Routines
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C1B9: Get Distance To Monster
Used by the routine at FindNearestMonster.
Works out the distance from the object at IX to a monster, which is the sum of the differences in their columns and rows.
Input
IX Object
IY Monster
Output
A The distance, or FF if the monster is dead
Return FF if the monster is dead.
GetDistanceToMonster C1B9 LD A,(IY+$00) Check whether the monster is alive, and jump to GetDistanceToMonster_Alive if it is.
C1BC AND A
C1BD JR NZ,GetDistanceToMonster_Alive
C1BF LD A,$FF The monster has been killed, so return a distance of FF, which is too far away to matter.
C1C1 RET Return.
Add the absolute differences in columns and rows.
GetDistanceToMonster_Alive C1C2 LD A,(IX+$02) Work out how many columns the object is from the monster.
C1C5 SUB (IY+$02)
C1C8 JP P,GetDistanceToMonster_Rows Jump to GetDistanceToMonster_Rows if the difference is not negative.
C1CB NEG Otherwise turn the distance round, so that it is positive.
GetDistanceToMonster_Rows C1CD LD C,A Keep the distance in columns.
C1CE LD A,(IX+$04) Work out how many rows the object is from the monster.
C1D1 SUB (IY+$04)
C1D4 JP P,GetDistanceToMonster_Add Jump to GetDistanceToMonster_Add if the difference is not negative.
C1D7 NEG Otherwise turn the distance round, so that it is positive.
GetDistanceToMonster_Add C1D9 ADD A,C Add the two distances together to get the distance from the monster.
C1DA RET Return.
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