02-27-2012, 06:57 PM

This might be another float bug. I wrote up the distance algorithm na_th_an showed us. It seems to work - but in this test program, it's the floating point values that go wrong...

If you set your emulator to full speed, you see it looking for the biggest error it can find, showing larger errors as it finds them. Eventually it hits distance (250,56) which my function says is > 255 (so returns 255). The value calculated from square rooting in floats is...er 0. So that's an error of 255, apparently. Except the correct answer would be 256.1952 Is the int (256) returning 0 for some reason? On a float?

When I'm happy with this funtion, it will go in the function library as a code snippet.

If you set your emulator to full speed, you see it looking for the biggest error it can find, showing larger errors as it finds them. Eventually it hits distance (250,56) which my function says is > 255 (so returns 255). The value calculated from square rooting in floats is...er 0. So that's an error of 255, apparently. Except the correct answer would be 256.1952 Is the int (256) returning 0 for some reason? On a float?

When I'm happy with this funtion, it will go in the function library as a code snippet.

Code:

`FUNCTION fastcall distance (a as ubyte, b as ubyte) as uByte`

' returns a fast approximation of SQRT (a^2 + b^2) - the distance formula, generated from taylor series expansion.

asm

POP DE ; return address

; First parameter in A

POP BC ; second parameter -> B

LD C,A ; Put second parameter in C for safekeeping.

LD HL,0

ADD A,B

LD L,A ; Put result in HL

RL H ; Pull in carry bit if necessary.

LD A,B ; get original

CP C ; compare with C

JP NC, distance_Cbigger

LD C,A ; B was smaller, so we replace the value in C with the smaller value in A

distance_Cbigger:

; C was smaller, so we leave it alone.

SRL C ; C=C/2

XOR A

LD B,A

SBC HL,BC ; take half our smallest from HL

SRL C

AND A ; Clear carry flag.

SBC HL,BC

SRL C

SRL C

AND A ; Clear carry flag.

ADD HL,BC

LD A,H

AND A

JP NZ, distance_toobig

LD A,L ; Get result.

EX DE,HL ; return address is in DE.

JP (HL)

distance_toobig:

; If the answer turns out to be > 255, we'll return 255.

LD A,255

EX DE,HL ; return address is in DE.

JP (HL)

END ASM

END FUNCTION

CLS

DIM i,imax as uByte

dim j,jmax as uByte

dim answer,realanswer as uByte

dim errorval,errorvalmax as float

imax=0

jmax=0

errorvalmax=0

for j=1 to 250

for i=1 to 250

answer=distance(i,j)

realanswer=SQR(CAST(float,i)*i+CAST(float,j)*j)

errorval=ABS(CAST(float,realanswer)-answer)

if errorval>errorvalmax then

PRINT i;" ";j;" ";answer;" ";INT realanswer; " ";int errorval

imax=i

jmax=j

errorvalmax=errorval

end if

border j

next i

next j