// Package frontmatter parses the key: value front-matter block that may
// open a mopac-pdf markdown input (delimited by --- lines).
package frontmatter

import (
	"fmt"
	"strings"
)

// Meta is the parsed front-matter. The well-known keys mopac-pdf templates
// consume are Title, Subtitle, Author, Date, Classification and Template;
// anything else found in the block is kept in Extra (never an error: the
// currency of this stack is loose markdown).
type Meta struct {
	Title          string
	Subtitle       string
	Author         string
	Date           string
	Classification string
	Template       string
	Extra          map[string]string
}

// Split separates an optional front-matter block from the markdown body and
// parses it. Input without a leading --- line is all body, empty Meta.
func Split(input string) (Meta, string, error) {
	var meta Meta
	first, rest, ok := strings.Cut(input, "\n")
	if strings.TrimRight(first, "\r") != "---" || !ok {
		return meta, input, nil
	}
	end := -1
	var block []string
	for i, line := range strings.Split(rest, "\n") {
		trimmed := strings.TrimRight(line, "\r")
		if trimmed == "---" || trimmed == "..." {
			end = i
			break
		}
		block = append(block, trimmed)
	}
	if end < 0 {
		return meta, input, fmt.Errorf("frontmatter: opening --- without closing ---")
	}
	body := strings.Join(strings.Split(rest, "\n")[end+1:], "\n")
	body = strings.TrimPrefix(body, "\n")
	meta.Extra = map[string]string{}
	for _, line := range block {
		line = strings.TrimSpace(line)
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}
		key, value, ok := strings.Cut(line, ":")
		if !ok {
			return meta, "", fmt.Errorf("frontmatter: not a key: value line: %q", line)
		}
		key = strings.TrimSpace(key)
		value = strings.Trim(strings.TrimSpace(value), `"'`)
		switch strings.ToLower(key) {
		case "title":
			meta.Title = value
		case "subtitle":
			meta.Subtitle = value
		case "author":
			meta.Author = value
		case "date":
			meta.Date = value
		case "classification":
			meta.Classification = value
		case "template":
			meta.Template = value
		default:
			meta.Extra[key] = value
		}
	}
	return meta, body, nil
}