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