Space Raiders: Routine at 26490

Check whether the player's bullet has hit an alien bullet. There are up to 4 alien bullets in play, so loop over them and check their screen locations. If there's a hit, play the click sound and remove the bullets from screen.

If there's no such interception jump off to the routine which checks whether the player's bullet has hit the underside of a barrier.

Used by the routine at is_alien_hit.

Check whether the player's bullet has hit an alien bullet. There are up to 4 alien bullets in play, so loop over them and check their screen locations. If there's a hit, play the click sound and remove the bullets from screen. If there's no such interception jump off to the routine which checks whether the player's bullet has hit the underside of a barrier. Used by the routine at is_alien_hit.
bullet_intercept_detect	26490	LD IX,_ALIEN_BULLET_ARRAY	Alien bullet array, 4 slots, x,y entries
	26494	LD A,4	Loop 4 times, once for each alien bullet
	26496	LD (_MOVE_BULLET_LOOP),A	Loop 4 times, once for each alien bullet
	26499	LD BC,(_PLAYER_BULLET_XPOS)	C=xpos, B=ypos of player bullet
bullet_intercept_detect_0	26503	LD A,(IX+1)	Alien bullet ypos, a zero value is used to indicate this bullet isn't in flight. Skip slot if not used.
	26506	OR A
	26507	JR Z,not_hit_player_bullet
	26509	LD A,(IX+0)	Pick up alien bullet xpos. Compare to player bullet xpos, jump forward if it's not the same since it can't be an intercept
	26512	CP C
	26513	JR NZ,not_hit_player_bullet
Alien bullets locations are stored upside down. i.e. the x,y pos on screen is the top of the bullet, not the leading point which is the bottom as drawn. So this ypos check is a bit fiddly.
	26515	LD A,(IX+1)	Pick up player bullet ypos again, subtract player bullet ypos, take 2s comp and compare with 6 (bullet length). NC means no hit.
	26518	SUB B
	26519	NEG
	26521	CP 6
	26523	JR NC,not_hit_player_bullet
Intercept!
alien_bullet_intercepted	26525	CALL clear_alien_bullet_sprite	Clears alien bullet
	26528	CALL sound_burbler	Sound burbler
	26531	DEFB 4,24,8,12,16,6,0	Hit alien bullet sound
	26538	JP clear_player_bullet	Jump to clear player bullet. The return exits this routine, which is fine since there's no player bullet now.
Alien bullet and player bullet haven't intecepted, so check the next alien bullet.
not_hit_player_bullet	26541	INC IX	Next alien bullet
	26543	INC IX
	26545	LD A,(_MOVE_BULLET_LOOP)	Back to check next alien bullet for interception
	26548	DEC A
	26549	LD (_MOVE_BULLET_LOOP),A
	26552	JR NZ,bullet_intercept_detect_0
If we get here the player's bullet hasn't intercepted an alien bullet, so we need to check if the player's bullet hit a barrier
	26554	LD BC,(_PLAYER_BULLET_XPOS)	Player bullet y,x in BC, move player bullet up the screen a pixel
	26558	DEC B
	26559	CALL bullet_barrier_hit	Check whether the player's bullet hit a barrier
	26562	RET Z	Return with Z mean no barrier collision, otherwise clear the player's bullet
	26563	JP clear_player_bullet