feat(logic): implements source_file/1 predicate

x/logic/interpreter/registry.go

@@ -118,6 +118,7 @@ var Registry = map[string]RegistryEntry{
 	"sha_hash/2":                {predicate.SHAHash, 1},
 	"hex_bytes/2":               {predicate.HexBytes, 1},
 	"bech32_address/2":          {predicate.Bech32Address, 1},
+	"source_file/1":             {predicate.SourceFile, 1},
 }
 
 // RegistryNames is the list of the predicate names in the Registry.

x/logic/predicate/file.go

@@ -0,0 +1,72 @@
+package predicate
+
+import (
+	"fmt"
+	"reflect"
+	"sort"
+
+	"github.com/ichiban/prolog/engine"
+)
+
+func SourceFile(vm *engine.VM, file engine.Term, cont engine.Cont, env *engine.Env) *engine.Promise {
+	loaded := getLoadedSources(vm)
+
+	inputFile, err := getFile(env, file)
+	if err != nil {
+		return engine.Error(fmt.Errorf("source_file/1: %w", err))
+	}
+
+	if inputFile != nil {
+		if _, ok := loaded[*inputFile]; ok {
+			return engine.Unify(vm, file, engine.NewAtom(*inputFile), cont, env)
+		}
+		return engine.Unify(vm, file, engine.List(), cont, env)
+	}
+
+	result := make([]engine.Term, 0, len(loaded))
+	for _, filename := range sortLoadedSources(loaded) {
+		result = append(result, engine.NewAtom(filename))
+	}
+
+	return engine.Unify(
+		vm,
+		file,
+		engine.List(result...),
+		cont,
+		env,
+	)
+}
+
+func getLoadedSources(vm *engine.VM) map[string]interface{} {
+	loadedField := reflect.ValueOf(vm).Elem().FieldByName("loaded").MapKeys()
+	loaded := make(map[string]interface{}, len(loadedField))
+	for _, value := range loadedField {
+		loaded[value.String()] = nil
+	}
+
+	return loaded
+}
+
+func sortLoadedSources(sources map[string]interface{}) []string {
+	result := make([]string, 0, len(sources))
+	for filename := range sources {
+		result = append(result, filename)
+	}
+	sort.SliceStable(result, func(i, j int) bool {
+		return result[i] < result[j]
+	})
+
+	return result
+}
+
+func getFile(env *engine.Env, term engine.Term) (*string, error) {
+	switch file := env.Resolve(term).(type) {
+	case engine.Variable:
+	case engine.Atom:
+		strFile := file.String()
+		return &strFile, nil
+	default:
+		return nil, fmt.Errorf("cannot unify file with %T", term)
+	}
+	return nil, nil
+}