package docs

import (
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"regexp"
	"strings"
	"time"
)

const (
	timestampLayout = "20060102T150405.000Z"
	maxSlug         = 80
)

var (
	nameRe = regexp.MustCompile(`^(\d{8}T\d{9}Z)-(.+)\.md$`)
	slugRe = regexp.MustCompile(`^[a-z0-9]+(?:-[a-z0-9]+)*$`)
)

// parseName validates a record filename and returns its stem, UTC timestamp
// and a problem message (empty when valid).
func parseName(name string) (stem string, at time.Time, msg string) {
	m := nameRe.FindStringSubmatch(name)
	if m == nil {
		return "", at, "name must be YYYYMMDDTHHMMSSmmmZ-slug.md"
	}
	at, err := time.Parse(timestampLayout, m[1][:15]+"."+m[1][15:])
	if err != nil {
		return "", at, "name has an invalid UTC timestamp"
	}
	switch {
	case !slugRe.MatchString(m[2]):
		return "", at, "slug must be lowercase a-z0-9 words joined by single hyphens"
	case len(m[2]) > maxSlug:
		return "", at, fmt.Sprintf("slug is longer than %d characters", maxSlug)
	}
	return strings.TrimSuffix(name, ".md"), at, ""
}

// slugify lowercases title and joins its ASCII letter and digit runs with hyphens.
func slugify(title string) string {
	var b strings.Builder
	for _, r := range strings.ToLower(title) {
		if r >= 'a' && r <= 'z' || r >= '0' && r <= '9' {
			b.WriteRune(r)
		} else if b.Len() > 0 && !strings.HasSuffix(b.String(), "-") {
			b.WriteByte('-')
		}
	}
	s := b.String()
	return strings.Trim(s[:min(len(s), maxSlug)], "-")
}

// New creates a record of the given kind ("adr" or "plan") under root and
// returns its root-relative slash path. The file is created exclusively; the
// timestamp advances by 1 ms on a collision.
func New(root, kind, title string, now time.Time) (string, error) {
	title = strings.TrimSpace(title)
	if title == "" || strings.ContainsAny(title, "\r\n") {
		return "", errors.New("title must be non-empty and a single line")
	}
	slug := slugify(title)
	if slug == "" {
		return "", errors.New("title needs at least one ASCII letter or digit for the file name")
	}
	var dir string
	for _, d := range dirs {
		if d.kind == kind {
			dir = d.dir
		}
	}
	tmpl, err := templates.ReadFile("templates/" + kind + ".md")
	if err != nil {
		return "", fmt.Errorf("unknown record type %q", kind)
	}
	quoted, err := json.Marshal(title) // JSON strings are valid YAML double-quoted scalars
	if err != nil {
		return "", err
	}
	used, err := stems(root)
	if err != nil {
		return "", err
	}
	if err := os.MkdirAll(filepath.Join(root, dir), 0o755); err != nil {
		return "", err
	}
	at := now.UTC().Truncate(time.Millisecond)
	for ; ; at = at.Add(time.Millisecond) {
		stem := strings.Replace(at.Format(timestampLayout), ".", "", 1) + "-" + slug
		if used[stem] {
			continue
		}
		rel := dir + "/" + stem + ".md"
		f, err := os.OpenFile(filepath.Join(root, filepath.FromSlash(rel)), os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o644)
		if errors.Is(err, os.ErrExist) {
			continue
		}
		if err != nil {
			return "", err
		}
		content := strings.NewReplacer("@TITLE_YAML@", string(quoted), "@TITLE@", title, "@DATE@", at.Format(time.DateOnly)).Replace(string(tmpl))
		_, werr := f.WriteString(content)
		return rel, errors.Join(werr, f.Close())
	}
}

// stems returns the IDs of every record-named file in both directories.
func stems(root string) (map[string]bool, error) {
	used := map[string]bool{}
	for _, d := range dirs {
		entries, err := os.ReadDir(filepath.Join(root, d.dir))
		if err != nil && !errors.Is(err, os.ErrNotExist) {
			return nil, err
		}
		for _, e := range entries {
			if stem, _, msg := parseName(e.Name()); msg == "" {
				used[stem] = true
			}
		}
	}
	return used, nil
}