Routines
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8598: Get Random Coordinate
Used by the routine at SpawnCreature.
Chooses a random position along one axis of the room. A random number is reduced to less than the size of the room less 08 and then it is either added to or taken away from 60. This is used by SpawnCreature to place a new creature.
Input
B The size of the room along the axis
HL The address of the previous byte of the position in the entity
Output
A The coordinate
HL The address of the next byte of the entity
GetRandomCoordinate 8598 LD A,B Fetch the size of the room along the axis.
8599 SUB $08 Take 08 off the size of the room.
859B LD B,A Keep the result in B as the limit for the random number.
859C LD A,R Fetch a random number (from the refresh register).
859E INC HL Move HL on to the next byte.
GetRandomCoordinate_Reduce 859F CP B Jump to GetRandomCoordinate_Offset if the random number is less than the limit.
85A0 JR C,GetRandomCoordinate_Offset
85A2 SUB B Subtract the limit from the random number.
85A3 JR GetRandomCoordinate_Reduce Jump back to GetRandomCoordinate_Reduce to keep reducing the number.
GetRandomCoordinate_Offset 85A5 LD C,A Keep the random offset in C.
85A6 LD A,R Fetch a random number (from the refresh register).
85A8 BIT 1,A Test bit 1 of the random number, to choose which side of 60 to use.
85AA LD A,$60 A=60.
85AC JR Z,GetRandomCoordinate_Negative Jump to GetRandomCoordinate_Negative if bit 1 was not set.
85AE ADD A,C Add the offset to 60.
85AF RET Return.
GetRandomCoordinate_Negative 85B0 SUB C Take the offset away from 60.
85B1 RET Return.
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