//
// Cforall Version 1.0.0 Copyright (C) 2015 University of Waterloo
//
// The contents of this file are covered under the licence agreement in the
// file "LICENCE" distributed with Cforall.
//
// ResolveAssertions.cc --
//
// Author           : Aaron B. Moss
// Created On       : Fri Oct 05 13:46:00 2018
// Last Modified By : Aaron B. Moss
// Last Modified On : Fri Oct 05 13:46:00 2018
// Update Count     : 1
//

#include "ResolveAssertions.h"

#include <cassert>                // for assertf
#include <list>                   // for list
#include <utility>                // for move
#include <vector>                 // for vector

#include "Alternative.h"          // for Alternative, AssertionItem
#include "Common/FilterCombos.h"  // for filterCombos
#include "SymTab/Indexer.h"       // for Indexer
#include "TypeEnvironment.h"      // for TypeEnvironment, etc.
#include "typeops.h"              // for adjustExprType
#include "Unify.h"                // for unify

namespace ResolvExpr {
	/// Unified assertion candidate
	struct AssnCandidate {
		SymTab::Indexer::IdData cdata;  ///< Satisfying declaration
		Type* adjType;                  ///< Satisfying type
		TypeEnvironment env;            ///< Post-unification environment
		AssertionSet have;              ///< Post-unification have-set
		AssertionSet need;              ///< Post-unification need-set
		OpenVarSet openVars;            ///< Post-unification open-var set

		AssnCandidate( const SymTab::Indexer::IdData& cdata, Type* adjType, TypeEnvironment&& env, 
			AssertionSet&& have, AssertionSet&& need, OpenVarSet&& openVars ) 
		: cdata(cdata), adjType(adjType), env(std::move(env)), have(std::move(have)), 
			need(std::move(need)), openVars(std::move(openVars)) {}
	};

	/// List of candidate assertion resolutions
	using CandidateList = std::vector<AssnCandidate>;

	/// Reference to single deferred item
	struct DeferRef {
		const DeclarationWithType* decl;
		const AssertionSetValue& info;
		const AssnCandidate& match;
	};

	/// Wrapper for the deferred items from a single assertion resolution.
	/// Acts like indexed list of DeferRef
	struct DeferItem {
		DeclarationWithType* decl;
		AssertionSetValue info;
		CandidateList matches;

		DeferItem( DeclarationWithType* decl, const AssertionSetValue& info, 
			CandidateList&& matches )
		: decl(decl), info(info), matches(std::move(matches)) {}

		bool empty() const { return matches.empty(); }

		CandidateList::size_type size() const { return matches.size(); }

		DeferRef operator[] ( unsigned i ) const { return { decl, info, matches[i] }; }
	};

	/// List of deferred resolution items
	using DeferList = std::vector<DeferItem>;

	/// Combo iterator that combines candidates into an output list, merging their environments. 
	/// Rejects an appended candidate if the environments cannot be merged.
	class CandidateEnvMerger {
		/// Current list of merged candidates
		std::vector<DeferRef> crnt;
		/// Stack of environments to support backtracking
		std::vector<TypeEnvironment> envs;
		/// Stack of open variables to support backtracking
		std::vector<OpenVarSet> varSets;
		/// Indexer to use for merges
		const SymTab::Indexer& indexer;
	
	public:
		/// The merged environment/open variables and the list of candidates
		struct OutType {
			TypeEnvironment env;
			OpenVarSet openVars;
			std::vector<DeferRef> assns;

			OutType( const TypeEnvironment& env, const OpenVarSet& openVars, 
				const std::vector<DeferRef>& assns )
			: env(env), openVars(openVars), assns(assns) {}
		};

		CandidateEnvMerger( const TypeEnvironment& env, const OpenVarSet& openVars, 
			const SymTab::Indexer& indexer )
		: crnt(), envs{ env }, varSets{ openVars }, indexer(indexer) {}

		bool append( DeferRef i ) {
			TypeEnvironment env = envs.back();
			OpenVarSet openVars = varSets.back();
			mergeOpenVars( openVars, i.match.openVars );

			if ( ! env.combine( i.match.env, openVars, indexer ) ) return false;

			crnt.emplace_back( i );
			envs.emplace_back( env );
			varSets.emplace_back( openVars );
			return true;
		}

		void backtrack() {
			crnt.pop_back();
			envs.pop_back();
			varSets.pop_back();
		}

		OutType finalize() { return { envs.back(), varSets.back(), crnt }; }
	};

	/// Flag for state iteration
	enum IterateFlag { IterateState };

	/// State needed to resolve a set of assertions
	struct ResnState {
		Alternative alt;                  ///< Alternative assertion is rooted on
		std::vector<AssertionItem> need;  ///< Assertions to find
		AssertionSet newNeed;             ///< New assertions for current resolutions
		DeferList deferred;               ///< Deferred matches
		SymTab::Indexer& indexer;         ///< Name lookup (depends on previous assertions)

		/// Initial resolution state for an alternative
		ResnState( Alternative& a, SymTab::Indexer& indexer )
		: alt(a), need(), newNeed(), deferred(), indexer(indexer) {
			need.swap( a.need );
		}

		/// Updated resolution state with new need-list
		ResnState( ResnState&& o, IterateFlag )
		: alt(std::move(o.alt)), need(o.newNeed.begin(), o.newNeed.end()), newNeed(), deferred(), 
		  indexer(o.indexer) {}
	};

	/// Binds a single assertion, updating resolution state
	void bindAssertion( const DeclarationWithType* decl, AssertionSetValue info, Alternative& alt, 
			AssnCandidate& match ) {
		
		DeclarationWithType* candidate = match.cdata.id;
		assertf( candidate->get_uniqueId(), "Assertion candidate does not have a unique ID: %s", toString( candidate ).c_str() );

		// everything with an empty idChain was pulled in by the current assertion.
		// add current assertion's idChain + current assertion's ID so that the correct 
		// inferParameters can be found.
		for ( auto& a : match.need ) {
			if ( a.second.idChain.empty() ) {
				a.second.idChain = info.idChain;
				a.second.idChain.push_back( decl->get_uniqueId() );
			}
		}

		Expression* varExpr = match.cdata.combine( alt.cvtCost );
		varExpr->result = match.adjType;

		// follow the current assertion's ID chain to find the correct set of inferred parameters 
		// to add the candidate o (i.e. the set of inferred parameters belonging to the entity 
		// which requested the assertion parameter)
		InferredParams* inferParams = &alt.expr->inferParams;
		for ( UniqueId id : info.idChain ) {
			inferParams = (*inferParams)[ id ].inferParams.get();
		}

		(*inferParams)[ decl->get_uniqueId() ] = ParamEntry{
				candidate->get_uniqueId(), match.adjType, decl->get_type(), varExpr };
	}

	/// Adds a captured assertion to the symbol table
	void addToIndexer( AssertionSet &assertSet, SymTab::Indexer &indexer ) {
		for ( AssertionSet::iterator i = assertSet.begin(); i != assertSet.end(); ++i ) {
			if ( i->second.isUsed ) {
				indexer.addId( i->first );
			}
		}
	}

	/// Resolve a single assertion, in context
	bool resolveAssertion( AssertionItem& assn, ResnState& resn ) {
		// skip unused assertions
		if ( ! assn.info.isUsed ) return true;

		// lookup candidates for this assertion
		std::list< SymTab::Indexer::IdData > candidates;
		resn.indexer.lookupId( assn.decl->name, candidates );

		// find the candidates that unify with the desired type
		CandidateList matches;
		for ( const auto& cdata : candidates ) {
			DeclarationWithType* candidate = cdata.id;

			// build independent unification context for candidate
			AssertionSet have, newNeed;
			TypeEnvironment newEnv{ resn.alt.env };
			OpenVarSet newOpenVars{ resn.alt.openVars };
			Type* adjType = candidate->get_type()->clone();
			adjustExprType( adjType, newEnv, resn.indexer );

			// keep unifying candidates
			if ( unify( assn.decl->get_type(), adjType, newEnv, newNeed, have, newOpenVars, 
					resn.indexer ) ) {
				matches.emplace_back( cdata, adjType, std::move(newEnv), std::move(have), 
					std::move(newNeed), std::move(newOpenVars) );
			}
		}

		// break if no suitable assertion
		if ( matches.empty() ) return false;

		// defer if too many suitable assertions
		if ( matches.size() > 1 ) {
			resn.deferred.emplace_back( assn.decl, assn.info, std::move(matches) );
			return true;
		}

		// otherwise bind current match in ongoing scope
		AssnCandidate& match = matches.front();
		addToIndexer( match.have, resn.indexer );
		resn.newNeed.insert( match.need.begin(), match.need.end() );
		resn.alt.env = std::move(match.env);
		resn.alt.openVars = std::move(match.openVars);

		bindAssertion( assn.decl, assn.info, resn.alt, match );
		return true;
	}

	///< Limit to depth of recursion of assertion satisfaction
	static const int recursionLimit = /* 10 */ 4;

	void resolveAssertions( Alternative& alt, const SymTab::Indexer& indexer, AltList& out ) {
		// finish early if no assertions to resolve
		if ( alt.need.empty() ) {
			out.emplace_back( alt );
			return;
		}

		// build list of possible resolutions
		using ResnList = std::vector<ResnState>;
		SymTab::Indexer root_indexer{ indexer };
		ResnList resns{ ResnState{ alt, root_indexer } };
		ResnList new_resns{};

		// resolve assertions in breadth-first-order up to a limited number of levels deep
		for ( unsigned level = 0; level < recursionLimit; ++level ) {
			// scan over all mutually-compatible resolutions
			for ( auto& resn : resns ) {
				// make initial pass at matching assertions
				for ( auto& assn : resn.need ) {
					// fail early if any assertion is not resolvable
					if ( ! resolveAssertion( assn, resn ) ) goto nextResn;
				}

				if ( resn.deferred.empty() ) {
					// either add successful match or push back next state
					if ( resn.newNeed.empty() ) {
						out.emplace_back( resn.alt );
					} else {
						new_resns.emplace_back( std::move(resn), IterateState );
					}
				} else {
					// resolve deferred assertions by mutual compatibility
					std::vector<CandidateEnvMerger::OutType> compatible = filterCombos(
						resn.deferred, 
						CandidateEnvMerger{ resn.alt.env, resn.alt.openVars, resn.indexer } );
					
					for ( auto& compat : compatible ) {
						ResnState new_resn = resn;

						// add compatible assertions to new resolution state
						for ( DeferRef r : compat.assns ) {
							AssnCandidate match = r.match;
							addToIndexer( match.have, new_resn.indexer );
							new_resn.newNeed.insert( match.need.begin(), match.need.end() );

							bindAssertion( r.decl, r.info, new_resn.alt, match );
						}

						// set mutual environment into resolution state
						new_resn.alt.env = std::move(compat.env);
						new_resn.alt.openVars = std::move(compat.openVars);

						// either add sucessful match or push back next state
						if ( new_resn.newNeed.empty() ) {
							out.emplace_back( new_resn.alt );
						} else {
							new_resns.emplace_back( std::move(new_resn), IterateState );
						}
					}
				}
			nextResn:; }

			// finish or reset for next round
			if ( new_resns.empty() ) return;
			resns.swap( new_resns );
			new_resns.clear();
		}
		
		// exceeded recursion limit if reaches here
		if ( out.empty() ) {
			SemanticError( alt.expr->location, "Too many recursive assertions" );
		}
	}
} // namespace ResolvExpr

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// tab-width: 4 //
// mode: c++ //
// compile-command: "make install" //
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