Space Game

Space Game is a two-dimensional shooter in which the player maneuvers a ship along the bottom of the screen to destroy descending aliens while avoiding being hit. The player moves left and right using keys 5 and 8, and collision detection is performed by comparing the ship’s column position against the alien’s position at specific row thresholds (lines 200 and 210). Block graphics characters are used to render the ship, aliens, and a missile trail, and SOUND commands provide engine noise and explosion effects. The score, ship count, and alien (lives-lost) counter are displayed in an inverse-video status bar at the top of the screen, updated in real time during play.

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Program Structure

The program is organized into a small number of functional blocks:

1. Initialization (lines 20–60): Sets colors, clears the screen, seeds variables for lives (c), score (s), and alien-hit counter (a), and populates the starfield with 20 randomly placed dots.
2. Main outer loop (lines 70–260): Iterates up to 100 waves (FOR i=0 TO 100). Each wave picks a new alien trajectory and drives the inner movement loop.
3. Inner scroll loop (lines 110–240): Steps the alien down the screen two rows at a time (FOR h=4 TO 21 STEP 2), reads player input, draws sprites, checks collisions, and clears old sprite positions.
4. Alien-destroyed handler (lines 270–370): Awards 10 points, runs a multi-frame explosion animation with random ink colors, then resumes the outer loop.
5. Ship-destroyed handler (lines 380–500): Increments the alien counter, plays a descending tone burst, flashes an expanding explosion, checks for game over, resets the screen, and jumps back into the wave loop.

Sprite Rendering and Movement

All sprites are drawn entirely with block graphic characters inside PRINT AT statements. The player ship appears at row 21 and the alien at row h; a “missile” (asterisk) is printed at row 24-h, creating a symmetric counter-moving tracer. Old sprite positions are erased by printing spaces over them (line 230) before advancing h.

The alien’s horizontal position is governed by a string expression built at line 100: p$ is assigned "p" concatenated with "+1" or "-1" depending on the random direction r, then evaluated each frame with VAL p$ at line 130. This is a compact idiom for choosing between three movement modes (drift left, stay, drift right) without an IF/GOTO chain.

Collision Detection

Collision is tested at two specific rows. Line 200 checks h=12 (mid-screen) and triggers the alien-destroyed branch if the ship column m is within ±2 of the alien column p. Line 210 performs the same test at h=20 (near the bottom) for the ship-destroyed branch. Using fixed row thresholds rather than a general per-frame test keeps the logic simple but means collisions are only registered at those two rows.

SOUND Usage

SOUND statements appear in several roles:

• Lines 180–190: continuous engine tone during alien approach (channels 7 and 8 set to a fixed pitch, channel 0 modulated).
• Line 220: silences the engine after the inner loop body.
• Line 290: multi-channel explosion chord on alien destruction (channels 6, 7, 8, 10, 12, 13).
• Lines 350, 380: channel cleanup and ship-hit alarm.
• Lines 420–440: descending sweep (FOR j=0 TO 30: SOUND 0,j) on ship destruction, producing a rising-pitch effect.

Key BASIC Idioms

Several idiomatic ZX BASIC / TS2068 techniques appear throughout:

• Boolean arithmetic: Line 90 uses 12 AND r=0 and 12 AND r=2 — since Boolean expressions evaluate to 1 (true) or 0 (false), multiplying by 12 either adds or subtracts 12 from the base column, biasing the alien’s starting position left or right.
• String concatenation for computed expressions: p$ at line 100 constructs a self-modifying expression string evaluated by VAL, avoiding multiple conditional branches.
• Inline color attribute changes: A single PRINT statement at line 160 sets INK multiple times and mixes AT and TAB to draw all three sprites in one pass.
• Color cycling: c cycles 1–6 at line 390–460 and is applied via BORDER c, giving the screen a color flash on each ship loss.

Notable Variables

Variable	Role
c	Border color cycle counter (1–6)
s	Score (incremented by 10 per alien kill)
a	Number of times the player’s ship has been hit
m	Player ship column (range ~2–28, step 2)
p	Alien column, updated each frame via VAL p$
p$	Expression string ("p+1", "p", or "p-1") for alien drift
h	Current alien row in the inner scroll loop
r	Random direction selector (0, 1, or 2)
i	Wave counter (outer loop)

Bugs and Anomalies

• Line 360 contains a stray comma in PRINT AT 21,m-1,; — the comma after the column argument is syntactically unusual and may cause an error on some interpreter versions, though some accept the redundant separator before the semicolon.
• Line 470 contains an embedded color-control token (\{17}\{0}) before the PRINT keyword, which appears inside the FOR…NEXT loop. This is likely a stray attribute byte in the original listing rather than intentional code.
• The game over condition at line 450 (a >= 4) branches to GO TO 260, which is STOP, ending execution silently rather than displaying a “Game Over” message.
• The score variable s is reset to 0 at line 60 at the start of each wave iteration inside the outer loop, meaning the score displayed (line 280) accumulates within a wave but is zeroed at the start of the next — the displayed score never carries across waves except via the status-bar print at line 490.