Land-Mine

Land Mine is a turn-based strategy game in which the player navigates a 10×20 grid of hidden score values, choosing moves of 1–9 steps at a time to accumulate points while avoiding mines. The 200-cell array A() is populated with random integers in the range −10 to +10, then nine cells are zeroed to act as “booby traps.” The player’s position is tracked as a linear index into the array, displayed as an asterisk (inverse video) on the grid using the formula row = INT(J/20), column = J MOD 20. A running average score per move (S/M) is compared against a high-score variable SS, and a congratulations message is shown if the player matches or beats it.

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Program Analysis

Program Structure

The program is organized into a main initialization block, several subroutines, and a game loop. Lines 0–19 handle setup; line 40 launches the game loop via GOSUB 200, which is re-entered repeatedly via GOTO 200 at line 280. Subroutine 100 draws the grid header and row labels; subroutine 200 is the main turn handler; subroutine 300 updates the player marker on the grid; subroutine 800 draws the move counter sidebar. Lines 560–582 handle the BOMB (mine) event, and lines 519–540 handle the booby-trap (zero-cell) penalty.

Line Range	Purpose
0–5	Title display, variable initialization, array allocation
12–19	Draw border graphics, call grid subroutine, populate A()
20–24	Zero out nine random cells as booby traps
35–50	Switch to SLOW mode, enter game loop
100–150	Subroutine: draw column numbers and row labels
200–280	Subroutine: main turn — input, move, score update
300–333	Subroutine: redraw grid and place player marker
500–540	Booby-trap handler (reset position, warn player)
560–582	BOMB animation and end-of-game branch
590–700	Score averaging, high-score check, end screen
800–810	Subroutine: draw “MOVE N” sidebar
9996–9999	High-score save/load scaffold

Game Mechanics

The game board is a linear array A(200) whose cells hold values from −10 to +10, generated by INT(RND*20)+1-10. Nine cells are subsequently set to 0 at random positions (lines 20–24) to serve as booby traps. The player’s position J advances by 1–9 cells per turn; reaching a cell with value 0 resets position to the start (line 519), while a cell with value 44 — which can never occur under normal initialization — is used as a mine trigger sentinel. In practice the mine trigger at line 265 (IF A(J)=44) is unreachable since values are bounded to −10…+10 and zeroed cells are the only special case; the intended mine event is therefore only triggered by falling off the end of the array at line 260 (IF J>=200 THEN GOTO 583), but line 583 does not exist — control falls through to line 590 instead.

Grid Display and Position Tracking

The grid is drawn at screen rows 3–12, columns 4–24, giving a 10-row × 20-column visual map of the 200-element array. The player marker (inverse asterisk %*) is placed using the conversion at lines 320–330:

• K = INT(J/20) — grid row (0-based)
• L = J-(K*20)+1 — grid column (1-based)
• PRINT AT K+3, L+4; "%*" — offset to screen coordinates

Row labels at lines 110–130 use CHR$(163+I), which on the ZX81 produces graphics characters for rows 3–12, giving a simple visual row indicator along the left edge of the grid.

Move Counter Sidebar

Subroutine 800 prints the word “MOVE” vertically (one letter per row at column 30, rows 3–6) and “NO” at rows 8–9, then prints the move count M at row 11. Lines 806–807 select column 29 or 30 depending on whether M is greater or less than 99, providing crude right-alignment. Note the condition uses M>99 and M<99, so the value 99 exactly prints at column 30, which is correct for a two-digit number, but the logic is slightly asymmetric.

Scoring and High Score

Score S accumulates the raw cell values (positive or negative). At game end, the average score per move SM = S/M is computed and compared to SS. SS is never initialized in the main game path — it is only set at line 9997 (LET SS=0), which is part of the save scaffold and not reached during normal play. This means SS will be undefined on first run, likely defaulting to 0, which could cause erratic high-score comparisons if SM is negative.

Notable Techniques and Idioms

• FAST/SLOW switching: FAST at line 1 speeds up array initialization and grid drawing; SLOW at lines 35 and 140 restores display for interactive play.
• Busy-wait input loop: Lines 220–230 poll INKEY$ continuously and validate the range “1”–”9″ before accepting input, a standard ZX81 input idiom.
• BOMB flash animation: Lines 560–580 alternate between printing and immediately clearing inverse-video “BOMB” text in a loop of 25 iterations, creating a flicker effect.
• Save scaffold: Lines 9996–9999 are a developer utility — STOP halts execution, LET SS=0 resets the high score, and SAVE "LAND MINE" preserves the program with SS at zero, after which GOTO 1 restarts.
• Unreachable code: Lines 500 and 519–540 contain a dead branch — line 500 jumps directly to 560, bypassing the booby-trap reset logic at 519–540 entirely. The intended booby-trap path (zero-cell handling at line 266) calls GOSUB 510, but line 510 does not exist, which would cause an error in practice.