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| // RUN: %clang_cc1 -verify -std=c++2a %s
// expected-no-diagnostics
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() = default;
constexpr ~Tape() {
if (l) { l->r = nullptr; delete l; }
if (r) { r->l = nullptr; delete r; }
}
constexpr Tape *left() {
if (!l) { l = new Tape; l->r = this; }
return l;
}
constexpr Tape *right() {
if (!r) { r = new Tape; r->l = this; }
return r;
}
Tape *l = nullptr;
bool val = false;
Tape *r = nullptr;
};
// Run turing machine 'tm' on tape 'tape' from state 'state'. Return number of
// steps taken until halt.
constexpr unsigned run(const State *tm) {
Tape *tape = new Tape;
unsigned state = 0;
unsigned steps = 0;
for (state = 0; state != halt; ++steps) {
auto [val, dir, next_state] = tm[state][tape->val];
tape->val = val;
tape = (dir == L ? tape->left() : tape->right());
state = next_state;
}
delete tape;
return steps;
}
// 3-state busy beaver. S(bb3) = 21.
constexpr State bb3[] = {
{ { true, R, 1 }, { true, R, halt } },
{ { true, L, 1 }, { false, R, 2 } },
{ { true, L, 2 }, { true, L, 0 } }
};
static_assert(run(bb3) == 21, "");
// 4-state busy beaver. S(bb4) = 107.
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) == 107, "");
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