package main

import (
	"bytes"
	"errors"
	"fmt"
	"html/template"
	"log"
	"os"
	"path/filepath"
	"regexp"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"

	"github.com/yuin/goldmark"
)

// Post is one rendered blog post.
type Post struct {
	Slug    string
	Title   string
	Date    time.Time
	Summary string
	Author  string
	Draft   bool
	Pinned  bool
	HTML    template.HTML
}

// index is an immutable snapshot of the published posts.
type index struct {
	posts   []*Post        // newest first; feed and newer/older links use this
	listing []*Post        // index page order: pinned first, each group newest first
	bySlug  map[string]int // slug -> position in posts
}

// Store reads posts from a directory and caches them until the directory's
// contents (file names, sizes or modification times) change.
type Store struct {
	dir string
	md  goldmark.Markdown

	mu  sync.Mutex
	sig string
	idx *index
}

func NewStore(dir string, md goldmark.Markdown) *Store {
	return &Store{dir: dir, md: md}
}

// Index returns the current published posts, reloading them if any file in
// the content directory has changed. Files that fail to parse are logged and
// skipped so one bad post cannot take the site down.
func (s *Store) Index() (*index, error) {
	s.mu.Lock()
	defer s.mu.Unlock()

	entries, err := os.ReadDir(s.dir)
	if err != nil {
		return nil, err
	}
	var sig strings.Builder
	var files []os.DirEntry
	for _, e := range entries {
		if !e.Type().IsRegular() || filepath.Ext(e.Name()) != ".md" {
			continue
		}
		info, err := e.Info()
		if err != nil {
			return nil, err
		}
		fmt.Fprintf(&sig, "%s\x00%d\x00%d\n", e.Name(), info.Size(), info.ModTime().UnixNano())
		files = append(files, e)
	}
	if s.idx != nil && sig.String() == s.sig {
		return s.idx, nil
	}

	idx := &index{bySlug: map[string]int{}}
	for _, e := range files {
		path := filepath.Join(s.dir, e.Name())
		src, err := os.ReadFile(path)
		if err != nil {
			return nil, err
		}
		info, err := e.Info()
		if err != nil {
			return nil, err
		}
		p, err := parsePost(s.md, e.Name(), src, info.ModTime())
		if err != nil {
			log.Printf("skipping %s: %v", path, err)
			continue
		}
		if p.Draft {
			continue
		}
		if _, dup := idx.bySlug[p.Slug]; dup {
			log.Printf("skipping %s: duplicate slug %q", path, p.Slug)
			continue
		}
		idx.bySlug[p.Slug] = -1
		idx.posts = append(idx.posts, p)
	}
	sort.SliceStable(idx.posts, func(i, j int) bool {
		a, b := idx.posts[i], idx.posts[j]
		if !a.Date.Equal(b.Date) {
			return a.Date.After(b.Date)
		}
		return a.Slug < b.Slug
	})
	for i, p := range idx.posts {
		idx.bySlug[p.Slug] = i
	}
	idx.listing = make([]*Post, 0, len(idx.posts))
	for _, pinned := range []bool{true, false} {
		for _, p := range idx.posts {
			if p.Pinned == pinned {
				idx.listing = append(idx.listing, p)
			}
		}
	}

	s.sig, s.idx = sig.String(), idx
	return idx, nil
}

var (
	datedName = regexp.MustCompile(`^(\d{4}-\d{2}-\d{2})-([a-z0-9][a-z0-9-]*)\.md$`)
	plainName = regexp.MustCompile(`^([a-z0-9][a-z0-9-]*)\.md$`)
)

// slugFromName extracts the slug and, for YYYY-MM-DD-slug.md names, the date.
func slugFromName(name string) (slug string, date time.Time, err error) {
	if m := datedName.FindStringSubmatch(name); m != nil {
		date, err = time.Parse(time.DateOnly, m[1])
		if err != nil {
			return "", time.Time{}, fmt.Errorf("bad date in filename: %w", err)
		}
		return m[2], date, nil
	}
	if m := plainName.FindStringSubmatch(name); m != nil {
		return m[1], time.Time{}, nil
	}
	return "", time.Time{}, errors.New("filename must be YYYY-MM-DD-slug.md or slug.md using lowercase letters, digits and hyphens")
}

// parsePost builds a Post from a file's name and contents. modTime is the
// last-resort date when neither front matter nor the filename gives one.
func parsePost(md goldmark.Markdown, name string, src []byte, modTime time.Time) (*Post, error) {
	slug, fileDate, err := slugFromName(name)
	if err != nil {
		return nil, err
	}
	meta, body, err := splitFrontMatter(src)
	if err != nil {
		return nil, err
	}

	p := &Post{Slug: slug, Title: meta["title"], Summary: meta["summary"], Author: meta["author"]}

	switch {
	case meta["date"] != "":
		if p.Date, err = parseDate(meta["date"]); err != nil {
			return nil, err
		}
	case !fileDate.IsZero():
		p.Date = fileDate
	default:
		p.Date = modTime.UTC().Truncate(24 * time.Hour)
	}

	if v := meta["draft"]; v != "" {
		if p.Draft, err = strconv.ParseBool(v); err != nil {
			return nil, fmt.Errorf("draft: %q is not true or false", v)
		}
	}
	if v := meta["pinned"]; v != "" {
		if p.Pinned, err = strconv.ParseBool(v); err != nil {
			return nil, fmt.Errorf("pinned: %q is not true or false", v)
		}
	}

	if p.Title == "" {
		p.Title, body = takeHeading(body)
	}
	if p.Title == "" {
		p.Title = slug
	}

	var buf bytes.Buffer
	if err := md.Convert(body, &buf); err != nil {
		return nil, fmt.Errorf("rendering markdown: %w", err)
	}
	p.HTML = template.HTML(buf.String())
	return p, nil
}

func parseDate(s string) (time.Time, error) {
	for _, layout := range []string{time.DateOnly, time.RFC3339} {
		if t, err := time.Parse(layout, s); err == nil {
			return t, nil
		}
	}
	return time.Time{}, fmt.Errorf("date: %q is not YYYY-MM-DD", s)
}

// splitFrontMatter separates an optional leading block of "key: value" lines
// fenced by "---" lines from the Markdown body.
func splitFrontMatter(src []byte) (map[string]string, []byte, error) {
	src = bytes.ReplaceAll(src, []byte("\r\n"), []byte("\n"))
	meta := map[string]string{}
	lines := strings.SplitAfter(string(src), "\n")
	if strings.TrimSpace(lines[0]) != "---" {
		return meta, src, nil
	}
	for i := 1; i < len(lines); i++ {
		line := strings.TrimSpace(lines[i])
		if line == "---" {
			return meta, []byte(strings.Join(lines[i+1:], "")), nil
		}
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}
		key, value, ok := strings.Cut(line, ":")
		if !ok {
			return nil, nil, fmt.Errorf("front matter line %d: expected key: value", i+1)
		}
		meta[strings.ToLower(strings.TrimSpace(key))] = unquote(strings.TrimSpace(value))
	}
	return nil, nil, errors.New("front matter is missing its closing ---")
}

func unquote(s string) string {
	if len(s) >= 2 && (s[0] == '"' || s[0] == '\'') && s[len(s)-1] == s[0] {
		return s[1 : len(s)-1]
	}
	return s
}

// takeHeading removes a leading "# Title" line from body and returns the title.
func takeHeading(body []byte) (string, []byte) {
	rest := bytes.TrimLeft(body, " \t\n")
	line, after, _ := bytes.Cut(rest, []byte("\n"))
	if title, ok := bytes.CutPrefix(line, []byte("# ")); ok {
		return strings.TrimSpace(string(title)), after
	}
	return "", body
}