package iris

import (
	"bytes"
	"crypto/sha1"
	"encoding/base64"
	"encoding/json"
	"fmt"
	"io"
	"net/mail"
	"net/textproto"
	"os"
	"os/exec"
	"path"
	"sort"
	"strconv"
	"strings"
	"time"

	"github.com/dustin/go-nntp"
	nntpserver "github.com/dustin/go-nntp/server"
	"github.com/go-kit/log"
	"github.com/go-kit/log/level"
)

var group = &nntp.Group{
	Name:        "ctrl-c.iris",
	Description: "The iris message board",
	Posting:     nntp.PostingPermitted,
	Low:         1,
}

const DefaultWaitTime = 30 * time.Second
const msgfile = ".iris.messages"

// NewBackend builds an iris nntp backend.
//
// The provided waitTime may be <= 0, in which case DefaultWaitTime will be used.
func NewBackend(logger log.Logger, waitTime time.Duration) (nntpserver.Backend, error) {
	if waitTime <= 0 {
		waitTime = DefaultWaitTime
	}

	b := &backend{logger: logger, waitTime: waitTime}
	if err := b.refresh(); err != nil {
		return nil, err
	}

	return b, nil
}

type backend struct {
	logger   log.Logger
	waitTime time.Duration
	lastRead time.Time
	messages []*nntp.Article
}

func (b backend) debug(keyvals ...any) error { return level.Debug(b.logger).Log(keyvals...) }
func (b backend) info(keyvals ...any) error  { return level.Info(b.logger).Log(keyvals...) }
func (b backend) warn(keyvals ...any) error  { return level.Warn(b.logger).Log(keyvals...) }
func (b backend) err(keyvals ...any) error   { return level.Error(b.logger).Log(keyvals...) }

func (b backend) ListGroups(max int) ([]*nntp.Group, error) {
	return []*nntp.Group{group}, nil
}

func (b *backend) GetGroup(name string) (*nntp.Group, error) {
	if name != group.Name {
		return nil, nntpserver.ErrNoSuchGroup
	}
	if err := b.refresh(); err != nil {
		return nil, err
	}

	return group, nil
}

func (b *backend) GetArticles(_ *nntp.Group, from, to int64) ([]nntpserver.NumberedArticle, error) {
	if err := b.refresh(); err != nil {
		return nil, err
	}

	numbered := make([]nntpserver.NumberedArticle, 0, len(b.messages))
	for i, msg := range b.messages {
		num := int64(i + 1)
		if num >= from && num <= to {
			numbered = append(numbered, nntpserver.NumberedArticle{
				Num:     num,
				Article: copyArticle(msg),
			})
		}
	}

	return numbered, nil
}

func (b *backend) GetArticle(_ *nntp.Group, messageID string) (*nntp.Article, error) {
	if err := b.refresh(); err != nil {
		return nil, err
	}

	for _, msg := range b.messages {
		if msg.Header.Get("Message-Id") == messageID {
			return msg, nil
		}
	}

	num, err := strconv.Atoi(messageID)
	if err == nil && num <= len(b.messages) {
		return copyArticle(b.messages[num-1]), nil
	}

	return nil, nntpserver.ErrInvalidMessageID
}

func (b backend) Post(article *nntp.Article) error {
	// iris replies are all made to a top-level post, there is no grandchild nesting.
	//
	// but NNTP supports this, so collapse the provided "References" header up to the OP.
	parent := b.findOP(article.Header.Get("References"))
	if parent != nil {
		article.Header.Set("References", parent.MessageID())
	}

	msg, err := msgToIris(article)
	if err != nil {
		return err
	}
	return appendMessage(msg)
}

func (b backend) Authorized() bool                                     { return true }
func (b backend) AllowPost() bool                                      { return true }
func (b backend) Authenticate(_, _ string) (nntpserver.Backend, error) { return nil, nil }

func (b backend) findOP(ref string) *nntp.Article {
	if ref == "" {
		return nil
	}
	// all references should have the same OP so just take the first
	msgID := strings.SplitN(ref, " ", 2)[0]

	// traverse backwards expecting most reply activity concentrated late in the total history
	for i := len(b.messages) - 1; i >= 0; i-- {
		article := b.messages[i]
		if article.MessageID() != msgID {
			continue
		}

		gpID := article.Header.Get("References")
		if gpID != "" {
			return b.findOP(gpID)
		}
		return article
	}

	return nil
}

func (b *backend) refresh() error {
	now := time.Now()
	if b.lastRead.IsZero() || now.Sub(b.lastRead) > b.waitTime {
		b.lastRead = now
	} else {
		return nil
	}

	binpath, err := exec.LookPath("iris")
	if err != nil {
		return err
	}
	cmd := exec.Command(binpath, "-d")
	buf := &bytes.Buffer{}
	cmd.Stdout = buf

	if err := cmd.Run(); err != nil {
		return err
	}

	msgs := irisDump{}
	if err := json.NewDecoder(buf).Decode(&msgs); err != nil {
		return err
	}

	b.messages, err = msgs.Articles()
	if err != nil {
		return err
	}
	group.High = int64(len(b.messages))
	group.Count = int64(len(b.messages))
	return nil
}

func copyArticle(article *nntp.Article) *nntp.Article {
	out := *article
	out.Body = bytes.NewBuffer(article.Body.(*bytes.Buffer).Bytes())
	return &out
}

type irisMsg struct {
	Hash      string  `json:"hash"`
	EditHash  *string `json:"edit_hash"`
	IsDeleted *bool   `json:"is_deleted"`
	Data      struct {
		Author    string  `json:"author"`
		Parent    *string `json:"parent"`
		Timestamp string  `json:"timestamp"`
		Message   string  `json:"message"`
	} `json:"data"`
}

func (m irisMsg) calcHash() (string, error) {
	/*
		Careful coding here to match ruby's hash calculation:
		```
		Base64.encode64(Digest::SHA1.digest(m["data"].to_json))
		```

		* have to use an encoder rather than json.Marshal so we can
			turn off the default HTML escaping (ruby doesn't do this)
		* strip trailing newline from JSON encoding output
		* add a trailing newline to base64 encoded form
	*/

	b := &bytes.Buffer{}
	enc := json.NewEncoder(b)
	enc.SetEscapeHTML(false)
	if err := enc.Encode(m.Data); err != nil {
		return "", err
	}

	arr := sha1.Sum(bytes.TrimSuffix(b.Bytes(), []byte("\n")))
	s := base64.StdEncoding.EncodeToString(arr[:])
	if !strings.HasSuffix(s, "\n") {
		s += "\n"
	}
	return s, nil
}

func msgToIris(article *nntp.Article) (*irisMsg, error) {
	postTime := time.Now().UTC().Format(time.RFC3339)

	body, err := io.ReadAll(article.Body)
	if err != nil {
		return nil, err
	}

	var msg irisMsg
	msg.Data.Author = irisAuthor(article.Header.Get("From"))
	msg.Data.Timestamp = postTime
	refs := article.Header.Get("References")
	if refs != "" {
		spl := strings.SplitN(refs, " ", 2)
		ref := fromMsgID(spl[0])
		msg.Data.Parent = &ref
		msg.Data.Message = string(body)
	} else {
		msg.Data.Message = irisBody(article.Header.Get("Subject"), string(body))
	}

	hash, err := msg.calcHash()
	if err != nil {
		return nil, err
	}
	msg.Hash = hash

	return &msg, nil
}

func irisBody(subject, body string) string {
	firstline, _, _ := strings.Cut(body, "\n")
	if subject != "" && subject != firstline {
		body = subject + "\n\n" + body
	}
	return body
}

func irisAuthor(nntpAuthor string) string {
	addr, err := mail.ParseAddress(nntpAuthor)
	if err != nil {
		return nntpAuthor
	}

	return addr.Address
}

type irisDump []irisMsg

func (dump irisDump) Articles() ([]*nntp.Article, error) {
	// calculate the article replacements due to edits
	//
	// note: this is only a single "hop", and because there can be edits-of-edits
	// and edits-of-edits-of-edits, we must actually resolve replacements with a loop.
	//
	// we need to keep all the hops though because there could have been replies to
	// the original or to any intermediate edits.
	replacements := make(map[string]string)
	for _, msg := range dump {
		if msg.EditHash != nil {
			replacements[*msg.EditHash] = msg.Hash
		}
	}

	articles := make([]*nntp.Article, 0, len(dump)-len(replacements))

	// index iris hashes -> nntp Articles for reference lookups
	idx := make(map[string]*nntp.Article)

	sort.SliceStable(dump, func(i, j int) bool {
		return dump[i].Data.Timestamp < dump[j].Data.Timestamp
	})

outer:
	for _, msg := range dump {
		if _, ok := replacements[msg.Hash]; ok {
			continue
		}
		if msg.EditHash != nil && *msg.EditHash == msg.Hash {
			continue
		}

		msgID := msgIDFor(&msg)
		ts, err := time.Parse(time.RFC3339, msg.Data.Timestamp)
		if err != nil {
			return nil, err
		}

		article := &nntp.Article{
			Header: textproto.MIMEHeader{
				"Message-Id": []string{msgID},
				"From":       []string{msg.Data.Author},
				"Newsgroups": []string{group.Name},
				"Date":       []string{ts.Format(time.RFC1123Z)},
			},
		}

		if msg.IsDeleted != nil && *msg.IsDeleted {
			article.Header.Set("Subject", "**TOPIC DELETED BY AUTHOR**")
			article.Body = &bytes.Buffer{}
			article.Bytes = 0
			article.Lines = 0
		} else {
			article.Body = bytes.NewBufferString(msg.Data.Message)
			article.Bytes = len(msg.Data.Message)
			article.Lines = strings.Count(msg.Data.Message, "\n")

			if msg.Data.Parent == nil {
				article.Header.Set("Subject", strings.SplitN(msg.Data.Message, "\n", 2)[0])
			} else {
				parentHash := *msg.Data.Parent
				for {
					if p, ok := replacements[parentHash]; ok {
						if parentHash == p {
							continue outer
						}
						parentHash = p
					} else {
						break
					}
				}
				msg.Data.Parent = &parentHash
				parent, ok := idx[parentHash]
				if !ok {
					continue
				}
				parentSubj := strings.TrimPrefix(parent.Header.Get("Subject"), "Re: ")
				article.Header.Set("Subject", "Re: "+parentSubj)
			}
		}

		if msg.Data.Parent != nil {
			parent := idx[*msg.Data.Parent]
			if parent == nil {
				continue
			}
			parentRefs := parent.Header.Get("References")
			if parentRefs != "" {
				article.Header.Set("References", parentRefs)
			} else {
				article.Header.Set("References", parent.MessageID())
			}
		}

		articles = append(articles, article)
		idx[msg.Hash] = article
	}

	return articles, nil
}

func (id irisDump) MarshalJSON() ([]byte, error) {
	buf := &bytes.Buffer{}
	enc := json.NewEncoder(buf)
	enc.SetEscapeHTML(false)

	out := bytes.NewBufferString("[\n  ")

	for i, msg := range id {
		if err := enc.Encode(msg); err != nil {
			return nil, err
		}

		if i > 0 {
			_, _ = out.WriteString(",\n  ")
		}
		_, _ = out.Write(bytes.TrimSuffix(buf.Bytes(), []byte("\n")))
		buf.Reset()
	}
	_, _ = out.WriteString("\n]")

	return out.Bytes(), nil
}

func msgIDFor(msg *irisMsg) string {
	return fmt.Sprintf("<%s.%s>",
		strings.TrimSuffix(msg.Hash, "=\n"),
		msg.Data.Author,
	)
}

func fromMsgID(nntpID string) string {
	hash, _, _ := strings.Cut(strings.TrimSuffix(strings.TrimPrefix(nntpID, "<"), ">"), ".")
	return hash + "=\n"
}

func appendMessage(msg *irisMsg) error {
	home, err := os.UserHomeDir()
	if err != nil {
		return err
	}

	msgFile, err := os.Open(path.Join(home, msgfile))
	if err != nil {
		return err
	}

	var msgs irisDump
	if err := json.NewDecoder(msgFile).Decode(&msgs); err != nil {
		_ = msgFile.Close()
		return err
	}
	_ = msgFile.Close()
	msgs = append(msgs, *msg)

	msgFile, err = os.Create(path.Join(home, msgfile))
	if err != nil {
		return err
	}
	defer func() { _ = msgFile.Close() }()

	out, err := msgs.MarshalJSON()
	if err != nil {
		return err
	}
	_, err = msgFile.Write(out)
	return err
}