Alien Lure

Alien Lure is a grid-based strategy game in which the player manoeuvres a piece (the “lure,” displayed as an asterisk) around a 5×9 sub-grid to attract and trap alien ships. Two player pieces and three alien pieces move simultaneously on rows 17–21, with column positions constrained to even values (0, 2, 4, 6, 8). The aliens use a simple probabilistic AI governed by RND comparisons to chase the player’s lure position. A second “gun” mode (G=2) fires a separate marker at an adjacent cell, destroying any alien it hits for 100 or 200 points, while extra lives are awarded every 3000 points. Skill level A (Amateur) or P (Professional) affects alien movement decisions at certain board positions.

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Program Analysis

Program Structure

The program is organised into several functional blocks:

1. Lines 1–22: Initialisation — skill level input, high score display, score and lives setup.
2. Lines 40–170: Game variable initialisation — player position (U, V), two player-ship positions (X,Y and A,B), three alien positions (C,D; E,F; GG,HH), and a gun-sight position (HX,HY).
3. Lines 195–770: Main game loop — draws all pieces, then cycles through each entity by saving its coordinates into M,N, calling the AI movement subroutine at 2000, then jumping through the dispatch mechanism at 6000.
4. Lines 2003–2420: Movement/AI subroutine — reads keys 1/2 for mode switching, computes RND-driven alien movement, detects player capture.
5. Lines 2430–3040: Player movement subroutine — handles keys 5/6/7/8, enforces grid constraints, manages the gun sight (mode 2), and checks for collisions.
6. Lines 6000–6060: Collision and scoring dispatch — checks whether the moved entity coincides with any other entity, awards points, and routes back to the correct continuation line via the variable W.
7. Lines 7000–7040: Life-loss handler — decrements SO, pauses, clears screen, restarts round if lives remain.
8. Lines 8000–8200: Game-over sequence — scrolls screen, updates high score, prints “GAME OVER” message, returns to line 2 for a new game.

Movement and Dispatch Mechanism

Rather than using a large IF–THEN chain or ON–GOTO, the program uses a computed-return technique. Before each AI subroutine call, the variable W is set to the line number that should execute after the entity’s position has been applied (e.g. LET W=345 before the first entity, LET W=410 for the second, and so on). The dispatch at line 6000 then either jumps to W (on collision) or to W-10 (normal movement). This means each entity’s “write-back and redraw” lines are exactly 10 lines apart from the corresponding “no collision” target — a deliberate and compact layout strategy.

Grid Constraints

All entities are restricted to a sub-grid spanning rows 17–21 and columns 0–8, but column movement is further constrained to even values only (0, 2, 4, 6, 8), and row movement to odd rows within 17–21. The conditions like V=0 OR V=2 OR V=4 OR V=6 OR V=8 and U=17 OR U=19 OR U=21 appear repeatedly in both the player movement and alien AI subroutines to enforce this checkerboard-style grid.

Alien AI

The alien movement subroutine (lines 2003–2420) uses two random values R and Z generated by RND. The basic strategy is: if certain board conditions are met (edge proximity, Professional skill level, parity of column), the alien moves vertically toward the lure’s row; otherwise it moves horizontally toward the lure’s column. The threshold 0.5 vs 0.7 for Z biases movement in one direction, producing slightly asymmetric drift. Skill level P enables additional directional biasing at lines 2090, 2120, and 2122.

Dual-Mode Gun System

Pressing key 2 switches to gun mode (G=2); key 1 reverts to lure mode. In gun mode (subroutine from line 2590), the lure’s current position is used as the origin, and one of the directional keys fires a marker O to an adjacent cell stored in HX,HY. The dispatch at line 6005 checks whether any alien occupies the gun-sight cell and awards points. A base hit scores 100; if the entity was the first player ship (Q=141), an additional 100 is awarded (lines 6020–6035), giving 200 total. After a hit, HX is reset to 15 (off the playfield) at line 6050.

Scoring and Extra Lives

Score is displayed live at column 21 of row 18. Extra lives are granted at line 6044 using the condition S/3000 = INT(S/3000) — every exact multiple of 3000 points — with a second clause (S-100)/3000 = INT((S-100)/3000) to catch the case where the score was at 2900 before a 100-point hit, preventing a missed boundary.

Notable Bugs and Anomalies

• Line 2520 — Missing key check: The condition reads IF INKEY$<>"5" AND V>0 … but the intent is clearly IF INKEY$="5". As written, pressing any key other than 5 can decrement V, making left-movement behaviour erratic.
• Line 2600 — Typo in condition: UL21 is almost certainly a typo for U=21, causing the condition to evaluate differently than intended and potentially breaking rightward gun movement.
• Line 3000 — Missing OR operator: The condition HX=A AND HY=BWHX=C appears to be a truncated/corrupted expression — BWHX is not a valid variable; this should read HY=B OR HX=C. This would prevent correct detection of gun hits on the second player ship and first alien.
• Line 2560 — Variable O vs 0: The condition V=O uses what appears to be the letter O rather than the digit 0. If O is not initialised, this evaluates as 0 anyway on ZX BASIC (unset numeric variables default to 0), so it is effectively harmless but is a readability issue.

Variable Summary

Variable	Role
U, V	Player lure row, column
X, Y	First player ship row, column
A, B	Second player ship row, column
C, D	Alien 1 row, column
E, F	Alien 2 row, column
GG, HH	Alien 3 row, column
HX, HY	Gun sight row, column
M, N	Temporary working position for current entity
W	Dispatch target line number
G	Gun mode (1=lure, 2=gun)
Q	CHR$ code for scoring display (141 or 189)
S	Current score
SO	Spare men (lives)
BEST	High score (persists across games)
R, Z	Random values for alien AI decisions
A$	Skill level (“A” or “P”)