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Copy path6_loop_move_tail.asm
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6_loop_move_tail.asm
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; /////////////////////////////////////////////////////////////////////////////
; Znake (ZX Spectrum 48K)
; -----------------------------------------------------------------------------
; 6_loop_move_tail.asm
; -----------------------------------------------------------------------------
; Copyright (C) 2016, Chris Wyatt
; All rights reserved
; Distributed under the Apache 2 license (see LICENSE)
; /////////////////////////////////////////////////////////////////////////////
update_tail_history:
; Move pointer to start of snake history
ld h,(TBL_SNAKE_HISTORY >> 8) & $FF
ld a,(snake_history_tail_offset)
ld l,a
; Load current tail location to register b
ld d,(hl)
; Low bit of graphics (blank)
ld e,0x00
; Draw later
push de
; Move pointer to turn (or head) before tail
inc l
; Load current turn location to register e
ld e,(hl)
ld a,e
xor d
cp 0x10
jr c,tail_moving_vertically
; Tail moving horizontally
; Restore current turn location
xor d
and 0xf0
ld c,a
ld a,d
and 0xf0
sub c
jr c,tail_moving_right
jp tail_moving_left
tail_moving_vertically:
; Restore current turn location
xor d
and 0x0f
ld c,a
ld a,d
and 0x0f
sub c
jr c,tail_moving_down
macro move_tail, chop_comparitor, graphics_low_addr, direction
cp chop_comparitor
jr z,chop_off_tail
if direction = 0x08
; Shift location up
dec d
endif
if direction = 0x04
; Shift location down
inc d
endif
if direction = 0x02
; Get tail location again and shift it left
ld a,d
sub 0x10
ld d,a
endif
if direction = 0x01
; Get tail location again and shift it right
ld a,d
add a,0x10
ld d,a
endif
; Low bit of graphics
ld e,graphics_low_addr
; Draw later
push de
; Move pointer back to tail
dec l
; And update the existing tail location in our table
ld (hl),d
endm
; Tail moving up
move_tail 0x01, 0x68, 0x08
jp collision_detection
tail_moving_down:
move_tail 0xff, 0x58, 0x04
jp collision_detection
tail_moving_left:
move_tail 0x10, 0x60, 0x02
jp collision_detection
tail_moving_right:
move_tail 0xf0, 0x70, 0x01
jp collision_detection
chop_off_tail:
; Remove the reference to the old tail, and let the next turn in the
; snake's body be the new tail
ld a,(snake_history_tail_offset)
inc a
ld (snake_history_tail_offset),a
; Load new tail location to register d
ld d,e
; Move pointer to the turn after the new tail
inc l
; Load current turn location to register e
ld e,(hl)
ld a,e
xor d
cp 16
jr c,new_tail_moving_vertically
; New tail moving horizontally
; Restore current turn location
xor d
and 0xf0
ld e,a
ld a,d
and 0xf0
sub e
jr c,new_tail_moving_right
jp new_tail_moving_left
new_tail_moving_vertically:
; Restore current turn location
xor d
and 0x0f
ld e,a
ld a,d
and 0x0f
sub e
jr c,new_tail_moving_down
; New tail moving up
; Low bit of graphics
ld e,0x68
; Draw later
push de
jp collision_detection
new_tail_moving_down:
; Low bit of graphics
ld e,0x58
; Draw later
push de
jp collision_detection
new_tail_moving_left:
; Low bit of graphics
ld e,0x60
; Draw later
push de
jp collision_detection
new_tail_moving_right:
; Low bit of graphics
ld e,0x70
; Draw later
push de