Method EvalExternalQuery

engine/topdown.go:82–137

EvalExternalQuery evaluates an external query. See ExternalPredicateCallback for more details.

(query ast.Atom, mode []ast.ArgMode,
	ext ExternalPredicateCallback, pushdown []ast.Term, cb func(fact ast.Atom) error)

// EvalExternalQuery evaluates an external query.
// See ExternalPredicateCallback for more details.
func (q QueryContext) EvalExternalQuery(query ast.Atom, mode []ast.ArgMode,
	ext ExternalPredicateCallback, pushdown []ast.Term, cb func(fact ast.Atom) error) error {
	// Step 1. Check if we already have the facts.
	err := q.Store.GetFacts(query, func(fact ast.Atom) error {
		return errFound
	})
	if err != nil {
		if errors.Is(err, errFound) {
			return nil
		}
		return err
	}
	// Step 2. Check if we need to query.
	var inputs []ast.Constant
	var filters []ast.BaseTerm
	for i, arg := range query.Args {
		if mode[i] == ast.ArgModeInput {
			inputs = append(inputs, arg.(ast.Constant))
		} else {
			filters = append(filters, arg)
		}
	}
	if !ext.ShouldQuery(inputs, filters, pushdown) {
		return nil
	}
	// Step 3. Query and add facts.
	var hasFilters bool
	for _, filter := range filters {
		if _, ok := filter.(ast.Variable); !ok {
			hasFilters = true
			break
		}
	}
	return ext.ExecuteQuery(inputs, filters, pushdown, func(output []ast.BaseTerm) {
		j := 0
		args := make([]ast.BaseTerm, query.Predicate.Arity)
		for i, arg := range query.Args {
			if mode[i] == ast.ArgModeInput {
				args[i] = arg
			} else if !hasFilters {
				args[i] = output[j]
				j++
			} else { // check if filters match
				res := output[j]
				c, ok := arg.(ast.Constant)
				if !ok || c.Equals(res) {
					args[i] = res
					j++
				} else {
					return // skip this output
				}
			}
		}
		cb(ast.Atom{query.Predicate, args})
	})
}

// EvalPremise evaluates a single premise top-down.