The littleendian function

The definitions below can be found in LittleEndian.hs and the testing code can be found in Test_LittleEndian.hs.

Inputs and Outputs

To quote Bernstein: If b is a 4-byte sequence then littleendian(b) is a word, and, hence, its Haskell type is:

littleendian :: Quad (W 8) -> W 32     -- i.e., (W 8, W 8, W 8, W 8) -> W 32

Bernstein also notes that littleendian is invertible, meaning that the following function can be defined:

inv_littleendian :: W 32 -> Quad (W 8)

Note that the (++) operator below is vector concatenation (rather than list append from the Haskell prelude). So, in the definition of littleendian below, four (W 8) bit vectors (i.e., b3, b2, b1, and b0) are concatenated to make a single (W 32).

revbytes :: Quad (W 8) -> Quad (W 8) 
revbytes (b0,b1,b2,b3) = (b3,b2,b1,b0)

littleendian :: Quad (W 8) -> W 32   
littleendian w8x4 = b3 ++ b2 ++ b1 ++ b0
  where
    b0 , b1 , b2 , b3 :: W 8
    (b3 , b2 , b1 , b0) = revbytes w8x4

Bernstein also notes that littleendian is invertible:

inv_littleendian :: W 32 -> Quad (W 8)
inv_littleendian w = revbytes (slice0 w , slice1 w , slice2 w , slice3 w)

Here, the operation of the slice functions (slice0, etc.) is illustrated by the following test:

slicetest :: P.Bool
slicetest = (slice0 x , slice1 x , slice2 x , slice3 x) P.== (lit 0xde , lit 0xad , lit 0xbe , lit 0xef)
  where
    x :: W 32
    x = lit 0xdeadbeef

These functions are defined in LittleEndian.hs and are uninteresting.

Examples

alltests :: [P.Bool]
alltests = [test1 , test2 , test3] P.++ invertible_tests

test1 , test2 , test3 :: P.Bool 
test1 = littleendian (lit 0,lit 0,lit 0,lit 0)         == lit 0x00000000
test2 = littleendian (lit 86,lit 75,lit 30,lit 9)      == lit 0x091e4b56
test3 = littleendian (lit 255,lit 255,lit 255,lit 250) == lit 0xfaffffff

invertible_tests :: [P.Bool]
invertible_tests = [ invert q0 P.== q0
                   , invert q1 P.== q1
                   , invert q2 P.== q2
                   ]
   where
     invert :: Quad (W 8) -> Quad (W 8)
     invert q = inv_littleendian (littleendian q)

λ> slicetest
True
λ> alltests
[True,True,True,True,True,True]

ReWire by Example