feat: replace mapping generation with checked copy utilities

copyx/options.go

@@ -0,0 +1,169 @@
+// Package copyx copies Go values without JSON, generated code or global registration.
+package copyx
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+)
+
+// NilPolicy controls nil input. NilError preserves nil for nullable targets and
+// rejects nil to non-nullable targets. NilSkip retains the existing target.
+type NilPolicy uint8
+
+const (
+	NilError NilPolicy = iota
+	NilZero
+	NilSkip
+)
+
+type pair struct{ from, to reflect.Type }
+type converter func(reflect.Value) (reflect.Value, error)
+type fields struct {
+	names                      map[string]string
+	ignoreSource, ignoreTarget map[string]bool
+}
+type settings struct {
+	deep, skipZero, strict bool
+	nilPolicy              NilPolicy
+	converters             map[pair]converter
+	fields                 map[pair]*fields
+}
+
+// Option configures one immutable Copier. Options never change global behavior.
+type Option func(*settings) error
+
+func WithDeepCopy() Option { return func(s *settings) error { s.deep = true; return nil } }
+
+// WithSkipZero explicitly skips Go zero values, including false, 0 and "".
+func WithSkipZero() Option { return func(s *settings) error { s.skipZero = true; return nil } }
+
+// WithStrictFields additionally rejects unconsumed exported source fields.
+// Target fields must have a source by default, unless explicitly ignored.
+func WithStrictFields() Option { return func(s *settings) error { s.strict = true; return nil } }
+func WithNilPolicy(p NilPolicy) Option {
+	return func(s *settings) error {
+		if p > NilSkip {
+			return errors.New("copyx: invalid nil policy")
+		}
+		s.nilPolicy = p
+		return nil
+	}
+}
+
+// WithConverter registers an exact source/target type pair. Typed nils are
+// delivered to converters before default nil handling. The function must not
+// mutate its input and must be safe for concurrent calls when Copier is shared.
+func WithConverter[S, T any](fn func(S) (T, error)) Option {
+	return func(s *settings) error {
+		if fn == nil {
+			return errors.New("copyx: nil converter")
+		}
+		key := pair{reflect.TypeFor[S](), reflect.TypeFor[T]()}
+		if _, exists := s.converters[key]; exists {
+			return fmt.Errorf("copyx: duplicate converter %s -> %s", key.from, key.to)
+		}
+		s.converters[key] = func(v reflect.Value) (reflect.Value, error) {
+			var input S
+			reflect.ValueOf(&input).Elem().Set(v)
+			out, err := fn(input)
+			return reflect.ValueOf(&out).Elem(), err
+		}
+		return nil
+	}
+}
+
+// WithFieldNames declares source -> target Go field names for one struct pair.
+// It is not a JSON tag mapping. Only actual, unambiguous exported fields qualify.
+func WithFieldNames[S, T any](names map[string]string) Option {
+	copyNames := make(map[string]string, len(names))
+	for k, v := range names {
+		copyNames[k] = v
+	}
+	return fieldOption[S, T](func(f *fields) error {
+		for from, to := range copyNames {
+			if _, exists := f.names[from]; exists {
+				return fmt.Errorf("copyx: duplicate field rule %s", from)
+			}
+			f.names[from] = to
+		}
+		return nil
+	})
+}
+func WithIgnoreSource[S, T any](names ...string) Option {
+	names = append([]string(nil), names...)
+	return fieldOption[S, T](func(f *fields) error {
+		for _, n := range names {
+			f.ignoreSource[n] = true
+		}
+		return nil
+	})
+}
+
+// WithIgnoreTarget preserves the corresponding existing target fields in Copy.
+// Deep mode rejects that preservation if it would retain a source reference.
+func WithIgnoreTarget[S, T any](names ...string) Option {
+	names = append([]string(nil), names...)
+	return fieldOption[S, T](func(f *fields) error {
+		for _, n := range names {
+			f.ignoreTarget[n] = true
+		}
+		return nil
+	})
+}
+func fieldOption[S, T any](apply func(*fields) error) Option {
+	return func(s *settings) error {
+		key := pair{reflect.TypeFor[S](), reflect.TypeFor[T]()}
+		if key.from.Kind() != reflect.Struct || key.to.Kind() != reflect.Struct {
+			return errors.New("copyx: field options require struct types")
+		}
+		f := s.fields[key]
+		if f == nil {
+			f = &fields{map[string]string{}, map[string]bool{}, map[string]bool{}}
+			s.fields[key] = f
+		}
+		return apply(f)
+	}
+}
+
+// Copier is immutable after construction. Each Copy owns its traversal state.
+type Copier struct{ options settings }
+
+func New(options ...Option) (*Copier, error) {
+	s := settings{converters: map[pair]converter{}, fields: map[pair]*fields{}}
+	for _, option := range options {
+		if option == nil {
+			return nil, errors.New("copyx: nil option")
+		}
+		if err := option(&s); err != nil {
+			return nil, err
+		}
+	}
+	for key, f := range s.fields {
+		from, to := exportedFields(key.from), exportedFields(key.to)
+		used := map[string]bool{}
+		for src, dst := range f.names {
+			if _, ok := from[src]; !ok {
+				return nil, fmt.Errorf("copyx: unknown source field %s.%s", key.from, src)
+			}
+			if _, ok := to[dst]; !ok {
+				return nil, fmt.Errorf("copyx: unknown target field %s.%s", key.to, dst)
+			}
+			if used[dst] || f.ignoreSource[src] || f.ignoreTarget[dst] {
+				return nil, fmt.Errorf("copyx: conflicting field rule %s", dst)
+			}
+			used[dst] = true
+		}
+		for n := range f.ignoreSource {
+			if _, ok := from[n]; !ok {
+				return nil, fmt.Errorf("copyx: unknown source field %s", n)
+			}
+		}
+		for n := range f.ignoreTarget {
+			if _, ok := to[n]; !ok {
+				return nil, fmt.Errorf("copyx: unknown target field %s", n)
+			}
+		}
+	}
+	return &Copier{options: s}, nil
+}