Clover

This program draws a geometric pattern called “Clover,” building overlapping diamond and square shapes arranged in a circular layout using PLOT and DRAW commands with arc parameters. It selects random ink, paper, and border colors at startup, avoiding colors 3–5 for the paper to ensure visibility. Three pattern modes (controlled by variable U) determine whether shapes are drawn as arcs, combined arcs and rectangles, or rectangles alone. After drawing the petal arrangement, the program floods the screen with vertical and horizontal fill lines in four passes before looping back to restart with an increased petal count.

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Program Analysis

Program Structure

The program is organized into three logical phases that loop indefinitely via GO TO 545 at line 680:

1. Initialization (lines 542–567): Sets brightness, chooses random colors, selects a pattern mode U and step increment F, draws a border rectangle.
2. Main drawing routine (lines 580–625): Iterates over L “petals” arranged around a central circle, drawing expanding diamond/square shapes at each angular position.
3. Fill lines (lines 635–680): Draws vertical and horizontal scan lines across the screen in four directional passes, pausing between each, then restarts the whole program.

Color Selection

The paper color A1 is chosen randomly at line 546 but colors 3, 4, and 5 (cyan, green, magenta) are rejected at line 547 via a retry loop (GO TO 546), leaving black (0), blue (1), red (2), and white (7) as valid paper colors. INK 9 selects transparent/contrast ink. BORDER is set to a random value each cycle.

Pattern Modes

Variable U is set to 1, 2, or 3 and controls which geometric primitives are drawn at each petal position:

U value	Shape drawn
1	Diamond arcs only (four-quadrant DRAW with angle W=0)
2	Diamond arcs plus axis-aligned rectangles
3	Axis-aligned rectangles only (lines 606–607 force V=42, skipping the arc loop)

Arc Drawing Technique

The four-quadrant diamond at each petal is drawn at line 610 using DRAW dx,dy,angle with W=0, which produces straight lines (zero-radian arc). Each call plots a side of a rotated diamond: DRAW V,-V,W, DRAW -V,-V,W, DRAW -V,V,W, DRAW V,V,W. Lines 616–617 repeat the pattern with V incremented by 1 to add a slightly larger outline, giving a double-trace effect.

Petal Positioning

Each petal center is calculated using polar coordinates at lines 595–600. The angle A steps from 0 to 2π over L iterations: A = T / (L/2) * PI. The center is placed at a fixed radius of 42 pixels from screen center (127, 87). The inner loop at line 605 expands V from R (a random starting radius 5–39) down to 42 in steps of F (1–6), creating layered concentric shapes.

Mode 3 Single-Iteration Trick

When U=3, lines 606–607 replace the normal FOR V=R TO 42 STEP F loop body with FOR V=42 TO 42, effectively forcing a single iteration at V=42. This is a clever inline conditional loop override — the outer FOR at line 605 is already entered, and the inner FOR at line 607 shadows V for one step, but because line 606 uses GO TO 610 to skip line 607 when U<3, the inner loop only runs in mode 3. However, the NEXT V at line 620 will serve the outer loop in modes 1 and 2 and the inner loop in mode 3, which may cause the outer loop to continue unexpectedly after the inner FOR completes — a potential behavioral anomaly.

Screen Fill Effect

Lines 640–670 draw lines across the entire screen in four passes — left-to-right verticals, top-to-bottom horizontals, right-to-left verticals, and bottom-to-top horizontals — each separated by a PAUSE G (where G=100 frames). The step size B=2 leaves every other column/row undrawn, creating a striped overlay effect over the clover pattern.

Loop Growth

At line 630, L is incremented by 2 each cycle, beginning at 6. This increases the number of petals drawn (and therefore the angular resolution of the circular arrangement) with each iteration. Since there is no cap on L, the petal count grows indefinitely, though in practice the pattern repeats visually once L becomes large relative to screen resolution.

Notable Variables

• L — petal count, starts at 6, increments by 2 each loop
• U — drawing mode (1=arcs, 2=arcs+rects, 3=rects)
• R — random starting radius for inner loop (5–39)
• F — step size for expanding shapes (1–6)
• W — arc angle parameter, fixed at 0 (straight lines)
• G — pause duration in frames (100)
• B — fill line spacing (2)