Copyright	(c) Levent Erkok
License	BSD3
Maintainer	erkokl@gmail.com
Stability	experimental
Safe Haskell	None
Language	Haskell2010

Documentation.SBV.Examples.BitPrecise.MultMask

Description

An SBV solution to the bit-precise puzzle of shuffling the bits in a 64-bit word in a custom order. The idea is to take a 64-bit value:

1.......2.......3.......4.......5.......6.......7.......8.......

And turn it into another 64-bit value, that looks like this:

12345678........................................................

We do not care what happens to the bits that are represented by dots. The problem is to do this with one mask and one multiplication.

Apparently this operation has several applications, including in programs that play chess of all things. We use SBV to find the appropriate mask and the multiplier.

Note that this is an instance of the program synthesis problem, where we "fill in the blanks" given a certain skeleton that satisfy a certain property, using quantified formulas.

Synopsis

maskAndMult :: IO ()

Documentation

maskAndMult :: IO () Source #

Find the multiplier and the mask as described. We have:

>>> maskAndMult
Satisfiable. Model:
  mask = 0x8080808080808080 :: Word64
  mult = 0x0002040810204081 :: Word64

That is, any 64 bit value masked by the first and multipled by the second value above will have its bits at positions [7,15,23,31,39,47,55,63] moved to positions [56,57,58,59,60,61,62,63] respectively.

NB. Depending on your z3 version, you might also get the following multiplier as the result: 0x8202040810204081. That value works just fine as well!

Note that we have to send z3 custom configuration for this problem as otherwise it takes too long. See http://github.com/Z3Prover/z3/issues/2075 for details.