Routines
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D002: Check Overlap Along Y
Used by the routines at CheckWallsVerticalFirst and CheckWallsHorizontalFirst.
Checks whether the vertical range of an object, after it moves by VerticalSpeed, overlaps the vertical range of a rectangle of a wall. The distance between the middles is compared with half of the height of the object plus the half height of the rectangle.
Input
D The vertical 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 heights.
CheckOverlapAlongY D002 LD A,(IX+$09) Find the sum of half of the height of the object and the half height of the rectangle, in C.
D005 SRL A
D007 ADD A,(IY+$03)
D00A LD C,A
Find the distance between the middles.
D00B LD A,($A215) Fetch VerticalSpeed.
D00E SRA A Add half of it to the position, then find the distance from the middle of the rectangle.
D010 ADD A,D
D011 SUB (IY+$01)
Make the distance positive, then subtract the sum.
D014 JP P,CheckOverlapAlongY_Depth Make the distance positive.
D017 NEG
CheckOverlapAlongY_Depth D019 SUB C Subtract the sum.
D01A RET Return.
View the equivalent code in;
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