Initial data commit

Task/Entropy/Fortran/entropy.f

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+!-*- mode: compilation; default-directory: "/tmp/" -*-
+!Compilation started at Tue May 21 21:43:12
+!
+!a=./f && make $a && OMP_NUM_THREADS=2 $a 1223334444
+!gfortran -std=f2008 -Wall -ffree-form -fall-intrinsics f.f08 -o f
+! Shannon entropy of 1223334444 is    1.84643936
+!
+!Compilation finished at Tue May 21 21:43:12
+
+program shannonEntropy
+  implicit none
+  integer :: num, L, status
+  character(len=2048) :: s
+  num = 1
+  call get_command_argument(num, s, L, status)
+  if ((0 /= status) .or. (L .eq. 0)) then
+    write(0,*)'Expected a command line argument with some length.'
+  else
+    write(6,*)'Shannon entropy of '//(s(1:L))//' is ', se(s(1:L))
+  endif
+
+contains
+  !     algebra
+  !
+  ! 2**x = y
+  ! x*log(2) = log(y)
+  ! x = log(y)/log(2)
+
+  !   NB. The j solution
+  !   entropy=:  +/@:-@(* 2&^.)@(#/.~ % #)
+  !   entropy '1223334444'
+  !1.84644
+
+  real function se(s)
+    implicit none
+    character(len=*), intent(in) :: s
+    integer, dimension(256) :: tallies
+    real, dimension(256) :: norm
+    tallies = 0
+    call TallyKey(s, tallies)
+    ! J's #/. works with the set of items in the input.
+    ! TallyKey is sufficiently close that, with the merge, gets the correct result.
+    norm = tallies / real(len(s))
+    se = sum(-(norm*log(merge(1.0, norm, norm .eq. 0))/log(2.0)))
+  end function se
+
+  subroutine TallyKey(s, counts)
+    character(len=*), intent(in) :: s
+    integer, dimension(256), intent(out) :: counts
+    integer :: i, j
+    counts = 0
+    do i=1,len(s)
+      j = iachar(s(i:i))
+      counts(j) = counts(j) + 1
+    end do
+  end subroutine TallyKey
+
+end program shannonEntropy