Routines
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BA21: Check Terrain Collision
Used by the routines at NudgeObject and RunFrame.
Checks whether an object has run into the terrain and bounces it when it has. The tiles come from StartTerrainCheck and SelectNextTile. A tile record starts with the number of boxes, and each box is five bytes: its position, its width, its height and its flags. Bits 2 to 5 clear means a slope (TestSlopeCollision); otherwise the box is a rectangle (TestBoxesOverlap). A rectangle with bits 4 and 5 clear reverses, halves and limits the vertical speed. Any other rectangle does the same to the horizontal speed, and skips the halving when the collision mode is zero. The mode in HorizontalStepSize is FF for the ship, 80 when the object should stop, and zero otherwise. The ship also plays the crash or bump sound and changes the scroll speed.
Input
B Height of the object
C Width of the object
IX Address of the object
Output
$5C1C Non-zero if the object hit something
CheckTerrainCollision BA21 XOR A Clear the hit flag.
BA22 LD ($5C1C),A The hit flag at ScratchValue starts clear.
BA25 LD ($BA09),IX Keep the address of the object.
BA29 CALL StartTerrainCheck Find the tiles the object overlaps.
BA2C JR CheckTerrainCollision_Tile Test the first of those tiles.
CheckTerrainCollision_NextTiles BA2E CALL SelectNextTile The next overlapping tile.
CheckTerrainCollision_Tile BA31 LD A,(IX+$00) The number of boxes in this tile.
BA34 AND A None: on to the next tile.
BA35 JR Z,CheckTerrainCollision_Done
BA37 INC IX The first box of the tile.
CheckTerrainCollision_Box BA39 PUSH IX Keep the address of this box.
BA3B EX AF,AF' The box count is in the alternate accumulator.
BA3C XOR A Clear the hit flag for this box.
BA3D LD ($5C1C),A The hit flag starts clear.
BA40 LD A,(IX+$04) The flags of this box.
BA43 LD ($5C20),A Keep the flags.
BA46 AND %00111100 Bits 2 to 5 mark a rectangle.
BA48 JR NZ,CheckTerrainCollision_Rectangle A rectangle is tested below.
BA4A JP CheckTerrainCollision_Slope Otherwise the box is a slope.
CheckTerrainCollision_Check BA4D LD A,($5C35) The collision status.
BA50 BIT 6,A Jump to CheckTerrainCollision_Stop if bit 6 of A is set.
BA52 JR NZ,CheckTerrainCollision_Stop
CheckTerrainCollision_NextBox BA54 POP IX Back to this box.
BA56 LD DE,$0005 A box is five bytes.
BA59 ADD IX,DE The next box.
BA5B EX AF,AF' Back to the box count.
BA5C DEC A One box has been tested.
BA5D JR NZ,CheckTerrainCollision_Box Repeat while boxes remain.
CheckTerrainCollision_Done BA5F LD A,($5C1D) The tiles still to test.
BA62 DEC A One tile has been tested.
BA63 LD ($5C1D),A Remember how many tiles remain.
BA66 JP NZ,CheckTerrainCollision_NextTiles Jump to CheckTerrainCollision_NextTiles if the zero flag is clear.
BA69 LD IX,($BA09) Fetch the word at BA09 into IX.
BA6D LD A,($5C25) The collision mode.
BA70 INC A The ship is mode FF which becomes zero here.
BA71 RET NZ Return if the zero flag is clear.
BA72 LD ($BC12),A The ship has finished this check.
BA75 RET Return.
CheckTerrainCollision_Stop BA76 LD A,($8F53) The ship state.
BA79 CP $0A Jump to CheckTerrainCollision_Return if A is less than 0A.
BA7B JR C,CheckTerrainCollision_Return
BA7D LD A,($5C25) The collision mode.
BA80 AND A Jump to CheckTerrainCollision_Return if A is not zero.
BA81 JR NZ,CheckTerrainCollision_Return
BA83 JP CheckTerrainCollision_NextBox Jump to CheckTerrainCollision_NextBox.
CheckTerrainCollision_Return BA86 POP IX Discard the box address.
BA88 LD IX,($BA09) Fetch the word at BA09 into IX.
BA8C RET Return.
BA8D NOP Do nothing.
BA8E NOP Do nothing.
CheckTerrainCollision_Rectangle BA8F LD A,($5C35) The collision status.
BA92 BIT 7,A Jump to CheckTerrainCollision_Check if bit 7 of A is set.
BA94 JP NZ,CheckTerrainCollision_Check
BA97 LD HL,($CE3A) Fetch the word at ScratchValues into HL.
BA9A LD A,($5C25) The collision mode.
BA9D CP $80 Jump to CheckTerrainCollision_Test if A is 80.
BA9F JR Z,CheckTerrainCollision_Test
BAA1 LD A,($5C25) The collision mode again.
BAA4 INC A The ship is the only object tested here.
BAA5 JP NZ,CheckTerrainCollision_Flags Jump to CheckTerrainCollision_Flags if the zero flag is clear.
BAA8 LD A,L The low byte of the position in the tile.
BAA9 ADD A,$04 Add 04 to A.
BAAB CP $E8 Jump to CheckTerrainCollision_Limit if A is less than E8.
BAAD JR C,CheckTerrainCollision_Limit
BAAF LD A,$E8 Do not test past column E8.
CheckTerrainCollision_Limit BAB1 LD D,A The near edge of the test.
BAB2 ADD A,$24 Add 24 to A.
BAB4 JR NC,CheckTerrainCollision_Lower Jump to CheckTerrainCollision_Lower if the carry flag is clear.
BAB6 LD A,$F8 Do not test past column F8.
CheckTerrainCollision_Lower BAB8 LD E,A The far edge of the test.
BAB9 BIT 2,(IX+$04) Jump to CheckTerrainCollision_Flags if bit 2 of offset 04 of IX is clear.
BABD JR Z,CheckTerrainCollision_Flags
BABF LD A,($F5EF) The second part flag.
BAC2 AND A Jump to CheckTerrainCollision_Flags if A is not zero.
BAC3 JR NZ,CheckTerrainCollision_Flags
BAC5 LD A,(IX+$01) The box's other coordinate.
BAC8 SUB H Subtract H from A.
BAC9 JR C,CheckTerrainCollision_Flags Jump to CheckTerrainCollision_Flags if the carry flag is set.
BACB CP $29 Jump to CheckTerrainCollision_Flags if A is 29 or more.
BACD JR NC,CheckTerrainCollision_Flags
BACF LD A,(IX+$00) The box's first coordinate.
BAD2 CP E Jump to CheckTerrainCollision_Flags if A is E or more.
BAD3 JR NC,CheckTerrainCollision_Flags
BAD5 ADD A,(IX+$02) Add the byte at offset 02 of IX to A.
BAD8 CP D Jump to CheckTerrainCollision_Flags if A is less than D.
BAD9 JR C,CheckTerrainCollision_Flags
BADB LD ($C6D0),A The ship has landed on this box.
CheckTerrainCollision_Flags BADE LD A,(IX+$04) The box flags.
BAE1 AND %00110000 Bits 4 and 5 select a sideways face.
BAE3 JR NZ,CheckTerrainCollision_Test2 Jump to CheckTerrainCollision_Test2 if the zero flag is clear.
BAE5 LD A,($BE8E) The saved vertical speed.
BAE8 AND A Jump to CheckTerrainCollision_Test if A is zero.
BAE9 JR Z,CheckTerrainCollision_Test
BAEB BIT 2,(IX+$04) Jump to CheckTerrainCollision_Skip if bit 2 of offset 04 of IX is set.
BAEF JR NZ,CheckTerrainCollision_Skip
BAF1 NEG Negate A.
CheckTerrainCollision_Skip BAF3 BIT 7,A Jump to CheckTerrainCollision_Check if bit 7 of A is set.
BAF5 JP NZ,CheckTerrainCollision_Check
BAF8 JR CheckTerrainCollision_Test Jump to CheckTerrainCollision_Test.
CheckTerrainCollision_Test2 BAFA BIT 4,A Test bit 4 of A.
BAFC LD A,($BE8D) The saved horizontal speed.
BAFF JR NZ,CheckTerrainCollision_Check2 Jump to CheckTerrainCollision_Check2 if the zero flag is clear.
BB01 NEG Negate A.
CheckTerrainCollision_Check2 BB03 AND A Jump to CheckTerrainCollision_Test if A is zero.
BB04 JR Z,CheckTerrainCollision_Test
BB06 BIT 7,A Jump to CheckTerrainCollision_Check if bit 7 of A is clear.
BB08 JP Z,CheckTerrainCollision_Check
BB0B JR CheckTerrainCollision_Test Jump to CheckTerrainCollision_Test.
CheckTerrainCollision_Test BB0D LD DE,($CE3C) Fetch the word at CE3C into DE.
BB11 CALL TestBoxesOverlap Call TestBoxesOverlap.
BB14 LD A,($5C1C) The hit flag from the rectangle test.
BB17 AND A Jump to CheckTerrainCollision_Check if A is zero.
BB18 JP Z,CheckTerrainCollision_Check
BB1B LD A,($5C25) The collision mode.
BB1E CP $80 Jump to CheckTerrainCollision_Stop if A is 80.
BB20 JP Z,CheckTerrainCollision_Stop
BB23 LD A,$80 A rectangle hit is marked with 80.
BB25 LD ($5C35),A Remember that hit in the collision status.
BB28 LD A,(IX+$01) The box's second coordinate.
BB2B SUB $14 Subtract 14 from A.
BB2D LD ($DEDC),A Keep that coordinate.
BB30 LD A,(IX+$04) The box flags again.
BB33 LD ($DEDD),A Keep the box flags.
BB36 LD B,A The flags, kept for the bounce.
BB37 AND %00110000 Bits 4 and 5 mean the face is sideways.
BB39 LD IX,($BA09) Fetch the word at BA09 into IX.
BB3D JP NZ,CheckTerrainCollision_Wall Jump to CheckTerrainCollision_Wall if the zero flag is clear.
BB40 LD A,($5C25) The collision mode.
BB43 AND A Jump to CheckTerrainCollision_Free if A is zero.
BB44 JR Z,CheckTerrainCollision_Free
BB46 BIT 3,B Jump to CheckTerrainCollision_Vertical if bit 3 of B is clear.
BB48 JR Z,CheckTerrainCollision_Vertical
BB4A LD A,($F5F6) The ninth part flag.
BB4D AND A Jump to CheckTerrainCollision_Vertical if A is not zero.
BB4E JR NZ,CheckTerrainCollision_Vertical
BB50 LD A,$04 Four frames of the hit flash.
BB52 LD ($C7C1),A Start the hit flash.
BB55 JR CheckTerrainCollision_Vertical Jump to CheckTerrainCollision_Vertical.
CheckTerrainCollision_Free BB57 CALL ClearShot Call ClearShot.
BB5A BIT 3,B Jump to CheckTerrainCollision_Vertical if bit 3 of B is set.
BB5C JR NZ,CheckTerrainCollision_Vertical
BB5E LD A,($DEDC) The coordinate the object should stand on.
BB61 LD (IX+$05),A Put the object on that row.
CheckTerrainCollision_Vertical BB64 LD H,(IX+$09) The high byte of the vertical speed.
BB67 LD L,(IX+$0A) The low byte of the vertical speed.
BB6A CALL NegateNumber Call NegateNumber.
BB6D CALL HalveNumber Call HalveNumber.
BB70 CALL LimitSpeed Call LimitSpeed.
BB73 CALL ZeroSmallSpeed Call ZeroSmallSpeed.
CheckTerrainCollision_StoreVertical BB76 LD (IX+$09),H The bounced vertical speed.
BB79 LD (IX+$0A),L The low byte of the bounced vertical speed.
BB7C LD A,H The high byte of that speed.
BB7D LD ($BE8E),A Keep it for the next box.
BB80 OR L Set the bits of A that are set in L.
BB81 JR Z,CheckTerrainCollision_Still Jump to CheckTerrainCollision_Still if the zero flag is set.
BB83 LD A,($5C35) The collision status.
BB86 OR %01000000 Set the bits of A that are set in 40.
BB88 LD ($5C35),A Note that the object is still moving vertically.
BB8B JR CheckTerrainCollision_Ship Jump to CheckTerrainCollision_Ship.
CheckTerrainCollision_Still BB8D LD A,($5C25) The collision mode.
BB90 INC A See whether this is the ship.
BB91 JR Z,CheckTerrainCollision_Ship Jump to CheckTerrainCollision_Ship if the zero flag is set.
BB93 BIT 3,B Jump to CheckTerrainCollision_Check if bit 3 of B is clear.
BB95 JP Z,CheckTerrainCollision_Check
BB98 LD (IX+$09),$FE Store FE at offset 09 of IX.
CheckTerrainCollision_Ship BB9C LD A,($5C25) The collision mode.
BB9F INC A The ship is mode FF.
BBA0 JP NZ,CheckTerrainCollision_Check Jump to CheckTerrainCollision_Check if the zero flag is clear.
BBA3 LD A,L The low byte of the bounced speed.
BBA4 OR H Set the bits of A that are set in H.
BBA5 CALL NZ,PlayCrashSound Call PlayCrashSound if the zero flag is clear.
BBA8 LD DE,$0000 No fractional vertical scroll.
BBAB LD ($B320),DE Clear the vertical scroll fraction.
BBAF CALL UpdateVerticalScrollSpeed Call UpdateVerticalScrollSpeed.
BBB2 LD A,($DEDD) The box flags that were hit.
BBB5 CP $04 Jump to CheckTerrainCollision_Check if A is not 04.
BBB7 JP NZ,CheckTerrainCollision_Check
BBBA LD A,($C8EA) The ship's map zone.
BBBD LD B,A Keep the map zone.
BBBE LD A,($DEDC) The box coordinate.
BBC1 CP B Jump to CheckTerrainCollision_Check if A is B or more.
BBC2 JP NC,CheckTerrainCollision_Check
BBC5 LD A,$02 A small downward scroll.
BBC7 LD ($CCBA),A Scroll the world down by two.
BBCA JP CheckTerrainCollision_Check Jump to CheckTerrainCollision_Check.
CheckTerrainCollision_Wall BBCD LD H,(IX+$07) The high byte of the horizontal speed.
BBD0 LD L,(IX+$08) The low byte of the horizontal speed.
BBD3 CALL NegateNumber Call NegateNumber.
BBD6 LD A,($5C25) The collision mode.
BBD9 AND A Jump to CheckTerrainCollision_Update if A is not zero.
BBDA JR NZ,CheckTerrainCollision_Update
BBDC CALL ClearShot Call ClearShot.
BBDF JR CheckTerrainCollision_StoreHorizontal Jump to CheckTerrainCollision_StoreHorizontal.
CheckTerrainCollision_Update BBE1 CALL HalveNumber Call HalveNumber.
BBE4 CALL LimitSpeed Call LimitSpeed.
BBE7 CALL ZeroSmallSpeed Call ZeroSmallSpeed.
CheckTerrainCollision_StoreHorizontal BBEA LD (IX+$07),H The bounced horizontal speed.
BBED LD (IX+$08),L The low byte of the bounced horizontal speed.
BBF0 LD A,H The high byte of that speed.
BBF1 LD ($BE8D),A Keep it for the next box.
BBF4 LD A,($5C25) The collision mode.
BBF7 INC A The ship is mode FF.
BBF8 JP NZ,CheckTerrainCollision_Check Jump to CheckTerrainCollision_Check if the zero flag is clear.
BBFB LD A,L The low byte of the bounced speed.
BBFC OR H Set the bits of A that are set in H.
BBFD CALL NZ,PlayBumpSound Call PlayBumpSound if the zero flag is clear.
BC00 LD DE,$0000 No fractional horizontal scroll.
BC03 LD ($C7FA),DE Clear the horizontal scroll fraction.
BC07 CALL UpdateHorizontalScrollSpeed Call UpdateHorizontalScrollSpeed.
BC0A LD A,$01 The ship has hit a wall.
BC0C LD ($D0BC),A The collision kind is the terrain.
BC0F JP CheckTerrainCollision_Check Jump to CheckTerrainCollision_Check.
BC12 NOP Do nothing.
CheckTerrainCollision_Slope BC13 LD A,($5C35) The collision status.
BC16 BIT 4,A Jump to CheckTerrainCollision_Check if bit 4 of A is set.
BC18 JP NZ,CheckTerrainCollision_Check
BC1B CALL TestSlopeCollision Call TestSlopeCollision.
BC1E LD A,($5C1C) The hit flag from the slope test.
BC21 AND A Jump to CheckTerrainCollision_Check if A is zero.
BC22 JP Z,CheckTerrainCollision_Check
BC25 LD A,($5C35) The collision status.
BC28 OR %00010000 Set the bits of A that are set in 10.
BC2A LD ($5C35),A Note that a slope has been hit.
BC2D LD A,($5C25) The collision mode.
BC30 CP $80 Jump to CheckTerrainCollision_Stop if A is 80.
BC32 JP Z,CheckTerrainCollision_Stop
BC35 AND A Jump to CheckTerrainCollision_Kind if A is zero.
BC36 JR Z,CheckTerrainCollision_Kind
BC38 DEC A One less than the mode.
BC39 LD ($D0BC),A The collision kind follows the mode.
CheckTerrainCollision_Kind BC3C LD A,(IX+$04) The slope flags.
BC3F LD ($5C20),A Keep the slope flags.
BC42 AND %01111111 The steepness and the facing, without bit 7.
BC44 CP $02 Jump to CheckTerrainCollision_Reflect if A is 02.
BC46 JP Z,CheckTerrainCollision_Reflect
BC49 AND %00000111 The steepness.
BC4B CP $02 Jump to CheckTerrainCollision_Three if A is 02.
BC4D JP Z,CheckTerrainCollision_Three
BC50 LD A,(IX+$04) The slope flags again.
BC53 BIT 6,A Jump to CheckTerrainCollision_Rebound if bit 6 of A is clear.
BC55 JP Z,CheckTerrainCollision_Rebound
BC58 JP CheckTerrainCollision_ReboundBoth Jump to CheckTerrainCollision_ReboundBoth.
CheckTerrainCollision_Bounce BC5B LD IX,($BA09) Fetch the word at BA09 into IX.
BC5F LD A,($5C25) The collision mode.
BC62 INC A See whether this is the ship.
BC63 JR Z,CheckTerrainCollision_Clear Jump to CheckTerrainCollision_Clear if the zero flag is set.
BC65 LD L,(IX+$07) The high byte of the horizontal speed, kept in L.
BC68 LD H,(IX+$08) The low byte of the horizontal speed, kept in H.
BC6B LD ($BA8D),HL Keep the old horizontal speed.
BC6E LD A,($5C1A) How far the object went into the slope.
BC71 NEG Negate A.
BC73 LD (IX+$07),A The horizontal speed becomes that depth, reversed.
BC76 BIT 7,A Jump to CheckTerrainCollision_Speed if bit 7 of A is clear.
BC78 JR Z,CheckTerrainCollision_Speed
BC7A NEG Negate A.
CheckTerrainCollision_Speed BC7C CP $10 Jump to CheckTerrainCollision_Position if A is less than 10.
BC7E JR C,CheckTerrainCollision_Position
BC80 SET 3,(IX+$10) Set bit 3 of offset 10 of IX.
CheckTerrainCollision_Position BC84 LD (IX+$08),$00 Store 00 at offset 08 of IX.
BC88 CALL FindHorizontalMove Call FindHorizontalMove.
BC8B LD (IX+$01),H The map column after the bounce.
BC8E LD (IX+$02),L The horizontal position after the bounce.
BC91 LD HL,($BA8D) Fetch the word at BA8D into HL.
BC94 LD (IX+$07),L Restore the high byte of the old horizontal speed.
BC97 LD (IX+$08),H Restore the low byte of the old horizontal speed.
CheckTerrainCollision_Clear BC9A CALL ClearShot Call ClearShot.
BC9D LD H,(IX+$09) The high byte of the vertical speed.
BCA0 LD L,(IX+$0A) The low byte of the vertical speed.
BCA3 CALL ZeroSmallSpeed Call ZeroSmallSpeed.
BCA6 LD A,L The low byte of the vertical speed.
BCA7 OR H Set the bits of A that are set in H.
BCA8 JR NZ,CheckTerrainCollision_Finish Jump to CheckTerrainCollision_Finish if the zero flag is clear.
BCAA LD H,(IX+$07) The high byte of the horizontal speed.
BCAD LD L,(IX+$08) The low byte of the horizontal speed.
BCB0 CALL ZeroSmallSpeed Call ZeroSmallSpeed.
BCB3 LD (IX+$08),L The low byte, once a tiny speed has been dropped.
BCB6 LD A,H The high byte of the horizontal speed.
BCB7 OR L Set the bits of A that are set in L.
BCB8 JR NZ,CheckTerrainCollision_Finish Jump to CheckTerrainCollision_Finish if the zero flag is clear.
BCBA CALL CheckTerrainCollision_Flags2 Call CheckTerrainCollision_Flags2.
BCBD JR CheckTerrainCollision_Finish Jump to CheckTerrainCollision_Finish.
CheckTerrainCollision_Flags2 BCBF LD A,($5C20) The slope flags.
BCC2 LD (IX+$07),$01 Store 01 at offset 07 of IX.
BCC6 AND %11000000 The two facing bits.
BCC8 RET Z Return if the zero flag is set.
BCC9 XOR %11000000 Flip the bits of A that are set in C0.
BCCB RET Z Return if the zero flag is set.
BCCC LD (IX+$07),$FF Store FF at offset 07 of IX.
BCD0 RET Return.
CheckTerrainCollision_Finish BCD1 LD A,($5C25) The collision mode.
BCD4 AND A Jump to CheckTerrainCollision_Stop if A is zero.
BCD5 JP Z,CheckTerrainCollision_Stop
BCD8 XOR A Clear A.
BCD9 LD ($CCBB),A Stop the horizontal scroll.
BCDC LD ($CCBA),A Stop the vertical scroll.
BCDF LD A,($5C40) The ship was just hit.
BCE2 AND A Jump to CheckTerrainCollision_Pending if A is not zero.
BCE3 JR NZ,CheckTerrainCollision_Pending
BCE5 LD A,($BC12) The terrain-check flag.
BCE8 AND A Call PlayBumpSound if A is zero.
BCE9 CALL Z,PlayBumpSound
CheckTerrainCollision_Pending BCEC LD A,($BC12) The terrain-check flag.
BCEF AND A Jump to CheckTerrainCollision_Set if A is zero.
BCF0 JR Z,CheckTerrainCollision_Set
BCF2 CALL CheckTerrainCollision_Flags2 Call CheckTerrainCollision_Flags2.
BCF5 LD A,(IX+$07) The high byte of the horizontal speed.
BCF8 ADD A,A Twice that speed.
BCF9 LD ($CCBB),A The scroll follows the bounce.
CheckTerrainCollision_Set BCFC LD A,$80 The check is finished.
BCFE LD ($BC12),A Mark the check as finished.
BD01 JP CheckTerrainCollision_Stop Jump to CheckTerrainCollision_Stop.
CheckTerrainCollision_Reflect BD04 XOR A Clear A.
CheckTerrainCollision_Patch BD05 LD ($BD16),A Choose whether the first speed is reversed.
BD08 LD ($BD2B),A Choose whether the second speed is reversed.
BD0B LD IX,($BA09) Fetch the word at BA09 into IX.
BD0F LD H,(IX+$07) The high byte of the horizontal speed.
BD12 LD L,(IX+$08) The low byte of the horizontal speed.
CheckTerrainCollision_PatchFirst BD15 JR CheckTerrainCollision_PatchFirst Jump to CheckTerrainCollision_PatchFirst.
BD17 CALL CheckTerrainCollision_Negate Call CheckTerrainCollision_Negate.
BD1A CALL LimitSpeed Call LimitSpeed.
BD1D EX DE,HL Swap DE and HL.
BD1E LD H,(IX+$09) The high byte of the vertical speed.
BD21 LD L,(IX+$0A) The low byte of the vertical speed.
BD24 LD (IX+$09),D The vertical speed takes the adjusted horizontal speed.
BD27 LD (IX+$0A),E The low byte of that new vertical speed.
CheckTerrainCollision_PatchSecond BD2A JR CheckTerrainCollision_PatchSecond Jump to CheckTerrainCollision_PatchSecond.
BD2C CALL CheckTerrainCollision_Negate Call CheckTerrainCollision_Negate.
CheckTerrainCollision_Horizontal BD2F CALL LimitSpeed Call LimitSpeed.
BD32 LD (IX+$07),H The horizontal speed takes the adjusted vertical speed.
BD35 LD (IX+$08),L The low byte of that new horizontal speed.
BD38 JP CheckTerrainCollision_Bounce Jump to CheckTerrainCollision_Bounce.
CheckTerrainCollision_Three BD3B LD A,$03 Steepness two does not reverse the speeds.
BD3D JR CheckTerrainCollision_Patch Jump to CheckTerrainCollision_Patch.
CheckTerrainCollision_Negate BD3F LD A,H The high byte of the speed.
BD40 NEG Negate A.
BD42 LD H,A The speed is reversed.
BD43 RET Return.
CheckTerrainCollision_Rebound BD44 XOR A Clear A.
BD45 LD ($BD85),A Choose whether this speed is reversed.
BD48 LD ($BD74),A Choose whether the other speed is reversed.
CheckTerrainCollision_ReboundStart BD4B LD A,(IX+$04) The slope flags.
BD4E LD C,A Keep the slope flags.
BD4F AND %00000111 The steepness.
BD51 PUSH AF Keep the steepness.
BD52 LD IX,($BA09) Fetch the word at BA09 into IX.
BD56 CP $03 Call SwapSpeeds if A is 03.
BD58 CALL Z,SwapSpeeds
BD5B LD H,(IX+$07) The high byte of the horizontal speed.
BD5E LD L,(IX+$08) The low byte of the horizontal speed.
BD61 CALL TakeThreeEighths Call TakeThreeEighths.
BD64 LD ($5C15),HL Three eighths of the horizontal speed.
BD67 LD H,(IX+$07) The horizontal speed again.
BD6A LD L,(IX+$08) The low byte again.
BD6D CALL DoubleNumber Call DoubleNumber.
BD70 CALL TakeThreeEighths Call TakeThreeEighths.
CheckTerrainCollision_PatchThird BD73 JR CheckTerrainCollision_PatchThird Jump to CheckTerrainCollision_PatchThird.
BD75 CALL CheckTerrainCollision_ReboundNegate Call CheckTerrainCollision_ReboundNegate.
BD78 LD ($5C17),HL The adjusted fraction of the doubled horizontal speed.
BD7B LD H,(IX+$09) The high byte of the vertical speed.
BD7E LD L,(IX+$0A) The low byte of the vertical speed.
BD81 CALL TakeThreeEighths Call TakeThreeEighths.
CheckTerrainCollision_PatchFourth BD84 JR CheckTerrainCollision_PatchFourth Jump to CheckTerrainCollision_PatchFourth.
BD86 CALL CheckTerrainCollision_ReboundNegate Call CheckTerrainCollision_ReboundNegate.
BD89 LD DE,($5C15) Fetch the word at 5C15 into DE.
BD8D ADD HL,DE Add the saved fraction of the horizontal speed.
BD8E CALL LimitSpeed Call LimitSpeed.
BD91 LD (IX+$07),H The new horizontal speed.
BD94 LD (IX+$08),L The low byte of the new horizontal speed.
BD97 LD H,(IX+$09) The vertical speed.
BD9A LD L,(IX+$0A) The low byte of the vertical speed.
BD9D CALL DoubleNumber Call DoubleNumber.
BDA0 CALL TakeThreeEighths Call TakeThreeEighths.
BDA3 CALL CheckTerrainCollision_ReboundNegate Call CheckTerrainCollision_ReboundNegate.
BDA6 LD DE,($5C17) Fetch the word at 5C17 into DE.
BDAA ADD HL,DE Add the saved fraction of the doubled horizontal speed.
BDAB CALL LimitSpeed Call LimitSpeed.
BDAE LD (IX+$09),H The new vertical speed.
BDB1 LD (IX+$0A),L The low byte of the new vertical speed.
BDB4 POP AF Back to the steepness.
BDB5 CP $03 Call SwapSpeeds if A is 03.
BDB7 CALL Z,SwapSpeeds
BDBA JP CheckTerrainCollision_Bounce Jump to CheckTerrainCollision_Bounce.
CheckTerrainCollision_ReboundBoth BDBD LD A,$03 This facing does not reverse the speeds.
BDBF LD ($BD85),A This facing skips the first reverse.
BDC2 LD ($BD74),A This facing skips the second reverse.
BDC5 JP CheckTerrainCollision_ReboundStart Jump to CheckTerrainCollision_ReboundStart.
CheckTerrainCollision_ReboundNegate BDC8 BIT 7,C Jump to NegateNumber if bit 7 of C is set.
BDCA JP NZ,NegateNumber
BDCD LD A,C The slope flags.
BDCE AND %00000111 The steepness.
BDD0 CP $03 Jump to NegateNumber if A is 03.
BDD2 JP Z,NegateNumber
BDD5 JP CheckTerrainCollision_Negate Jump to CheckTerrainCollision_Negate.
SwapSpeeds BDD8 LD H,(IX+$07) The high byte of the horizontal speed.
BDDB LD L,(IX+$08) The low byte of the horizontal speed.
BDDE LD D,(IX+$09) The high byte of the vertical speed.
BDE1 LD E,(IX+$0A) The low byte of the vertical speed.
BDE4 LD (IX+$07),D Swap the speeds: horizontal takes the vertical.
BDE7 LD (IX+$08),E The low byte of the new horizontal speed.
BDEA LD (IX+$09),H Vertical takes the old horizontal speed.
BDED LD (IX+$0A),L The low byte of the new vertical speed.
BDF0 RET Return.
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