Concentration

A one-player Concentration card game using a full 52-card deck. Cards are arranged face-down in a 10×5 grid (plus two extras), each identified by a position number from 1 to 52. On each turn the player enters two positions; matching card values are removed and the pair count increments, while non-matching cards are re-covered after a brief pause. Four UDG suit symbols — Hearts, Clubs, Diamonds, Spades — are defined and printed in red or black according to the card’s suit. The green PAPER 4 background simulates a card table, framed by a PLOT/DRAW border. Matching is by face value only, yielding 26 pairs across 13 ranks; the game ends when all pairs are found and reports the total number of turns taken.

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UDG definitions (lines 20–30):

Four suit symbols are defined by POKEing 8 bytes each into named UDG slots:

Letter	Suit	UDG char
H	Hearts	\h (151)
C	Clubs	\c (146)
D	Diamonds	\d (147)
S	Spades	\s (162)

Line 60 stores the corresponding CHR$ codes in array S() using CODE S$(I)+79 — an elegant trick to convert the ASCII letter to its UDG character code without needing a lookup table.

Deck and shuffle (lines 70–90):

C() is filled 1–52, then shuffled by swapping each card with a random position across the full 52 (a slightly biased but functionally adequate algorithm). Suit T(I) and value V(I) are decoded from card number: suit = INT((C(I)-1)/13)+1, value = remainder within that suit group. Value string V$="A23456789TJQK" maps values 1–13 to display characters.

Grid layout (subroutine 390):

Card positions are determined by treating N as a two-digit number: tens digit → screen row (L1=1+3*X10), units digit → column (C1=3*X1-2), with a correction for units=0 (card 10, 20, etc.). This produces a grid of 10 cards per row across 5 full rows, with cards 51–52 in a partial sixth row.

Background (lines 100–120):

Line 100 prints a green (PAPER 4) tiled ██ pattern to simulate a card table baize. Line 120 overlays a white (PAPER 7) bordered play area using PLOT/DRAW — seven line segments forming a rectangle.

Card reveal (subroutine 340):

Displays value character and suit UDG in two rows. Ink color is determined by suit parity: odd suits (Hearts=1, Diamonds=3) get INK 2 (red); even suits (Clubs=2, Spades=4) get INK 0 (black). A box is drawn around the revealed card using PLOT/DRAW. Matching is by value only — any two cards sharing the same face value (e.g. any two Queens) form a valid pair regardless of suit, giving 4 cards of each value and 26 possible pairs in the deck.

Cover (subroutine 440):

Re-covers a card with INVERSE 1; N. For single-digit N, appends a █ block to maintain consistent 2-character width.

Game loop (lines 140–300):

Prompts for two positions with input validation (integer, 1–52, not already matched, not the same card twice). On a correct match, V(P1) and V(P2) are set to 0 (the “already paired” sentinel). On a mismatch, both cards are re-covered after a pause. Choice and pair counts are updated after each turn. At 26 pairs the game congratulates the player and offers a restart.