machine.go

// Golog aspires to be an ISO Prolog interpreter.  It currently
// supports a small subset of the standard.  Any deviations from
// the standard are bugs.  Typical usage looks something like this:
//
//      m := NewMachine().Consult(`
//          father(john).
//          father(jacob).
//
//          mother(sue).
//
//          parent(X) :-
//              father(X).
//          parent(X) :-
//              mother(X).
//      `)
//      if m.CanProve(`father(john).`) {
//          fmt.Printf("john is a father\n")
//      }
//
//      solutions := m.ProveAll(`parent(X).`)
//      for _, solution := range solutions {
//          fmt.Printf("%s is a parent\n", solution.ByName_("X"))
//      }
//
// This sample highlights a few key aspects of using Golog.  To start,
// Golog data structures are immutable.  NewMachine() creates an empty
// Golog machine containing just the standard library.
// Consult() creates another new machine with some extra
// code loaded.  The original, empty machine is untouched.
// It's common to build a large Golog machine during init()
// and then add extra rules to it at runtime.
// Because Golog machines are immutable,
// multiple goroutines can access, run and "modify" machines in parallel.
// This design also opens possibilities for and-parallel and or-parallel
// execution.
//
// Most methods, like Consult(), can accept Prolog code in several forms.
// The example above shows Prolog as a string.  We could have used any
// io.Reader instead.
//
// Error handling is one oddity.  Golog methods follow Go convention by
// returning an error value to indicate that something went wrong.  However,
// in many cases the caller knows that an error is highly improbable and
// doesn't want extra code to deal with the common case.
// For most
// methods, Golog offers one with a trailing underscore, like ByName_(),
// which panics on error instead of returning an error value.
//
// See also:
//  * Golog's architecture: https://github.com/mndrix/golog/blob/master/doc/architecture.md
//  * Built in and foreign predicates: See func Builtin...
//  * Standard library: See golog/prelude package
package golog

import . "github.com/mndrix/golog/term"
import . "github.com/mndrix/golog/util"

import (
	"bufio"
	"bytes"
	"fmt"
	"os"
	"strconv"
	"strings"

	"github.com/mndrix/golog/prelude"
	"github.com/mndrix/golog/read"
	"github.com/mndrix/ps"
)

// NoBarriers error is returned when trying to access a cut barrier that
// doesn't exist.  See MostRecentCutBarrier
var NoBarriers = fmt.Errorf("There are no cut barriers")

// MachineDone error is returned when a Golog machine has been stepped
// as far forward as it can go.  It's unusual to need this variable.
var MachineDone = fmt.Errorf("Machine can't step any further")

// EmptyDisjunctions error is returned when trying to pop a disjunction
// off an empty disjunction stack.  This is mostly useful for those hacking
// on the interpreter.
var EmptyDisjunctions = fmt.Errorf("Disjunctions list is empty")

// EmptyConjunctions error is returned when trying to pop a conjunction
// off an empty conjunction stack.  This is mostly useful for those hacking
// on the interpreter.
var EmptyConjunctions = fmt.Errorf("Conjunctions list is empty")

// Golog users interact almost exclusively with a Machine value.
// Specifically, by calling one of the three methods Consult, CanProve and
// ProveAll.  All others methods are for those hacking on the interpreter or
// doing low-level operations in foreign predicates.
type Machine interface {
	// A Machine is an acceptable return value from a foreign predicate
	// definition.  In other words, a foreign predicate can perform low-level
	// manipulations on a Golog machine and return the result as the new
	// machine on which future execution occurs.  It's unlikely that you'll
	// need to do this.
	ForeignReturn

	// Temporary.  These will eventually become functions rather than methods.
	// All three accept Prolog terms as strings or as io.Reader objects from
	// which Prolog terms can be read.
	CanProve(interface{}) bool
	Consult(interface{}) Machine
	ProveAll(interface{}) []Bindings

	String() string

	// Bindings returns the machine's most current variable bindings.
	//
	// This method is typically only used by ChoicePoint implementations
	Bindings() Bindings

	// SetBindings returns a new machine like this one but with the given
	// bindings
	SetBindings(Bindings) Machine

	// PushConj returns a machine like this one but with an extra term
	// on front of the conjunction stack
	PushConj(Callable) Machine

	// PopConj returns a machine with one less item on the conjunction stack
	// along with the term removed.  Returns err = EmptyConjunctions if there
	// are no more conjunctions on that stack
	PopConj() (Callable, Machine, error)

	// ClearConjs replaces the conjunction stack with an empty one
	ClearConjs() Machine

	// ClearDisjs replaces the disjunction stack with an empty one
	ClearDisjs() Machine

	// DemandCutBarrier makes sure the disjunction stack has a cut barrier
	// on top.  If not, one is pushed.
	// This marker can be used to locate which disjunctions came immediately
	// before the marker existed.
	DemandCutBarrier() Machine

	// MostRecentCutBarrier returns an opaque value which uniquely
	// identifies the most recent cut barrier on the disjunction stack.
	// Used with CutTo to remove several disjunctions at once.
	// Returns NoBarriers if there are no cut barriers on the disjunctions
	// stack.
	MostRecentCutBarrier() (int64, error)

	// CutTo removes all disjunctions stacked on top of a specific cut barrier.
	// It does not remove the cut barrier itself.
	// A barrier ID is obtained from MostRecentCutBarrier.
	CutTo(int64) Machine

	// PushDisj returns a machine like this one but with an extra ChoicePoint
	// on the disjunctions stack.
	PushDisj(ChoicePoint) Machine

	// PopDisj returns a machine with one fewer choice points on the
	// disjunction stack and the choice point that was removed.  Returns
	// err = EmptyDisjunctions if there are no more disjunctions on
	// that stack
	PopDisj() (ChoicePoint, Machine, error)

	// RegisterForeign registers Go functions to implement Golog predicates.
	// When Golog tries to prove a predicate with one of these predicate
	// indicators, it executes the given function instead.
	// Calling RegisterForeign with a predicate indicator that's already
	// been registered replaces the predicate implementation.
	RegisterForeign(map[string]ForeignPredicate) Machine

	// Step advances the machine one "step" (implementation dependent).
	// It produces a new machine which can take the next step.  It might
	// produce a proof by giving some variable bindings.  When the machine
	// has done as much work as it can do, it returns err=MachineDone
	Step() (Machine, Bindings, error)
}

// Golog allows Prolog predicates to be defined in Go.  The foreign predicate
// mechanism is implemented via functions whose type is ForeignPredicate.
type ForeignPredicate func(Machine, []Term) ForeignReturn

const smallThreshold = 4

type machine struct {
	db    Database
	env   Bindings
	disjs ps.List // of ChoicePoint
	conjs ps.List // of Term

	smallForeign [smallThreshold]ps.Map // arity => functor => ForeignPredicate
	largeForeign ps.Map                 // predicate indicator => ForeignPredicate
}

func (*machine) IsaForeignReturn() {}

// NewMachine creates a new Golog machine.  This machine has the standard
// library already loaded and is typically the way one obtains
// a machine.
func NewMachine() Machine {
	return NewBlankMachine().
		Consult(prelude.Prelude).
		RegisterForeign(map[string]ForeignPredicate{
		"!/0":             BuiltinCut,
		"$cut_to/1":       BuiltinCutTo,
		",/2":             BuiltinComma,
		"->/2":            BuiltinIfThen,
		";/2":             BuiltinSemicolon,
		"=/2":             BuiltinUnify,
		"=:=/2":           BuiltinNumericEquals,
		"==/2":            BuiltinTermEquals,
		"\\==/2":          BuiltinTermNotEquals,
		"@</2":            BuiltinTermLess,
		"@=</2":           BuiltinTermLessEquals,
		"@>/2":            BuiltinTermGreater,
		"@>=/2":           BuiltinTermGreaterEquals,
		`\+/1`:            BuiltinNot,
		"atom_codes/2":    BuiltinAtomCodes2,
		"atom_number/2":   BuiltinAtomNumber2,
		"call/1":          BuiltinCall,
		"call/2":          BuiltinCall,
		"call/3":          BuiltinCall,
		"call/4":          BuiltinCall,
		"call/5":          BuiltinCall,
		"call/6":          BuiltinCall,
		"downcase_atom/2": BuiltinDowncaseAtom2,
		"fail/0":          BuiltinFail,
		"findall/3":       BuiltinFindall3,
		"ground/1":        BuiltinGround,
		"is/2":            BuiltinIs,
		"listing/0":       BuiltinListing0,
		"msort/2":         BuiltinMsort2,
		"printf/1":        BuiltinPrintf,
		"printf/2":        BuiltinPrintf,
		"printf/3":        BuiltinPrintf,
		"succ/2":          BuiltinSucc2,
		"var/1":           BuiltinVar1,
	})
}

// NewBlankMachine creates a new Golog machine without loading the
// standard library (prelude)
func NewBlankMachine() Machine {
	var m machine
	m.db = NewDatabase()
	m.env = NewBindings()
	m.disjs = ps.NewList()
	m.conjs = ps.NewList()

	for i := 0; i < smallThreshold; i++ {
		m.smallForeign[i] = ps.NewMap()
	}
	m.largeForeign = ps.NewMap()
	return (&m).DemandCutBarrier()
}

func (m *machine) clone() *machine {
	m1 := *m
	return &m1
}

func (m *machine) Consult(text interface{}) Machine {
	terms := read.TermAll_(text)
	m1 := m.clone()
	for _, t := range terms {
		if IsDirective(t) {
			// ignore all directives, for now
			continue
		}
		m1.db = m1.db.Assertz(t)
	}
	return m1
}

func (m *machine) RegisterForeign(fs map[string]ForeignPredicate) Machine {
	m1 := m.clone()
	for indicator, f := range fs {
		parts := strings.SplitN(indicator, "/", 2)
		functor := parts[0]
		arity, err := strconv.Atoi(parts[1])
		MaybePanic(err)

		if arity < smallThreshold {
			m1.smallForeign[arity] = m1.smallForeign[arity].Set(functor, f)
		} else {
			m1.largeForeign = m1.largeForeign.Set(indicator, f)
		}
	}
	return m1
}

func (m *machine) String() string {
	var buf bytes.Buffer
	fmt.Fprintf(&buf, "disjs:\n")
	m.disjs.ForEach(func(v interface{}) {
		fmt.Fprintf(&buf, "  %s\n", v)
	})
	fmt.Fprintf(&buf, "conjs:\n")
	m.conjs.ForEach(func(v interface{}) {
		fmt.Fprintf(&buf, "  %s\n", v)
	})
	fmt.Fprintf(&buf, "bindings: %s", m.env)
	return buf.String()
}

// CanProve returns true if goal can be proven from facts and clauses
// in the database.  Once a solution is found, it abandons other
// solutions (like once/1).
func (self *machine) CanProve(goal interface{}) bool {
	var answer Bindings
	var err error

	goalTerm := self.toGoal(goal)
	m := self.PushConj(goalTerm)
	for {
		m, answer, err = m.Step()
		if err == MachineDone {
			return answer != nil
		}
		MaybePanic(err)
		if answer != nil {
			return true
		}
	}
	return false
}

func (self *machine) ProveAll(goal interface{}) []Bindings {
	var answer Bindings
	var err error
	answers := make([]Bindings, 0)

	goalTerm := self.toGoal(goal)
	vars := Variables(goalTerm) // preserve incoming human-readable names
	m := self.PushConj(goalTerm)
	for {
		m, answer, err = m.Step()
		if err == MachineDone {
			break
		}
		MaybePanic(err)
		if answer != nil {
			answers = append(answers, answer.WithNames(vars))
		}
	}

	return answers
}

// advance the Golog machine one step closer to proving the goal at hand.
// at the end of each invocation, the top item on the conjunctions stack
// is the goal we should next try to prove.
func (self *machine) Step() (Machine, Bindings, error) {
	var m Machine = self
	var goal Callable
	var err error
	var cp ChoicePoint

	//Debugf("stepping...\n%s\n", self)
	if false { // for debugging. commenting out needs import changes
		_, _ = bufio.NewReader(os.Stdin).ReadString('\n')
	}

	// find a goal other than true/0 to prove
	arity := 0
	functor := "true"
	for arity == 0 && functor == "true" {
		var mTmp Machine
		goal, mTmp, err = m.PopConj()
		if err == EmptyConjunctions { // found an answer
			answer := m.Bindings()
			Debugf("  emitting answer %s\n", answer)
			m = m.PushConj(NewAtom("fail")) // backtrack on next Step()
			return m, answer, nil
		}
		MaybePanic(err)
		m = mTmp
		arity = goal.Arity()
		functor = goal.Name()
	}

	// are we proving a foreign predicate?
	f, ok := m.(*machine).lookupForeign(goal)
	if ok { // foreign predicate
		args := m.(*machine).resolveAllArguments(goal)
		Debugf("  running foreign predicate %s with %s\n", goal, args)
		ret := f(m, args)
		switch x := ret.(type) {
		case *foreignTrue:
			return m, nil, nil
		case *foreignFail:
			// do nothing. continue to iterate disjunctions below
		case *machine:
			return x, nil, nil
		case *foreignUnify:
			terms := []Term(*x) // guaranteed even number of elements
			env := m.Bindings()
			for i := 0; i < len(terms); i += 2 {
				env, err = terms[i].Unify(env, terms[i+1])
				if err == CantUnify {
					env = nil
					break
				}
				MaybePanic(err)
			}
			if env != nil {
				return m.SetBindings(env), nil, nil
			}
		}
	} else { // user-defined predicate, push all its disjunctions
		goal = goal.ReplaceVariables(m.Bindings()).(Callable)
		Debugf("  running user-defined predicate %s\n", goal)
		clauses, err := m.(*machine).db.Candidates(goal)
		MaybePanic(err)
		m = m.DemandCutBarrier()
		for i := len(clauses) - 1; i >= 0; i-- {
			clause := clauses[i]
			cp := NewHeadBodyChoicePoint(m, goal, clause)
			m = m.PushDisj(cp)
		}
	}

	// iterate disjunctions looking for one that succeeds
	for {
		cp, m, err = m.PopDisj()
		if err == EmptyDisjunctions { // nothing left to do
			Debugf("Stopping because of EmptyDisjunctions\n")
			return nil, nil, MachineDone
		}
		MaybePanic(err)

		// follow the next choice point
		Debugf("  trying to follow CP %s\n", cp)
		mTmp, err := cp.Follow()
		switch err {
		case nil:
			Debugf("  ... followed\n")
			return mTmp, nil, nil
		case CantUnify:
			Debugf("  ... couldn't unify\n")
			continue
		case CutBarrierFails:
			Debugf("  ... skipping over cut barrier\n")
			continue
		}
		MaybePanic(err)
	}
}

func (m *machine) lookupForeign(goal Callable) (ForeignPredicate, bool) {
	var f interface{}
	var ok bool

	arity := goal.Arity()
	if arity < smallThreshold {
		f, ok = m.smallForeign[arity].Lookup(goal.Name())
	} else {
		f, ok = m.largeForeign.Lookup(goal.Indicator())
	}

	if ok {
		return f.(ForeignPredicate), ok
	}
	return nil, ok
}

func (m *machine) toGoal(thing interface{}) Callable {
	switch x := thing.(type) {
	case Term:
		return x.(Callable)
	case string:
		return m.readTerm(x).(Callable)
	}
	msg := fmt.Sprintf("Can't convert %#v to a term", thing)
	panic(msg)
}

func (m *machine) resolveAllArguments(goal Callable) []Term {
	Debugf("resolving all arguments: %s\n", goal)
	env := m.Bindings()
	args := goal.Arguments()
	resolved := make([]Term, len(args))
	for i, arg := range args {
		if IsVariable(arg) {
			a, err := env.Resolve(arg.(*Variable))
			if err == nil {
				resolved[i] = a
				continue
			}
		} else if IsCompound(arg) {
			resolved[i] = arg.ReplaceVariables(env)
			continue
		}
		resolved[i] = arg
	}

	return resolved
}

func (m *machine) readTerm(src interface{}) Term {
	return read.Term_(src)
}

func (m *machine) Bindings() Bindings {
	return m.env
}

func (m *machine) SetBindings(env Bindings) Machine {
	m1 := m.clone()
	m1.env = env
	return m1
}

func (m *machine) PushConj(t Callable) Machine {
	// change all !/0 goals into '$cut_to'(RecentBarrierId) goals
	barrierID, err := m.MostRecentCutBarrier()
	if err == nil {
		t = resolveCuts(barrierID, t)
	}

	m1 := m.clone()
	m1.conjs = m.conjs.Cons(t)
	return m1
}

func (m *machine) PopConj() (Callable, Machine, error) {
	if m.conjs.IsNil() {
		return nil, nil, EmptyConjunctions
	}

	t := m.conjs.Head().(Callable)
	m1 := m.clone()
	m1.conjs = m.conjs.Tail()
	return t, m1, nil
}

func (m *machine) ClearConjs() Machine {
	m1 := m.clone()
	m1.conjs = ps.NewList()
	return m1
}

func (m *machine) ClearDisjs() Machine {
	m1 := m.clone()
	m1.disjs = ps.NewList()
	return m1
}

func (m *machine) PushDisj(cp ChoicePoint) Machine {
	m1 := m.clone()
	m1.disjs = m.disjs.Cons(cp)
	return m1
}

func (m *machine) PopDisj() (ChoicePoint, Machine, error) {
	if m.disjs.IsNil() {
		return nil, nil, EmptyDisjunctions
	}

	cp := m.disjs.Head().(ChoicePoint)
	m1 := m.clone()
	m1.disjs = m.disjs.Tail()
	return cp, m1, nil
}

func (m *machine) DemandCutBarrier() Machine {
	// is the top choice point already a cut barrier?
	if !m.disjs.IsNil() {
		topCP := m.disjs.Head().(ChoicePoint)
		_, ok := BarrierId(topCP)
		if ok {
			return m
		}
	}

	// nope, push a new cut barrier
	barrier := NewCutBarrier(m)
	return m.PushDisj(barrier)
}

func (m *machine) MostRecentCutBarrier() (int64, error) {
	ds := m.disjs
	for {
		if ds.IsNil() {
			return -1, NoBarriers
		}

		id, ok := BarrierId(ds.Head().(ChoicePoint))
		if ok {
			return id, nil
		}

		ds = ds.Tail()
	}
}

func (m *machine) CutTo(want int64) Machine {
	ds := m.disjs
	for {
		if ds.IsNil() {
			msg := fmt.Sprintf("No cut barrier with ID %d", want)
			panic(msg)
		}

		found, ok := BarrierId(ds.Head().(ChoicePoint))
		if ok && found == want {
			m1 := m.clone()
			m1.disjs = ds
			return m1
		}

		ds = ds.Tail()
	}
}

func resolveCuts(id int64, t Callable) Callable {
	switch t.Arity() {
	case 0:
		if t.Name() == "!" {
			return NewCallable("$cut_to", NewInt64(id))
		}
	case 2:
		switch t.Name() {
		case ",", ";":
			args := t.Arguments()
			t0 := resolveCuts(id, args[0].(Callable))
			t1 := resolveCuts(id, args[1].(Callable))
			if t0 == args[0] && t1 == args[1] {
				return t
			}
			return NewCallable(t.Name(), t0, t1)
		case "->":
			args := t.Arguments()
			t0 := args[0] // don't resolve cuts in Condition
			t1 := resolveCuts(id, args[1].(Callable))
			if t1 == args[1] { // no changes. don't create a new term
				return t
			}
			return NewCallable(t.Name(), t0, t1)
		}
	}

	// leave any other cuts unresolved
	return t
}