Squash – Basic

This program implements a single-player Squash game in which a ball bounces around an enclosed court and the player controls a bat at the bottom using the P and Q keys. The court walls are drawn using inverse-video characters for the sides and block graphics (▄) for the top, while the bat is rendered with a block graphic character. Ball movement is tracked with direction variables L and M, and the collision logic at line 130 uses a chained comparison idiom to detect wall bounces. Scores are tracked separately for the Sinclair (computer) and the player, with the first to reach 10 winning the match.

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Program Analysis

Program Structure

The program is divided into four logical sections:

1. Initialisation (lines 2–9): Sets up all game variables and calls the title/intro subroutine at line 1000.
2. Court drawing (lines 15–75): Draws the side walls, top wall, and score labels using PRINT AT.
3. Main game loop (lines 80–210): Handles bat movement, ball movement, ball rendering, and collision detection in a tight GOTO 80 loop.
4. Subroutines (lines 1000–3110): Intro screen (1000), Sinclair scores a point (2000), player scores a point (3050).

Variable Usage

Variable	Purpose
C	Sinclair (computer) score
F	Player score
X, Y	Ball column and row offset within court
L, M	Ball horizontal and vertical direction (±1)
H	Bat horizontal position (column 8–17)

Court Rendering

The side walls (lines 15–35) are drawn using the inverse-video space character "% " (inverse space), producing solid vertical bars. The top wall (lines 40–60) uses the block graphic \".." (▄, lower-half block) to form a horizontal ceiling. The bat at line 80 is rendered with "\ '' " (a combination of block graphic characters), giving it a two-cell wide appearance.

Main Game Loop Mechanics

The loop begins at line 80, where the bat position H is updated using a single-line Boolean arithmetic expression — a classic Sinclair BASIC idiom. INKEY$="P" and INKEY$="Q" evaluate to 1 (true) or 0 (false), allowing movement in either direction without branching:

LET H=H+(INKEY$="P" AND H<17)-(INKEY$="Q" AND H>8)

This both reads input and constrains the bat within the court boundaries (columns 8–17) in a single statement.

Ball Movement and Wall Bouncing

The previous ball position is erased at line 120 by printing a space, then new coordinates are computed and the ball is redrawn at line 170. Horizontal bouncing (line 130) uses a chained comparison expression:

IF L+X>9<=L+X<0 THEN LET L=-L

This is an idiomatic ZX81/TS1000 BASIC technique exploiting left-to-right Boolean chaining: the expression fires if the new X would be out of bounds in either direction. Vertical bouncing (line 140) uses a more conventional OR check.

Collision Detection

Lines 190 and 200 check whether the ball has reached the bottom row (Y=8). If ABS(H-X)>=9, the ball has missed the bat entirely and the Sinclair scores (subroutine 2000). If ABS(H-X)=8, the ball is touching or just beside the bat and the player scores (subroutine 3050). The boundary of 8 here corresponds to the bat’s position relative to the court origin.

Scoring Subroutines

Both scoring subroutines (2000 and 3050) follow the same pattern: increment the relevant counter, print it at the correct screen position, check if the score has reached 10, and either return to the game loop or end the game with a congratulatory/commiserating message and STOP. The player’s score label reads “YOUR SCORE IS” and the Sinclair’s label reads “SINCLAIR SCORE”, updated in-place using PRINT AT.

Notable Techniques and Anomalies

• Boolean arithmetic for bat movement and boundary clamping in a single LET statement (line 90) — avoids multiple IF branches.
• Chained comparison in the horizontal bounce condition (line 130) — a compact but non-obvious idiom.
• The intro loop at line 1140 uses IF INKEY$<>"P" THEN GOTO 1140, a standard busy-wait for a key press.
• Lines 3110–3130 (CLEAR, SAVE, RUN) are unreachable dead code following STOP at line 3100; they would only execute if somehow reached by fall-through, which cannot happen.
• There is no mechanism for the ball to actually “bounce” off the bat — the player scoring subroutine only awards a point without reversing the ball’s vertical direction, meaning a successful return is not simulated; the ball always travels in one direction vertically until it reaches the bottom.
• The REM statements at lines 10, 65, and 180 use inverse-video characters to display readable labels when the program is listed, a common ZX81 documentation technique.