package main

import (
	"bufio"
	"bytes"
	"errors"
	"flag"
	"fmt"
	"io"
	"io/ioutil"
	"net/mail"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"time"
)

const (
	sendmailPathDefault = "/usr/sbin/sendmail"
	defaultMaxSize      = 6 * 1024 * 1024 // Bytes
	explDomain          = "the domain of your mail server (used for List-Id)"
	explName            = "name of list (used for List-Id)"
	explMaxsize         = "max valid size of received mail (bytes)"
	explOpen            = "allow non-members to post to the list"
	explNewsletter      = "newsletter mode (few-to-many, implies \"not open\" and an owner file in 'list'@'domain'.pico.owners)"
	explDebug           = "log even successful execution"
	explSendmail        = "path to sendmail"
	program             = "picolist"
	maxRcpts            = 50
	logFile             = "picolist.log"
)

type flags struct {
	domain     string
	name       string
	maxsize    int64
	open       bool
	newsletter bool
}

func (f flags) listname() string {
	return f.name + "@" + f.domain
}

func (f flags) returnPath() string {
	return f.name + "-bounces" + "@" + f.domain
}

func parseFlags() flags {
	domain := flag.String("domain", "", explDomain)
	name := flag.String("list", "", explName)
	maxsize := flag.Int("maxsize", defaultMaxSize, explMaxsize)
	open := flag.Bool("open", false, explOpen)
	newsletter := flag.Bool("newsletter", false, explNewsletter)
	tmpDebug := flag.Bool("debug", false, explDebug)
	tmpSendmail := flag.String("sendmail", sendmailPathDefault, explSendmail)
	flag.Parse()
	if *domain == "" || *name == "" {
		log(nil, "Wrong usage")
	}
	debug = *tmpDebug
	sendmailPath = *tmpSendmail
	return flags{domain: *domain, name: *name, maxsize: int64(*maxsize), open: *open, newsletter: *newsletter}
}

func loadMembers(listname string) (hashset, error) {
	return readAddressFile(recipientFile(listname))
}

func loadOwners(listname string, f flags, members hashset) (hashset, error) {
	if f.newsletter {
		return readAddressFile(ownerFile(listname))
	} else {
		return members, nil
	}
}

func originator(mail *mail.Message) (string, error) {
	header := mail.Header
	if sender, ok := header["Sender"]; ok && len(sender) == 1 {
		return parseAddress(sender[0])
	} else {
		if from, ok := header["From"]; ok && len(from) == 1 {
			return parseAddress(from[0])
		} else {
			return "", err("malformed message as per RFC 5322 section-3.6.2, originator unclear", nil)
		}
	}
}

func isAllowed(mail *mail.Message, f flags, owners hashset) (bool, error) {
	if !f.newsletter && f.open {
		return true, nil
	}
	orig, err := originator(mail)
	if err != nil {
		return false, err
	}
	return owners.contains(orig), nil
}

func setHeaderFields(mail *mail.Message, listname string, f flags) {
	sender := f.returnPath()
	mail.Header["Precedence"] = []string{"list"}
	mail.Header["Sender"] = []string{"<" + sender + ">"}
	mail.Header["Errors-To"] = []string{sender}
	mail.Header["List-Id"] = []string{"<" + listname + ">"}
	if !f.newsletter {
		mail.Header["List-Post"] = []string{"<mailto:" + listname + ">"}
	}
}

func recipients(members hashset) ([]bulkRecipients, error) {
	rcpts := members.elements()
	return splitRecipients(rcpts), nil
}

func sendmail(original *mail.Message, mail string, returnPath string, recps string) error {
	if debug {
		log(original, sendmailPath+" -f "+returnPath+" "+recps+"\n")
	}
	cmd := exec.Command(sendmailPath, "-f "+returnPath, recps)
	cmd.Stdin = strings.NewReader(mail)
	e := cmd.Run()
	if e != nil {
		return err("error on call to sendmail", e)
	}
	return nil
}

func forward(mail *mail.Message, returnPath string, members hashset) []error {
	errs := []error{}
	var err error
	flatmail := asText(mail)
	rcpts, err := recipients(members)
	errs = appendErr(errs, err)
	if len(errs) > 0 {
		return errs
	}
	for _, bulk := range rcpts {
		err = sendmail(mail, flatmail, returnPath, bulk.String())
		errs = appendErr(errs, err)
	}
	return errs
}

func readMail(maxsize int64) (*mail.Message, error) {
	var m *mail.Message
	var e error
	reader := bufio.NewReader(io.LimitReader(os.Stdin, maxsize))
	// discard first line (envelope header from MTA)
	_, e = reader.ReadBytes('\n')
	if e == nil {
		m, e = mail.ReadMessage(reader)
	}
	if e != nil {
		return nil, err("could not read mail from stdin", e)
	}
	return m, e
}

func spreadMessage(f flags) {
	listname := f.listname()
	mail, e := readMail(f.maxsize)
	optLogFatal(mail, e)
	members, e := loadMembers(listname)
	optLogFatal(mail, e)
	owners, e := loadOwners(listname, f, members)
	optLogFatal(mail, e)
	allowed, e := isAllowed(mail, f, owners)
	optLogFatal(mail, e)
	returnPath := f.returnPath()
	if allowed {
		setHeaderFields(mail, listname, f)
		errs := forward(mail, returnPath, members)
		optLogAll(mail, errs)
	} else if !f.newsletter {
		log(mail, "not allowed to post to "+listname)
	}
}

func main() {
	exePath = filepath.Dir(os.Args[0])
	flags := parseFlags()
	spreadMessage(flags)
}

//
/*--------- helpers ---------*/
//

var exePath string
var debug bool
var sendmailPath string

func err(msg string, e error) error {
	if e != nil {
		msg += " - " + e.Error()
	}
	return errors.New(msg)
}

func appendErr(errs []error, e error) []error {
	if e != nil {
		errs = append(errs, e)
	}
	return errs
}

type hashset map[string]interface{}

func makeHashset() hashset {
	return make(map[string]interface{})
}

func (s hashset) add(member string) error {
	if s == nil {
		return err("uninitialized hashset", nil)
	}
	s[member] = nil
	return nil
}

func (s hashset) contains(member string) bool {
	if s == nil {
		return false
	}
	if _, ok := s[member]; ok {
		return true
	} else {
		return false
	}
}

func (set hashset) elements() []string {
	keys := []string{}
	for k, _ := range set {
		keys = append(keys, k)
	}
	return keys
}

var logfile *os.File

func fullPath(file string) string {
	return filepath.Join(exePath, file)
}

func ensureLogfile() bool {
	if logfile == nil {
		flags := os.O_APPEND | os.O_CREATE | os.O_WRONLY
		logfile, _ = os.OpenFile(fullPath(logFile), flags, 0644)
		if logfile == nil {
			return false
		}
	}
	return true
}

func log(mail *mail.Message, s string) {
	if !ensureLogfile() {
		// be quietly sad
		return
	}
	if mail != nil {
		header := mail.Header
		from := header.Get("From")
		to := header.Get("To")
		subject := header.Get("Subject")
		fmt.Fprintf(logfile, "%v FROM=%s TO=%s SUBJECT=%s -> %s\n", time.Now(), from, to, subject, s)
	} else {
		fmt.Fprintf(logfile, "%s%s", s, "\n")
	}
}

func optLogFatal(mail *mail.Message, err error) {
	if err != nil {
		log(mail, err.Error())
		// don't complain to the caller
		// so that we don't accidentally spill
		// information
		os.Exit(0)
	}
}

func optLogAll(mail *mail.Message, errs []error) {
	for _, err := range errs {
		log(mail, err.Error())
	}
}

func recipientFile(listname string) string {
	return fullPath(listname + ".pico")
}

func ownerFile(listname string) string {
	return recipientFile(listname) + ".owners"
}

func readAddressFile(filename string) (hashset, error) {
	addrs := makeHashset()
	addrFile, e := ioutil.ReadFile(filename)
	if e != nil {
		return addrs, err("could not read address file "+filename, e)
	}
	tmpAddrs := strings.Split(string(addrFile), "\n")
	for _, addr := range tmpAddrs {
		addr = strings.TrimSpace(addr)
		if addr != "" && !strings.HasPrefix(addr, "#") {
			addrs.add(addr)
		}
	}
	return addrs, nil
}

func writeHeader(buf *bytes.Buffer, key string, value string) {
	if buf == nil {
		return
	}
	buf.WriteString(key)
	buf.WriteString(": ")
	buf.WriteString(value)
	buf.WriteString("\r\n")
}

func asText(mail *mail.Message) string {
	if mail == nil {
		return ""
	}
	var buf bytes.Buffer
	header := mail.Header
	for key, values := range header {
		if key == "Received" {
			for _, rcv := range values {
				writeHeader(&buf, key, rcv)
			}
		} else {
			writeHeader(&buf, key, strings.Join(values, ","))
		}
	}
	buf.WriteString("\r\n")
	buf.ReadFrom(mail.Body)
	s := buf.String()
	if debug {
		log(nil, s)
	}
	return s
}

func parseAddress(s string) (string, error) {
	addr, e := mail.ParseAddress(s)
	if e != nil {
		return "", err("could not parse originator address"+s, e)
	} else {
		return addr.Address, nil
	}
}

type bulkRecipients []string

func (rcpts bulkRecipients) String() string {
	return strings.Join([]string(rcpts), " ")
}

func splitRecipients(rcpts []string) []bulkRecipients {
	all := []bulkRecipients{}
	bulkRcpts := bulkRecipients([]string{})
	for i := range rcpts {
		bulkRcpts = append(bulkRcpts, rcpts[i])
		if len(bulkRcpts)%maxRcpts == 0 || i == len(rcpts)-1 {
			all = append(all, bulkRcpts)
			bulkRcpts = []string{}
		}
	}
	return all
}