Collect something worth the risk

A safe route keeps the ship alive. A star gives the player a reason to leave it. We will let an object’s kind choose its picture and contact consequence.

The byte at record offset 2 now means kind: 0 is free, 1 is meteor, 2 is star. Nonzero still means occupied, so the existing pool search and occupied-object count keep their meaning. We have extended a concrete representation rather than adding a second pool.

The event table regains all 120 accepted entries: one hundred meteors and twenty stars. Events now contain X, speed, next delay, drift and kind. Their stride is five, so waves now adds the event index to HL five times. It copies the fifth field into a new byte, spawn_kind, and spawn stores that at offset 2 where it used to store 1. The object stride stays five because kind replaces active rather than adding another field.

One contact test, two consequences

Both kinds move down at their stored speed and use the same contact region. After the shared distance test, kind 2 branches to the star path: it adds one to score and retires the star without damaging the ship. Any other kind reaches the new label meteor_contact, where the existing code clears hull as before.

advance_meteors_absolute:
 cp 16
 jp nc,advance_meteors_skip
 ld a,(ix+2)
 cp 2
 jr nz,meteor_contact
 ld c,1
 ld a,(score)
 add a,c
 ld (score),a
 jr advance_meteors_remove
meteor_contact:
 ; One contact ends this attempt; no recovery state is needed.
 xor a
 ld (hull),a
 push bc
 call impact_sound
 pop bc

The star path puts the award in C with ld c,1, reads score into A, adds C and stores the sum. It then jumps to advance_meteors_remove, the label just after this excerpt, which frees the record.

Stars bypass horizontal drift before the drift gate. Their event record can contain a drift byte, but their kind determines that it will not be used. Watch a star’s X in the active-record table: it should stay fixed as Y increases.

draw_meteor now selects meteor_sprites or star_sprites from kind; both choices meet at the new label draw_object, which sets the height and draws. Its name has survived the first implementation, but it draws either falling object. The caller must not clear kind before erasing: the old kind selects the image that must be XORed away.

draw_meteor:
 ld hl,meteor_sprites
 ld a,(ix+2)
 cp 2
 jr nz,draw_object
 ld hl,star_sprites
draw_object:
 ld a,12
 jp draw_sprite

Award a pickup only once

After adding a point unit, the star takes the same removal path used when an object leaves the play area. That clears kind to zero. On the next visit, the slot is free and its old coordinates cannot award more points.

The score byte stores tens of points: 1 means 10 points, 2 means 20. Each star adds one unit. score sits inside the run block, so every run starts it at zero. This program does not print the score yet: read it in the inspector above, which shows 1 after the first star. The next lesson follows that byte into its printed decimal representation.

Try changing one star’s event kind from 2 to 1 in the companion file, then reassemble. It should use meteor artwork, take the drift path and become harmful. Change all three expectations before running; a data edit can affect several branches that agree on the meaning of kind. Revert restores the companion file as well as the main source.