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 //===- Automaton.td ----------------------------------------*- tablegen -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file defines the key top-level classes needed to produce a reasonably
 // generic finite-state automaton.
 //
 //===----------------------------------------------------------------------===//
 
 // Define a record inheriting from GenericAutomaton to generate a reasonably
 // generic finite-state automaton over a set of actions and states.
 //
 // This automaton is defined by:
 //   1) a state space (explicit, always bits<32>).
 //   2) a set of input symbols (actions, explicit) and
 //   3) a transition function from state + action -> state.
 //
 // A theoretical automaton is defined by <Q, S, d, q0, F>:
 //   Q: A set of possible states.
 //   S: (sigma) The input alphabet.
 //   d: (delta) The transition function f(q in Q, s in S) -> q' in Q.
 //   F: The set of final (accepting) states.
 //
 // Because generating all possible states is tedious, we instead define the
 // transition function only and crawl all reachable states starting from the
 // initial state with all inputs under all transitions until termination.
 //
 // We define F = S, that is, all valid states are accepting.
 //
 // To ensure the generation of the automaton terminates, the state transitions
 // are defined as a lattice (meaning every transitioned-to state is more
 // specific than the transitioned-from state, for some definition of specificity).
 // Concretely a transition may set one or more bits in the state that were
 // previously zero to one. If any bit was not zero, the transition is invalid.
 //
 // Instead of defining all possible states (which would be cumbersome), the user
 // provides a set of possible Transitions from state A, consuming an input
 // symbol A to state B. The Transition object transforms state A to state B and
 // acts as a predicate. This means the state space can be discovered by crawling
 // all the possible transitions until none are valid.
 //
 // This automaton is considered to be nondeterministic, meaning that multiple
 // transitions can occur from any (state, action) pair. The generated automaton
 // is determinized, meaning that is executes in O(k) time where k is the input
 // sequence length.
 //
 // In addition to a generated automaton that determines if a sequence of inputs
 // is accepted or not, a table is emitted that allows determining a plausible
 // sequence of states traversed to accept that input.
 class GenericAutomaton {
   // Name of a class that inherits from Transition. All records inheriting from
   // this class will be considered when constructing the automaton.
   string TransitionClass;
 
   // Names of fields within TransitionClass that define the action symbol. This
   // defines the action as an N-tuple.
   //
   // Each symbol field can be of class, int, string or code type.
   //   If the type of a field is a class, the Record's name is used verbatim
   //     in C++ and the class name is used as the C++ type name.
   //   If the type of a field is a string, code or int, that is also used
   //     verbatim in C++.
   //
   // To override the C++ type name for field F, define a field called TypeOf_F.
   // This should be a string that will be used verbatim in C++.
   //
   // As an example, to define a 2-tuple with an enum and a string, one might:
   //   def MyTransition : Transition {
   //     MyEnum S1;
   //     int S2;
   //   }
   //   def MyAutomaton : GenericAutomaton }{
   //     let TransitionClass = "Transition";
   //     let SymbolFields = ["S1", "S2"];
   //     let TypeOf_S1 = "MyEnumInCxxKind";
   //   }
   list<string> SymbolFields;
 }
 
 // All transitions inherit from Transition.
 class Transition {
   // A transition S' = T(S) is valid if, for every set bit in NewState, the
   // corresponding bit in S is clear. That is:
   //   def T(S):
   //     S' = S | NewState
   //     return S' if S' != S else Failure
   //
   // The automaton generator uses this property to crawl the set of possible
   // transitions from a starting state of 0b0.
   bits<32> NewState;
 }

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