package finger

import (
	"bufio"
	"errors"
	"io"
	"net/url"
	"strings"

	sr "tildegit.org/tjp/sliderule"
)

// ForwardingDenied is returned in response to requests for forwarding service.
var ForwardingDenied = errors.New("Finger forwarding service denied.")

// InvalidFingerQuery is sent when a client doesn't properly format the query.
var InvalidFingerQuery = errors.New("Invalid finger query.")

// ParseRequest builds a sliderule.Request by reading a finger protocol request.
//
// At the time of writing, there is no firm standard on how to represent finger
// queries as URLs (the finger protocol itself predates URLs entirely), but there
// are a few helpful resources to go from.
//  - The lynx browser supports finger URLs and documents the forms they may take:
//    https://lynx.invisible-island.net/lynx_help/lynx_url_support.html#finger_url
//  - There is an IETF draft:
//    https://datatracker.ietf.org/doc/html/draft-ietf-uri-url-finger
//
// As this function builds a *sliderule.Request (which is mostly a wrapper around a URL)
// from nothing but an io.Reader, it doesn't have the context of the hostname which
// the receiving server was hosting. So it only has the host component if it
// arrived in the body of the query in the form username@hostname. Bear in mind that
// in sliderule handlers, request objects will also carry a reference to the server so
// that hostname is always available as request.Server.Hostname().
//
// The primary deviation from the IETF draft is that a query-specified host becomes
// the Host section of the URL, rather than remaining in the Path. Where the IETF draft
// would consider a query of "tjp@ctrl-c.club\r\n" to be "finger:/tjp@ctrl-c.club", this
// function will parse it into "finger://ctrl-c.club/tjp". This decision to separate the
// query-specified host from the username is intended to make it easier to avoid
// inadvertently acting as a jump host for example with:
// `exec.Command("/usr/bin/finger", request.Path[1:])`.
//
// Consistent with the IETF draft, the /W whois switch is dropped and not represented
// in the URL at all.
//
// In accordance with the recommendation of RFC 1288 section 3.2.1
// (https://datatracker.ietf.org/doc/html/rfc1288#section-3.2.1), any queries which
// include a jump-host (user@host1@host2) are rejected with the ForwardingDenied error.
func ParseRequest(rdr io.Reader) (*sr.Request, error) {
	line, err := bufio.NewReader(rdr).ReadString('\n')
	if err != nil {
		return nil, err
	}

	if line[len(line)-2] != '\r' {
		return nil, InvalidFingerQuery
	}

	line = strings.TrimSuffix(line, "\r\n")
	line = strings.TrimPrefix(line, "/W")
	line = strings.TrimLeft(line, " ")

	username, hostname, _ := strings.Cut(line, "@")
	if strings.Contains(hostname, "@") {
		return nil, ForwardingDenied
	}

	return &sr.Request{URL: &url.URL{
		Scheme:   "finger",
		Host:     hostname,
		Path:     "/" + username,
		OmitHost: true,
	}}, nil
}