package parser

import (
	"fmt"
	"html/template"
	"os"
	"path/filepath"
	"sort"
	"unicode"
)

// Slide is a single parsed slide: its frontmatter metadata plus the
// rendered HTML of its Markdown body.
type Slide struct {
	// Filename is the base name of the source file, e.g. "010-intro.md".
	Filename string
	// Title is the frontmatter "title" field.
	Title string
	// Class is the frontmatter "class" field, applied as a CSS class on the
	// slide's <section> element.
	Class string
	// Notes is the frontmatter "notes" field (speaker notes).
	Notes string
	// Incremental is the frontmatter "incremental" field. When true, the
	// slide's top-level list items are revealed one at a time as
	// navigation "fragments" instead of all at once.
	Incremental bool
	// HTML is the slide body rendered from Markdown to HTML.
	HTML template.HTML
}

// ParseDir discovers Markdown slide files in dir, sorted by filename, parses
// each one, and returns the resulting slides. Files whose frontmatter sets
// skip: true are omitted from the result.
func ParseDir(dir string) ([]Slide, error) {
	names, err := discoverSlideFiles(dir)
	if err != nil {
		return nil, fmt.Errorf("discovering slide files in %s: %w", dir, err)
	}

	slides := make([]Slide, 0, len(names))
	for _, name := range names {
		path := filepath.Join(dir, name)
		data, err := os.ReadFile(path)
		if err != nil {
			return nil, fmt.Errorf("reading slide file %s: %w", name, err)
		}

		slide, skip, err := parseSlide(name, data)
		if err != nil {
			return nil, fmt.Errorf("parsing slide file %s: %w", name, err)
		}
		if skip {
			continue
		}
		slides = append(slides, slide)
	}
	return slides, nil
}

// discoverSlideFiles lists the ".md" files directly inside dir, sorted by
// filename so that a numeric filename prefix (e.g. "010-intro.md",
// "020-...") determines slide order.
func discoverSlideFiles(dir string) ([]string, error) {
	entries, err := os.ReadDir(dir)
	if err != nil {
		return nil, err
	}

	var names []string
	for _, entry := range entries {
		if entry.IsDir() {
			continue
		}
		if filepath.Ext(entry.Name()) != ".md" {
			continue
		}
		names = append(names, entry.Name())
	}
	sort.Slice(names, func(i, j int) bool { return lessNatural(names[i], names[j]) })
	return names, nil
}

// lessNatural compares two filenames the way a human would: runs of digits
// are compared by numeric value rather than lexicographically, so
// "9-x.md" sorts before "10-x.md" and "999-x.md" before "1000-x.md"
// regardless of digit count. Non-digit runs are compared as-is.
func lessNatural(a, b string) bool {
	for len(a) > 0 && len(b) > 0 {
		aDigit, bDigit := unicode.IsDigit(rune(a[0])), unicode.IsDigit(rune(b[0]))
		if aDigit && bDigit {
			aNum, aRest := leadingDigits(a)
			bNum, bRest := leadingDigits(b)
			aVal, bVal := trimLeadingZeros(aNum), trimLeadingZeros(bNum)
			if len(aVal) != len(bVal) {
				return len(aVal) < len(bVal)
			}
			if aVal != bVal {
				return aVal < bVal
			}
			a, b = aRest, bRest
			continue
		}
		if a[0] != b[0] {
			return a[0] < b[0]
		}
		a, b = a[1:], b[1:]
	}
	return len(a) < len(b)
}

// leadingDigits splits s into its leading run of ASCII digits and the rest.
func leadingDigits(s string) (digits, rest string) {
	i := 0
	for i < len(s) && s[i] >= '0' && s[i] <= '9' {
		i++
	}
	return s[:i], s[i:]
}

// trimLeadingZeros strips leading zeros from a digit string, keeping at
// least one digit, so numeric values compare correctly by length first.
func trimLeadingZeros(digits string) string {
	i := 0
	for i < len(digits)-1 && digits[i] == '0' {
		i++
	}
	return digits[i:]
}

// parseSlide parses a single slide file's raw content into a Slide. The
// second return value reports whether the slide's frontmatter sets
// skip: true, in which case the Slide should be discarded by the caller.
func parseSlide(filename string, data []byte) (Slide, bool, error) {
	fm, body, err := parseFrontmatter(data)
	if err != nil {
		return Slide{}, false, err
	}
	if fm.Skip {
		return Slide{}, true, nil
	}

	htmlBody, err := renderMarkdown(body)
	if err != nil {
		return Slide{}, false, fmt.Errorf("rendering markdown: %w", err)
	}

	return Slide{
		Filename:    filename,
		Title:       fm.Title,
		Class:       fm.Class,
		Notes:       fm.Notes,
		Incremental: fm.Incremental,
		HTML:        template.HTML(htmlBody), //nolint:gosec // slide content is trusted local input, not user-supplied.
	}, false, nil
}