package main

import (
	"bytes"
	"fmt"
	"os"
	"path/filepath"
	"strings"
	"time"
)

func writeMaildir(folderPath string, msg []byte) error {
	for _, dir := range []string{"tmp", "new"} {
		if err := os.MkdirAll(filepath.Join(folderPath, dir), 0755); err != nil {
			return err
		}
	}

	now := time.Now()
	filename := fmt.Sprintf("%d.M%06dP%d.%s",
		now.Unix(),
		now.Nanosecond()/1000,
		os.Getpid(),
		maildirHostname(),
	)

	tmpPath := filepath.Join(folderPath, "tmp", filename)
	newPath := filepath.Join(folderPath, "new", filename)

	if err := os.WriteFile(tmpPath, msg, 0600); err != nil {
		return err
	}
	return os.Rename(tmpPath, newPath)
}

func maildirHostname() string {
	h, err := os.Hostname()
	if err != nil {
		return "localhost"
	}
	return h
}

// maildirHasMessageID reports whether any file in folderPath/new or folderPath/cur
// contains msgID in its headers.
func maildirHasMessageID(folderPath, msgID string) bool {
	needle := []byte(msgID)
	for _, sub := range []string{"new", "cur"} {
		dir := filepath.Join(folderPath, sub)
		entries, err := os.ReadDir(dir)
		if err != nil {
			continue
		}
		for _, e := range entries {
			if e.IsDir() {
				continue
			}
			if fileContainsInHeaders(filepath.Join(dir, e.Name()), needle) {
				return true
			}
		}
	}
	return false
}

// headerBytes returns the header section of an RFC 5322 message (everything
// before the first blank line). Returns the full slice if no blank line found.
func headerBytes(b []byte) []byte {
	if i := bytes.Index(b, []byte("\r\n\r\n")); i >= 0 {
		return b[:i]
	}
	if i := bytes.Index(b, []byte("\n\n")); i >= 0 {
		return b[:i]
	}
	return b
}

// fileContainsInHeaders reads only the header section of an RFC 5322 message
// (up to the first blank line) and reports whether needle appears in it.
func fileContainsInHeaders(path string, needle []byte) bool {
	f, err := os.Open(path)
	if err != nil {
		return false
	}
	defer f.Close()
	buf := make([]byte, 8192)
	n, _ := f.Read(buf)
	return bytes.Contains(headerBytes(buf[:n]), needle)
}

// extractMessageID returns the value of the Message-ID header, or "" if absent.
func extractMessageID(msg []byte) string {
	for _, line := range bytes.Split(headerBytes(msg), []byte("\n")) {
		line = bytes.TrimRight(line, "\r")
		key, val, ok := bytes.Cut(line, []byte(":"))
		if !ok {
			continue
		}
		if strings.EqualFold(strings.TrimSpace(string(key)), "message-id") {
			return strings.TrimSpace(string(val))
		}
	}
	return ""
}