Routines
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D3EB: Test Entity Collisions
Used by the routine at MoveEntity.
Tests the entity against each of the other entities with TestEntityPair. A puff and a shot are not tested. A player skips puffs and sparkles, and a saucer skips shots. A disc, a free slot and a hidden entity are skipped. The direction bytes are 20 during the tests. Afterwards the low bytes of x and y, relative to the patched origins, replace them for TestEntityCells. The step in MoveEntityBack is removed for the tests and then put back.
Input
IX The entity
The direction bytes start at 20 so a hit during the tests has a neutral component. The type of this entity is patched into the skip below.
TestEntityCollisions D3EB LD A,$04 Mark the entity as only touching.
D3ED LD ($DD4C),A
D3F0 LD A,(IX+$00) Remember this entity's type for the skips below.
D3F3 LD ($D41F),A
D3F6 CP $05 A puff is not tested against the others.
D3F8 JR Z,TestEntityCollisions_Finish
D3FA CP $0E A shot is not tested against the others.
D3FC JR Z,TestEntityCollisions_Finish
D3FE LD (IX+$18),$20 Set both direction bytes to 20.
D402 LD (IX+$19),$20
D406 XOR A Clear the object flags and remove the move-back step.
D407 LD ($DDE4),A
D40A LD ($5CD8),A
D40D LD B,$0C There are 0C entities.
D40F LD IY,$B540 Start at the first entity.
TestEntityCollisions_Loop D413 PUSH BC Remember how many entities remain.
D414 LD A,(IY+$00) Skip an entity that is free.
D417 AND A
D418 JR Z,TestEntityCollisions_Next
D41A CP $0C Skip a disc.
D41C JR Z,TestEntityCollisions_Next
D41E LD A,$00 The type patched above selects the next skips.
D420 LD L,A
D421 DEC A
D422 JR NZ,TestEntityCollisions_Saucer A player does not test a puff or a sparkle.
D424 LD A,(IY+$00)
D427 CP $05
D429 JR Z,TestEntityCollisions_Next
D42B CP $0D
D42D JR Z,TestEntityCollisions_Next
TestEntityCollisions_Saucer D42F LD A,L A saucer does not test a shot.
D430 CP $09
D432 JR NZ,TestEntityCollisions_Frame
D434 LD A,(IY+$00) Skip the shot.
D437 CP $0E
D439 JR Z,TestEntityCollisions_Next
TestEntityCollisions_Frame D43B LD A,(IY+$01) Read the other entity's frame.
D43E AND %11000000 Use bits 6 and 7 of the frame.
D440 JR NZ,TestEntityCollisions_Next Skip an entity with either bit set.
D442 PUSH IY It is not tested against itself.
D444 POP HL
D445 PUSH IX
D447 POP DE
D448 AND A Test the pair when the two entities differ.
D449 SBC HL,DE
D44B CALL NZ,TestEntityPair
TestEntityCollisions_Next D44E LD BC,$0026 Move on to the next entity.
D451 ADD IY,BC
D453 POP BC Bring the count back.
D454 DJNZ TestEntityCollisions_Loop Repeat for the remaining entities.
TestEntityCollisions_Finish D456 LD DE,$0000 Keep the low byte of x relative to the patched origin.
D459 LD H,(IX+$02)
D45C LD L,(IX+$03)
D45F AND A
D460 SBC HL,DE
D462 LD (IX+$18),L
D465 LD DE,$0000 Keep the low byte of y relative to the patched origin.
D468 LD H,(IX+$05)
D46B LD L,(IX+$06)
D46E AND A
D46F SBC HL,DE
D471 LD (IX+$19),L
D474 LD A,$2D Put the step back into the move-back routine.
D476 LD ($DDE4),A
D479 RET Return.
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