Tollbooth

This program simulates a toll booth traffic scene, animating vehicles moving down the screen toward toll booths and then being directed into lanes. Each vehicle is randomly assigned as either a car (“%C”) or a truck (“%T”), and the program tracks cumulative toll revenue — 30 cents per car and 60 cents per truck — stored in array element S(32). The road scene is drawn using block graphics characters to depict booth structures, lane dividers, and a curved on-ramp plotted pixel by pixel with PLOT and SIN. After all vehicles have been processed, the simulation pauses and then restarts with RUN.

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Program Structure

The program is divided into clearly separated routines accessed via GOSUB:

1. Initialisation (lines 10–180): Sets display mode, prompts for vehicle count, draws the static scene (booths, road markings, curved ramp).
2. Main loop (lines 185–399): Iterates HH times, each iteration spawning one vehicle, animating it down the screen, directing it to a lane, and updating the score.
3. Vehicle approach from top (lines 2000–2550): Animates the vehicle from row 20 down to row 9, with a random or type-forced lane-change decision at the midpoint.
4. Lane wander during descent (lines 300–360): Moves the vehicle from row 8 to row 2 with bounded random horizontal drift.
5. Toll lane assignment (lines 4000–4130): Snaps the vehicle’s column to the nearest lane, increments that lane’s counter, flashes a symbol, and updates the revenue display.
6. Revenue formatting (lines 5000–5030): Converts the running total to a string and pads it to two decimal places.
7. Reset subroutine (line 3000): Resets position variables I and II to column 15 for each new vehicle.

Scene Construction

The static road scene is assembled entirely from block graphics literals embedded in PRINT AT statements. The three booth structures (lines 75–90 and 120) use ▌, ▐, ▄, ▀, and ▟ characters to create box outlines. The curved on-ramp is drawn pixel-by-pixel in two loops (lines 130–180) using PLOT 24-24*SIN(N/72*PI),N and PLOT 39+24*SIN(N/72*PI),N, producing sinusoidal merge curves for each carriageway.

Vehicle Representation

Vehicles are represented as two-character strings stored in A$. The token %T represents a truck and %C a car. These are printed at a screen position and the previous position is erased by printing a space at the trailing coordinate stored in II.

Token	Type	Toll	Lane behaviour
%T	Truck	£0.60	Always forced right (RR=1)
%C	Car	£0.30	Random left or right lane

Lane Boundary Enforcement

During the descent (lines 305–308) the column variable I is clamped with four conditional guards:

• IF I<4 THEN LET I=4 — left road edge
• IF RR=0 AND I>13 THEN LET I=13 — right edge of left carriageway
• IF I<18 AND RR=1 THEN LET I=18 — left edge of right carriageway
• IF I>27 THEN LET I=27 — right road edge

This dual-carriageway model means lane direction (RR) is decided before the wander phase and then enforced throughout.

Toll Lane Snapping

At line 4000–4002, the vehicle’s column is snapped to one of several discrete lane positions using arithmetic rounding:

• Left lanes: I = 3*INT(I/3 + 1.5) - 2
• Right lanes: I = 3*INT(I/3 + 1.5) - 3

This maps any column within a 3-cell band to the same lane peg, giving deterministic booth assignment from approximate positions. The per-lane count is stored in array S(), indexed by column, and printed at row 0 above the booth.

Revenue Formatting

The subroutine at lines 5000–5030 converts the float S(32) (the accumulator element) to a string and patches missing decimal places manually: if the string has length 1 it appends ".00"; if the second-to-last character is "." it appends "0". This is necessary because STR$ does not guarantee two decimal digits for monetary values like 0.3, 0.6, 0.9.

Notable Techniques

• FAST/SLOW switching: FAST is used for drawing the static scene; SLOW is restored before animation so the display refreshes visibly.
• Accumulator in array tail: Using S(32) as a revenue accumulator within the same lane-count array is compact but relies on column indices never legitimately reaching 32.
• Flash effect: Lines 4050–4080 print %* then a space at the booth position, with short busy-wait loops (lines 4030–4070), simulating a flash to acknowledge payment.
• Auto-restart: Lines 396–399 pause, switch to FAST, clear the screen, and issue RUN to restart continuously without user intervention.