Writing bits

Is there any way to write bits to a file in Blitz? So, if you were just storing a true or false value, for example, and you didn't want to take up a whole 8 bits would it be possible to just store it as one bit in a file?

Storage works in bytes, so really if you want to handle at the bit level, you have to separate your writes into bunches of 8 bits.

Why not dedicate a byte of storage to say 8 bits? I mean, if you have for instance other instances when a bit needs to be stored, group them all together, in groups of 8.

Ok, so if I wanted to store bits, then I would have to combine them into bunches of 8 bits which would make 1 byte that could be stored. But then how would I read that byte and be able to take from it the individual value of each bit? Maybe this is a stupid question, but I only half understand how the whole bit-byte system works so please excuse my ignorance.

Function GetBit(value,position)
	Return (value Shr (position - 1)) And 1
End Function 

value is the byte, position is 8 to 1, left to right

and PutBit()

Function PutBit(value, position, bit = True)
	Return value Or (bit Shl (position - 1))
End Function 

here's a test

a = 128
Print Right$(Bin$(a),8)

Print GetBit(a,7)

a = PutBit(a,4)
Print a
Print Right$(Bin$(a),8)

WaitKey
End 

voila, you can now put up to 8 booleans into one byte.

(there's actually another way to do this, like the way bitflags are done for file attributes, don't feel like getting into it.)

While these GetBit and PutBIt Functions are well done, I have to say he probably will understand nothing of it, esp. cause of SHR.

I suggest to read some quick tutorials about BOOLEAN algebra.

When you know each bit has a binary value (1,2,4,8,16,32 etc.) you can test a bit this way (eg. for the bit number 4, that represents a value of 8):

my_byte=rand(0,255)

is_set=(my_byte AND 8)
if is_set=8 then print "Bit 4 is set"

Now to set a bit you will use the OR command

my_byte=my_byte OR 8

and finally to clear a bit you'll use AND NOT()

my_byte=my_byte AND ~(8)

(BTW Snarkbait PutBit lacks of this...if bit=0 then unset etc)

So these are the underlying basics of something rather sophisticated like "Return (value Shr (position - 1)) And 1"

EDIT: the Boolen command NOT doesn't seem to work as desired, instead use the operator "~" that does the same job.

Well, yeah I realized the put bit didn't work to clear a bit...

How bout this:

Function GetBit(value,position)
	Return (value Shr (position - 1)) And 1
End Function 

Function SetBit(value, position)
	Return value Or (1 Shl (position - 1))
End Function 

Function UnSetBit(value, position)
	Return value And ~(1 Shl (position - 1)) 
End Function 

Function ToggleBit(value,position)
	Return (1 Shl (position - 1)) Xor value
End Function 

He doesn't have to understand the bitwise math to use them as black box functions ;)

Ok, but you still can't clear it, can you?

I'd suggest to

Function SetBit(value, position, bit=1)
	If bit<>0
		Return value Or (1 Shl (position - 1))
	Endif
	Return value And ~(1 Shl (position - 1))
End Function 

Edit uh, you've edited yours :) so now it'S complete.

BTW - even I don't know (or forgot) about the "~" operator o_O. Truely black box technology.

Yep ;)

and blitzplus at least won't let me use 'and not' you have to use the bitwise complement ~

Oops - right now it seems I can't get NOT to work at all!!! But I am absolutely sure it worked before! Probably somehthing new in version 1.99??

~ works, NOT no longer!

a=Not(8)
print a

will give zero!

Can't believe this...

Wow. I just tested it back to Blitz3D Version 1.91... No way. It never worked. I apologize for all my "supercompetent help".

Well, simply replace the NOT by a ~
(So I'll edit my code)

You know what is really strange: I used NOT in a program to control a CNC Mill, and the program worked...weird...spooky.

Wow, thanks for all the replies everyone. I think I semi-understand what's going on but I'm going to check out some tutorials like jfk suggested. Those functions look really useful too, just hope I can figure out the mechanics behind them.

Well, the simplest i think i can put it, is a byte is as follows:

00000000

8 bits of data stored together

Now, simply speaking each bit has a value, and when turned to 1, starting from the right, their values are:

1,2,4,8,16,32,64,128

Using combinations of these bits, you can make any number between 0 and 255. The computer stores these as voltages. Above a certain voltage means that a 1 is stored. Below a certain voltage, a 0 is stored.

When you use AND NOT OR... etc you are basically filtering the data against other data. If you read a byte of data and it contained the number:

168

That would break down to:

128+32+8

or

10101000

128   64   32   16    8    4    2    1
---------------------------------------
 1     0    1    0    1    0    0    0

(used a code box to keep the formatting)
I hope this helps a little and adds to what the other guys have been saying.

what about

a=1
b=0

if a and (NOT b)
; this actually works
end if

Oh thanks Ross, that explanation really makes sense. I just had one of those 'aha' moments!