Patterns

This program lets the user draw a 16×16 binary pixel pattern on screen, then animates it by repeatedly rotating the pattern 90 degrees and encoding each row into UDG (User Defined Graphics) characters. The 16×16 grid is entered cell by cell using key “1” for a filled pixel or any other key for blank, with FLASH and BRIGHT attributes highlighting the current cell. The encoded pattern is displayed as a 2×2 block of UDGs that can be scrolled around the screen using keys 5, 6, 7, and 8. The rotation is performed in BASIC by transposing the matrix into a second string array B$ and copying back into A$, repeating four times to cycle through all orientations. SAVE at line 440 uses the expression `PI` (≈3.14159, truncated to 3) as the auto-start line number, which is a known memory-optimisation trick.

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Program Structure

The program is organised into several logical phases controlled by GO SUB calls:

1. Initialisation (line 280 → 360–430): Clears the screen, prints instructions, draws a 16×16 grid with PLOT/DRAW, and sets up variables including N1=1, DIM A$(16,16), and DIM B$(16,16).
2. Pattern entry (lines 300–350): Iterates over every cell of the 16×16 grid, flashes the cursor, waits for a keypress with PAUSE 0, records “1” or space, and prints a block character if “1” was pressed.
3. UDG encoding and rotation loop (lines 20–120): Iterates four times (B = 144, 148, 152, 156), encodes eight rows of A$ into four UDGs per iteration using FN C, then rotates the grid 90° using B$ as a buffer.
4. Playback / movement (lines 130–230): Plays a brief ascending/descending beep sequence, then enters an event loop reading keys 5/6/7/8 to move and redisplay the 2×2 UDG block.

UDG Encoding — FN C

The function defined at line 380 converts an 8-character substring of A$ into a single UDG byte:DEF FN C(L$) = (128 AND L$(1)="1") + (64 AND L$(2)="1") + ... + (1 AND L$(8)="1")

Each of the eight characters in the substring contributes its positional bit-weight if and only if it equals "1". This exploits the Spectrum BASIC convention that boolean AND returns the left operand if the condition is true, or 0 otherwise, producing a clean bitmap byte without any explicit branching.

90-Degree Rotation

Lines 90–120 perform an in-place 90° clockwise rotation of the 16×16 string matrix. The algorithm uses B$ as a scratch buffer:

• LET B$(I,J) = A$(J, 17-I) — reads the transposed and column-reversed element (equivalent to a 90° clockwise rotation).
• LET A$(I) = B$(I) — copies each row of B$ back into A$ immediately after computing it.

This rotation is applied inside the outer FOR B=144 TO 159 STEP 4 loop, so all four 90° rotations (0°, 90°, 180°, 270°) are encoded into UDG slots 144–159 successively before the display phase begins.

UDG Layout

Each orientation of the pattern occupies four consecutive UDGs covering a 2×2 character cell (16×16 pixels):

B value	UDGs used	Orientation
144	\a \b / \c \d (144–147)	0°
148	148–151	90°
152	152–155	180°
156	156–159	270°

The encoding fills the top-left quadrant (rows 0–7, cols 0–7) into the first UDG, top-right into the second, bottom-left into the third, and bottom-right into the fourth, using the 9 TO string slice to select the right half of each row.

Movement Dispatch — Computed GO SUB

Lines 180–200 implement a four-way keypress dispatcher without an IF chain:LET B=(10 AND T$="5")+(20 AND T$="6")+(30 AND T$="7")+(40 AND T$="8")

Only one term is non-zero at a time, giving B a value of 10, 20, 30, or 40. GO SUB 230+B then jumps to line 240, 250, 260, or 270 respectively — each subroutine sets the active UDG base (CH) and adjusts the screen position (R, C) with clamped arithmetic before returning.

Notable Idioms and Techniques

• NOT PI evaluates to NOT 3.14159… = 0, used as a zero literal in FOR I=NOT PI TO 7 and elsewhere. This saves bytes over typing 0 in some tokenised forms.
• N1=1 is set once at line 370 and reused throughout as a numeric constant, saving tokenised space compared to repeated literal 1s, particularly inside the long DEF FN at line 380.
• Boundary clamping uses boolean arithmetic: e.g. C=C-2*(C>2) subtracts 2 only when C is already greater than 2, avoiding IF statements.
• SAVE "PATTERNS" LINE PI at line 440 saves with auto-start line 3 (INT of PI), which does not correspond to a program line and will simply run from line 10 — a harmless quirk.
• The grid-drawing routine (lines 390–420) uses PLOT/DRAW for the border lines and PRINT … OVER 1 for the horizontal interior lines, mixing pixel and character graphics.