Odd error

[britlion]

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.

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