atan128

atan:
;D.E is x
;returns atan(D.E)*128/pi
;if DE is negative, return -atan(-D.E)
  ld a,d
  or a
  jr nc,+_
    sbc hl,hl
    sbc hl,de
    ex de,hl
    call +_
    xor a
    sub l
    ld l,a
    sbc a,a
    sub h
    ld h,a
    ret
_:
;at this point, DE is positive
;if D!=0, then we need to return 64-atan(1.0/D.E)
  inc d
  dec d
  jr z,+_
    call InvDE
    call +_
    ex de,hl
    ld hl,64
    or a
    sbc hl,de
    ret
_:
  ld hl,atan28LUT
  add hl,de
  ld e,(hl)
  ex de,hl
  ret
InvDE:
;Special case, D.E >= 1.0
;returns 1.0/D.E
;Note: In the case D.E=1.0, actually returns 255/256, but in this context it is fine for the atan routine. Otherwise, we'd just need to add a .db $20 to the end of the LUT, but that's just a copy of 255/256 :P
  ld hl,256
//  or a
//  sbc hl,de
//  ret z
//  add hl,de
  ld b,8
_:
  add.s hl,hl \ sbc.s hl,de \ jr nc,$+3 \ add hl,de \ rla \ djnz -_
  cpl
  ld e,a
  ld d,b
  ret

atan128LUT:
.db $00,$00,$00,$00,$01,$01,$01,$01,$01,$01,$02,$02,$02,$02,$02,$02
.db $03,$03,$03,$03,$03,$03,$03,$04,$04,$04,$04,$04,$04,$05,$05,$05
.db $05,$05,$05,$06,$06,$06,$06,$06,$06,$06,$07,$07,$07,$07,$07,$07
.db $08,$08,$08,$08,$08,$08,$08,$09,$09,$09,$09,$09,$09,$0a,$0a,$0a
.db $0a,$0a,$0a,$0a,$0b,$0b,$0b,$0b,$0b,$0b,$0b,$0c,$0c,$0c,$0c,$0c
.db $0c,$0c,$0d,$0d,$0d,$0d,$0d,$0d,$0d,$0e,$0e,$0e,$0e,$0e,$0e,$0e
.db $0f,$0f,$0f,$0f,$0f,$0f,$0f,$10,$10,$10,$10,$10,$10,$10,$11,$11
.db $11,$11,$11,$11,$11,$11,$12,$12,$12,$12,$12,$12,$12,$13,$13,$13
.db $13,$13,$13,$13,$13,$14,$14,$14,$14,$14,$14,$14,$14,$15,$15,$15
.db $15,$15,$15,$15,$15,$15,$16,$16,$16,$16,$16,$16,$16,$16,$17,$17
.db $17,$17,$17,$17,$17,$17,$17,$18,$18,$18,$18,$18,$18,$18,$18,$18
.db $19,$19,$19,$19,$19,$19,$19,$19,$19,$19,$1a,$1a,$1a,$1a,$1a,$1a
.db $1a,$1a,$1a,$1b,$1b,$1b,$1b,$1b,$1b,$1b,$1b,$1b,$1b,$1c,$1c,$1c
.db $1c,$1c,$1c,$1c,$1c,$1c,$1c,$1c,$1d,$1d,$1d,$1d,$1d,$1d,$1d,$1d
.db $1d,$1d,$1d,$1e,$1e,$1e,$1e,$1e,$1e,$1e,$1e,$1e,$1e,$1e,$1f,$1f
.db $1f,$1f,$1f,$1f,$1f,$1f,$1f,$1f,$1f,$1f,$20,$20,$20,$20,$20,$20