Routines
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40430: 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 4 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 40430 LD A,H Fetch the vertical position of the box.
40431 CP (IX+3) Compare it with the lower limit of the object.
40434 RET C Return if it is below.
40435 BIT 7,(IX+4) Jump to FindSlopeOverlap_Reverse if bit 7 is set.
40439 JR NZ,FindSlopeOverlap_Reverse
40441 CP (IX+1) Compare the position with the upper limit of the object.
40444 JR C,FindSlopeOverlap_Horizontal Jump to FindSlopeOverlap_Horizontal if it is below.
40446 SUB E Subtract the height of the box.
40447 CP (IX+1) Compare it with the upper limit.
40450 RET NC Return if the box is entirely above the object.
40451 LD H,(IX+1) Clip the vertical position to the upper limit.
40454 JR FindSlopeOverlap_Horizontal Jump to FindSlopeOverlap_Horizontal.
FindSlopeOverlap_Reverse 40456 SUB E Subtract the height of the box.
40457 LD H,A Store it as the vertical position.
40458 CP (IX+1) Compare it with the upper limit of the object.
40461 RET NC Return if it is above.
40462 CP (IX+3) Compare it with the lower limit of the object.
40465 JR NC,FindSlopeOverlap_Horizontal Jump to FindSlopeOverlap_Horizontal if it is above.
40467 LD H,(IX+3) Clip the vertical position to the lower limit.
FindSlopeOverlap_Horizontal 40470 LD A,L Fetch the horizontal position of the box.
40471 CP (IX+2) Compare it with the horizontal limit of the object.
40474 JR Z,FindSlopeOverlap_Found Jump to FindSlopeOverlap_Found if they are the same.
40476 RET NC Return if the box is beyond the limit.
FindSlopeOverlap_Found 40477 LD A,1 Set the result to 1.
40479 LD (23580),A Store it.
40482 LD A,(IX+1) Fetch the upper limit of the object.
40485 SUB H Subtract the vertical position to get the distance up the slope.
40486 LD B,A Store it in B.
40487 LD A,C Fetch the steepness of the slope.
40488 CP 2 Is it 2?
40490 RET Z Return if so.
40491 SRL B Halve the distance.
40493 CP 3 Is the steepness 3?
40495 RET Z Return if so.
40496 SLA B Otherwise multiply the halved distance by four.
40498 SLA B
40500 RET Return.
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