###########################################
#
#  Vertical Clip
#
#  Sprite drawing routines which are capable
#  of clipping a sprite along the top and
#  bottom edges of the screen.
#
#  These routines are designed with 'lores'
#  64x32 pixel graphics in mind.
#
#  Sprite y coordinates must range between
#  17 and 64- at either extreme the sprite
#  is completely invisible. If only a partial
#  clip along one edge is necessary or
#  other constraints can be added these
#  routines could be simplified.
#
###########################################

:const HEIGHT   15 # sprite height
:const N_EDGE   32
:calc  N_LIMIT  { N_EDGE - HEIGHT }
:const S_EDGE   64
:calc  S_LIMIT  { S_EDGE - HEIGHT }

:alias offset  vd # could be a temp register
:alias height  vc # could be a temp register
:alias y       vb
:alias x       va

: flower
	0x3C 0x66 0x57 0xB7 0xFB 0x7A 0x1A
	0xC4 0x64 0x79 0x12 0x26 0x2C 0x10 0x08

: draw-sprite
	# If our sprite is below the north edge,
	# we don't need to perform top clipping:
	if y >= N_EDGE then jump clip-s

	# Calculate the number of pixels we should
	# draw and the corresponding offset into
	# the sprite data which should be used:
	offset := N_EDGE
	offset -= y
	height := HEIGHT
	height -= offset

	# If we're entirely off the top edge,
	# we don't need to draw anything:
	if height == 0 then return

	# Overwrite the lowest nybble of the 'sprite instruction
	# to reflect the desired height:
	v0 := height
	i := nrows
	save v0
	v0 := 0

	i := flower
	i += offset
	:next nrows sprite x v0 HEIGHT
;

: clip-s
	# Calculate the number of pixels we
	# should draw based on the height of the screen:
	height := S_EDGE
	height -= y

	# Drawing a 0-height sprite has the unintended
	# consequence of drawing a SuperChip 16x16 sprite,
	# so we must break if the sprite is fully offscreen:
	if height == 0 then return

	# On the other hand, if we're far enough away from the
	# bottom edge we should display the full sprite.
	# We could skip this step if the sprite was always
	# against the bottom edge:
	if y < S_LIMIT then height := HEIGHT

	# Overwrite the lowest nybble of the 'sprite' instruction
	# to parameterize the sprite height on the fly.
	# We must also preserve the y register setting, but
	# that is known statically:
	v0 := 0xB0
	v0 += height
	i := srows
	save v0

	i := flower
	:next srows sprite x y HEIGHT
;

: main
	x := 28
	y := 32
	loop
		clear
		draw-sprite
		v0 := key
		if v0 == 5 then y += -1
		if v0 == 8 then y +=  1
	again