Slalom

This ZX81/TS1000 BASIC program implements a horizontal slalom skiing game in which the player steers a dot through six gates displayed across the screen. Gates are randomly positioned using an array of six Y-coordinates, each drawn as two pixels with a one-pixel gap between them. The player uses keys “7” and “6” to move the dot up and down respectively, with movement handled by subtracting the INKEY$ comparisons directly in arithmetic. Line 100 tests gate collisions using a compact Boolean expression: `30 AND V>=C(B/10) AND V<=C(B/10)+1` routes to line 110 (continue) or line 138 (a non-existent line, triggering a BASIC error/stop) depending on whether the dot passes cleanly through the gate gap.

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Program Analysis

Program Structure

The program is divided into three logical phases: setup, play loop, and restart.

1. Setup (lines 10–70): Dimensions array C(6), clears the screen, then randomly places six gates. Each gate occupies column A*10 and consists of two pixels at C(A)-1 and C(A)+2, leaving a one-pixel gap at C(A) and C(A)+1 for the player to pass through.
2. Play loop (lines 80–150): Iterates B from 3 to 63 (the horizontal pixel range), moving the player dot column by column. At every gate column (B/10 is an integer), a collision check is performed.
3. Restart (line 160): RUN loops the game indefinitely with fresh random gates each time.

Gate Drawing

Each gate is drawn with two PLOT calls: one at C(A)-1 (below the gap) and one at C(A)+2 (above the gap). This leaves exactly two pixels of clear passage at Y positions C(A) and C(A)+1, which is the window the player’s dot must occupy. Gate X positions are 10, 20, 30, 40, 50, 60, all within the ZX81’s 64×44 pixel display.

Collision Detection Technique

Line 100 uses a compact boolean arithmetic trick for gate collision:

100 IF B/10=INT(B/10) THEN GOTO 80+(30 AND V>=C(B/10) AND V<=C(B/10)+1)

On the ZX81, boolean expressions evaluate to 1 (true) or 0 (false). The sub-expression 30 AND V>=C(B/10) AND V<=C(B/10)+1 evaluates to 30 if the player is inside the gate gap, or 0 if not. This is added to 80, producing either GOTO 110 (pass — continue plotting) or GOTO 80… wait, actually 80+0=80 sends execution back to line 80 which resets V to C(1), effectively resetting the player to the first gate’s Y position rather than stopping play. When the player is safely through the gap, the target is 80+30=110, which falls through to the next line normally.

Player Movement

Line 130 handles input using arithmetic on INKEY$ comparisons:

LET V=V+(INKEY$="7")-(INKEY$="6")

This is a standard ZX81 BASIC idiom. If “7” is held, the boolean true (1) is added, moving the dot up. If “6” is held, 1 is subtracted, moving it down. Both keys pressed simultaneously cancel out. There is no boundary clamping, so the dot can wander off-screen.

Animation Method

The dot is animated by plotting at the new position (PLOT B,V) and then unplotting the previous position (UNPLOT B,X) using the saved value X=V. Since B advances by one pixel per iteration, this produces a smooth single-pixel trail-free movement across the screen.

Variable Summary

Variable	Role
C(6)	Array of gate Y-coordinates
A	Gate index (1–6) during setup
B	Current horizontal pixel column (3–63)
V	Current vertical position of player dot
X	Previous vertical position (for UNPLOT)

Notable Anomalies

• When a gate is missed (GOTO 80), rather than ending the game, the player’s Y position is silently reset to C(1) (the first gate’s Y value) and the loop continues. This may be an unintended consequence of the 80+0 arithmetic rather than a deliberate design choice — a “game over” would more typically jump to a non-existent line.
• Gate Y-coordinates range from 13 to 25 (INT(RND*12)+14), keeping them well within the lower half of the 44-pixel-high display.
• The play loop starts at B=3, giving the player a small lead before the first gate at column 10.