Sniper

Sniper is a reaction-based shooting game where a figure appears at a random horizontal position on screen and the player must press the correct number key before it disappears. The figure is drawn using block graphics and inverse-video characters across four rows, giving it a simple but recognizable silhouette. The target position variable X is multiplied by 3 to space figures across the screen in columns, while T is computed as (X/3 + 1) × SGN(9 − X/3) to map each position to a unique digit key the player must press. The window of opportunity shrinks as the counter C increases, since PAUSE 90−C gives progressively less time to react. POKE 16437,255 is used repeatedly to clear the keyboard buffer before checking INKEY$, preventing stale keypresses from registering as hits.

---

Program Analysis

Program Structure

The program is structured as a single main loop running from line 70 to line 300. Initialization (score and counter) happens once at lines 50–60, and the game resets when the counter C reaches 40 at line 260, which loops back to line 40 to re-initialize everything. The overall flow is:

1. Initialize score S=0 and counter C=0 (lines 50–60)
2. Choose a random column position X and compute the expected key T (lines 70–80)
3. Draw the target figure at column X*3 (lines 90–130)
4. Pause for a decreasing time window, then test for the correct keypress (lines 140–160)
5. If correct, draw a “hit” figure, update and display score, and pause (lines 170–240)
6. Clear screen, increment counter, pause a random interval, and repeat (lines 250–300)

Position and Key Mapping

The random column index is generated as X = INT(RND * 9.9), giving values 0–9. This is then scaled to screen columns with X = X * 3, spacing figures across 30 columns in steps of 3. The expected key digit T is calculated at line 80 as (X+1) * SGN(9-X) (using the pre-scaled X). For values 0–8 this yields digits 1–9 since SGN(9−X) = 1; for X=9 it yields 0 since SGN(0)=0. This neatly maps each of the ten positions to a unique digit key 1–9 and 0.

Difficulty Progression

The reaction window is controlled by PAUSE 90-C at line 140, where C increments by 1 each round (line 290) and resets to 0 every 40 rounds. This means the display time shrinks from approximately 1.5 seconds at the start of a cycle down to about 0.8 seconds by round 40, creating a gradual difficulty ramp within each 40-round cycle.

Keyboard Buffer Management

POKE 16437,255 appears at lines 150, 240, and 280. Address 16437 is the LAST K system variable, which holds the last key pressed. Poking it with 255 effectively invalidates the stored keypress, ensuring that INKEY$ only returns a fresh keypress made during the current window. This prevents a key held down from a previous round from accidentally scoring a hit.

Graphics Techniques

The target figure spans four PRINT AT rows and uses a combination of block graphics escapes and inverse-video characters. The head is drawn as " O", the body uses inverse-space characters (shown as filled blocks) for a torso, and the legs use block graphic characters. The “hit” animation at lines 170–200 replaces the figure with a different arrangement suggesting a falling or exploding character, providing visual feedback for a successful shot.

Line	Row offset	Content description
100	Row 4	Head: space + “O”
110	Row 5	Shoulders: block graphics with inverse space (torso)
120	Row 6	Body: space + inverse-space + two spaces
130	Row 7	Legs: block graphics with inverse space

Notable Idioms and Anomalies

• STR$ T at line 160 converts the numeric digit T to a string for comparison with INKEY$, which always returns a string — a necessary type-matching technique.
• The inter-target pause at line 270, PAUSE INT(RND*120)+30, adds 30–149 frames of random delay between figures, preventing the player from anticipating timing.
• The REM "SNIPER" at line 10 serves as the program title; lines 40–300 form the actual code, with the gap from 10 to 40 being cosmetic.
• Line 310 (SAVE "1016%7") and line 320 (LIST) are utility lines appended after the main program body and are not part of normal execution flow.
• There is no explicit “Game Over” message or final score display when the 40-round cycle completes — the game simply resets silently at line 260.