Time Bomb

“Time Bomb” is a reaction and knowledge game that displays analogue clock faces on screen and challenges the player to read the time correctly within a countdown. The program generates random times using RND, filters them by difficulty level (1–5), where level 1 only allows times on the hour, level 2 on the half-hour, and so on down to five-minute and one-minute increments. Clock hands are drawn using trigonometric PLOT loops: a minute hand of length 14 units and an hour hand of length 10 units, both calculated with SIN and COS from the centre point (33, 22). A custom line-drawing subroutine at line 1000 steps along the dominant axis to plot each hand pixel by pixel. The player has five seconds on the counter and wins by reaching 10 correct answers; a wrong answer costs one count, a correct answer adds one, and reaching zero triggers an animated “BOOM” loop.

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Program Analysis

Program Structure

The program is divided into four logical sections:

1. Lines 1–20: Introduction, rules display, and difficulty level input.
2. Lines 30–410: Main game loop — time generation, clock rendering, and response checking.
3. Lines 1000–1120: Subroutine to draw a clock hand as a plotted line.
4. Lines 2000–2120: Win and lose animations (flashing inverse-video loops).

Time Generation and Difficulty Filtering

A random integer between 100 and 1299 is generated at line 40 and treated as an HHMM time string. The minutes portion M$ is extracted by taking characters 3 onwards of the zero-padded string T$. Lines 80–120 reject invalid or unsuitable values in a retry loop:

• Line 80: Rejects minutes ≥ 60.
• Line 90: Level 1 — only exact hours (minutes must be 0).
• Line 100: Level 2 — minutes must be a multiple of 30.
• Line 110: Level 3 — multiples of 15.
• Line 120: Level 4 — multiples of 5.
• Level 5 (implicit): any valid time passes through.

This rejection-sampling approach is simple but can loop many times at lower levels since only a small fraction of random values satisfy the constraints.

Clock Face Rendering

The clock is drawn in two passes. First, hour numerals 1–12 are placed with PRINT AT using the formula AT 10-8*COS(D/6*PI), 16+8*SIN(D/6*PI) (line 160), positioning each digit on a circle of radius 8 character cells. Second, the clock rim is plotted as 60 individual points on a circle of radius 20 pixels centred at (33, 22) using PLOT with SIN/COS (lines 190–200).

Hand Calculation

The minute hand endpoint is computed at lines 240–260 using L = VAL M$ / 30 * PI, giving a full circle across 60 minutes, with hand length 14 pixels. Hour hand position is calculated at lines 210–300: the hour value H starts as twice the integer hours and is incremented at lines 220–230 if minutes are past the quarter or three-quarter marks (a coarse but functional approximation of hour-hand sweep). The angle G = H/12*PI drives a hand of length 10 pixels.

Line-Drawing Subroutine (Lines 1000–1120)

Rather than using a Bresenham algorithm, the subroutine draws each hand by stepping from the clock centre (33, 22) toward the computed endpoint (D, E). The dominant axis difference is found at line 1070:

C = (A AND ABS A >= ABS B) + (B AND ABS B > ABS A)

This uses ZX81 BASIC’s numeric AND to select whichever of the horizontal or vertical span is larger as the step count, ensuring a roughly continuous line. The guard at line 1080 prevents division by zero when the endpoint equals the centre. Each point is plotted by stepping F from 0 to C and interpolating both coordinates.

Response Handling

The player enters the time as a plain integer (e.g., 130 for 1:30). The response is compared directly to TIME (the raw random integer) at line 350. A correct answer increments COUNT; reaching 10 wins. A wrong answer decrements it; reaching 0 loses. The game then loops back to line 40 for a new time.

Win/Lose Animations

Both end states use infinite GOTO loops. The win screen (line 2010) alternates a normal and inverse-video “YOU DID IT” on each pass. The lose screen (lines 2110–2120) similarly alternates “BOOM” in normal and inverse video, creating a flashing effect.

Bugs and Anomalies

• The hour-hand approximation (incrementing H at the 15- and 45-minute marks) means the hour hand only takes two intermediate positions per hour rather than sweeping continuously, which can make some times ambiguous to read.
• The difficulty level filter has no upper bound check; entering a level above 5 will accept any valid time (same as level 5) since no rule rejects it.
• The INPUT RESPONSE at line 340 expects a numeric value; entering a non-numeric string would cause an error rather than a graceful wrong-answer response.
• Variable D is reused as both the loop variable in the numeral-drawing FOR loop (line 150) and as the x-endpoint of clock hands (lines 260, 300), which is not a problem in practice because the hand calculations happen after the loop exits.