Routines
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47638: Find Group
Used by the routines at DrawLevelObjects and TestGroupObjects.
Finds the group of objects at a position of the level, using the level map at the address in LevelHeaderAddress and the table of groups at GroupTable.
Input
H The vertical position, which starts at 32
L The horizontal position, which starts at 32
Output
IY The address of the group
F Carry set if there is no group, or it has no objects
Check that the position is inside the map, and find the cell of the map.
FindGroup 47638 LD IY,(23806) Fetch the address of the header of the level into IY.
47642 LD A,L Copy the horizontal position into A.
47643 SUB 32 Take away the border of 32 to make it a column of the map.
47645 RET C Return if the position is left of the map.
47646 LD C,A Keep the column in C.
47647 CP (IY+5) Compare the column with the width of the map.
47650 JR NC,SetCarryAndReturn Return with no group if the position is right of the map.
47652 LD A,H Copy the vertical position into H.
47653 SUB 32 Take away the border of 32 to make it a row of the map.
47655 RET C Return if the position is above the map.
47656 LD B,A Keep the row in B.
47657 JR Z,FindGroup_Row Jump to FindGroup_Row if it is the first row.
47659 CP (IY+6) Compare the row with the height of the map.
47662 JR NC,SetCarryAndReturn Return with no group if the position is below the map.
Find the number of the cell by adding the width of the map once for each row, and then add that number of cells to the start of the map.
47664 LD A,C Start with the column.
47665 LD D,(IY+5) Fetch the width of the map.
FindGroup_Loop 47668 ADD A,D Add the width of the map.
47669 DJNZ FindGroup_Loop Decrease the row counter and loop back to FindGroup_Loop until all the rows have been added.
47671 LD C,A Keep the number of the cell in C.
FindGroup_Row 47672 SLA C Double it, as there are two bytes for each cell.
47674 ADD IY,BC Add it to the address of the header, to reach the cell.
Read the cell. Its first byte is returned as an offset, and its second is the number of the group.
47676 LD E,B Clear E.
47677 LD A,(IY+7) Fetch the first byte of the cell, which starts 7 bytes after the header.
47680 LD D,A Keep it in D.
47681 ADD A,2 Add 2 which sets the carry flag if the byte is 254 or 255.
47683 RET C Return with no group if the cell has none.
47684 LD C,(IY+8) Fetch the number of the group from the second byte of the cell.
47687 SLA C Double it, as the table has two bytes for each group.
47689 LD HL,35761 Point HL at GroupTable.
47692 ADD HL,BC Add the offset to find the entry for the group.
47693 LD C,(HL) Fetch the low byte of the address of the group.
47694 INC HL Move onto the high byte.
47695 LD B,(HL) Fetch the high byte of the address of the group.
47696 PUSH BC Put the address of the group in BC onto the stack...
47697 POP IY ...and take it off into IY.
47699 SRL D Multiply the first byte of the cell by 128 by shifting it...
47701 RR E ...across DE, ready to return it as an offset.
47703 LD A,(IY+0) Fetch the first byte of the group.
47706 AND A Test it.
47707 RET NZ Return with the carry flag reset if the group has objects.
47708 SCF Set the carry flag as the group is empty.
47709 RET Return.
Prev: 47636 Up: Map Next: 47710