package model

import (
	"errors"
	"fmt"
	"net/http"
)

var (
	ErrNotFound       = errors.New("resource not found")
	ErrDuplicate      = errors.New("resource already exists")
	ErrUnauthorized   = errors.New("unauthorized")
	ErrForbidden      = errors.New("forbidden")
	ErrUploadTooLarge = errors.New("upload exceeds maximum size")
)

// AppError is a service-layer error that carries an HTTP status, a user-safe
// message, and an optional internal cause for logging. Handlers unpack it
// transparently via api.RespondError.
type AppError struct {
	Status  int    // HTTP status code
	Message string // safe for API response
	Err     error  // internal cause (nil = user-caused, skip logging)
}

func (e *AppError) Error() string { return e.Message }
func (e *AppError) Unwrap() error { return e.Err }

// NewBadRequestError creates a 400 AppError.
func NewBadRequestError(message string) *AppError {
	return &AppError{Status: http.StatusBadRequest, Message: message}
}

// NewConflictError creates a 409 AppError.
func NewConflictError(message string) *AppError {
	return &AppError{Status: http.StatusConflict, Message: message}
}

// NewNotFoundError creates a 404 AppError.
func NewNotFoundError(message string, cause error) *AppError {
	return &AppError{Status: http.StatusNotFound, Message: message, Err: cause}
}

// NewForbiddenError creates a 403 AppError.
func NewForbiddenError(message string, cause error) *AppError {
	return &AppError{Status: http.StatusForbidden, Message: message, Err: cause}
}

// NewPayloadTooLargeError creates a 413 AppError.
func NewPayloadTooLargeError(message string, cause error) *AppError {
	return &AppError{Status: http.StatusRequestEntityTooLarge, Message: message, Err: cause}
}

// NewInternalError creates a 500 AppError with a wrapped internal cause.
// msg describes the operation that failed; cause is the underlying error.
func NewInternalError(msg string, cause error) *AppError {
	return &AppError{
		Status:  http.StatusInternalServerError,
		Message: "internal server error",
		Err:     fmt.Errorf("%s: %w", msg, cause),
	}
}