Lander

This program simulates a lunar lander descent game where the player manages thrust and burn duration to safely land a spacecraft. Each turn the player inputs thrust level and a duration (capped at 10 cycles), and the simulation updates height, velocity, and fuel over that many animation frames. The display uses AT coordinates to render a crude spacecraft sprite that moves vertically on screen, with height scaled to fit the 22-row display by dividing by 100. Outcomes include a successful landing (velocity under 10 at low altitude with fuel remaining), a crash landing with a crater depth calculation, fuel exhaustion, or explosion if the craft somehow exceeds 1600 units altitude.

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Program Analysis

Program Structure

The program is self-contained within lines 2005–2360, looping back via RUN at line 2330. The overall flow is:

1. Initialise game variables (lines 2005–2030).
2. Wait for ENTER to begin (lines 2040–2047).
3. Inner animation/physics loop over D cycles (FOR A=1 TO D, lines 2095–2228).
4. Prompt for thrust and duration after each burst (lines 2230–2270).
5. Outcome reporting and restart (lines 2280–2360).

Variable Roles

Variable	Role
H	Height (starts ~1350–1549, decreases each cycle)
V	Velocity (downward; increases with gravity term 17-T/3)
F	Fuel remaining (drains by T/5 per cycle)
T	Thrust level entered by player
D	Duration (number of cycles to simulate, max 10)
R	Result code: 0=in progress, 1=safe, 2=crash, 3=no fuel
K1	Horizontal column of the sprite, drifts randomly
A	Loop counter for current burst

Physics Model

Each cycle the velocity is updated as V = V + 17 - T/3. A thrust of roughly 51 exactly cancels gravity, making that the neutral point. Height decreases by the current velocity: H = H - V. Fuel drains by T/5 per cycle, so higher thrust is more fuel-hungry. The horizontal position K1 drifts by a random increment each cycle, wrapping back to column 20 if it exceeds 26.

Display and Sprite Rendering

The spacecraft sprite is drawn at PRINT AT 16-H/100, K1, mapping the full height range (~0–1600) to screen rows 0–16. The sprite characters \.:\''\:. represent a block-graphic shape. Lines 2140 and 2145 test the even/odd parity of A with 2*(INT(A/2))=A, but both branches print the identical sprite string — no actual alternation occurs. A status line at row 0 shows HGT, VEL, FL (fuel), and a progress bar at row 16 shows elapsed time within the burst.

Rounding Idiom

Lines 2101–2106 apply a multiply-by-10 / INT / divide-by-10 sequence to F, H, and V every cycle. This truncates each value to one decimal place, preventing floating-point accumulation from producing untidy display strings. It is a common Sinclair BASIC technique for controlling numeric display width.

Early Loop Exit

When a landing result is detected mid-burst (R>0 or H<1), lines 2107–2108 force H=0 and set A=D, which causes the FOR…NEXT loop to terminate on its next iteration, jumping to the outcome detection block at line 2280. This avoids a direct GOTO out of the loop.

Outcome Logic

Code	Condition	Message
R=1	H<100 and F>0 and V<10	SUCCESSFUL LANDING
R=2	H<100 and V>9	CRASH LANDING + crater depth INT(V*4.5) feet
R=3	F<1	YOU HAVE RUN OUT OF FUEL
—	H>1600	YOUR SHIP EXPLODED

Note that R=3 is set at line 2223 but the program also reaches line 2300 directly from line 2195 via GOTO 2300 when fuel runs out mid-burst, bypassing the R assignment. The PRINT at 2300 is therefore reachable by two separate paths.

Notable Anomalies

• Lines 2140 and 2145 are intended to alternate two sprite frames but print the same string — no visual flicker is produced.
• The duration input at line 2260 is validated only for exceeding 10 (IF D>10 THEN GOTO 2260); a value of 0 or a negative number is accepted and would produce a zero-iteration or backward FOR loop.
• The H>1600 check at line 2125 branches to 2305, which only prints if H>1600 — this is consistent, but upward motion is physically implausible given the gravity-dominated velocity update, so this path is rarely reachable in practice.
• Line 2350 contains SAVE "1027%3"; the %3 escape denotes an inverse digit 3, functioning as an auto-run flag on load.