package application

import (
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"image"
	"reflect"
	"strings"
)

const (
	MinIconDPI = 48
	MaxIconDPI = 768
)

var ErrInvalidIconEvent = errors.New("invalid icon application event")

// IconFailureCode is a stable, non-sensitive reason exposed to UI adapters.
type IconFailureCode string

const (
	IconFailureUnavailable IconFailureCode = "unavailable"
	IconFailureInvalid     IconFailureCode = "invalid_content"
	IconFailureUnsafe      IconFailureCode = "unsafe_cache"
)

// Valid reports whether code is part of the documented icon event contract.
func (code IconFailureCode) Valid() bool {
	switch code {
	case IconFailureUnavailable, IconFailureInvalid, IconFailureUnsafe:
		return true
	default:
		return false
	}
}

// IconEventIdentity correlates one background request with one catalog icon.
type IconEventIdentity struct {
	RequestID string
	AppID     string
	Reference string
	DPI       int
}

// NewIconEventIdentity validates and canonicalizes an icon event identity.
func NewIconEventIdentity(
	requestID string,
	appID string,
	reference string,
	dpi int,
) (IconEventIdentity, error) {
	if strings.TrimSpace(requestID) == "" {
		return IconEventIdentity{}, fmt.Errorf(
			"%w: empty request ID",
			ErrInvalidIconEvent,
		)
	}
	if strings.TrimSpace(appID) == "" {
		return IconEventIdentity{}, fmt.Errorf(
			"%w: empty app ID",
			ErrInvalidIconEvent,
		)
	}
	canonicalReference, err := NormalizeIconReference(reference)
	if err != nil {
		return IconEventIdentity{}, err
	}
	if dpi < MinIconDPI || dpi > MaxIconDPI {
		return IconEventIdentity{}, fmt.Errorf(
			"%w: DPI %d outside %d..%d",
			ErrInvalidIconEvent,
			dpi,
			MinIconDPI,
			MaxIconDPI,
		)
	}
	return IconEventIdentity{
		RequestID: requestID,
		AppID:     appID,
		Reference: canonicalReference,
		DPI:       dpi,
	}, nil
}

// NormalizeIconReference returns the canonical sha256:<lower-hex> form.
func NormalizeIconReference(reference string) (string, error) {
	const prefix = "sha256:"
	if !strings.HasPrefix(reference, prefix) {
		return "", fmt.Errorf(
			"%w: icon reference must use sha256",
			ErrInvalidIconEvent,
		)
	}
	digest := strings.ToLower(strings.TrimPrefix(reference, prefix))
	decoded, err := hex.DecodeString(digest)
	if err != nil || len(decoded) != sha256.Size || len(digest) != sha256.Size*2 {
		return "", fmt.Errorf(
			"%w: malformed icon digest",
			ErrInvalidIconEvent,
		)
	}
	return prefix + digest, nil
}

// IconReadyPayload contains an image decoded outside the Gio UI goroutine.
type IconReadyPayload struct {
	Reference string
	DPI       int
	Image     image.Image
}

// IconFailedPayload contains a stable failure classification, never a raw URL.
type IconFailedPayload struct {
	Reference string
	DPI       int
	ErrorCode IconFailureCode
}

// IconEvent is the validated representation consumed by UI adapters.
type IconEvent struct {
	Type      EventType
	Identity  IconEventIdentity
	Image     image.Image
	ErrorCode IconFailureCode
}

// NewIconReadyEvent builds a validated ready event.
func NewIconReadyEvent(
	identity IconEventIdentity,
	icon image.Image,
) (Event, error) {
	validated, err := NewIconEventIdentity(
		identity.RequestID,
		identity.AppID,
		identity.Reference,
		identity.DPI,
	)
	if err != nil {
		return Event{}, err
	}
	if isNilImage(icon) {
		return Event{}, fmt.Errorf("%w: nil ready image", ErrInvalidIconEvent)
	}
	return Event{
		Type:      EventIconReady,
		RequestID: validated.RequestID,
		AppID:     validated.AppID,
		Payload: IconReadyPayload{
			Reference: validated.Reference,
			DPI:       validated.DPI,
			Image:     icon,
		},
	}, nil
}

// NewIconFailedEvent builds a validated failure event.
func NewIconFailedEvent(
	identity IconEventIdentity,
	code IconFailureCode,
) (Event, error) {
	validated, err := NewIconEventIdentity(
		identity.RequestID,
		identity.AppID,
		identity.Reference,
		identity.DPI,
	)
	if err != nil {
		return Event{}, err
	}
	if !code.Valid() {
		return Event{}, fmt.Errorf(
			"%w: unknown failure code %q",
			ErrInvalidIconEvent,
			code,
		)
	}
	return Event{
		Type:      EventIconFailed,
		RequestID: validated.RequestID,
		AppID:     validated.AppID,
		Payload: IconFailedPayload{
			Reference: validated.Reference,
			DPI:       validated.DPI,
			ErrorCode: code,
		},
	}, nil
}

// ParseIconEvent validates an icon envelope. Non-icon events return handled=false.
func ParseIconEvent(event Event) (parsed IconEvent, handled bool, err error) {
	switch event.Type {
	case EventIconReady:
		payload, ok := event.Payload.(IconReadyPayload)
		if !ok {
			return IconEvent{}, true, fmt.Errorf(
				"%w: ready payload has type %T",
				ErrInvalidIconEvent,
				event.Payload,
			)
		}
		identity, identityErr := NewIconEventIdentity(
			event.RequestID,
			event.AppID,
			payload.Reference,
			payload.DPI,
		)
		if identityErr != nil {
			return IconEvent{}, true, identityErr
		}
		if isNilImage(payload.Image) {
			return IconEvent{}, true, fmt.Errorf(
				"%w: nil ready image",
				ErrInvalidIconEvent,
			)
		}
		return IconEvent{
			Type:     event.Type,
			Identity: identity,
			Image:    payload.Image,
		}, true, nil
	case EventIconFailed:
		payload, ok := event.Payload.(IconFailedPayload)
		if !ok {
			return IconEvent{}, true, fmt.Errorf(
				"%w: failed payload has type %T",
				ErrInvalidIconEvent,
				event.Payload,
			)
		}
		identity, identityErr := NewIconEventIdentity(
			event.RequestID,
			event.AppID,
			payload.Reference,
			payload.DPI,
		)
		if identityErr != nil {
			return IconEvent{}, true, identityErr
		}
		if !payload.ErrorCode.Valid() {
			return IconEvent{}, true, fmt.Errorf(
				"%w: unknown failure code %q",
				ErrInvalidIconEvent,
				payload.ErrorCode,
			)
		}
		return IconEvent{
			Type:      event.Type,
			Identity:  identity,
			ErrorCode: payload.ErrorCode,
		}, true, nil
	default:
		return IconEvent{}, false, nil
	}
}

func isNilImage(icon image.Image) bool {
	if icon == nil {
		return true
	}
	value := reflect.ValueOf(icon)
	switch value.Kind() {
	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map,
		reflect.Ptr, reflect.Slice:
		return value.IsNil()
	default:
		return false
	}
}