General Computer Science; Problems and Solutions for ... - Kwarc

Problem 1.12 (Compressing binary lists)
Define a data type of binary digits. Write a function that takes a list of binary digits and returns
an int list that is a compressed version of it and the first binary digit of the list (needed for
reconversion). For example,
ZIPit([zero,zero,zero, one,one,one,one,
zero,zero,zero, one, zero,zero]) -> (0,[3,4,3,1,2]),
because the binary list begins with 3 zeros, followed by 4 ones etc.
Solution:
datatype bin = zero | one;
local fun ZIP(nil,_,cnt) = [cnt] |
ZIP(hd::tl, last, cnt) =
if hd=last then ZIP(tl, hd, cnt+1)
else cnt::ZIP(tl, hd, 1);
in
fun ZIPit(hd::tl) = (hd, ZIP(tl, hd, 1))
end;
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