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:: CORDIC demo 256 ::
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256 byte education intro for ZX Spectrum 48k + AY

  Code: Busy soft
  Music: Noro soft
  Create: 08.02.2025
  Release: LoveByte 2025

This intro draws concentric circles
by using simplified CORDIC algoritm and
then play simple color music on these circles.


Draw init screen:

  FOR y = 175 TO 0
    FOR x = 0 TO 255
      LET distance = SQR((X-128)^2+(Y-88)^2)
      LET circle_index = INT(d/10)
      PLOT INVERSE circle_index MOD 2; x,y
      Store pixel[x,y] into set of circle[circle_index]


Play color music:

  For every note from music:
    circle_base = F(note_pitch)
    FOR i = 7 TO 1
      LET circle_index = circle_base + 7 - i
      Set all pixes of cicle[circle_index] to color i

  Every 40 ms do:
   For each pixel on screen do:
    IF color > 0 THEN LET color = color - 1


For drawing init screen, there is needed to determine distane
between processed point [X,Y] and the middle of screen [128,88]:

  distance = SQR(dx^2+dy^2)

where dx = X - 128
      dy = Y - 88

Since there is a mandatory condition X > 0 and Y > 0,
coordinates cannot be negative, so we must compute
absolute value of dx and dy. And for sure,
we decided increment coordinate in case of zero.


Source code of evaluation SQR(dx^2+dy^2):

  Input: C = coordinate X
         B = coordinate Y

        ld      a,b             ;; dY processing
        sub     #58             ;; Convert interval 0..175 to -88..0..+87
        jr      nc,skipy        ;; If not negative result, skip the cpl
        cpl                     ;; cpl:inc = neg  changes the signum of value
 skipy: inc     a               ;; but inc only incremens positive value to avoid zero
        ld      b,a             ;; Result dY interval will be symmetrical +87..1,1..+87

        ld      a,c             ;; dX processing
        sub     #80             ;; Convert interval 0..255 to -128..0..+127
        jr      nc,skipx        ;; If not negative result, skip the cpl
        cpl                     ;; cpl:inc = neg  changes the signum of value
 skipx: inc     a               ;; but inc only incremens positive value to avoid zero
        ld      c,a             ;; Result dX interval will be symmetrical +128..1,1..+128
        cp      b
        jr      nc,L1           ;; If X < Y
        ld      c,b             ;; Then swap X and Y
        ld      b,a

        ld      a,c             ;; Simplified CORDIC
    L1: sub     b
        jr      nc,L2
        inc     c
        add     c
    L2: djnz    L1

  Output: C = distance = SQR(dx^2+dy^2)

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