56 lines
1.7 KiB
C++
56 lines
1.7 KiB
C++
// RUN: %clang_cc1 -verify -std=c++11 %s
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// expected-no-diagnostics
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// A direct proof that constexpr is Turing-complete, once DR1454 is implemented.
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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() : l(0), val(false), r(0) {}
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constexpr Tape(const Tape &old, bool write) :
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l(old.l), val(write), r(old.r) {}
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constexpr Tape(const Tape &old, Dir dir) :
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l(dir == L ? old.l ? old.l->l : 0 : &old),
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val(dir == L ? old.l ? old.l->val : false
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: old.r ? old.r->val : false),
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r(dir == R ? old.r ? old.r->r : 0 : &old) {}
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const Tape *l;
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bool val;
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const Tape *r;
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};
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constexpr Tape update(const Tape &old, bool write) { return Tape(old, write); }
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constexpr Tape move(const Tape &old, Dir dir) { return Tape(old, dir); }
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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, const Tape &tape, unsigned state) {
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return state == halt ? 0 :
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run(tm, move(update(tape, tm[state][tape.val].tape),
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tm[state][tape.val].dir),
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tm[state][tape.val].next) + 1;
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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, Tape(), 0) == 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, Tape(), 0) == 107, "");
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