Galaxian

This program implements a simple Galaxian-style shoot-’em-up in BASIC. The player controls a cannon (rendered with block graphics) at the bottom of the screen using Z and C keys to move left and right, and M to fire. A single alien descends from the top of the screen, occasionally firing back at the player. The program uses PI/PI as a constant 1 throughout, a common memory-saving idiom, and employs block graphic characters to render the cannon, alien, and laser bolts. The score variable S increments each time an alien is hit, and the game ends—displaying the score and restarting—when the player is destroyed or five aliens are defeated.

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Program Analysis

Program Structure

The program is a compact, single-file action game with no subroutines. Execution flows mostly through a main loop centred on lines 19–30, with branching to an end/reset block at lines 36–41. The overall structure is:

1. Initialisation (lines 5–14): Sets up speed constant V=10 (player row), score S=1 (via PI/PI), gun column G=5, and alien position X, Y.
2. Draw alien row header (lines 15–18): Prints a row of block-graphic shapes at the top.
3. Main loop (lines 19–30): Reads keys, moves the gun, moves the alien down, optionally fires an alien shot, and checks collisions.
4. Player fire (lines 31–34): Animates a bullet upward when M is pressed and tests for a hit.
5. Score/reset block (lines 35–41): Increments score, clears screen, checks win/death condition, prints score, pauses, and restarts.

Key Variables

Variable	Role
V	Player’s fixed row (10)
S	Score / kill count (starts at 1 via PI/PI)
G	Gun column position
X	Alien column position
Y	Alien row (counts down from 0 to V)
I	Loop counter / bullet row

PI/PI Idiom

The constant 1 is never written as a numeric literal; instead PI/PI is used throughout (lines 11, 13, 16, 22, 26, 31, 35, 37). On this platform, numeric literals are stored with a 5-byte floating-point value inline in the BASIC line, whereas a variable reference or expression reuses the existing FP value. Using PI/PI avoids storing multiple copies of the float value for 1, saving a small amount of RAM per occurrence. This is a well-known Sinclair BASIC size-optimisation idiom.

Movement and Control Logic

Line 19 updates the gun column in a single expression:

LET G=G+(INKEY$="C")-(INKEY$="Z")*(G>PI/PI)

Boolean expressions evaluate to 1 (true) or 0 (false), so pressing C increments G and pressing Z decrements it, but only when G>1, preventing movement off the left edge. There is no equivalent right-edge guard, so the gun can move off-screen to the right.

Line 21 steers the alien horizontally: LET X=X+(Y>3)-(Y<4). When Y>3 and Y<4 are simultaneously true (impossible for integer Y), the alien moves diagonally. For integer Y, if Y>3 it moves right; if Y<4 (i.e., Y≤3) it moves left. This creates a simple triangular zigzag as Y increases through 4.

Block Graphic Rendering

All sprites are built from ZX Spectrum block graphics characters:

• "\.'\. " — the decorative alien row at the top (lines 17–18)
• "\.:\. " — the player’s cannon sprite (line 20)
• "\'.\' " — the descending alien sprite (line 23)
• "\ :" — the laser bolt (lines 27, 32), immediately overwritten with " " to animate movement

Alien Fire Mechanic

Line 25 gives the alien a 20% chance each iteration to fire: IF RND<.8 THEN GOTO 30. When it does fire (the 20% case), lines 26–28 animate a descending bolt from Y+1 to V using a FOR loop, printing and immediately erasing each row. Line 29 then checks if the bolt’s column X matches the gun column G; if so, it jumps to line 38 (game over / score display).

Player Fire Mechanic

Line 30 checks for the M key. If pressed, lines 31–33 animate a bolt upward from V-1 to 1 (step -PI/PI = -1). Line 34 tests a hit: IF G<>X AND G<>X+PI/PI THEN GOTO 19. The two-column hit test (G=X or G=X+1) accounts for the alien sprite being two characters wide.

Win and Death Conditions

Line 37 is the critical branching point after a shot event:

IF S<>5 AND (ABS(X-G)>PI/PI OR Y<>V) THEN GOTO 13

This continues the game (spawning a new alien at line 13) if the score is not yet 5 AND the player was not hit (the alien column is not adjacent to the gun, or the alien hasn’t reached the player row). If S=5, or if the alien reached row V directly above the gun, execution falls through to line 38 to display the score.

Bugs and Anomalies

• No right-edge guard on gun: The left-movement guard (G>PI/PI) on line 19 protects the left boundary, but there is no equivalent for the right, allowing G to exceed the screen width.
• Score starts at 1: S is initialised to PI/PI (=1) on line 11, so the first kill gives S=2 and the win condition S=5 is reached after four kills, not five.
• Alien fire collision uses alien column, not bullet column: Line 29 tests X=G, where X is the alien’s column — which is also where the shot was fired from — so this check is correctly positioned for a vertically falling shot.
• Initial TAB on line 15 is not cleaned up: Line 15 prints TAB X without a following PRINT statement to finish the line, meaning the cursor is left mid-line before the FOR loop on lines 16–18 begins printing.