Routines
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9654: Handler: Drifting Alien
Count the alien, then drift it relative to the scroll in Jetman_Position.
Input
IX Actor slot
Handler_DriftingAlien 9654 LD HL,$5E5E HL=AlienCount.
9657 INC (HL) Increase AlienCount by one.
Compare the alien with Jetman_Position and pick a drift direction.
9658 LD H,(IX+$03) Fetch the high byte of the world X.
965B LD L,(IX+$02) Fetch the low byte of the world X.
965E LD DE,($607A) Fetch Jetman_Position.
9662 AND A Subtract the scroll position.
9663 SBC HL,DE
9665 BIT 4,H Jump to Handler_DriftingAlien_DriftRight if the alien is left of the scroll.
9667 JP NZ,Handler_DriftingAlien_DriftRight
Speed the alien up towards the left, up to a limit.
966A LD C,$00 Clear the reverse-animation flag.
966C LD A,(IX+$05) Jump to Handler_DriftingAlien_Decelerate if the horizontal speed is positive.
966F AND A
9670 JP P,Handler_DriftingAlien_Decelerate
9673 CP $D1 Jump to Handler_DriftingAlien_Loop if the speed is already below D1.
9675 JR C,Handler_DriftingAlien_Loop
Handler_DriftingAlien_Decelerate 9677 SUB $02 Subtract 02 from the speed.
Handler_DriftingAlien_Loop 9679 LD (IX+$05),A Write the speed back.
Aim the vertical thrust at Jetman, or at mid-screen when Jetman is in the rover.
967C LD A,($6078) Fetch the Jetman type from Jetman_Slot.
967F CP $11 Jump to Handler_DriftingAlien_FetchJetmanY if Jetman is on foot.
9681 JR C,Handler_DriftingAlien_FetchJetmanY
9683 LD A,$68 A=68.
9685 JR Handler_DriftingAlien_CompareY Jump to Handler_DriftingAlien_CompareY.
Handler_DriftingAlien_FetchJetmanY 9687 LD A,($607C) Fetch Jetman_Y.
Compare with the alien's Y position and, if the alien is above the target, increase the downward thrust, up to a limit.
Handler_DriftingAlien_CompareY 968A SUB (IX+$04) Subtract the alien's Y position.
968D JP M,Handler_DriftingAlien_ThrustUp Jump to Handler_DriftingAlien_ThrustUp if the alien is below the target.
9690 LD A,(IX+$06) Jump to Handler_DriftingAlien_AddThrust if the vertical thrust is negative.
9693 AND A
9694 JP M,Handler_DriftingAlien_AddThrust
9697 CP $30 Jump to Handler_DriftingAlien_ApplyThrust if the thrust is already at least 30.
9699 JR NC,Handler_DriftingAlien_ApplyThrust
Handler_DriftingAlien_AddThrust 969B ADD A,$02 Add 02 to the thrust.
Apply the thrust to the alien's Y position, bouncing at the top and bottom.
Handler_DriftingAlien_ApplyThrust 969D LD (IX+$06),A Write the thrust back.
96A0 SRA A Divide it by 10.
96A2 SRA A
96A4 SRA A
96A6 SRA A
96A8 ADD A,(IX+$04) Add it to the alien's Y position.
96AB CP $38 Jump to Handler_DriftingAlien_BounceTop if the alien is above the play area.
96AD JR C,Handler_DriftingAlien_BounceTop
96AF CP $B0 Jump to Handler_DriftingAlien_BounceGround if the alien has hit the ground line.
96B1 JR NC,Handler_DriftingAlien_BounceGround
96B3 LD (IX+$04),A Write the Y position back.
Animate the frame and colour, then continue at PlaceSpawnedAlien_Age.
Handler_DriftingAlien_Animate 96B6 INC (IX+$11) Increase the animation timer.
96B9 LD A,(IX+$11) Fetch the animation timer.
96BC AND %00000111 Keep only bits 0-2.
96BE BIT 7,C Jump to Handler_DriftingAlien_WriteFrame if the reverse-animation flag is set.
96C0 JR NZ,Handler_DriftingAlien_WriteFrame
96C2 XOR %00000111 Flip bits 0-2.
Sprite IDs begin at 48 ("The Blob: Frame 1").
Handler_DriftingAlien_WriteFrame 96C4 ADD A,$48 Add 48 for the drifting frames.
96C6 LD (IX+$00),A Write it to the actor type.
Alternate the colour using bit 0 of GameTimer.
96C9 LD A,($5E42) Fetch GameTimer.
96CC RLA Rotate it left once.
96CD AND %00000010 Keep only bit 1.
96CF ADD A,$44 Add 44 for the colour.
96D1 LD (IX+$07),A Write it to the actor colour.
96D4 JP PlaceSpawnedAlien_Age Jump to PlaceSpawnedAlien_Age.
When the alien is left of the scroll, drift right.
Handler_DriftingAlien_DriftRight 96D7 LD C,$80 Set the reverse-animation flag.
96D9 LD A,(IX+$05) Jump to Handler_DriftingAlien_AddSpeed if the horizontal speed is negative.
96DC AND A
96DD JP M,Handler_DriftingAlien_AddSpeed
96E0 CP $30 Jump to Handler_DriftingAlien_Loop if the speed is already at least 30.
96E2 JR NC,Handler_DriftingAlien_Loop
Handler_DriftingAlien_AddSpeed 96E4 ADD A,$02 Add 02 to the speed.
96E6 JR Handler_DriftingAlien_Loop Jump to Handler_DriftingAlien_Loop.
When the alien is below the target, apply upward thrust.
Handler_DriftingAlien_ThrustUp 96E8 LD A,(IX+$06) Jump to Handler_DriftingAlien_SubtractThrust if the vertical thrust is positive.
96EB AND A
96EC JP P,Handler_DriftingAlien_SubtractThrust
96EF CP $D1 Jump to Handler_DriftingAlien_ApplyThrust if the thrust is already below D1.
96F1 JR C,Handler_DriftingAlien_ApplyThrust
Handler_DriftingAlien_SubtractThrust 96F3 SUB $02 Subtract 02 from the thrust.
96F5 JR Handler_DriftingAlien_ApplyThrust Jump to Handler_DriftingAlien_ApplyThrust.
Bounce off the top of the play area.
Handler_DriftingAlien_BounceTop 96F7 LD A,(IX+$06) Jump to Handler_DriftingAlien_Animate if the vertical thrust is positive.
96FA AND A
96FB JP P,Handler_DriftingAlien_Animate
Handler_DriftingAlien_NegateThrust 96FE NEG Negate the thrust.
9700 LD (IX+$06),A Write it back.
9703 JR Handler_DriftingAlien_Animate Jump to Handler_DriftingAlien_Animate.
Bounce off the ground line.
Handler_DriftingAlien_BounceGround 9705 LD A,(IX+$06) Jump to Handler_DriftingAlien_Animate if the vertical thrust is negative.
9708 AND A
9709 JP M,Handler_DriftingAlien_Animate
970C JR Handler_DriftingAlien_NegateThrust Jump to Handler_DriftingAlien_NegateThrust.
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