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query.go
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query.go
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//
// query.go
//
// Copyright (c) 2018 Markku Rossi
//
// All rights reserved.
//
package datalog
import (
"fmt"
"strconv"
)
const debug bool = false
// A `program' is a finite set of clauses of the form:
//
// A :- L1,...,Ln
//
// Where A is an atom and L1,...,Ln are literals. When n=0, a clause,
// possibly containing variables, is called a `fact'. By a `subgoal',
// we mean an atom. Subgoals (and literals) that are variants of each
// other are considered syntactically identical.
//
// - Extensional database predicates (EDB) – source tables
// - Intensional database predicates (IDB) – derived tables
//
// Execute executes a datalog query on the data, limited by limits.
func Execute(q *Atom, db DB, limits Predicates) []*Clause {
query := &Query{
atom: q,
db: db,
limits: limits,
bindings: NewBindings(),
table: NewTable(),
}
query.Search()
return query.result
}
// Query implements a datalog query.
type Query struct {
atom *Atom
db DB
limits Predicates
bindings *Bindings
table *Table
result []*Clause
parent *Query
level int
parentHead *Atom
parentRest []*Atom
parentBindings *Bindings
}
// Printf prints the formatted message, indented by the query level.
func (q *Query) Printf(format string, a ...interface{}) {
for i := 0; i < q.level*4; i++ {
fmt.Print(" ")
}
fmt.Printf(format, a...)
}
// Equals tests if the queries are equal.
func (q *Query) Equals(o *Query) bool {
return q.atom.Equals(o.atom)
}
// Search executes the query.
func (q *Query) Search() {
if debug {
q.Printf("Search %s\n", q.atom)
}
found, entry := q.table.Add(q)
if found {
q.searchResult(entry.q.result)
return
}
for _, clause := range q.db.Get(q.atom, q.limits) {
env := q.bindings.Clone()
if clause.IsFact() {
if q.atom.Unify(clause.Head, env) {
r := &Clause{
Timestamp: clause.Timestamp,
Head: q.atom.Clone().Substitute(env),
}
if debug {
q.Printf("Fact: %s\n", r.Head)
}
q.addResult(r)
}
} else {
// Iterate rules
renamed := clause.Rename()
if !q.atom.Unify(renamed.Head, env) {
continue
}
renamed.Substitute(env)
q.rule(q.atom.Clone().Substitute(env),
renamed.Body[0], renamed.Body[1:], NewBindings())
}
}
q.searchResult(q.result)
// Notify waiters.
start := 0
end := len(q.result)
for start < end {
for _, waiter := range entry.waiters {
if debug {
q.Printf("->%s %v\n", entry.q.atom, q.result[start:end])
}
waiter.searchResult(q.result[start:end])
}
start = end
end = len(q.result)
}
}
func (q *Query) searchResult(result []*Clause) {
if q.parent != nil {
q.parent.subQueryResult(q.parentHead, q.atom, q.parentRest,
q.parentBindings, result)
}
}
func (q *Query) rule(head, atom *Atom, rest []*Atom, bindings *Bindings) {
if debug {
q.Printf("<Rule %s :- %s,%v\n", head, atom, rest)
}
if atom.Predicate.IsExpr() {
// XXX Move eval to term interface methods.
expr := atom.Terms[0].(*TermExpression)
_, err := expr.Expr.Eval(bindings)
if err == nil {
q.exprResult(head, atom, rest, bindings)
}
} else {
subQuery := &Query{
atom: atom,
db: q.db,
limits: q.limits,
bindings: bindings,
table: q.table,
parent: q,
level: q.level + 1,
parentHead: head,
parentRest: rest,
parentBindings: bindings,
}
subQuery.Search()
}
}
func (q *Query) subQueryResult(head, atom *Atom, rest []*Atom,
bindings *Bindings, clauses []*Clause) {
if debug {
q.Printf(">Rule %s :- %s,%v <= %v\n", head, atom, rest, clauses)
}
for _, clause := range clauses {
env := bindings.Clone()
if !atom.Unify(clause.Head, env) {
continue
}
if len(rest) == 0 {
// Unified is part of the solution, and env contains
// the bindings for the rule head. Expand head with
// env and add to results.
r := &Clause{
Timestamp: clause.Timestamp,
Head: head.Clone().Substitute(env),
}
if debug {
q.Printf("Fact: %s, env=%s\n", r.Head, env)
}
q.addResult(r)
} else {
// Sideways information passing strategies (SIPS)
if debug {
q.Printf("sips: %s\n", env)
}
expanded := rest[0].Clone().Substitute(env)
q.rule(head, expanded, rest[1:], env)
}
}
}
func (q *Query) exprResult(head, atom *Atom, rest []*Atom, bindings *Bindings) {
if len(rest) == 0 {
r := &Clause{
Head: head.Clone().Substitute(bindings),
}
q.addResult(r)
} else {
expanded := rest[0].Clone().Substitute(bindings)
q.rule(head, expanded, rest[1:], bindings)
}
}
func (q *Query) addResult(result *Clause) {
if debug {
q.Printf("=> %s\n", result)
}
for _, r := range q.result {
if r.Equals(result) {
return
}
}
q.result = append(q.result, result)
}
// NewTable creates a new table for remembered queries.
func NewTable() *Table {
return &Table{
strings: make(map[string]int),
entries: make(map[string]*TableEntry),
}
}
// Table implements remembered queries.
type Table struct {
strings map[string]int
nextStringID int
entries map[string]*TableEntry
}
// MakeID creates an unique ID for the atom.
func (table *Table) MakeID(atom *Atom) string {
var nextSymbol = 0
vars := make(map[Symbol]int)
result := strconv.Itoa(int(atom.Predicate))
for _, term := range atom.Terms {
sym := term.Variable()
if sym != SymNil {
// Variable.
id, ok := vars[sym]
if !ok {
id = nextSymbol
nextSymbol++
vars[sym] = id
}
result += "S"
result += strconv.Itoa(id)
} else {
// Constant.
id, ok := table.strings[term.String()]
if !ok {
id = table.nextStringID
table.nextStringID++
table.strings[term.String()] = id
}
result += "s"
result += strconv.Itoa(id)
}
}
return result
}
// Add adds a query to remembered queries.
func (table *Table) Add(q *Query) (bool, *TableEntry) {
id := table.MakeID(q.atom)
entry, ok := table.entries[id]
if ok {
entry.waiters = append(entry.waiters, q)
return true, entry
}
entry = &TableEntry{
q: q,
}
table.entries[id] = entry
return false, entry
}
// TableEntry implements remembered queries.
type TableEntry struct {
q *Query
waiters []*Query
}