Stumper

This program implements a two-player number-comparison card game called “Stumper,” in which the human player and the computer each hold ten randomly generated numbers (up to 7 digits, produced by multiplying four RND values to skew the distribution toward smaller numbers). On each turn the human selects one of their numbers by pressing a key (0–9), and the computer automatically picks its best counter-move: the smallest computer number that still beats the player’s choice, or if none exists, it uses any remaining positive value. Wins are tracked with counters AA and BB, and at the end of ten rounds the program randomly selects one of three taunting messages for each outcome. The subroutine at line 9000 draws a pixel grid using PLOT to create a scoreboard background, and right-aligned number display is achieved by padding with spaces from the pre-dimensioned empty string X$(32).

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Program Analysis

Program Structure

The program is divided into four logical sections:

1. Initialisation (lines 10–36): Sets up string and numeric arrays, zeroes score counters, and calls the board-drawing subroutine.
2. Round setup (lines 40–120): Runs in FAST mode to generate and display ten random numbers for each side.
3. Game loop (lines 200–590): Ten turns of human input, computer response, comparison, and score update.
4. End-of-game (lines 600–870): Evaluates totals, branches to randomised taunt messages, then restarts.
5. Board subroutine (lines 9000–9160): Draws the pixel grid background and column headers using PLOT.
6. Save line (9998–9999): SAVE "STUMPER" followed by RUN, a common ZX81/TS1000 pattern to relaunch after saving.

Random Number Generation and Distribution

Each number is generated as INT(10000000*RND*RND*RND*RND). Multiplying four independent uniform random variables produces a heavily right-skewed (Beta-like) distribution concentrated near zero. This means most values are small, making it less likely the computer will always find a winning counter, and keeping games competitive despite the computer’s greedy strategy.

Computer Strategy

The computer’s move selection (lines 300–360) is a greedy “smallest winning value” approach:

1. Scan B(I) for the first value greater than the player’s chosen A(O); if found, use it (line 320).
2. If no winning value exists, fall through to lines 340–360 and pick any remaining positive value.

This is a simple but not optimal strategy; it finds the first (not the smallest) computer value that beats the human, because the array is unsorted.

Right-Aligned Number Display

Numbers are right-aligned in an 8-character field using a clean idiom. X$(32) is dimensioned but never filled, so it contains 32 spaces. The expression X$( TO 8-LEN C$)+C$ pads C$ on the left with spaces to exactly 8 characters, which is then stored in A$(N) or B$(N) (lines 85–97). This avoids any explicit loop or TAB arithmetic.

Human Input Handling

Input (lines 220–240) reads a single keypress via INKEY$ and validates it by checking that its character code falls between 28 and 37 (the ZX81/TS1000 codes for digits ‘0’–’9′). VAL O$+1 converts the digit character to a 1-based array index. Line 250 also guards against re-selecting an already-played number by checking whether the slot has been blanked to eight spaces.

Randomised Taunt Messages

Instead of a single end-of-game message, the program selects one of three taunts for wins and losses using a computed GOTO:

• LET D=20*(INT(3*RND)+1) produces 20, 40, or 60.
• GOTO 700+D (or 800+D) jumps to lines 720, 740, or 760 (human win) and 820, 840, or 860 (computer win).
• Lines 710 and 810 are never directly executed; they exist solely as jump-over code between the branch instruction and the first target.

Board Drawing Subroutine

The subroutine at lines 9000–9160 uses PLOT loops to draw horizontal lines every 4 pixels (STEP 4) and vertical lines every 18 pixels (STEP 18) to create a grid background. This runs in FAST mode to speed up the plotting. Column headers “YOU” and “STUMPER” and row numbers 0–9 are then printed over the graphics.

Victory Indicator

Lines 450–460 print an inverse-video asterisk (%* — a block graphic rendered as a highlighted star) at the row of the winning player’s number. The % prefix denotes inverse video in the source encoding, giving a clear visual indicator of who won each round without clearing the board.

Notable Variables

Variable	Purpose
A(N) / B(N)	Numeric values for human/computer hands
A$(N) / B$(N)	Right-aligned 8-char string representations
X$(32)	Space-padding source string
D$	Animated/decorative string of block graphics (▖▄▖▄…) used as a “played” marker
AA / BB	Human/computer round-win counters
O / I	Human-chosen index / computer-chosen index

Bugs and Anomalies

• The computer’s greedy scan (lines 300–320) finds the first array entry that beats the human, not the smallest such value. Because the array is in random order, this is neither the strongest nor the weakest valid play.
• Line 25 (POKE 16418,0) writes to the ZX81 system variable MARGIN, setting the vertical blank interrupt margin to zero. This suppresses the normal display interrupt, a common technique to eliminate flicker or speed up display in certain modes.
• PAUSE 40000 is used at lines 640 and 998 to wait for the Enter key; on a ZX81/TS1000 this pauses for approximately 40,000 frames (roughly 11 minutes at 50 Hz) unless interrupted by a keypress, functioning as a practical “press any key” wait.
• Lines 998–999 (bare PAUSE 40000 / GOTO 999) are unreachable dead code; program flow never reaches them in normal play.