Routines
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9DEE: Find Slope Overlap
Used by the routines at TestSlopeCollision and FindMirroredSlopeOverlap.
Part of the test for a box touching an object with a sloping face. It checks the position of the box against the vertical limits of the object and works out how far up the slope the box reaches. Bit 7 of the byte at offset 04 of the object selects which way the slope runs. Purpose not yet known beyond this.
Input
C Steepness of the slope
E Height of the box
H Vertical position of the box
L Horizontal position of the box
IX Address of the object
Output
B Distance up the slope
FindSlopeOverlap 9DEE LD A,H Fetch the vertical position of the box.
9DEF CP (IX+$03) Compare it with the lower limit of the object.
9DF2 RET C Return if it is below.
9DF3 BIT 7,(IX+$04) Jump to FindSlopeOverlap_Reverse if bit 7 is set.
9DF7 JR NZ,FindSlopeOverlap_Reverse
9DF9 CP (IX+$01) Compare the position with the upper limit of the object.
9DFC JR C,FindSlopeOverlap_Horizontal Jump to FindSlopeOverlap_Horizontal if it is below.
9DFE SUB E Subtract the height of the box.
9DFF CP (IX+$01) Compare it with the upper limit.
9E02 RET NC Return if the box is entirely above the object.
9E03 LD H,(IX+$01) Clip the vertical position to the upper limit.
9E06 JR FindSlopeOverlap_Horizontal Jump to FindSlopeOverlap_Horizontal.
FindSlopeOverlap_Reverse 9E08 SUB E Subtract the height of the box.
9E09 LD H,A Store it as the vertical position.
9E0A CP (IX+$01) Compare it with the upper limit of the object.
9E0D RET NC Return if it is above.
9E0E CP (IX+$03) Compare it with the lower limit of the object.
9E11 JR NC,FindSlopeOverlap_Horizontal Jump to FindSlopeOverlap_Horizontal if it is above.
9E13 LD H,(IX+$03) Clip the vertical position to the lower limit.
FindSlopeOverlap_Horizontal 9E16 LD A,L Fetch the horizontal position of the box.
9E17 CP (IX+$02) Compare it with the horizontal limit of the object.
9E1A JR Z,FindSlopeOverlap_Found Jump to FindSlopeOverlap_Found if they are the same.
9E1C RET NC Return if the box is beyond the limit.
FindSlopeOverlap_Found 9E1D LD A,$01 Set the result to 01.
9E1F LD ($5C1C),A Store it.
9E22 LD A,(IX+$01) Fetch the upper limit of the object.
9E25 SUB H Subtract the vertical position to get the distance up the slope.
9E26 LD B,A Store it in B.
9E27 LD A,C Fetch the steepness of the slope.
9E28 CP $02 Is it 02?
9E2A RET Z Return if so.
9E2B SRL B Halve the distance.
9E2D CP $03 Is the steepness 03?
9E2F RET Z Return if so.
9E30 SLA B Otherwise multiply the halved distance by four.
9E32 SLA B
9E34 RET Return.
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