// 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
}