Routines
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9AE2: Handler: Shooter
Fire a shot when the Shooter is on screen, then chase Jetman's Y position.
Input
IX Actor slot
Handler_Shooter 9AE2 LD H,(IX+$03) Fetch the high byte of the world X.
9AE5 LD L,(IX+$02) Fetch the low byte of the world X.
9AE8 PUSH HL Stash the world X on the stack.
9AE9 LD DE,($607A) Fetch Jetman_Position.
9AED AND A Subtract the scroll position.
9AEE SBC HL,DE
9AF0 BIT 7,(IX+$05) Jump to Handler_Shooter_ChaseStillOnScreen if the horizontal speed is negative.
9AF4 JP NZ,Handler_Shooter_ChaseStillOnScreen
9AF7 LD A,H Fetch the high byte of the gap.
9AF8 AND %00011111 Keep only bits 0-4.
9AFA CP $01 Jump to Handler_Shooter_ChaseClearType if the alien has left the screen.
9AFC JP Z,Handler_Shooter_ChaseClearType
Skip firing and chase Jetman if the shooter is not on this screen.
Handler_Shooter_ShotTolerance 9AFF LD DE,$0078 Add a 78 tolerance.
9B02 ADD HL,DE
9B03 LD A,H Fetch the high byte.
9B04 POP DE Restore the world X into DE from the stack.
9B05 AND A Jump to Handler_Shooter_ChaseJetmanY if the alien is not on this screen.
9B06 JR NZ,Handler_Shooter_ChaseJetmanY
9B08 PUSH DE Stash the world X on the stack again.
Set the speed to 50 or B0 if bit 7 of *IX+01 differs from the direction of travel.
9B09 LD A,(IX+$01) Fetch the screen column.
9B0C XOR (IX+$05) XOR it with the horizontal speed.
9B0F AND %10000000 Keep only bit 7.
9B11 JR Z,Handler_Shooter_SpawnShot Jump to Handler_Shooter_SpawnShot if they already match.
9B13 LD A,(IX+$05) Jump to Handler_Shooter_ShotSpeedLeft if the horizontal speed is negative.
9B16 AND A
9B17 JP M,Handler_Shooter_ShotSpeedLeft
9B1A LD (IX+$05),$50 Write 50 to the horizontal speed.
9B1E JR Handler_Shooter_SpawnShot Jump to Handler_Shooter_SpawnShot.
Handler_Shooter_ShotSpeedLeft 9B20 LD (IX+$05),$B0 Write B0 to the horizontal speed.
Spawn a shot in a free alien slot.
Handler_Shooter_SpawnShot 9B24 CALL FindSlotAlien Call FindSlotAlien.
9B27 POP DE Restore the world X into DE from the stack.
9B28 JR NZ,Handler_Shooter_ChaseJetmanY Jump to Handler_Shooter_ChaseJetmanY if no alien slot is free.
9B2A LD (HL),$B6 Write type B6 to the new slot.
9B2C BIT 7,(IX+$05) Jump to Handler_Shooter_CopyShotX if the shot is moving left.
9B30 JR NZ,Handler_Shooter_CopyShotX
9B32 LD (HL),$B7 Write type B7.
Fill in the new shot: its position, speed, thrust and colour.
Handler_Shooter_CopyShotX 9B34 INC HL Move onto the world X.
9B35 INC HL
9B36 LD (HL),E Write the shooter's world X into the slot.
9B37 INC HL
9B38 LD (HL),D
9B39 INC HL Move onto the Y position.
9B3A LD A,(IX+$04) Copy the shooter's Y position.
9B3D LD (HL),A
Give the shot a speed in the direction the shooter is moving.
9B3E INC HL Move onto the horizontal speed.
9B3F LD A,(IX+$05) Jump to Handler_Shooter_ShotSpeedRight if the shooter is moving left.
9B42 AND A
9B43 JP M,Handler_Shooter_ShotSpeedRight
9B46 LD (HL),$70 Write 70 to the shot speed.
9B48 JR Handler_Shooter_InitialiseShotSlot Jump to Handler_Shooter_InitialiseShotSlot.
Handler_Shooter_ShotSpeedRight 9B4A LD (HL),$90 Write 90 to the shot speed.
Zero the thrust, set the colour and clear the rest of the slot.
Handler_Shooter_InitialiseShotSlot 9B4C INC HL Write 00 to the thrust.
9B4D LD (HL),$00
9B4F INC HL
9B50 LD (HL),$47 Write INK:WHITE, PAPER:BLACK (BRIGHT) to the colour.
9B52 LD B,$0F Set a byte counter in B for 0F loops.
Handler_Shooter_ClearShotTail 9B54 INC HL Clear the next slot byte.
9B55 LD (HL),$00
9B57 DJNZ Handler_Shooter_ClearShotTail Decrease the byte counter by one and loop back to Handler_Shooter_ClearShotTail until the slot is cleared.
Chase Jetman's Y position.
Handler_Shooter_ChaseJetmanY 9B59 LD A,($607C) Fetch Jetman_Y.
9B5C SUB (IX+$04) Subtract the alien's Y position.
9B5F JP P,Handler_Shooter_ChaseAddThrustTest Jump to Handler_Shooter_ChaseAddThrustTest if Jetman is below the alien.
9B62 LD A,(IX+$06) Jump to Handler_Shooter_ChaseSubtractThrust if the vertical thrust is positive.
9B65 AND A
9B66 JP P,Handler_Shooter_ChaseSubtractThrust
9B69 CP $EF Jump to Handler_Shooter_ChaseApplyThrust if the thrust is already below EF.
9B6B JR C,Handler_Shooter_ChaseApplyThrust
Handler_Shooter_ChaseSubtractThrust 9B6D SUB $02 Subtract 02 from the thrust.
Apply the thrust to the Y position unless it would leave the play area.
Handler_Shooter_ChaseApplyThrust 9B6F LD (IX+$06),A Write the thrust back.
9B72 SRA A Divide it by 10.
9B74 SRA A
9B76 SRA A
9B78 SRA A
9B7A ADD A,(IX+$04) Add it to the alien's Y position.
9B7D CP $38 Jump to Handler_Shooter_ChaseTickAnimation if the alien is above the play area.
9B7F JR C,Handler_Shooter_ChaseTickAnimation
9B81 CP $B0 Jump to Handler_Shooter_ChaseTickAnimation if the alien has hit the ground line.
9B83 JR NC,Handler_Shooter_ChaseTickAnimation
9B85 LD (IX+$04),A Write the Y position back.
Step the animation timer and pick the animation frame.
Handler_Shooter_ChaseTickAnimation 9B88 INC (IX+$11) Increase the animation timer.
9B8B LD A,(IX+$11) Fetch the animation timer.
9B8E CP $07 Jump to Handler_Shooter_ChaseWriteFrame if it is still below 07.
9B90 JR C,Handler_Shooter_ChaseWriteFrame
9B92 XOR A Write 00 to the animation timer.
9B93 LD (IX+$11),A
Sprite IDs begin at B8 ("Shooter: Frame 1").
Handler_Shooter_ChaseWriteFrame 9B96 ADD A,$B8 Add B8 for these frames.
9B98 LD (IX+$00),A Write it to the actor type.
9B9B JP PlaceSpawnedAlien_Age Jump to PlaceSpawnedAlien_Age.
Test whether a left-moving shooter has gone off the left edge.
Handler_Shooter_ChaseStillOnScreen 9B9E LD A,H Fetch the high byte of the gap.
9B9F OR %11100000 Set bits 5-7.
9BA1 CP $FE Jump to Handler_Shooter_ShotTolerance if the alien has not left the left edge.
9BA3 JP NZ,Handler_Shooter_ShotTolerance
Handler_Shooter_ChaseClearType 9BA6 LD (IX+$00),$00 Write 00 to the actor type.
9BAA POP HL Restore the world X from the stack.
9BAB RET Return.
When Jetman is below the shooter, thrust downward.
Handler_Shooter_ChaseAddThrustTest 9BAC LD A,(IX+$06) Jump to Handler_Shooter_ChaseAddThrust if the vertical thrust is negative.
9BAF AND A
9BB0 JP M,Handler_Shooter_ChaseAddThrust
9BB3 CP $20 Jump to Handler_Shooter_ChaseApplyThrust if the thrust is already at least 20.
9BB5 JR NC,Handler_Shooter_ChaseApplyThrust
Handler_Shooter_ChaseAddThrust 9BB7 ADD A,$02 Add 02 to the thrust.
9BB9 JR Handler_Shooter_ChaseApplyThrust Jump to Handler_Shooter_ChaseApplyThrust.
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