Routines
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CFC5: Check Overlap Along X
Used by the routines at CheckWallsVerticalFirst and CheckWallsHorizontalFirst.
Checks whether the horizontal range of an object, after it moves by HorizontalSpeed, overlaps the horizontal range of a rectangle of a wall. The distance between the middles is compared with half of the width of the object plus the half width of the rectangle.
Input
E The horizontal position of the object in the cell
IX The address of the object
IY The address of the rectangle
Output
A The depth into the rectangle, negative if they overlap
F The carry flag is set if the ranges overlap
Find the sum of the half widths.
CheckOverlapAlongX CFC5 LD A,(IX+$08) Find the sum of half of the width of the object and the half width of the rectangle, in C.
CFC8 SRL A
CFCA ADD A,(IY+$02)
CFCD LD C,A
Find the distance between the middles.
CFCE LD A,($A214) Fetch HorizontalSpeed.
CFD1 SRA A Add half of it to the position, then find the distance from the middle of the rectangle.
CFD3 ADD A,E
CFD4 SUB (IY+$00)
Make the distance positive, then subtract the sum.
CFD7 JP P,CheckOverlapAlongX_Depth Make the distance positive.
CFDA NEG
CheckOverlapAlongX_Depth CFDC SUB C Subtract the sum.
CFDD RET Return.
View the equivalent code in;
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