’s name.

Key-to-Subroutine Mapping

Key	Subroutine	Indices rotated
1	1000	2→13→4→5→2
2	1120	3→14→1→6→3
3	1130	4→15→2→7→4
4	1140	1→16→3→8→1
Q	1150	6→1→8→9→6
W	1160	7→2→5→10→7
E	1170	8→3→6→11→8
R	1180	5→4→7→12→5
A	1190	10→5→12→13→10
S	1200	11→6→9→14→11
D	1210	12→7→10→15→12
F	1220	9→8→11→16→9
Z	1230	14→9→16→1→14
X	1240	15→10→13→2→15
C	1250	16→11→10→3→16
V	1260	13→12→15→4→13

Notable Techniques

• Single-character string array: DIM A$(16,1) exploits the ZX81’s fixed-length string array rows to store exactly one character per tile, making indexing straightforward with A$(N) notation.
• Inline INKEY$ polling: Each key check is a separate IF INKEY$="x" THEN GOSUB line rather than using a variable, avoiding the overhead of storing the key value but requiring 16 consecutive INKEY$ reads per frame, which can miss fast keypresses.
• Symmetric keyboard layout: The keys 1–4 occupy a physical row, Q–R the next, A–F the next, and Z–V the bottom — matching the four columns and four rows of the spinner grid in a spatially intuitive way.
• Auto-run save: Line 1280 uses an inverse-video digit in the filename to set the auto-run flag, so the program restarts automatically when loaded.

Anomalies and Notes

• Line 1250 rotates indices 16→11→10→3→16. The middle step goes 11→10 rather than 11→14 as would be expected for a clean 2×2 spinner — this may be an intentional asymmetric move or a transcription error in the original.
• The subroutine numbering is inconsistent: the first subroutine is at 1000 while subsequent ones skip to 1120, leaving a gap of 120 lines that could have accommodated the earlier subroutine’s lines more naturally at 1100.
• PAUSE 60000 inside the main loop means the screen freezes for a very long time if no key is pressed during the INKEY$ checks at the top of each iteration; the effective frame rate depends entirely on how quickly lines 75–260 are scanned.