// Package di is a minimal reflection-based dependency injection helper.
//
// Usage pattern:
//
//	type Config struct {
//	    Logger di.Provide[*slog.Logger]
//	    Server func(logger *slog.Logger) (*http.Server, error)
//	}
//
//	type Result struct {
//	    Server *http.Server
//	}
//
//	cfg := Config{ /* constructors here ... */ }
//	var res Result
//	if err := di.Build(cfg, &res); err != nil { ... }\
//
// Or with New(cfg):
//
// server, err := di.New[*http.Server](cfg)
//
// See the doc comments for Build and New for more details
package di

import (
	"errors"
	"fmt"
	"reflect"
	"strings"
)

type Provide[Out any] struct {
	fOrV any
}

// SideEffect is a sentinel value representing a constructor used only for side
// effects such as introducing two components together without a circular
// dependency.
//
// It's idiomatic to have your config declare a SideEffects field of type
// []di.MustProvide[di.SideEffect], and have a "_" field in your result struct
// of type []di.SideEffect.
type SideEffect struct{}

func NewSideEffect(f any) (Provide[SideEffect], error) {
	return NewProvide[SideEffect](f)
}

func MustSideEffect(f any) Provide[SideEffect] {
	return Must(NewSideEffect(f))
}

type provideI interface {
	diOutType() reflect.Type
	diPayload() any
}

func (p Provide[Out]) diOutType() reflect.Type { return typeOf[Out]() }
func (p Provide[Out]) diPayload() any          { return p.fOrV }

func Must[T any](t T, err error) T {
	if err != nil {
		panic(err)
	}
	return t
}

func MustProvide[Out any](ctorOrVal any) Provide[Out] {
	return Must(NewProvide[Out](ctorOrVal))
}

func NewProvide[Out any](ctorOrVal any) (Provide[Out], error) {
	outType := typeOf[Out]()

	if ctorOrVal == nil {
		if canBeNil(outType) {
			return Provide[Out]{fOrV: nil}, nil
		}
		return Provide[Out]{}, fmt.Errorf("Provide[%v]: nil not valid for non-nilable type", outType)
	}

	t := reflect.TypeOf(ctorOrVal)

	// Case 1: function returning Out or (Out, error). Arbitrary args allowed.
	if t.Kind() == reflect.Func {
		nout := t.NumOut()
		switch nout {
		case 1:
			if !t.Out(0).AssignableTo(outType) {
				return Provide[Out]{}, fmt.Errorf("Provide[%v]: function return %v is not assignable to %v",
					outType, t.Out(0), outType)
			}
			return Provide[Out]{fOrV: ctorOrVal}, nil

		case 2:
			if !t.Out(0).AssignableTo(outType) {
				return Provide[Out]{}, fmt.Errorf("Provide[%v]: first return %v is not assignable to %v",
					outType, t.Out(0), outType)
			}
			if !isErrorType(t.Out(1)) {
				return Provide[Out]{}, fmt.Errorf("Provide[%v]: second return must be error, got %v",
					outType, t.Out(1))
			}
			return Provide[Out]{fOrV: ctorOrVal}, nil

		default:
			return Provide[Out]{}, fmt.Errorf("Provide[%v]: function must return Out or (Out, error); got %d returns",
				outType, nout)
		}
	}

	// Case 2: value assignable to Out (covers interface satisfaction)
	if !t.AssignableTo(outType) {
		return Provide[Out]{}, fmt.Errorf("Provide[%v]: value of type %v is not assignable to %v", outType, t, outType)
	}
	return Provide[Out]{fOrV: ctorOrVal}, nil
}

// Helpers
func typeOf[T any]() reflect.Type {
	return reflect.TypeOf((*T)(nil)).Elem()
}

func canBeNil(t reflect.Type) bool {
	switch t.Kind() {
	case reflect.Interface, reflect.Chan, reflect.Func, reflect.Map, reflect.Pointer, reflect.Slice:
		return true
	default:
		return false
	}
}

type ctor struct {
	name string
	fn   reflect.Value
	out  reflect.Type
}

type collector struct {
	// Singular instances & providers
	values    map[reflect.Type]reflect.Value // exact type -> instance
	providers map[reflect.Type][]ctor        // out type -> ctors

	// List providers for []T
	listValues    map[reflect.Type][]reflect.Value // elem dynamic type -> instances
	listProviders map[reflect.Type][]ctor          // elem out type -> ctors
	listPresence  map[reflect.Type]bool            // elem T present explicitly (even if empty)

	// Cycle detection
	resolving map[reflect.Type]bool
}

func New[R any, C any](config C) (R, error) {
	var r R
	err := Build(config, &r)
	return r, err
}

// Build resolves only what's needed to populate exported fields in result.
// Supports arbitrarily nested provider namespaced structs inside config.
//
// # TODO rewrite this doc
//
// Supported providers in config (at any nesting depth):
//   - Provide[T]                    // single constructor or value for T
//   - []Provide[T]                  // list of constructors/values contributing to []T
//   - func(...Deps) T / (T,error)   // singular constructor for T
//   - value of type T               // preprovided singular value
//   - []func(...Deps) T/(T,error)   // contributes to []T
//   - []T                           // contributes to []T
//
// Non-func, non-zero exported fields remain prebound instances.
// result must be a pointer to a struct or value. If it is a value the config
// must define how to construct it.
func Build[C any, R any](config C, result R) error {
	cfgV := reflect.ValueOf(config)
	for cfgV.IsValid() && cfgV.Kind() == reflect.Pointer {
		if cfgV.IsNil() {
			return errors.New("config pointer is nil")
		}
		cfgV = cfgV.Elem()
	}
	if !cfgV.IsValid() || cfgV.Kind() != reflect.Struct {
		return errors.New("config must be a struct or pointer to struct")
	}

	resV := reflect.ValueOf(result)
	if !resV.IsValid() || resV.Kind() != reflect.Pointer {
		return errors.New("result must be a pointer to struct or a pointer to a value")
	}

	c := &collector{
		values:        make(map[reflect.Type]reflect.Value),
		providers:     make(map[reflect.Type][]ctor),
		listValues:    make(map[reflect.Type][]reflect.Value),
		listProviders: make(map[reflect.Type][]ctor),
		listPresence:  make(map[reflect.Type]bool),
		resolving:     make(map[reflect.Type]bool),
	}

	// Provide access to the Config value itself
	c.values[cfgV.Type()] = cfgV
	if err := c.collect(cfgV, ""); err != nil {
		return err
	}

	// Can we just resolve the direct result type?
	if v, err := c.resolve(resV.Elem().Type()); err == nil {
		resV.Elem().Set(v)
		return nil
	}

	if resV.Elem().Kind() != reflect.Struct {
		return fmt.Errorf("couldn't build result direct, and can not fill result as it is not a struct")
	}

	// Populate result fields of the struct
	var missing []string
	resStruct := resV.Elem()
	resT := resStruct.Type()
	for i := 0; i < resT.NumField(); i++ {
		sf := resT.Field(i)
		if sf.Name != "_" && sf.PkgPath != "" {
			continue
		}
		fv := resStruct.Field(i)
		if !fv.IsZero() {
			continue
		}
		v, err := c.resolve(sf.Type)
		if err != nil {
			missing = append(missing, fmt.Sprintf("%s (%s): %v", sf.Name, sf.Type, err))
			continue
		}

		// Evaluate, but do not set underscore field names
		if sf.Name != "_" {
			fv.Set(v)
		}
	}
	if len(missing) > 0 {
		return fmt.Errorf("failed to build result fields:\n  - %s", strings.Join(missing, "\n  - "))
	}
	return nil
}

func (c *collector) collect(v reflect.Value, path string) error {
	if !v.IsValid() {
		return nil
	}
	// Deref pointers
	for v.Kind() == reflect.Pointer {
		if v.IsNil() {
			return nil
		}
		v = v.Elem()
	}
	if v.Kind() != reflect.Struct {
		return nil
	}

	t := v.Type()
	for i := 0; i < t.NumField(); i++ {
		sf := t.Field(i)
		if sf.PkgPath != "" { // unexported
			continue
		}
		fv := v.Field(i)
		name := sf.Name
		if path != "" {
			name = path + "." + sf.Name
		}

		// Provide[T] (singular) must be recognized BEFORE treating structs as namespaces.
		if pi, ok := asProvide(fv); ok {
			outT := pi.diOutType()
			payload := pi.diPayload()
			if payload == nil {
				c.values[outT] = reflect.Zero(outT)
				continue
			}
			pt := reflect.TypeOf(payload)
			if pt.Kind() == reflect.Func {
				if err := validateCtorSignature(pt, name); err != nil {
					return err
				}
				c.providers[pt.Out(0)] = append(c.providers[pt.Out(0)], ctor{name: name, fn: reflect.ValueOf(payload), out: pt.Out(0)})
			} else {
				c.values[pt] = reflect.ValueOf(payload)
			}
			continue
		}

		// Namespace recursion for embedded structs
		if sf.Anonymous && sf.Type.Kind() == reflect.Struct {
			// Provide access to the nested config value itself
			c.values[sf.Type] = fv

			if err := c.collect(fv, name); err != nil {
				return err
			}
			continue
		}

		// []Provide[T] (list)
		if fv.Kind() == reflect.Slice && fv.Type().Elem().Kind() == reflect.Struct {
			if provideElem, ok := reflect.New(fv.Type().Elem()).Elem().Interface().(provideI); ok {
				if fv.Len() == 0 && !fv.IsNil() {
					// Set presence of empty list
					outT := provideElem.diOutType()
					c.listPresence[outT] = true
				}

				for j := 0; j < fv.Len(); j++ {
					pi := fv.Index(j).Interface().(provideI)
					outT := pi.diOutType()
					c.listPresence[outT] = true
					payload := pi.diPayload()
					if payload == nil {
						c.listValues[outT] = append(c.listValues[outT], reflect.Zero(outT))
						continue
					}
					pt := reflect.TypeOf(payload)
					if pt.Kind() == reflect.Func {
						if err := validateCtorSignature(pt, fmt.Sprintf("%s[%d]", name, j)); err != nil {
							return err
						}
						c.listProviders[pt.Out(0)] = append(c.listProviders[pt.Out(0)], ctor{
							name: fmt.Sprintf("%s[%d]", name, j),
							fn:   reflect.ValueOf(payload),
							out:  pt.Out(0),
						})
					} else {
						c.listValues[pt] = append(c.listValues[pt], reflect.ValueOf(payload))
					}
				}
				continue
			}
		}

		// Fallback to earlier forms (singular func/value; list of funcs/values)
		switch sf.Type.Kind() {
		case reflect.Func:
			ft := fv.Type()
			if err := validateCtorSignature(ft, name); err != nil {
				return err
			}
			c.providers[ft.Out(0)] = append(c.providers[ft.Out(0)], ctor{name: name, fn: fv, out: ft.Out(0)})

		case reflect.Slice:
			elemT := sf.Type.Elem()
			if elemT.Kind() == reflect.Func {
				for j := 0; j < fv.Len(); j++ {
					fn := fv.Index(j)
					ft := fn.Type()
					if err := validateCtorSignature(ft, fmt.Sprintf("%s[%d]", name, j)); err != nil {
						return err
					}
					c.listProviders[ft.Out(0)] = append(c.listProviders[ft.Out(0)], ctor{
						name: fmt.Sprintf("%s[%d]", name, j),
						fn:   fn, out: ft.Out(0),
					})
				}
			} else {
				if fv.Len() == 0 {
					c.listPresence[elemT] = true // explicit empty list present
				}
				for j := 0; j < fv.Len(); j++ {
					vj := fv.Index(j)
					c.listValues[vj.Type()] = append(c.listValues[vj.Type()], vj)
				}
			}

		default:
			// preprovided singular instance (non-zero only)
			if !fv.IsZero() {
				c.values[fv.Type()] = fv
			}
		}
	}
	return nil
}

func (c *collector) resolve(t reflect.Type) (reflect.Value, error) {
	// Cached exact?
	if v, ok := c.values[t]; ok {
		return v, nil
	}

	// Slice resolution []T
	if t.Kind() == reflect.Slice {
		elem := t.Elem()
		var elems []reflect.Value
		found := false

		// Explicit list values (concrete types assignable to elem)
		for haveT, vals := range c.listValues {
			if isAssignableOrImpl(haveT, elem) {
				found = true
				elems = append(elems, vals...)
			}
		}
		// From list constructors whose out is assignable to elem
		for outT, ctors := range c.listProviders {
			if !isAssignableOrImpl(outT, elem) {
				continue
			}
			found = true
			for _, ctor := range ctors {
				if c.resolving[t] {
					return reflect.Value{}, fmt.Errorf("dependency cycle detected at %s", t)
				}
				c.resolving[t] = true
				ft := ctor.fn.Type()
				args := make([]reflect.Value, ft.NumIn())
				for i := 0; i < ft.NumIn(); i++ {
					paramT := ft.In(i)
					arg, err := c.resolve(paramT)
					if err != nil {
						delete(c.resolving, t)
						return reflect.Value{}, fmt.Errorf("%s depends on %s: %w", ctor.name, paramT, err)
					}
					if !isAssignableOrImpl(arg.Type(), paramT) {
						delete(c.resolving, t)
						return reflect.Value{}, fmt.Errorf("%s: cannot use %s as %s", ctor.name, arg.Type(), paramT)
					}
					args[i] = arg
				}
				outs := ctor.fn.Call(args)
				delete(c.resolving, t)
				if len(outs) == 2 && !outs[1].IsNil() {
					return reflect.Value{}, fmt.Errorf("%s error: %w", ctor.name, outs[1].Interface().(error))
				}
				elems = append(elems, outs[0])
			}
		}

		// If an explicit provider for elem T exists (e.g., []Provide[T] or []T present but empty),
		// we should still succeed with an empty slice.
		if !found {
			if c.listPresence[elem] {
				c.values[t] = reflect.MakeSlice(t, 0, 0)
				return c.values[t], nil
			}
			return reflect.Value{}, fmt.Errorf("no provider for %s", t)
		}

		slice := reflect.MakeSlice(t, 0, len(elems))
		for _, e := range elems {
			slice = reflect.Append(slice, e)
		}
		c.values[t] = slice
		return slice, nil
	}

	// Try existing instances for interface targets (singular)
	if t.Kind() == reflect.Interface {
		for haveT, v := range c.values {
			if haveT.Implements(t) {
				return v, nil
			}
		}
	}

	// Cycle detection (singular)
	if c.resolving[t] {
		return reflect.Value{}, fmt.Errorf("dependency cycle detected at %s", t)
	}
	c.resolving[t] = true
	defer delete(c.resolving, t)

	// Pick singular provider(s)
	var candidates []ctor
	if ps, ok := c.providers[t]; ok {
		candidates = append(candidates, ps...)
	} else if t.Kind() == reflect.Interface {
		for outT, ps := range c.providers {
			if outT.Implements(t) {
				candidates = append(candidates, ps...)
			}
		}
	}

	if len(candidates) == 0 {
		return reflect.Value{}, fmt.Errorf("no provider for %s", t)
	}
	if len(candidates) > 1 {
		var names []string
		for _, candidate := range candidates {
			names = append(names, candidate.name+" -> "+candidate.out.String())
		}
		return reflect.Value{}, fmt.Errorf("ambiguous providers for %s: %s", t, strings.Join(names, ", "))
	}

	impl := candidates[0]
	ft := impl.fn.Type()
	args := make([]reflect.Value, ft.NumIn())
	for i := 0; i < ft.NumIn(); i++ {
		paramT := ft.In(i)
		arg, err := c.resolve(paramT)
		if err != nil {
			return reflect.Value{}, fmt.Errorf("%s depends on %s: %w", impl.name, paramT, err)
		}
		if !isAssignableOrImpl(arg.Type(), paramT) {
			return reflect.Value{}, fmt.Errorf("%s: cannot use %s as %s", impl.name, arg.Type(), paramT)
		}
		args[i] = arg
	}
	outs := impl.fn.Call(args)
	if len(outs) == 2 {
		if !outs[1].IsNil() {
			return reflect.Value{}, fmt.Errorf("%s error: %w", impl.name, outs[1].Interface().(error))
		}
		c.values[impl.out] = outs[0]
		return outs[0], nil
	}
	c.values[impl.out] = outs[0]
	return outs[0], nil
}

// --- helpers ---

func validateCtorSignature(ft reflect.Type, name string) error {
	if ft.IsVariadic() {
		return fmt.Errorf("constructor %q: variadics not supported", name)
	}
	nout := ft.NumOut()
	if nout == 1 {
		return nil
	}
	if nout == 2 && isErrorType(ft.Out(1)) {
		return nil
	}
	return fmt.Errorf("constructor %q: must return (T) or (T, error); got %d returns", name, nout)
}

func isAssignableOrImpl(have, want reflect.Type) bool {
	return have.AssignableTo(want) || (want.Kind() == reflect.Interface && have.Implements(want))
}

func isErrorType(t reflect.Type) bool {
	return t == reflect.TypeOf((*error)(nil)).Elem()
}

// asProvide tries to view v as a Provide[*]. Returns (iface, true) if so.
func asProvide(v reflect.Value) (provideI, bool) {
	if !v.IsValid() {
		return nil, false
	}
	x := v.Interface()
	pi, ok := x.(provideI)
	return pi, ok
}