package parser

import (
	"bytes"
	"fmt"

	"gopkg.in/yaml.v3"
)

// frontmatterDelim is the delimiter line that opens and closes a YAML
// frontmatter block at the top of a slide file.
var frontmatterDelim = []byte("---")

// frontmatter holds the metadata fields recognized in a slide's YAML
// frontmatter block.
type frontmatter struct {
	Title       string `yaml:"title"`
	Class       string `yaml:"class"`
	Notes       string `yaml:"notes"`
	Skip        bool   `yaml:"skip"`
	Incremental bool   `yaml:"incremental"`
}

// splitFrontmatter separates a leading YAML frontmatter block (delimited by
// "---" lines) from the remaining Markdown body. If data has no frontmatter
// block, it returns a nil frontmatter slice and the original data as body.
func splitFrontmatter(data []byte) (fm []byte, body []byte) {
	rest := bytes.TrimLeft(data, "\r\n")
	if !bytes.HasPrefix(rest, frontmatterDelim) {
		return nil, data
	}
	afterOpen := rest[len(frontmatterDelim):]
	afterOpen = bytes.TrimLeft(afterOpen, " \t")
	if len(afterOpen) > 0 && afterOpen[0] != '\n' && afterOpen[0] != '\r' {
		// The "---" is followed by other content on the same line, so it is
		// not a frontmatter delimiter.
		return nil, data
	}
	nlIdx := bytes.IndexByte(afterOpen, '\n')
	if nlIdx == -1 {
		return nil, data
	}
	remainder := afterOpen[nlIdx+1:]

	closeIdx := bytes.Index(remainder, []byte("\n---"))
	if closeIdx == -1 {
		return nil, data
	}
	fm = remainder[:closeIdx]

	after := remainder[closeIdx+len("\n---"):]
	after = bytes.TrimLeft(after, "\r")
	if nl := bytes.IndexByte(after, '\n'); nl != -1 {
		after = after[nl+1:]
	} else {
		after = nil
	}
	return fm, after
}

// parseFrontmatter extracts and parses the frontmatter block from data,
// returning the parsed metadata and the remaining Markdown body.
func parseFrontmatter(data []byte) (frontmatter, []byte, error) {
	raw, body := splitFrontmatter(data)
	var fm frontmatter
	if raw == nil {
		return fm, body, nil
	}
	if err := yaml.Unmarshal(raw, &fm); err != nil {
		return fm, body, fmt.Errorf("parsing frontmatter: %w", err)
	}
	return fm, body, nil
}