1// RUN: %clang_cc1 -verify -std=c++11 %s
2// expected-no-diagnostics
3
4// A direct proof that constexpr is Turing-complete, once DR1454 is implemented.
5
6const unsigned halt = (unsigned)-1;
7
8enum Dir { L, R };
9struct Action {
10  bool tape;
11  Dir dir;
12  unsigned next;
13};
14using State = Action[2];
15
16// An infinite tape!
17struct Tape {
18  constexpr Tape() : l(0), val(false), r(0) {}
19  constexpr Tape(const Tape &old, bool write) :
20    l(old.l), val(write), r(old.r) {}
21  constexpr Tape(const Tape &old, Dir dir) :
22    l(dir == L ? old.l ? old.l->l : 0 : &old),
23    val(dir == L ? old.l ? old.l->val : false
24                 : old.r ? old.r->val : false),
25    r(dir == R ? old.r ? old.r->r : 0 : &old) {}
26  const Tape *l;
27  bool val;
28  const Tape *r;
29};
30constexpr Tape update(const Tape &old, bool write) { return Tape(old, write); }
31constexpr Tape move(const Tape &old, Dir dir) { return Tape(old, dir); }
32
33// Run turing machine 'tm' on tape 'tape' from state 'state'. Return number of
34// steps taken until halt.
35constexpr unsigned run(const State *tm, const Tape &tape, unsigned state) {
36  return state == halt ? 0 :
37         run(tm, move(update(tape, tm[state][tape.val].tape),
38                      tm[state][tape.val].dir),
39             tm[state][tape.val].next) + 1;
40}
41
42// 3-state busy beaver. S(bb3) = 21.
43constexpr State bb3[] = {
44  { { true, R, 1 }, { true, R, halt } },
45  { { true, L, 1 }, { false, R, 2 } },
46  { { true, L, 2 }, { true, L, 0 } }
47};
48static_assert(run(bb3, Tape(), 0) == 21, "");
49
50// 4-state busy beaver. S(bb4) = 107.
51constexpr State bb4[] = {
52  { { true, R, 1 }, { true, L, 1 } },
53  { { true, L, 0 }, { false, L, 2 } },
54  { { true, R, halt }, { true, L, 3 } },
55  { { true, R, 3 }, { false, R, 0 } } };
56static_assert(run(bb4, Tape(), 0) == 107, "");
57