GHOTI

GHOTI is an arcade-style fish simulation game in which the player maneuvers a fish character around a swamp screen, eating rising bubble plankton while avoiding diving turtles. The player’s fish is moved using the arrow keys (5, 6, 7, 8), and three UDG characters—defined by POKEing 55 bytes of data into USR “a” through USR “f”—represent the fish and turtle sprites. A time-based life system ties difficulty to a skill level chosen at startup (1–9), where the multiplier of 40 produces time values from 40 to 360. Line 7011 contains a notable loop anomaly: a FOR loop with a STEP of 5 iterating from 0.01 to 0.100 executes only once due to the step size, effectively making it dead iteration code. Turtles appear at random intervals via line 360, momentarily freezing the swamp and potentially costing the player lives.

---

Program Analysis

Program Structure

The program is organized into several functional regions identified by their line number ranges:

1. Lines 1–8: Initialization — REMs, border/paper/ink setup, jump to setup routine at 9100 (via 6), then subroutine at 1000, then main loop entry at 500.
2. Lines 120–480: Main game loop — bubble movement, player input, collision detection, time management, and turtle triggering.
3. Lines 500–600: Bubble respawn and bubble-eaten scoring.
4. Lines 1000–1950: Initialization subroutine — draws the seabed, loads UDG data, and sets starting variables.
5. Lines 6100–6300: Bubble-eat animation and sound.
6. Lines 7000–7100: Turtle attack sequence.
7. Lines 7501–7530: Starvation (time=0) death sequence.
8. Lines 7910–8010: Eaten-alive death sequence and restart prompt.
9. Lines 8995–9030: Win sequence (10 bubbles eaten).
10. Lines 9132–9167: Title screen, instructions, skill level selection, and game launch.
11. Line 9999: SAVE with auto-run.

The main loop does not use a structured WHILE or FOR construct; instead it relies on GO TO 120 at line 480 to cycle continuously, with exits triggered by conditional branches throughout.

UDG Sprite Definition

Lines 1080–1140 define the custom character graphics. The subroutine POKEs 55 bytes of data (read via READ t) into memory starting at USR "a", defining characters \a through \f (approximately 6 UDGs, each 8 bytes). These are used for the fish sprite (\a\b or \d\e for left/right facing) and turtle sprite (\g\f). A RESTORE is called after loading to reset the DATA pointer for any subsequent reads.

Player Input and Movement

Player movement uses a two-iteration FOR loop (lines 135–153) that samples INKEY$ twice per game cycle, reducing missed keypresses. The arrow key mapping is:

Key	Action
5	Move left (decrease b)
8	Move right (increase b)
6	Move down (increase a)
7	Move up (decrease a)

Boundary clamping is enforced at lines 150–152, restricting the fish to rows 7–15 and columns 1–29. The fish sprite direction (left/right) is chosen based on a re-check of INKEY$ at line 154: if “5” is still pressed, the left-facing UDG pair \d\e is printed; otherwise the right-facing pair \a\b is used.

Bubble Mechanics

Bubbles are represented by the character “O” printed with INK 2. Each bubble spawns at a random column (y=INT(RND*25)) near the bottom of the screen and rises one row per game cycle (line 350: LET x=x-1). A slight horizontal drift is added each cycle at line 121 (LET y=y+RND*2), with clamping at line 125 to keep bubbles on screen. Collision between fish and bubble is detected at line 157 using ATTR comparison — a color-attribute trick rather than coordinate comparison, meaning ink/paper attribute equality triggers the pickup.

Turtle Attack Sequence

Turtles are spawned probabilistically at line 360 (IF INT(RND*20)=1), roughly a 5% chance per cycle. The turtle routine (lines 7000–7100) loops for a random number of turtles (1 to 5), printing the \g\f UDG sprite pair. Collision with a turtle (again via ATTR comparison at line 7035 and 7075) deducts a life. If lives reach zero, the game ends. The variable t at line 7012 caches the life count so that line 7085 (IF t<>lives THEN GO TO 120) can exit the turtle loop early if a life was lost.

Skill Level and Timing

At line 9155, the skill level is captured: LET time=VAL INKEY$*40. This maps key presses “1”–”9″ to time values 40–360. The validation at line 9156 rejects values outside 30–360, effectively requiring a key of 1–9. The time counter decrements each game cycle at line 156 and triggers a starvation death at line 164 when it reaches zero. Lines 162 apply an INK 3 blank (\ ) to erase trailing digits when time drops below 100 or 9, compensating for fixed-width PRINT positioning.

Notable Techniques

• ATTR-based collision detection (lines 157, 7035, 7075): Uses ATTR(row,col) equality rather than coordinate matching, leveraging the display file’s color attributes as a proxy for object identity.
• Double INKEY$ sampling (lines 135–153): A FOR loop of 2 iterations reads the key twice per frame to improve responsiveness without delaying the game loop.
• Seabed rendering (lines 1000–1075): The initialization subroutine fills the bottom portion of the screen with randomly colored “O” characters column by column using nested FOR loops, creating the primordial slime backdrop.
• VAL INKEY$ idiom (line 9155): Converts the skill key press directly to a numeric value in a single expression.

Bugs and Anomalies

• Line 6 references line 9100 which does not exist: GO TO 9100 will fall through to line 9132 (the title screen), which is the intended behavior — a well-known technique for jumping to the nearest following line.
• Line 7011 — degenerate FOR loop: FOR r=.01 TO .100 STEP 5 iterates only once because the STEP of 5 immediately exceeds the TO value of 0.1. The inner BEEP loop and the cr counter logic never accumulate beyond 1, and the IF cr=5 THEN GO TO 7012 branch is never taken. This renders the turtle sound effect loop effectively non-functional as a loop.
• Line 7070 — negative AT coordinates: PRINT AT -o,-20-h+o can produce negative row and column values depending on o and h. On Spectrum/TS2068, this will typically produce an “Invalid Argument” error or wrap unpredictably; this appears to be a logic error in the turtle exit animation.
• Line 7500 is missing: The starvation path at line 164 jumps to GO TO 7500, but line 7500 does not exist. Execution falls through to line 7501, which is the correct starvation handler — again an intentional line-skip technique.
• Line 8990 is referenced but absent: GO TO 8990 at line 160 (win condition) falls through to line 8995, which correctly suppresses key buffering before displaying the congratulations screen at line 9000.