CODEWITHSUNDEEP
haskellbinary
jaybee
jaybee

Reputation: 1925

Overwriting least-significant bit in a Word

I am trying to write a little tool to hide text in PGM image files by overwriting the LSB in each byte in the input. I need help writing a function such as

writeBit :: Bool -> Word8 -> Word8
writeBit b w = ...

where the ouput is equal to w with its LSB set to b. Can anyone help me get started with this?

Thanks

Upvotes: 2

Views: 185

Answers (3)

dfeuer
dfeuer

Reputation: 48591

I'll echo bmk by saying that Syd Kerckhove's answer is the correct one, but one very short option is

setLSB :: Integral n => Bool -> n -> n
setLSB b w = w - 1 + fromEnum b + fromEnum (even w)

Upvotes: 1

bmk
bmk

Reputation: 1568

Besides the right answer, that you should use the module Data.Bits, you could also write the function yourself if you only want to set/unset the least significant bit of the input byte. Using your function signature:

setLSB :: Bool -> Word8 -> Word8
setLSB b w
  | b && even w = w + 1
  | not b && odd w = w - 1
  | otherwise = w

Upvotes: 1

Syd Kerckhove
Syd Kerckhove

Reputation: 833

Have a look at setBit and clearBit

setBit :: a -> Int -> a
clearBit :: a -> Int -> a

Upvotes: 4

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