package main

import (
	"bytes"
	"io"
	"strings"
	"unicode"
	"unicode/utf8"
)

// isTitle returns the title level if the lines seem to form a title,
// zero otherwise. Input lines may inclide trailing newlines.
func isTitle(line1, line2 []byte) int {
	// This is a very naïve method, we should target graphemes (thus at least
	// NFC normalize the lines first) and account for wide characters.
	diff := utf8.RuneCount(line1) - utf8.RuneCount(line2)
	if len(line2) < 2 || diff < -1 || diff > 1 {
		return 0
	}

	// "Don't be fooled by back-to-back delimited blocks."
	// Still gets fooled by other things, though.
	if bytes.IndexFunc(line1, func(r rune) bool {
		return unicode.IsLetter(r) || unicode.IsNumber(r)
	}) < 0 {
		return 0
	}

	// The underline must be homogenous.
	for _, r := range bytes.TrimRight(line2, "\r\n") {
		if r != line2[0] {
			return 0
		}
	}
	return 1 + strings.IndexByte("=-~^+", line2[0])
}

func writeLine(w *io.PipeWriter, cur, next []byte) []byte {
	if level := isTitle(cur, next); level > 0 {
		w.Write(append(bytes.Repeat([]byte{'='}, level), ' '))
		next = nil
	}
	w.Write(cur)
	return next
}

// ConvertTitles converts AsciiDoc two-line (underlined) titles to single-line.
func ConvertTitles(w *io.PipeWriter, input []byte) {
	var last []byte
	for _, cur := range bytes.SplitAfter(input, []byte{'\n'}) {
		last = writeLine(w, last, cur)
	}
	writeLine(w, last, nil)
}