cython

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# mode: run
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# tag: closures
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# Cython version of Knuth's "man or boy" test -- "It separates the man
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# Algol 60 compilers from the boy Algol 60 compilers." Here's the
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# original (from wikipedia):
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#
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# begin
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#   real procedure A (k, x1, x2, x3, x4, x5);
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#   value k; integer k;
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#   begin
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#     real procedure B;
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#     begin k:= k - 1;
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#           B:= A := A (k, B, x1, x2, x3, x4);
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#     end;
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#     if k <= 0 then A:= x4 + x5 else B;
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#   end;
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#   outreal (A (10, 1, -1, -1, 1, 0));
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# end;
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#
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# and a table of values:
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#
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#   k           A
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#   0           1
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#   1           0
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#   2   	-2
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#   3   	0
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#   4   	1
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#   5   	0
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#   6   	1
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#   7   	-1
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#   8   	-10
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#   9   	-30
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#   10   	-67
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#
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# Past 10 or so, we blow the C stack -- can't just set a higher recursion limit
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# to get around that one.
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#
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def compute(val):
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    if isinstance(val, int):
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        return val
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    else:
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        return val()
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def a(in_k, x1, x2, x3, x4, x5):
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    """
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    >>> import sys
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    >>> old_limit = sys.getrecursionlimit()
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    >>> sys.setrecursionlimit(1350 if not getattr(sys, 'pypy_version_info', None) else 2700)
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    >>> a(10, 1, -1, -1, 1, 0)
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    -67
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    >>> sys.setrecursionlimit(old_limit)
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    """
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    k = [in_k]
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    def b():
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        k[0] -= 1
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        return a(k[0], b, x1, x2, x3, x4)
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    return compute(x4) + compute(x5) if k[0] <= 0 else b()
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