/*
Copyright (c) 2014 Microsoft Corporation. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.

Author: Leonardo de Moura
*/
#pragma once
#include <algorithm>
#include "util/lazy_list.h"
#include "util/list.h"
#include "util/name_generator.h"
#include "util/sequence.h"
#include "kernel/level.h"

namespace lean {
class expr;
class justification;
class substitution;

class delay_factor {
    optional<unsigned> m_value;
public:
    delay_factor() {}
    delay_factor(unsigned v):m_value(v) {}
    bool on_demand() const { return !m_value; }
    unsigned explict_value() const { lean_assert(!on_demand()); return *m_value; }
};

/**
   \brief The lean kernel type checker produces two kinds of constraints:

   - Equality constraint:          t ≈ s
        The terms t and s must be definitionally equal.
   - Universe level constaint:     l = m
        The universe level l must be less than or equal to m.

   \remark The constraints are only generated if the input term contains
   metavariables or level metavariables.

   Each constraint is associated with a justification object.

   \remark We also have choice constraints that are used by elaborator to specify
   the possible solutions for a metavariable. The choice constraints are not used by
   the kernel.
*/
enum class constraint_kind { Eq, LevelEq, Choice };
class constraint;
typedef list<constraint> constraints;
/**
   \brief A choice_fn is used to enumerate the possible solutions for a metavariable.
   The input arguments are:
        - metavariable that should be inferred
        - the metavariable type
        - substitution map (metavar -> value)
        - name generator
   The result is a lazy_list of constraints

   One application of choice constraints is overloaded notation.
*/
typedef std::function<lazy_list<constraints>(expr const &, expr const &, substitution const &, name_generator const &)> choice_fn;

struct constraint_cell;
class constraint {
    constraint_cell * m_ptr;
    constraint(constraint_cell * ptr);
public:
    constraint(constraint const & c);
    constraint(constraint && s);
    ~constraint();

    constraint_kind kind() const;
    justification const & get_justification() const;

    constraint & operator=(constraint const & c);
    constraint & operator=(constraint && c);

    friend bool is_eqp(constraint const & c1, constraint const & c2) { return c1.m_ptr == c2.m_ptr; }
    friend void swap(constraint & l1, constraint & l2) { std::swap(l1, l2); }

    friend constraint mk_eq_cnstr(expr const & lhs, expr const & rhs, justification const & j, bool relax_main_opaque);
    friend constraint mk_level_eq_cnstr(level const & lhs, level const & rhs, justification const & j);
    friend constraint mk_choice_cnstr(expr const & m, choice_fn const & fn, delay_factor const & f, bool owner,
                                      justification const & j, bool relax_main_opaque);

    constraint_cell * raw() const { return m_ptr; }
};

inline bool operator==(constraint const & c1, constraint const & c2) { return c1.raw() == c2.raw(); }
inline bool operator!=(constraint const & c1, constraint const & c2) { return !(c1 == c2); }

/** \brief Create a unification constraint lhs =?= rhs
    If \c relax_main_opaque is true, then opaque definitions from the main module are treated as transparent.
*/
constraint mk_eq_cnstr(expr const & lhs, expr const & rhs, justification const & j, bool relax_main_opaque);
constraint mk_level_eq_cnstr(level const & lhs, level const & rhs, justification const & j);

/** \brief Create a "choice" constraint m in fn(...), where fn produces a stream of possible solutions.
    \c delay_factor allows to control when the constraint is processed by the elaborator, bigger == later.
    If \c owner is true, then the elaborator should not assign the metavariable get_app_fn(m).
    The variable will be assigned by the choice constraint, and the elaborator should just check whether a solution
    produced by fn satisfies the other constraints or not.
    \c j is a justification for the constraint.
    If \c relax_main_opaque is true, then it signs that constraint was created in a context where
    opaque constants of the main module can be treated as transparent.
*/
constraint mk_choice_cnstr(expr const & m, choice_fn const & fn, delay_factor const & f,
                           bool owner, justification const & j, bool relax_main_opaque);

inline bool is_eq_cnstr(constraint const & c) { return c.kind() == constraint_kind::Eq; }
inline bool is_level_eq_cnstr(constraint const & c) { return c.kind() == constraint_kind::LevelEq; }
inline bool is_choice_cnstr(constraint const & c) { return c.kind() == constraint_kind::Choice; }

constraint update_justification(constraint const & c, justification const & j);

/** \brief Return the lhs of an equality constraint. */
expr const & cnstr_lhs_expr(constraint const & c);
/** \brief Return the rhs of an equality constraint. */
expr const & cnstr_rhs_expr(constraint const & c);
/** \brief Return true iff opaque definitions from the main module should be treated as transparent. */
bool relax_main_opaque(constraint const & c);
/** \brief Return the lhs of an level constraint. */
level const & cnstr_lhs_level(constraint const & c);
/** \brief Return the rhs of an level constraint. */
level const & cnstr_rhs_level(constraint const & c);
/** \brief Return the expression associated with the given choice constraint */
expr const & cnstr_expr(constraint const & c);
/** \brief Return the choice_fn associated with a choice constraint. */
choice_fn const & cnstr_choice_fn(constraint const & c);
/** \brief Return true iff the choice constraint should be solved as soon the type does not contains type variables */
bool cnstr_on_demand(constraint const & c);
/** \brief Return the choice constraint delay factor */
delay_factor const & cnstr_delay_factor_core(constraint const & c);
unsigned cnstr_delay_factor(constraint const & c);
/** \brief Return true iff the given choice constraints owns the right to assign the metavariable in \c c. */
bool cnstr_is_owner(constraint const & c);

typedef sequence<constraint> constraint_seq;
inline constraint_seq empty_cs() { return constraint_seq(); }
/** \brief Copy constraints in cs to r, and append justification j to them. */
void to_buffer(constraint_seq const & cs, justification const & j, buffer<constraint> & r);

/** \brief Printer for debugging purposes */
std::ostream & operator<<(std::ostream & out, constraint const & c);
}