// Package errors provides the unified application error type used throughout
// Cyrene v2.
//
// Design (DevDocs/02-development-constraints.md §四):
//
//   - An AppError carries a stable machine-readable Code, a user-safe Message,
//     an optional Cause, and an HTTP status code.
//   - Errors must be wrapped with %w semantics so errors.Is/errors.As can
//     traverse the chain, and the original Cause is preserved for callers that
//     need the raw error (logging, diagnostics, errors.Is).
//   - Internal errors are never exposed directly to users: Message is the only
//     user-facing text, while Cause keeps the underlying error out of the
//     client response (see ToJSON).
//   - Each module defines its own ErrXxx sentinels; the common ones live here.
package errors

import (
	"fmt"
	"net/http"
)

// AppError is the unified error type for Cyrene v2.
//
// It satisfies the error interface (Error returns Message) and the unwrap
// contract (Unwrap returns Cause), so errors.Is/errors.As descend through the
// chain. The zero value is a usable AppError with an empty Code and Message.
type AppError struct {
	// Code is a stable, machine-readable error code (e.g. "not_found").
	Code string

	// Message is the user-safe, readable text. Internal detail never goes here;
	// keep the raw cause in Cause instead.
	Message string

	// Cause is the underlying error that produced this AppError. It is preserved
	// for errors.Is/errors.As traversal and for logging. May be nil.
	Cause error

	// HTTPCode is the HTTP status to return with this error. 0 means "unset",
	// which HTTPStatus resolves to http.StatusInternalServerError (500).
	HTTPCode int
}

// Error returns the user-facing Message. It implements the error interface.
func (e *AppError) Error() string { return e.Message }

// Unwrap returns the Cause so errors.Is/errors.As can traverse the chain.
// It returns nil when there is no underlying cause.
func (e *AppError) Unwrap() error { return e.Cause }

// New creates an AppError with the given Code and Message and no cause.
// HTTPCode is left 0 (HTTPStatus maps it to 500 unless overridden).
func New(code, msg string) *AppError {
	return &AppError{Code: code, Message: msg}
}

// Wrap creates an AppError that wraps err as its Cause, with a formatted
// Message and the given Code. The Message is produced with fmt.Sprintf(format,
// args...) and is intended to be user-safe; the original error is preserved in
// Cause so errors.Is(err, original) and errors.As still work.
//
// The wrapped error inherits its HTTP status from err (see HTTPStatus), so
// wrapping a sentinel such as ErrNotFound keeps status 404.
func Wrap(err error, code, format string, args ...any) *AppError {
	return &AppError{
		Code:     code,
		Message:  fmt.Sprintf(format, args...),
		Cause:    err,
		HTTPCode: HTTPStatus(err),
	}
}

// newWithHTTP builds a sentinel AppError that carries an explicit HTTP status.
func newWithHTTP(code, msg string, httpCode int) *AppError {
	return &AppError{Code: code, Message: msg, HTTPCode: httpCode}
}

// Common application error sentinels, mirroring the HTTP status codes they map
// to. Modules should define their own ErrXxx and may wrap these as Causes.
var (
	// ErrInternal is the generic internal error (500).
	ErrInternal = newWithHTTP("internal", "internal error", http.StatusInternalServerError)

	// ErrNotFound means a requested resource does not exist (404).
	ErrNotFound = newWithHTTP("not_found", "not found", http.StatusNotFound)

	// ErrConflict means the request conflicts with the current state (409).
	ErrConflict = newWithHTTP("conflict", "conflict", http.StatusConflict)

	// ErrUnauthorized means the caller is not authenticated (401).
	ErrUnauthorized = newWithHTTP("unauthorized", "unauthorized", http.StatusUnauthorized)

	// ErrForbidden means the caller is authenticated but not allowed (403).
	ErrForbidden = newWithHTTP("forbidden", "forbidden", http.StatusForbidden)

	// ErrBadRequest means the request is malformed (400).
	ErrBadRequest = newWithHTTP("bad_request", "bad request", http.StatusBadRequest)

	// ErrPayloadTooLarge means the request body exceeds the allowed size (413).
	ErrPayloadTooLarge = newWithHTTP("payload_too_large", "payload too large", http.StatusRequestEntityTooLarge)

	// ErrUnsupportedMediaType means the request carries a media type the endpoint
	// does not accept (415).
	ErrUnsupportedMediaType = newWithHTTP("unsupported_media_type", "unsupported media type", http.StatusUnsupportedMediaType)

	// ErrRateLimited means the caller exceeded a rate limit (429).
	ErrRateLimited = newWithHTTP("rate_limited", "rate limited", http.StatusTooManyRequests)

	// ErrTimeout means the operation exceeded its deadline (503).
	ErrTimeout = newWithHTTP("timeout", "timeout", http.StatusServiceUnavailable)
)