package copyx

import (
	"errors"
	"fmt"
	"reflect"
	"sort"
)

// ErrDynamicType means a source interface requires a concrete runtime value.
// An exact converter can explicitly own that dynamic boundary.
var ErrDynamicType = errors.New("copyx: dynamic source type requires runtime validation or an exact converter")

// Check validates a source/target type pair using the same options as Copy.
// It never executes converters and requires no example values. Nil policies,
// converter failures, map-key collisions and deep reference isolation still
// require runtime checks. Options remain local; no mapping files or registration.
func Check[S, T any](options ...Option) error {
	c, err := New(options...)
	if err != nil {
		return err
	}
	return c.Check(reflect.TypeFor[S](), reflect.TypeFor[T]())
}

// Check validates types with an existing Copier's immutable settings. The target
// is the result type (not Copy's extra output pointer). Safe for concurrent use.
func (c *Copier) Check(source, target reflect.Type) error {
	if c == nil {
		return errors.New("copyx: nil Copier")
	}
	if source == nil || target == nil {
		return errors.New("copyx: source and target types are required")
	}
	return newTypeChecker(&c.options, false).check(source, target, "$", 0)
}

type typeChecker struct {
	options      *settings
	allowDynamic bool
	seen         map[pair]bool
	nodes        int
}

func newTypeChecker(options *settings, allowDynamic bool) *typeChecker {
	return &typeChecker{options: options, allowDynamic: allowDynamic, seen: map[pair]bool{}}
}

// Absent elements can reveal type/field mismatches, but contain no actual
// resource handles or dynamic interface values to validate. Runtime checks stay
// responsible for those value-dependent cases.
func (s *state) checkAbsent(from, to reflect.Type, path string) error {
	if s.checkedTypes == nil {
		s.checkedTypes = map[pair]bool{}
	}
	checker := typeChecker{options: s.options, allowDynamic: true, seen: s.checkedTypes, nodes: s.checkedNodes}
	err := checker.check(from, to, path, 0)
	s.checkedNodes = checker.nodes
	return err
}
func (c *typeChecker) check(from, to reflect.Type, path string, depth int) error {
	c.nodes++
	if depth > maxDepth || c.nodes > maxNodes {
		return at(path, ErrLimit)
	}
	key := pair{from, to}
	if c.seen[key] {
		return nil
	}
	c.seen[key] = true
	if !c.allowDynamic && !shallowSafe(to) {
		return at(path, ErrUnsupported)
	}
	if _, ok := c.options.converters[key]; ok {
		return nil
	}
	if from == to && len(c.options.converters) == 0 && len(c.options.fields) == 0 && !c.options.skipZero && c.options.nilPolicy != NilSkip {
		if !c.options.deep || simpleValue(to) {
			return nil
		}
	}
	if from.Kind() == reflect.Interface {
		if c.allowDynamic {
			return nil
		}
		return at(path, ErrDynamicType)
	}
	if to.Kind() == reflect.Interface {
		if !from.Implements(to) {
			return at(path, ErrType)
		}
		return c.check(from, from, path, depth+1)
	}
	if !c.allowDynamic && c.options.deep && forbidden(to) {
		return at(path, ErrUnsupported)
	}
	if from.Kind() == reflect.Pointer && to.Kind() != reflect.Pointer {
		return c.check(from.Elem(), to, path, depth+1)
	}
	if to.Kind() == reflect.Pointer {
		if !c.options.deep && from == to {
			return nil
		}
		if from.Kind() == reflect.Pointer {
			from = from.Elem()
		}
		return c.check(from, to.Elem(), path, depth+1)
	}
	if from.Kind() != to.Kind() {
		return at(path, fmt.Errorf("%w: %s -> %s", ErrType, from, to))
	}
	if to == timeType && from == to {
		return nil
	}
	switch to.Kind() {
	case reflect.Struct:
		for i := 0; i < to.NumField(); i++ {
			f := to.Field(i)
			if !c.allowDynamic && f.PkgPath != "" && (from != to || c.options.deep && privateMutable(f.Type)) {
				return at(path+"."+f.Name, ErrUnsupported)
			}
		}
		sources, targets := exportedFields(from), exportedFields(to)
		rules := c.options.fields[key]
		if rules == nil {
			rules = &fields{}
		}
		reverse, used := map[string]string{}, map[string]bool{}
		for a, b := range rules.names {
			reverse[b] = a
		}
		for _, name := range sortedFieldNames(targets) {
			if rules.ignoreTarget[name] {
				continue
			}
			sourceName := name
			if n, ok := reverse[name]; ok {
				sourceName = n
			}
			source, ok := sources[sourceName]
			if !ok || rules.ignoreSource[sourceName] {
				return at(path+"."+name, ErrField)
			}
			used[sourceName] = true
			if err := c.check(source.Type, targets[name].Type, path+"."+name, depth+1); err != nil {
				return err
			}
		}
		if c.options.strict {
			for _, name := range sortedFieldNames(sources) {
				if !used[name] && !rules.ignoreSource[name] {
					return at(path+"."+name, ErrField)
				}
			}
		}
		return nil
	case reflect.Slice:
		if !c.options.deep && from == to {
			return nil
		}
		return c.check(from.Elem(), to.Elem(), path+"[]", depth+1)
	case reflect.Array:
		if from.Len() != to.Len() {
			return at(path, ErrType)
		}
		return c.check(from.Elem(), to.Elem(), path+"[]", depth+1)
	case reflect.Map:
		if !c.options.deep && from == to {
			return nil
		}
		if err := c.check(from.Key(), to.Key(), path+"[key]", depth+1); err != nil {
			return err
		}
		return c.check(from.Elem(), to.Elem(), path+"[value]", depth+1)
	case reflect.Func, reflect.Chan, reflect.UnsafePointer:
		if c.options.deep && !c.allowDynamic {
			return at(path, ErrUnsupported)
		}
	}
	if from != to {
		return at(path, fmt.Errorf("%w: %s -> %s", ErrType, from, to))
	}
	return nil
}
func sortedFieldNames(fields map[string]reflect.StructField) []string {
	names := make([]string, 0, len(fields))
	for name := range fields {
		names = append(names, name)
	}
	sort.Strings(names)
	return names
}