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// RUN: %clang_cc1 -verify -std=c++11 %s
// A direct proof that constexpr is Turing-complete, once DR1454 is implemented.
const unsigned halt = (unsigned)-1;
enum Dir { L, R };
struct Action {
bool tape;
Dir dir;
unsigned next;
};
using State = Action[2];
// An infinite tape!
struct Tape {
constexpr Tape() : l(0), val(false), r(0) {}
constexpr Tape(const Tape &old, bool write) :
l(old.l), val(write), r(old.r) {}
constexpr Tape(const Tape &old, Dir dir) :
l(dir == L ? old.l ? old.l->l : 0 : &old),
val(dir == L ? old.l ? old.l->val : false
: old.r ? old.r->val : false),
r(dir == R ? old.r ? old.r->r : 0 : &old) {}
const Tape *l;
bool val;
const Tape *r;
};
constexpr Tape update(const Tape &old, bool write) { return Tape(old, write); }
constexpr Tape move(const Tape &old, Dir dir) { return Tape(old, dir); }
// Run turing machine 'tm' on tape 'tape' from state 'state'. Return number of
// steps taken until halt.
constexpr unsigned run(const State *tm, const Tape &tape, unsigned state) {
return state == halt ? 1 :
run(tm, move(update(tape, tm[state][tape.val].tape),
tm[state][tape.val].dir),
tm[state][tape.val].next) + 1;
}
// 3-state busy beaver. 14 steps.
constexpr State bb3[] = {
{ { true, R, 1 }, { true, L, 2 } },
{ { true, L, 0 }, { true, R, 1 } },
{ { true, L, 1 }, { true, R, halt } }
};
static_assert(run(bb3, Tape(), 0) == 14, "");
// 4-state busy beaver. 108 steps.
constexpr State bb4[] = {
{ { true, R, 1 }, { true, L, 1 } },
{ { true, L, 0 }, { false, L, 2 } },
{ { true, R, halt }, { true, L, 3 } },
{ { true, R, 3 }, { false, R, 0 } } };
static_assert(run(bb4, Tape(), 0) == 108, "");
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