Routines
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55268: Test Entity Cells
Used by the routine at MoveEntity.
Tests each cell of the level that the entity's box covers, by calling TestGroupObjects, and retests a cell when a collision moves the entity. When bit 6 of the flags is set, a z within one page of 255 is stored in field 31 after a zero top byte has been snapped up by one unit. A hit planted by HandleOverlap then turns the heading away from it. Purpose of bit 6 is not yet known.
Input
IX The entity
The cell is the high byte of x and the high byte of y. A collision can move those bytes, and the extent can make the entity cover the next cell.
TestEntityCells 55268 LD L,(IX+2) Take the cell from the high bytes of x and y.
55271 LD H,(IX+5)
55274 PUSH HL Test that cell.
55275 CALL TestGroupObjects
55278 POP HL Restore the cell.
55279 LD A,(IX+2) Retest the previous column when the high byte of x moved.
55282 CP L
55283 JR Z,TestEntityCells_SpanX
55285 DEC A Use the column below the new x.
55286 LD L,A
55287 JR TestEntityCells_Second
TestEntityCells_SpanX 55289 LD A,(IX+3) The entity covers the next column when the x extent carries.
55292 ADD A,(IX+11)
55295 JR NC,TestEntityCells_Moved
TestEntityCells_Second 55297 PUSH HL Test the next column.
55298 INC L
55299 CALL TestGroupObjects
55302 POP HL Restore the cell.
55303 LD A,(IX+5) Retest this row when the high byte of y moved.
55306 CP H
55307 JR Z,TestEntityCells_SpanY
55309 LD H,A Test the entity's row, then the row above.
55310 PUSH HL
55311 CALL TestGroupObjects
55314 POP HL
55315 INC H
55316 JR TestEntityCells_Next
TestEntityCells_SpanY 55318 LD A,(IX+6) The entity covers the next row when the y extent carries.
55321 ADD A,(IX+12)
55324 JR NC,TestEntityCells_Snap
55326 INC H Test the next row.
55327 PUSH HL
55328 CALL TestGroupObjects
55331 POP HL
TestEntityCells_Next 55332 INC L Test the diagonal cell too.
55333 JR TestEntityCells_Cell
TestEntityCells_Moved 55335 LD A,(IX+5) Use the entity's row when y has moved.
55338 CP H
55339 JR Z,TestEntityCells_SpanRow
55341 LD H,A Use that row.
55342 JR TestEntityCells_Cell
TestEntityCells_SpanRow 55344 LD A,(IX+6) Cover the next row when the y extent carries.
55347 ADD A,(IX+12)
55350 JR NC,TestEntityCells_Snap
55352 INC H Use the next row.
TestEntityCells_Cell 55353 CALL TestGroupObjects Test this cell.
Bit 6 of the flags stores a z that is within one page of 255 into field 31. A zero top byte is snapped up by one unit first. A pickup stores zero instead.
TestEntityCells_Snap 55356 LD C,0 Start with no hit. HandleOverlap plants one in this byte.
55358 BIT 6,(IX+30) Snap z when bit 6 of the flags is set.
55362 JR Z,TestEntityCells_Hit
55364 LD L,(IX+9) A zero top byte of z is snapped up by one unit.
55367 LD A,(IX+8)
55370 AND A
55371 JR NZ,TestEntityCells_Distance
55373 LD L,A Write that snap back into z.
55374 LD (IX+9),A
55377 INC A
55378 LD (IX+8),A
55381 LD A,(IX+0) A pickup stores zero in field 31.
55384 CP 15
55386 JR Z,TestEntityCells_Store
55388 LD A,L Otherwise measure from one.
55389 INC A
TestEntityCells_Distance 55390 LD H,A Store the distance when z is within one page of 255.
55391 XOR A
55392 LD DE,255
55395 SBC HL,DE
55397 OR H
55398 JR NZ,TestEntityCells_Hit
TestEntityCells_Store 55400 LD (IX+31),L Remember that distance in field 31.
A hit turns the heading. Fields 24 and 25 supply the two components, and the low bits of the hit choose which axis is turned.
TestEntityCells_Hit 55403 LD A,C Return when nothing was hit.
55404 AND A
55405 RET Z
55406 LD B,A Remember the hit and clear it.
55407 XOR A
55408 LD (55357),A
55411 BIT 5,(IX+1) Return when bit 5 of the frame is set.
55415 RET NZ
55416 LD A,(IX+24) Return when fields 24 and 25 are both zero.
55419 OR (IX+25)
55422 RET Z
55423 LD A,B Return when the hit was 255.
55424 CP 255
55426 RET Z
55427 LD A,(IX+24) Use field 24 as one component.
55430 AND A
55431 JR NZ,TestEntityCells_Component
55433 LD A,(IX+17) Read the heading.
55436 ADD A,16 Add 16 to the heading.
55438 AND %00111111 Use the low six bits of the heading.
55440 CP 32 Use 254 when those bits are 32 or more.
55442 JR C,TestEntityCells_Component
55444 LD A,254 Use 254 for that component.
TestEntityCells_Component 55446 LD L,A Keep the first component.
55447 LD A,(IX+25) Use field 25 as the other component.
55450 AND A
55451 JR NZ,TestEntityCells_Other
55453 LD A,(IX+17) Read the heading.
55456 AND %00111111 Use the low six bits of the heading.
55458 CP 32 Use 254 when those bits are 32 or more.
55460 JR C,TestEntityCells_Other
55462 LD A,254 Use 254 for that component.
TestEntityCells_Other 55464 LD H,A Keep the second component.
55465 LD A,B Use the low two bits of the hit.
55466 AND 3
55468 JR Z,TestEntityCells_Swap Swap the components when the hit was on x.
55470 LD B,H
55471 LD C,L
55472 JR TestEntityCells_Heading
TestEntityCells_Swap 55474 LD A,H Turn the second component to face the other way.
55475 NEG
55477 LD C,A
55478 LD A,B
55479 RRA
55480 RRA
55481 RRA
55482 LD B,L Keep the first component.
TestEntityCells_Heading 55483 RRCA Face the patched step away from the first component.
55484 LD E,A
55485 XOR B
55486 RLA
55487 LD B,254
55489 JR C,TestEntityCells_Signs
55491 LD A,B Face the other way when the signs differ.
55492 NEG
55494 LD B,A
TestEntityCells_Signs 55495 LD A,E Compare the signs of the two components.
55496 XOR C
55497 RLA
55498 JR NC,TestEntityCells_Apply Keep the turn when the signs agree.
55500 BIT 7,B A positive turn is 1.
55502 LD B,1
55504 JR Z,TestEntityCells_Apply
55506 LD B,255 A negative turn is 255.
TestEntityCells_Apply 55508 LD A,(IX+17) Read the heading.
55511 LD C,A Keep the old heading aside.
55512 ADD A,B Add the turn to the heading.
55513 AND %00111111 Use the low six bits of the new heading.
55515 LD B,A Keep the new heading aside.
55516 LD A,C Use bits 6-7 of the old heading.
55517 AND 192
55519 OR B Combine them with the new heading.
55520 LD (IX+17),A
55523 RET Return.
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