1 // RUN: %clang_cc1 -verify -std=c++11 %s
2 // RUN: %clang_cc1 -verify -std=c++11 %s -fexperimental-new-constant-interpreter
3 // expected-no-diagnostics
4
5 // A direct proof that constexpr is Turing-complete, once DR1454 is implemented.
6
7 const unsigned halt = (unsigned)-1;
8
9 enum Dir { L, R };
10 struct Action {
11 bool tape;
12 Dir dir;
13 unsigned next;
14 };
15 using State = Action[2];
16
17 // An infinite tape!
18 struct Tape {
TapeTape19 constexpr Tape() : l(0), val(false), r(0) {}
TapeTape20 constexpr Tape(const Tape &old, bool write) :
21 l(old.l), val(write), r(old.r) {}
TapeTape22 constexpr Tape(const Tape &old, Dir dir) :
23 l(dir == L ? old.l ? old.l->l : 0 : &old),
24 val(dir == L ? old.l ? old.l->val : false
25 : old.r ? old.r->val : false),
26 r(dir == R ? old.r ? old.r->r : 0 : &old) {}
27 const Tape *l;
28 bool val;
29 const Tape *r;
30 };
update(const Tape & old,bool write)31 constexpr Tape update(const Tape &old, bool write) { return Tape(old, write); }
move(const Tape & old,Dir dir)32 constexpr Tape move(const Tape &old, Dir dir) { return Tape(old, dir); }
33
34 // Run turing machine 'tm' on tape 'tape' from state 'state'. Return number of
35 // steps taken until halt.
run(const State * tm,const Tape & tape,unsigned state)36 constexpr unsigned run(const State *tm, const Tape &tape, unsigned state) {
37 return state == halt ? 0 :
38 run(tm, move(update(tape, tm[state][tape.val].tape),
39 tm[state][tape.val].dir),
40 tm[state][tape.val].next) + 1;
41 }
42
43 // 3-state busy beaver. S(bb3) = 21.
44 constexpr State bb3[] = {
45 { { true, R, 1 }, { true, R, halt } },
46 { { true, L, 1 }, { false, R, 2 } },
47 { { true, L, 2 }, { true, L, 0 } }
48 };
49 static_assert(run(bb3, Tape(), 0) == 21, "");
50
51 // 4-state busy beaver. S(bb4) = 107.
52 constexpr State bb4[] = {
53 { { true, R, 1 }, { true, L, 1 } },
54 { { true, L, 0 }, { false, L, 2 } },
55 { { true, R, halt }, { true, L, 3 } },
56 { { true, R, 3 }, { false, R, 0 } } };
57 static_assert(run(bb4, Tape(), 0) == 107, "");
58