Routines
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46186: Check Object Contact
Used by the routine at Check_Decoration_Contact.
Checks whether the object at IX touches the object at IY, which is an obstacle. The bytes 10 to 14 of the obstacle record are not used for movement. Instead they describe the size of the obstacle: its half width (byte 10), the width of its middle (byte 11), its height (byte 12), the depth from the top at which the object counts as being beside it (byte 13) and the offset of the middle from its x position (byte 14). The y position of an obstacle is the position of its bottom.
The result is returned in C. Bit 2 is set if the objects touch. Bit 7 is set if IX is on the right of the middle of the obstacle. Bit 0 is set if IX is outside the width of the middle. Bit 1 is set if the bottom of IX is in the top part of the obstacle (so that it is standing or landing on it).
When the object touches the top of the obstacle, bit 4 of its contact flags is set (so it is standing on it) unless it is a weapon pickup (see Handle_Weapon_Pickup) or it is outside the width of the middle. An obstacle of type 136 is handled by Check_Rock_Formation_Contact instead.
Input
IX Object record
IY Obstacle record
Output
C Contact result
AF' 160 less the y position of the top of the obstacle
Check_Object_Contact 46186 LD C,0 Clear the contact result.
46188 LD A,(IY+0) Fetch the type of the obstacle.
46191 AND A Return if the obstacle record is empty.
46192 RET Z
46193 BIT 0,(IY+15) Return if the obstacle has just been created.
46197 RET NZ
46198 CP 136 Jump to Check_Rock_Formation_Contact if the obstacle is a type 136 object.
46200 JP Z,Check_Rock_Formation_Contact
46203 LD A,(IY+1) Fetch the x position of the obstacle.
46206 ADD A,(IY+14) Add the offset of its middle.
46209 SUB (IX+1) Jump to Check_Object_Contact_Test_Distance if the object is on the left of the middle of the obstacle.
46212 JP P,Check_Object_Contact_Test_Distance
46215 NEG Make the distance positive.
46217 SET 7,C Set bit 7 of the result, as the object is on the right.
Check_Object_Contact_Test_Distance 46219 CP (IY+10) Jump to Check_Object_Contact_None if the distance is too great for them to touch.
46222 JR NC,Check_Object_Contact_None
46224 CP (IY+11) Jump to Check_Object_Contact_Test_Height if the object is inside the width of the middle.
46227 JR C,Check_Object_Contact_Test_Height
46229 SET 0,C Set bit 0 of the result, as the object is outside the middle.
Check_Object_Contact_Test_Height 46231 LD A,(IY+3) Fetch the y position of the obstacle.
46234 SUB (IX+3) Jump to Check_Object_Contact_None if the object is below the bottom of the obstacle.
46237 JP M,Check_Object_Contact_None
46240 PUSH AF Stash the distance.
46241 LD A,(IY+3) Work out the y position of the top of the obstacle.
46244 SUB (IY+12)
46247 SUB 160 Subtract it from 160.
46249 NEG
46251 EX AF,AF' Keep the result in AF'.
46252 POP AF Restore the distance.
46253 CP (IY+12) Jump to Check_Object_Contact_None if the distance is the height of the obstacle or more.
46256 JR NC,Check_Object_Contact_None
46258 SET 2,C Set bit 2 of the result, as the objects touch.
46260 CP (IY+13) Return if the distance is less than the depth in byte 13 of the obstacle.
46263 RET C
46264 SET 1,C Set bit 1 of the result, as the object is on top of the obstacle.
46266 LD A,(IX+0) Fetch the type of the object.
46269 AND %11111100 Reset the two low bits.
46271 CP 104 Return if the object is a weapon pickup.
46273 RET Z
46274 BIT 0,C Return if the object is outside the width of the middle.
46276 RET NZ
46277 SET 4,(IX+11) Set bit 4 of the contact flags of the object, as it is standing on the obstacle.
46281 RET Return.
This entry point is used by the routine at Check_Rock_Formation_Contact.
Check_Object_Contact_None 46282 LD C,0 Clear the result, as the objects do not touch.
46284 RET Return.
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