Cosmos Landing

You are in charge of Starship Astron and have just completed a mission in the outer limits of the galaxy. Having already landed your own craft safely you must land your radio controlled drones onto the constantly moving landing platform.

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Program Analysis

Cosmos Landing is a ZX81/TS1000 landing simulation game in which the player guides radio-controlled drones onto a moving platform scrolling across the bottom of the screen. The player presses “0” or “1” to move the drone left or right, aiming to align it with a platform that bounces back and forth across the display over ten attempts. Three difficulty levels (Hard, Medium, Easy) are implemented by varying the descent increment per iteration: 0.25, 0.5, or 1.0 rows per loop, giving the player more or fewer steps to correct their approach. The moving platform is rendered using PRINT AT with a star-field backdrop of 50 randomly placed dots drawn in FAST mode before switching back to SLOW for the interactive descent loop. After ten rounds, the program calculates a score ratio and assigns one of six pilot ratings stored in D$.

Program Structure

The program is organised into a main game loop with a subroutine for instructions. The top-level flow is as follows:

1. Lines 1–9: Initialise score variables P (safe landings) and Q (total attempts); offer instructions.
2. Lines 10–160: Set up each round — choose platform direction B, drone start column A, and read difficulty level into I.
3. Lines 170–340: Draw the star-field in FAST mode, then run the descent loop in SLOW mode.
4. Lines 350–440: Evaluate the landing, update scores, loop for up to 10 rounds.
5. Lines 450–580: Display final score and rating; offer another game via RUN.
6. Lines 585–720: Instructions subroutine, entered from line 8, returning at line 720.
7. Lines 730–750: CLEAR and SAVE utility lines.

Game Mechanics

The drone starts at a random column T (0–27) and descends row by row from row 0 to row 20. The platform moves horizontally, bouncing off columns 2 and 25 using the expression at line 250:

LET B=(A=2)-(A=25)+B*(A>2 AND A<25)

This single-line idiom reverses direction B at the boundaries without any IF statement. A landing is successful only if T=A+1 (line 350), requiring precise alignment. The player steers with “0” (right) and “1” (left) via INKEY$ polled inside the descent loop at line 270.

Difficulty and Descent Speed

Difficulty is implemented by setting I to 0.25, 0.5, or 1.0 for Hard, Medium, or Easy respectively. Row position accumulates as S=S+1/I per loop iteration, so Hard mode takes up to 80 iterations to reach row 20 while Easy takes only 20. This elegantly gives the player more reaction time at lower skill levels.

Key BASIC Idioms

• Boolean arithmetic for direction: B=(RND>.5)-(RND<.5) at line 10 produces −1, 0, or +1, though 0 is filtered out at line 15.
• Wrapping with Boolean: T=T+(T=0)-(T=28) at line 280 wraps the drone position without conditionals.
• FAST/SLOW toggling: Line 160 switches to FAST to draw 50 random stars quickly, then line 240 returns to SLOW for interactive play — a standard ZX81 technique to avoid slow screen rendering during setup.
• INKEY$ polling loop: Lines 7, 110, and 570 each use the IF INKEY$="" THEN GOTO self pattern to wait for any keypress.

Scoring and Rating System

After 10 rounds (Q=10), the ratio W=(P/Q)*10 is computed and matched to one of six rating strings. The ratings are:

W range	Rating
W = 10	SUPREME COMMANDER OF THE WORLD PILOTS ASSOCIATION
7 < W < 10	SUPREM AIR FLEET COMMANDER
5 < W < 8	PROFFESSIONAL AIRCRAFT LANDER
3 < W < 6	AMATEUR AIRCRAFT LANDER
1 < W < 4	I AM GLAD THIS IS ONLY A COMPUTER SIMULATION
W < 2	DANGEROUS UNCOORDINATED IDIOT

Note that the range 1 < W < 4 uses a different print path (lines 535–540) that skips the “YOUR RATING IS-” prefix, printing the string alone before jumping to line 560. This appears intentional for comedic effect at that tier.

Bugs and Anomalies

• Double INKEY$ read (lines 7–8): Line 7 waits until a key is pressed, then line 8 checks if that key is “Y”. However, by the time execution reaches line 8 the key may already have been released, causing the “Y” check to fail silently and skip instructions even when intended. A single read into a variable would be more reliable.
• Rating gap at W=2: The conditions W>1 AND W<4 and W<2 overlap slightly at non-integer values but leave W exactly equal to 2 uncovered by the lowest tier and ambiguously in the second-lowest tier only — an edge case unlikely in practice since W is always a multiple of 1 (integer landings out of 10).
• Spelling errors: “SUPREM” (line 490) and “PROFFESSIONAL” (line 500) are present in the original listing.
• Line 120 INKEY$ race: Like lines 7–8, line 120 checks INKEY$ immediately after line 110 releases the key, which may already be empty; the “0” option for viewing scores may be unreliable.
• Drone display character: The drone is rendered as <\~~> using the ZX81 block graphic tilde characters, giving a small spacecraft silhouette.