SNOW

Author: regregex
Category: Christmas Challenge
System: Sinclair ZX Spectrum 16K/48K/128K/+2/+2A
Language: Spectrum BASIC
Len source code: 292 bytes
Len exe file:    184 bytes
Len code only:   159 bytes
Instructions:
  Install and open Speccy ( https://fms.komkon.org/Speccy/ ).
  Click File -> Open, select autoload.tap.
  On hardware, enter:
  LOAD ""               J <SymbolShift+P> <SymbolShift+P> <Enter>
  and play the tape.  To run, enter:
  RUN                   R <Enter>

Description: The program displays a specific snowflake pattern on the
screen.
For each character cell in a large rectangle in the top left corner it
then calculates the Euclidean distance from the centre, and prints an
asterisk if the radius is within a prescribed limit AND in a lookup
table of radii, or if the cell lies on an axis.  The program waits for a
keypress before exiting.

Comments: An annotated dissection of the code follows.

1                       A leading line number puts the line in memory:
CLS :                   Clear the screen.
LET P=CODE "b":         Set P := 98, square of the radius of the shape
LET N=INT SQR P:        Set N := 9, length of each axis from the origin
LET E=SGN N:            Set E := 1
FOR Y=-N-E TO N:        Loop over rows -10 to 9, first row is blank:
  PRINT :               Move the cursor down, leaving row 1 also blank.
  FOR X=P-CODE "r" TO N:        Loop over cols -16 to 9, first 7 blank:
    LET H=Y*Y+X*X:      H := square of the Euclidean distance from (0,0)
    LET R=N+E+E         R := 11, last character of radius string
2   IF R AND H<P THEN   Skip if string exhausted ( ) or point=(7,7) (*)
      IF                Nested IFs act as a fast, lower-precedence AND
      CODE "EBW7b6i-m'o"(R)     Select R'th character and get its value
      -CODE "%"         Subtract 37 obtaining number in (2..74)
      <>H               If it doesn't match our radius
      AND X*Y THEN      and if the point is not on an axis (X*Y<>0)
        LET R=R-E:      then step to previous character
        GO TO E+E       and redo this line; else R remains >0
3   PRINT " *"          Print either space or asterisk
      (E                FALSE=0, TRUE=1 but string offsets start at 1
      +(R>=E AND H<=P));:       "*" if match found and within bounds
  NEXT X:               Loop until last column
NEXT Y:                 Loop until last row
PAUSE E-E               Wait for keypress (PAUSE 0) and exit

The radius table contains 11 entries but defines 12 stars per octant.
This is because:

  7^2 + 1^2 == 5^2 + 5^2.

As point (5,0) has an inconvenient alias, the lookup table is not used
for points on the axes.

Created with SpecIDE ( https://github.com/MartianGirl/SpecIde/releases )
which saves TAP files, but is a little trickier to use.