;------------ L I N E ----- BREZENHAIMUV ALGORITMUS ---------------------

                                 .Z80
                                 ASEG
                               ORG 100h

; -----------------------------------------------------------------------
LINE00:
     ld de,(X1)
     ld hl,(X2)
          call LINE10
     ld (DX),hl
     ld (S1),de
     ld (Z1),a
     ld de,(Y1)
     ld hl,(Y2)
          call LINE10
     ld (DY),hl          ; spocte  Delta X  a  Delta Y
     ld (S2),de           ; a  Sign 1[X]  a  Sign 2[Y]
     ld (Z2),a
     ld de,(S1)
     ld hl,(S2)
     ld a,(Z1)
     ld hl,Z2
               or (hl)   ; nejsou nahodou obe delty = 0 - chci jen bod ?
          jr nz,LINE01   ; ne
     ld hl,(X1)
     ld de,(Y1)
          jp PLOTXY      ; ano,takze ho nakresli a padej..
LINE01:   ; -----   vlastni Brezenhaim -----
     ld hl,(DY)
     ld de,(DX)
               or a
               sbc hl,de
     ld a,h
               cp 80h
          jr nc,LINE02
     ld (DX),hl
     ld (DY),de
     ld a,1
     ld (OBMEN),a   ; po jednicce se skace ve smeru osy X
          jr LINE03
LINE02:
               xor a
     ld (OBMEN),a   ; po jednicce se skace ve smeru osy Y
LINE03: ; ----------- if dy > dx then...
     ld hl,(DY)
               add hl,hl
     ld de,(DX)
               or a
               sbc hl,de
     ld (EPRUH),hl
     ld bc,(DX)
LINE90:   ; --------------   for i=1 to dx
                              push bc     
     ld hl,(XX)
     ld de,(YY)
          call PLOTXY ;
     ld hl,(EPRUH)
LINE91:             ; ----- while epruh >= 0
                    bit 7,h ; zaporne ?
          jr z,LINE92               
     ld a,(OBMEN)
               or a
          jr z,LINE93
     ld hl,(XX)
     ld de,(S1)
               add hl,de ; bud 1 nebo 0 nebo -1 (FFFF) 
     ld (XX),hl
          jr LINE94
LINE93:
     ld hl,(YY)
     ld de,(S2)
               add hl,de
     ld (YY),hl
LINE94:
     ld hl,(DX)
               add hl,hl
     ld de,(EPRUH)
                         ex de,hl
               or a
               sbc hl,de   ; ---- epruh = epruh - 2*dx
          jr LINE91    ; ---  end while
LINE92:
     ld a,(OBMEN)
               or a
          jr z,LINE95
     ld hl,(YY)
     ld de,(S2)
               add hl,de
     ld (YY),hl
          jr LINE96
LINE95:
     ld hl,(XX)
     ld de,(S1)
               add hl,de
     ld (XX),hl
LINE96:
     ld de,(EPRUH)
     ld hl,(DY)
               add hl,hl
               add hl,de
     ld (EPRUH),hl        ; epruh = epruh + 2*dy
                              pop bc
     dec bc
     ld a,b
               or c
          jr nz,LINE90
          ret
;--------------
LINE10:
               or a
               sbc hl,de  ; delta x = x2 - x1
     ld a,h
               cp 07Fh    ; uvazuji x = <0,32758> : podteklo ? { de>hl }
          jr nc,LINE11    ; podteklo,obrat poradi
     ld a,l
               or h
     ld e,l
     ld d,h
          ret z      ; sign = 0
     ld de,1         ; sign = 1
          ret
LINE11:
               add hl,de
                         ex de,hl
               or a
               sbc hl,de
     ld a,l
               or h
     ld e,l
     ld d,h
          ret z        ; sign = 0
     ld de,0FFFFh      ; sign = -1
          ret
;------------
PLOTXY:
     ld a,l
               add a," "
     ld (PLOT10),a
     ld a,e
               add a," "
     ld (PLOT9),a
     ld hl,PLOT00
PLOTW:
     ld a,(hl)
               cp 255
          ret z
     ld e,a
     ld c,2
                              push hl
          call 5
                              pop hl
          jr PLOTW
PLOT00:
        db 27,"="
PLOT9:  db " "
PLOT10: db " "
        db 255
X1: dw 10
X2: dw 3
Y1: dw 20
Y2: dw 3 ; vstupni promenne
DX: dw 0
DY: dw 0
XX: dw 0
YY: dw 0
S1: dw 0
S2: dw 0
Z1: db 0
Z2: db 0
OBMEN: db 0
EPRUH: dw 0
 end