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arctang.asm
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arctang.asm
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; quadrants
;
; 0 | 1
; --+--
; 2 | 3
; 24 angular regions total (0-23)
; 6 regions per quadrant
; direction
; 12 o'clock 6
; 3 o'clock 0
; 6 o'clock 18
; 9 o'clock 12
enemy_get_direction_of_player:
; references these zp vars
; collision_0_x
; collision_0_y
; player_x_hi
; player_y_hi
; uses these zp vars
; collision_0_w = x delta
; collision_0_h = y delta
; collision_1_x = "small"
; collision_1_y = "large"
; collision_1_h = "half"
; temp00 = quadrant
; temp01 = region
; temp02 = x register stash
; temp03 = y register stash
; returns a = direction (or region)
stx temp02
sty temp03
; find quadrant
lda #0
sta temp00
; y quadrants
lda player_y_hi
sec
sbc collision_0_y
bcc .top_quadrant
.bottom_quadrant
inc temp00
inc temp00
jmp .y_quadrant_done
.top_quadrant
lda collision_0_y
sec
sbc player_y_hi
.y_quadrant_done
sta collision_0_h
tay
; x quadrants
lda player_x_hi
sec
sbc collision_0_x
bcc .left_quadrant
.right_quadrant
inc temp00
jmp .x_quadrant_done
.left_quadrant
lda collision_0_x
sec
sbc player_x_hi
.x_quadrant_done
sta collision_0_w
tax
; quadrant is now in temp00
; start finding that region
cpx collision_0_h
bcs .x_greater_or_equal_y
.x_less_than_y
lda #16
sta temp01
stx collision_1_x
sty collision_1_y
bne .determine_region
.x_greater_or_equal_y
lda #0
sta temp01
stx collision_1_y
sty collision_1_x
.determine_region
lda collision_1_x
lsr
sta collision_1_h
lda collision_1_x
asl
bcs .q_smaller
clc
adc collision_1_h
bcs .q_smaller
cmp collision_1_y
bcc .q_larger
.q_smaller ; S * 2.5 > L
lsr collision_1_h
lda collision_1_x
clc
adc collision_1_h
cmp collision_1_y
bcc .region1
bcs .region0
.q_larger ; S * 2.5 < L
lda collision_1_x
asl
asl
asl
bcs .region2
sec
sbc collision_1_h
cmp collision_1_y
bcc .region3
jmp .region2
.region0 ; L / S < 1.25 ; d = 3,9,15,21
lda temp01
clc
bmi .result_lookup
.region1 ; 1.25 < L / S < 2.5 ; d = 2,4,8,10,14,16,20,22
lda temp01
clc
adc #4
bpl .result_lookup
.region2 ; 2.5 < L / S < 7.5 ; d = 1,5,7,11,13,17,19,23
lda temp01
clc
adc #8
bpl .result_lookup
.region3 ; 7.5 < L / S ; d = 0,6,12,18
lda temp01
clc
adc #12
.result_lookup
; temp01 should be ready in the accumulator
adc temp00
tax
lda ARCTANG_TRANSLATION_LOOKUP_TABLE,x
; restore enemy_ram_offset
ldx temp02
ldy temp03
rts
arctang_bound_dir: subroutine
; a = value to be bounded
; returns value 0..23 in a
bpl .check_above_24
.below_0
clc
adc #24
rts
.check_above_24
cmp #24
bcc .bounded
sec
sbc #24
.bounded
rts
; arctang movement is 16-bit
; oam ram x,y = high byte
; enemy ram x,y = low byte
arctang_update_x: subroutine
; temp00 = hi
; temp01 = lo
; temp02 = region
ldx temp02
lda ARCTANG_REGION_TO_X_VELOCITY_TABLE,x
asl
tay
lda ARCTANG_REGION_X_PLUS_OR_MINUS_TABLE,x
bpl arctang_16bit_maths
arctang_update_y: subroutine
; temp00 = hi
; temp01 = lo
; temp02 = region
ldx temp02
lda ARCTANG_REGION_TO_Y_VELOCITY_TABLE,x
asl
tay
lda ARCTANG_REGION_Y_PLUS_OR_MINUS_TABLE,x
bpl arctang_16bit_maths
arctang_16bit_maths: subroutine
; a = plus or minus
; y = velocity table offset
bne .velocity_add
.velocity_sub
; lo byte
lda temp01
sec
sbc (arctang_velocity_lo),y
sta temp01
; hi byte
lda temp00
iny
sbc (arctang_velocity_lo),y
sta temp00
jmp .done_x
.velocity_add
; lo byte
lda temp01
clc
adc (arctang_velocity_lo),y
sta temp01
; hi byte
lda temp00
iny
adc (arctang_velocity_lo),y
sta temp00
.done_x
rts
arctang_enemy_update:
; updates both x and y position given:
; - enemy_ram_ex holds region/direction
; - enemy_ram_x/y holds lo bytes
; - oam_ram_x/y holds hi bytes
; set region for arctang
lda enemy_ram_ex,x
sta temp02 ; region
; set x byte for arctang
lda oam_ram_x,y
sta temp00 ; hi byte
lda enemy_ram_x,x
sta temp01 ; lo byte
jsr arctang_update_x
ldy enemy_oam_offset
lda temp00
sta oam_ram_x,y
ldx enemy_ram_offset
lda temp01
sta enemy_ram_x,x
; set y byte for arctang
lda oam_ram_y,y
sta temp00 ; hi byte
lda enemy_ram_y,x
sta temp01 ; lo byte
jsr arctang_update_y
ldy enemy_oam_offset
lda temp00
jsr enemy_fix_y_visible
sta oam_ram_y,y
ldx enemy_ram_offset
lda temp01
sta enemy_ram_y,x
rts