Santa’s Dilemma

“Santa’s Dilemma” is a Christmas-themed herding game in which the player uses the cursor keys (5–8) to guide Santa around a screen full of randomly placed trees and bushes, nudging eight reindeer into a pen before a timer runs out. The program defines four UDG characters (slots a–d) loaded from DATA statements to represent a tree top, tree trunk, a bush/reindeer figure, and Santa himself. Screen attributes are used heavily for collision detection: ATTR values of 56 (plain paper 7/ink 0) indicate open ground, while 57 (ink 1) identifies a reindeer and 60 (ink 4) marks tree cells. A built-in music routine at line 600 plays a fragment of “We Wish You a Merry Christmas” using BEEP with rests encoded as data value 99 triggering PAUSE instead of a note. The score is derived from the system frames counter at addresses 23672–23673, converting elapsed time into a “presents” figure that starts at 65000 and counts down.

---

Program Analysis

Program Structure

The program is organized into clearly delineated blocks separated by REM statements:

1. Lines 60–120: UDG initialization — four characters loaded from DATA.
2. Lines 450–690: Subroutine block — tree printer (510), keypress wait (530–570), and carol player (600–690).
3. Lines 1000–1290: Title screen and two pages of instructions.
4. Lines 1300–1590: Variable initialization and screen setup.
5. Lines 1600–2070: Main game loop — timer display, Santa movement, reindeer herding, win/lose logic.
6. Lines 3000–3080: Keyboard-reading subroutine, mapping key codes to direction index z.
7. Lines 5000–5010: Direction and reindeer offset DATA.

Execution begins at line 10 via the LINE 10 parameter in the SAVE command, and line 450 immediately redirects to the title sequence at 1000 with GO TO 1000, bypassing the subroutine block.

UDG Characters

Four UDGs are defined at lines 60–120, occupying slots a–d (characters 144–147):

UDG	Use
\a	Tree top (printed in INK 4, cyan)
\b	Tree trunk (printed in INK 3, yellow)
\c	Reindeer / bush figure (INK 1, blue)
\d	Santa (INK 2, red)

Only 32 bytes (4 × 8) are poked, covering exactly four UDG slots starting at USR "a".

Collision Detection via ATTR

Rather than maintaining a separate map array, the game reads screen attribute bytes directly using the ATTR(y,x) function. The color combinations used as tokens are:

• 56 — PAPER 7, INK 0 (white background, black ink): passable empty ground.
• 57 — PAPER 7, INK 1 (white background, blue ink): a reindeer cell.
• 60 — PAPER 7, INK 4 (white background, cyan ink): a tree top cell, used to prevent stacking trees at line 1030/1500.

Santa can only move to cells with ATTR 56 (line 1690), and a reindeer is only shoved if the cell it would move into also has ATTR 56 (line 1780). This means visual appearance and game logic are tightly coupled to the color scheme chosen.

Timer and Scoring

The game uses the system frames counter held in addresses 23672–23673 as a real-time clock. At line 1610, b = PEEK 23672 + 256*PEEK 23673 gives elapsed frames, and the score p = 65000 - b counts down from 65000. If p exceeds 60000 it is capped there (line 1620), preventing an artificially high score if the game is reached very quickly. The game-over condition triggers when p drops below 30000 (line 1640), which at 50 frames per second corresponds to roughly 700 seconds of play. The POKE at line 1590 resets the frame counter to zero at the start of each game.

Direction and Movement Data

Line 5000 stores four direction vectors for Santa (up, down, left, right as row/column deltas). Line 5010 stores eight offset pairs used to check all eight surrounding cells for reindeer. The subroutine at line 3000 maps raw key codes to a direction index z (1–4):

• Keys 5–8 (codes 53–56): mapped by subtracting 52, yielding z = 1–4.
• Cursor keys (codes 8–11, i.e., left/right/down/up arrow CHR$ codes): explicitly mapped to z = 1–4.
• Any other key sets z=0, causing the loop to skip movement.

Reindeer AI

The “AI” is purely reactive: for each of the eight surrounding cells, if a reindeer occupies it (ATTR 57) and the cell directly beyond it in the same direction is empty (ATTR 56), the reindeer is moved one step away from Santa. This creates a simple repulsion effect. The eight offset vectors stored in m() (4 entries) and r() (8 entries) are loaded from DATA at lines 5000–5010 using RESTORE 5000 at line 1320. Note that DIM m(4,2) at line 1320 is read from only four pairs (for Santa movement), while DIM r(8,2) at line 1340 uses eight pairs for reindeer detection — both sourced from the same DATA block.

Carol Music Routine

The subroutine at lines 600–690 plays “We Wish You a Merry Christmas” using BEEP duration,pitch pairs read from DATA. A sentinel value of 99 in the duration field triggers a PAUSE b (using the pitch field as the pause duration) instead of a BEEP, implementing rests without a separate data format. The FOR i=1 TO 29 loop at line 620 counts data pairs rather than notes, so rests consume loop iterations alongside real notes; this means the loop count must be set carefully to match the DATA length.

Pen Gate and Win Condition

The pen is drawn on the left side of the screen (column 5, rows 7–13) using INVERSE characters, with a gap at row 10 (line 1440) acting as the entrance. The game detects a reindeer entering the pen by checking ATTR(10,5) = 57 at line 1810 — i.e., whether a reindeer (blue ink) occupies the gate cell. Each capture increments tr, erases that cell, and prints a reindeer icon in the pen interior (line 1870: AT 6+tr,0). When tr reaches 6 the win sequence fires. A subtle limitation is that only one reindeer can be counted per main loop iteration because only one gate cell is polled.

Keypress-Wait Idiom

Lines 550–560 implement a clean edge-triggered key wait: line 550 loops while INKEY$ is non-empty (flushing any held key), then line 560 loops while it is empty (waiting for a fresh press). This avoids the common problem of a held key from a previous screen immediately advancing the next screen.

Bugs and Anomalies

• Line 1700 checks bounds y<0 OR y>21 OR x<0 OR x>31 after the ATTR call at line 1690, so if the computed coordinates are off-screen, the ATTR call at 1690 executes first and may return an unexpected value rather than causing an error — this ordering could be reversed for robustness.
• The DIM r(8,2) array is filled with eight pairs from DATA at line 5010, but the DATA at 5000 already supplies four pairs; the RESTORE at 1320 resets the pointer to 5000, so both arrays are filled sequentially from the same stream — this works correctly but ties the two arrays’ DATA together implicitly.
• At line 1030/1500, trees are prevented from overlapping by checking ATTR of y+1,x (the trunk row) for value 60 (cyan/tree-top color), but the trunk itself uses INK 3 (yellow). This means the check detects the top of a previously placed tree at the candidate trunk position, not a trunk-on-trunk collision — an effective heuristic but not a fully rigorous overlap test.
• Line 1940 contains the word “alot” (should be “a lot”) — a typographical error in the original listing.