Mountie

This program draws a detailed illustration of a Royal Canadian Mounted Police officer on horseback using a compact run-length encoded DRAW command sequence. The image data is stored as strings of digit pairs in lines 40–90, where each pair encodes X and Y delta values for successive DRAW calls; a value of 10 signals a negative (two-digit) coordinate, decoded in the subroutine at lines 160–200. After rendering the horse and rider, the program animates the text “R.C.M.P.” scrolling from column 30 leftward across row 20 using INK 2 (red). The technique, credited to D.J. Currie in line 400, is described as a “map making method” — essentially a custom vector graphics encoder that compresses complex artwork into printable ASCII digit strings for compact BASIC storage.

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Program Analysis

Program Structure

The program divides into three logical phases:

1. Initialization (lines 10–30): Sets black border/paper with white ink, defines global variables w=10, q=10, e=0, and positions the initial PLOT point at (121,91).
2. Vector drawing (lines 40–150): Loads encoded path data into l$ in labeled segments (horse’s neck/head, forelegs, tail, hat, neck, mountie body), calls the decode subroutine at line 160, then draws additional details with explicit PLOT/DRAW calls.
3. Animation (lines 210–380): Scrolls the characters R, ., C, ., M, ., P, . rightward-to-leftward across row 20 in red (INK 2), each character in its own FOR loop ending one column further left than the last.

Encoding Scheme — The “Map Making Method”

The core technique, attributed to D.J. Currie in line 400, encodes a sequence of DRAW delta pairs as a string of decimal digits. The decode subroutine (lines 160–200) walks through l$ two characters at a time, reading an X delta and a Y delta for each DRAW call.

Because DRAW accepts signed values but individual digits are only 0–9, a sentinel value of 10 (the digit w, since w=10 at initialization) signals that the next digit should be negated. This allows negative offsets to be encoded using just two characters instead of a minus sign, keeping the string compact.

The variables w, q, and e are pre-assigned as:

Variable	Value	Role in encoding
w	10	Sentinel for negation (also a coordinate value of 10)
q	10	Same sentinel value — appears to be an alias; both w and q equal 10
e	0	Zero delta (used for purely horizontal or vertical steps)

Since VAL is applied character-by-character via VAL l$(n), each single character is interpreted as its numeric digit value (0–9). The variable names w, q, and e do not appear literally in the string — they are used only in the subroutine comparison IF x=10 and assignment logic, while the string data itself contains the raw digit characters 0–9.

Decode Subroutine (Lines 160–200)

The subroutine operates as a simple two-pointer loop:

• Line 170 checks termination: if n >= LEN l$, return. Note this uses >= rather than >, so a string of odd length would skip the final character rather than cause an error.
• Lines 180–190 read x and y from consecutive characters, applying negation if the digit is 10 (consuming an extra character in that case).
• Line 200 advances n past the Y character and issues DRAW x,y, then loops back.

A subtle issue: line 180 increments n by 1 when a negation sentinel is found, but the subsequent LET n=n+1 at the start of line 190 advances n again. This means the pointer correctly lands on the Y digit after consuming either a 1-digit or 2-digit X value. The same pattern repeats in line 190 for the Y value, with n advanced once more at the start of line 200.

Coordinate System and PLOT Usage

The program uses multiple anchor PLOT calls to reposition the drawing cursor between segments — line 30 sets (121,91) for the main horse/rider body, line 110 draws the reins explicitly, and line 140 draws the ground line. Line 120 re-anchors at (104,90) and loads a new string for what appears to be the rifle or lance detail. This segmented approach lets each l$ string describe relative offsets from a known starting point.

Animation Loop

Lines 210–370 animate “R.C.M.P.” by printing each character with a trailing space across progressively shorter column ranges (from column 30 down to 12, 13, 14, … 19 respectively), creating a left-scrolling marquee effect at screen row 20 using INK 2 (red on black paper). The trailing space effectively erases the previous character position as each loop advances.

Notable Techniques

• String concatenation across multiple lines (40–90) to build a long path while keeping individual line lengths manageable.
• Single-character VAL extraction (VAL l$(n)) as an efficient way to convert ASCII digit characters to integers without a lookup table.
• Use of pre-assigned single-letter variables (w, q, e) as named constants embedded within the data string interpretation logic, improving readability of the encoded data.
• SAVE "Mountie" LINE 10 at line 420 saves the program with an auto-run directive.

Potential Anomalies

• Both w and q are initialized to 10 (lines 20). Since the subroutine only tests IF x=10 and IF y=10, both variable names produce identical behavior. The dual naming likely reflects the author using q as a visually distinct glyph in the string data for readability, while both resolve to the same sentinel value.
• Line 390 is a blank REM-less line, serving only as a visual separator in the listing.
• The STOP at line 380 halts the program after animation; there is no loop back or prompt, making the program a one-shot display.