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thwill

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Joined: 16/09/2019
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Posted: 10:22am 04 Aug 2021

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Hi folks,

Setting, clearing and querying individual bits is relatively expensive in MMBasic so I was wondering if the following (which I don't believe will take up additional token 'slots') might be worthwhile and more performant:

' Sets 'offset' bit in the memory allocated to 'var' to 'value' (0 or 1).
POKE BIT var, offset, value

' Returns 'offset' bit from the memory allocated to 'var'.
PEEK(BIT var, offset)

Possibly for the POKE case it would be nicer to have POKE BITSET and POKE BITCLEAR instead of having the 'value' as a parameter ... and then we might have POKE BITFLIP too

.

And on a related note we have a VAR command that could presumably be overloaded to provide a VAR SWAP for swapping two variables which is something I believe I have seen requests for on TBS ... though I know it isn't a great match to the purpose of the other VAR sub-commands.

No worries if not, best wishes,

Tom
Edited 2021-08-04 20:25 by thwill

MMBasic for Linux, Game*Mite, CMM2 Welcome Tape, Creaky old text adventures

matherp
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Posted: 10:50am 04 Aug 2021

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Can't see any speed advantage over existing methods. Poke would be slower as three variables to parse
e.g.

INC myvar%,1<<offset
? myvar% and (1<<offset)

or if you need 0 or 1

? choice(myvar% and (1<<offset) ,1,0)

thwill

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Location: United Kingdom
Posts: 4311

Posted: 11:12am 04 Aug 2021

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matherp said Can't see any speed advantage over existing methods. Poke would be slower as three variables to parse
e.g.

INC myvar%,1<<offset
? myvar% and (1<<offset)

or if you need 0 or 1

? choice(myvar% and (1<<offset) ,1,0)

Using INC is buggy if the bit is already set isn't it ?

My current mechanism:

' Set bit i% of x%
x% = x% Or 1 << i%

' Clear bit i% of x%.
x% = (x% Or 1 << i%) Xor 1 << i%

' Get bit i% of x%.
y% = (x% And 1 << i%) > 0

Best wishes,

Tom

MMBasic for Linux, Game*Mite, CMM2 Welcome Tape, Creaky old text adventures

thwill

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Location: United Kingdom
Posts: 4311

Posted: 11:22am 04 Aug 2021

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In addition irrespective of performance I would argue this would be more readable:

POKE BITSET var, offset
POKE BITCLEAR var, offset
y% = PEEK(BIT var, offset)

And could also be written to work for var's other than 64-bit integers for those cases where a larger bit-set needs manipulating.

YMMV,

Tom

MMBasic for Linux, Game*Mite, CMM2 Welcome Tape, Creaky old text adventures

matherp
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Location: United Kingdom
Posts: 10310

Posted: 11:30am 04 Aug 2021

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clear bit as per standard C

x%= x% and inv (1<<offset)

thwill

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Joined: 16/09/2019
Location: United Kingdom
Posts: 4311

Posted: 11:51am 04 Aug 2021

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matherp said clear bit as per standard C

x%= x% and inv (1<<offset)

Thanks Peter, it had escaped my attention that we had a bitwise not, i.e. INV.

Best wishes,

Tom

MMBasic for Linux, Game*Mite, CMM2 Welcome Tape, Creaky old text adventures

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