package storage

import (
	"errors"
	"fmt"
	"net/url"
	"slices"
	"strings"
)

// Storage URL scheme names.
const (
	schemeFile   = "file"
	schemeS3     = "s3"
	schemeRclone = "rclone"
)

// Sentinel errors for storage URL parsing.
var (
	ErrEmptyStorageURL  = errors.New("storage URL is empty")
	ErrEmptyFilePath    = errors.New("file URL path is empty")
	ErrRelativeFilePath = errors.New(
		"file URL needs an absolute path, as in file:///path/to/backup")
	ErrMissingBucket     = errors.New("s3 URL missing bucket name")
	ErrMissingRemote     = errors.New("rclone URL missing remote name")
	ErrUnsupportedScheme = errors.New(
		"unsupported URL scheme: must start with s3://, file://, or rclone://")
	ErrUnsupportedStorage = errors.New("unsupported storage scheme")
	ErrURLCredentials     = errors.New(
		"storage URL must not carry credentials; " +
			"set s3.access_key_id and s3.secret_access_key in the config instead")
	ErrURLUnknownParam    = errors.New("unknown query parameter in storage URL")
	ErrRcloneRemoteParams = errors.New(
		"rclone URL must not carry parameters in the remote name; " +
			"set them on the remote with rclone config instead")
)

// URL represents a parsed storage URL.
type URL struct {
	Scheme       string // "s3", "file", or "rclone"
	Bucket       string // S3 bucket name (empty for file/rclone)
	Prefix       string // Path within bucket or filesystem base path
	Endpoint     string // S3 endpoint (optional, default AWS)
	Region       string // S3 region (optional)
	UseSSL       bool   // Use HTTPS for S3 (default true)
	RcloneRemote string // rclone remote name (for rclone:// URLs)
}

// ParseStorageURL parses a storage URL string.
// Supported formats:
//   - s3://bucket/prefix?endpoint=host&region=us-east-1&ssl=true
//   - file:///absolute/path/to/backup
//   - rclone://remote/path/to/backups
func ParseStorageURL(rawURL string) (*URL, error) {
	if rawURL == "" {
		return nil, ErrEmptyStorageURL
	}

	if after, ok := strings.CutPrefix(rawURL, "file://"); ok {
		return parseFileURL(after)
	}

	if strings.HasPrefix(rawURL, "s3://") {
		return parseS3URL(rawURL)
	}

	if strings.HasPrefix(rawURL, "rclone://") {
		return parseRcloneURL(rawURL)
	}

	return nil, ErrUnsupportedScheme
}

// parseFileURL parses the path that follows file://. The path must be
// absolute: a relative one would resolve against the working directory,
// so the destination would move with it. Percent-escapes are decoded, so
// my%20stick names the directory "my stick"; a literal space is kept.
func parseFileURL(rawPath string) (*URL, error) {
	if rawPath == "" {
		return nil, ErrEmptyFilePath
	}

	if !strings.HasPrefix(rawPath, "/") {
		return nil, ErrRelativeFilePath
	}

	path, err := url.PathUnescape(rawPath)
	if err != nil {
		return nil, fmt.Errorf("invalid file URL path: %w", err)
	}

	return &URL{Scheme: schemeFile, Prefix: path}, nil
}

// parseS3URL parses an s3://bucket/prefix URL. It rejects credentials in
// the userinfo or in the endpoint parameter's userinfo, and any query
// parameter other than endpoint, region and ssl, so a credential-bearing
// URL is never stored or echoed.
func parseS3URL(rawURL string) (*URL, error) {
	u, err := url.Parse(rawURL)
	if err != nil {
		return nil, wrapParseError(err)
	}

	if u.User != nil {
		return nil, ErrURLCredentials
	}

	bucket := u.Host
	if bucket == "" {
		return nil, ErrMissingBucket
	}

	query := u.Query()

	err = rejectUnknownParams(query, "endpoint", "region", "ssl")
	if err != nil {
		return nil, err
	}

	// A host name never contains "@", so one in an endpoint, with or
	// without a scheme, can only introduce userinfo.
	for _, endpoint := range query["endpoint"] {
		if strings.Contains(endpoint, "@") {
			return nil, ErrURLCredentials
		}
	}

	return &URL{
		Scheme:   schemeS3,
		Bucket:   bucket,
		Prefix:   strings.TrimPrefix(u.Path, "/"),
		Endpoint: query.Get("endpoint"),
		Region:   query.Get("region"),
		UseSSL:   query.Get("ssl") != "false",
	}, nil
}

// parseRcloneURL parses an rclone://remote/path URL. rclone:// takes no
// query parameters, so credentials in the userinfo, parameters in the
// remote name and any query parameter at all are rejected rather than
// silently ignored.
func parseRcloneURL(rawURL string) (*URL, error) {
	// The remote name runs up to the first "/", "?" or "#". It never
	// contains a comma; one starts rclone's remote,param=value form,
	// whose values can be credentials. This is checked before url.Parse,
	// whose error for such a name can print part of it.
	remote := strings.TrimPrefix(rawURL, "rclone://")
	if end := strings.IndexAny(remote, "/?#"); end >= 0 {
		remote = remote[:end]
	}

	if strings.Contains(remote, ",") {
		return nil, ErrRcloneRemoteParams
	}

	u, err := url.Parse(rawURL)
	if err != nil {
		return nil, wrapParseError(err)
	}

	if u.User != nil {
		return nil, ErrURLCredentials
	}

	remote = u.Host
	if remote == "" {
		return nil, ErrMissingRemote
	}

	err = rejectUnknownParams(u.Query())
	if err != nil {
		return nil, err
	}

	return &URL{
		Scheme:       schemeRclone,
		Prefix:       strings.TrimPrefix(u.Path, "/"),
		RcloneRemote: remote,
	}, nil
}

// rejectUnknownParams returns an error naming the first query parameter
// not in allowed. The parameter's name is included (so a misspelt
// endpoint= is caught), but never its value, which could be a secret,
// and never the whole URL.
func rejectUnknownParams(query url.Values, allowed ...string) error {
	for name := range query {
		if !slices.Contains(allowed, name) {
			return fmt.Errorf(
				"%w: %q; put credentials in s3.access_key_id and "+
					"s3.secret_access_key, not the URL",
				ErrURLUnknownParam, name)
		}
	}

	return nil
}

// wrapParseError wraps only the inner cause of a url.Parse failure. The
// *url.Error that url.Parse returns embeds the raw URL in its message, so
// wrapping it directly would echo a credential-bearing URL into logs.
func wrapParseError(err error) error {
	if uerr, ok := errors.AsType[*url.Error](err); ok {
		return fmt.Errorf("invalid URL: %w", uerr.Err)
	}

	return fmt.Errorf("invalid URL: %w", err)
}

// String returns a human-readable representation of the storage URL.
func (u *URL) String() string {
	switch u.Scheme {
	case schemeFile:
		return "file://" + u.Prefix
	case schemeS3:
		endpoint := u.Endpoint
		if endpoint == "" {
			endpoint = "s3.amazonaws.com"
		}

		if u.Prefix != "" {
			return fmt.Sprintf("s3://%s/%s (endpoint: %s)", u.Bucket, u.Prefix, endpoint)
		}

		return fmt.Sprintf("s3://%s (endpoint: %s)", u.Bucket, endpoint)
	case schemeRclone:
		if u.Prefix != "" {
			return fmt.Sprintf("rclone://%s/%s", u.RcloneRemote, u.Prefix)
		}

		return "rclone://" + u.RcloneRemote
	default:
		return u.Scheme + "://?"
	}
}