Geiger Counter

This program implements a grid-based treasure-hunt game where the player searches for hidden plutonium on a 10×10 grid. A random target position is set at lines 20–30, and the player navigates a cursor around the grid using keys 5, 6, 7, and 8, with a simulated “Geiger counter” reading at line 290 giving a proximity hint calculated as ABS(B-D)*7.29+C+A. The score counter S tracks the number of moves taken, and success triggers a congratulatory message before automatically restarting via RUN at line 950. Inverse-video characters are used throughout for the grid cells and cursor marker.

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Program Analysis

Program Structure

The program divides into four logical phases:

1. Initialisation (lines 10–50): Score reset, random target coordinates generated, screen cleared.
2. Grid drawing (lines 90–130): Nested FOR loops paint a 10×10 field of inverse-space characters.
3. Game loop (lines 140–300): Cursor placed, moved by key input, boundary-clamped, proximity hint printed, repeat.
4. Win sequence (lines 900–980): Victory message, pause, CLS, and RUN to restart.

Grid and Coordinate System

The playfield occupies screen rows 1–10 and columns 1–10, drawn with PRINT AT Y,X;"% " — an inverse-video space pair that fills each cell. The cursor is shown as an inverse X (line 170, "%X") and erased to an inverse + (line 190, "%+") before the next position is drawn. The target column A is chosen in the range 2–9 and target row B in the range 1–9, ensuring it always falls within the visible grid.

Movement and Input Handling

Key detection uses five consecutive IF INKEY$= checks (lines 200–235). Because each is a separate statement rather than ELSE IF, all five are evaluated every iteration regardless of which key is pressed. The mapping follows the conventional ZX layout:

Key	Action
7	Move up (C−1)
6	Move down (C+1)
5	Move left (D−1)
8	Move right (D+1)
S	Stop / break out

Boundary clamping at lines 240–270 keeps C and D in the range 1–10 after any move.

Proximity (“Geiger Counter”) Formula

The hint displayed at line 290 is ABS(B-D)*7.29+C+A. This is an asymmetric distance measure: vertical separation is heavily weighted by the factor 7.29, while horizontal separation has no multiplier. Notably, the formula adds the absolute target coordinates A and C (the current cursor row) rather than their difference, which means the reading does not approach zero as the cursor nears the target — it approaches A+B (roughly 3–17) rather than 0. This gives a relative rather than absolute proximity indicator.

Score Tracking

The variable S is incremented at line 175 on every iteration of the main loop, including the initial placement before any key is pressed. The pause at line 180 (PAUSE 40, approximately 1.6 seconds at 50 Hz) paces each turn. The win screen at line 900 reports S as the number of tries.

Notable Techniques and Anomalies

• Lines 40 and 50 are bare PRINT statements that output blank lines, providing a rough screen clear at the top without a full CLS.
• Line 960 (CLEAR) and line 980 (RUN) are unreachable dead code; after the win sequence the program branches to line 950 (RUN), which restarts immediately.
• The SAVE "1027%6" at line 970 uses an inverse-video digit in the filename, which acts as an auto-run flag on save.
• Using PRINT AT C,D with C as the row and D as the column is unconventional naming (one might expect row=R, col=C), which could cause confusion when reading the proximity formula where both C and A are row values.
• The erasing step at line 190 prints an inverse + rather than restoring the original grid cell character (inverse space), leaving a visible trail of + marks showing the player’s path — an intentional or incidental feature that aids navigation.