[Haskell-cafe] Flagstone problem

[michael rice]

Hi,

I've been looking at a "flagstone problem," where no two adjacent
n-tuples can be identical. I solved the problem with Icon using
an array of stacks and was going to explore how to do it in Haskell
when I saw another way to do it explained in the same text. Just
count the ones between the zeros in a(n) to get b(i), a non-repeating
sequence of zeros, ones, and twos. An a(n) is 0 if the number of
one bits in the binary representation of n is even, otherwise odd,
and the initial a(n) must be even. 


n                       a(n)                 b(i)
20 = 010100             0
21 = 010101             1
                                             2
22 = 010110             1
23 = 010111             0
                                             0
24 = 011000             0
25 = 011001             1
                                             2
26 = 011010             1
27 = 011011             0
                                             1
28 = 011100             1
29 = 011101             0
                                             0
30 = 011110             0

31 = 011111             1                    2

32 = 100000             1
33 = 100001             0
                                             0
34 = 100010             0
                                             1
35 = 100011             1
36 = 100100             0

========= Here's my Haskell code ========

import Data.Bits
import Data.List

flagstone n =  foldl (++) "" (take n (map show (f $ group [if even y then 0 else 1 | y <- [bitcount x  | x <- [20..]]])))

bitcount :: (Integral t) => t -> t
bitcount 0 = 0
bitcount n = let (q,r) = quotRem n 2
             in bitcount q + r

f [] = []
f ([0]:xs) = f xs
f ([0,0]:xs) = 0 : f xs
f ([1]:xs) = 1 : f xs
f ([1,1]:xs) = 2 : f xs

========= My question ========

A further exercise in the text:

"Exercise 5.-(a) Define a(n) as the sum of the binary
digits in the binary representation of n. Define b(i) as
the number of a's between successive zeros as before.
Then T = b(1) b(2) b(3) b(4) ... gives an infinite
sequence of *seven* symbols with no repeats. (b) Write
a routine to generate a sequence for seven colors of
beads on a string with no repeats."

I may be misreading, but does this make any sense?

There's a reference to The American Mathematical Monthly,
June-July 1963, p. 675, by C. H. Braunholtz.

Michael


 
                         



      
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