package braille

import (
	"errors"
	"strings"
	"unicode/utf8"
)

// Region is one semantic piece of a braille line. Cursor is a zero-based rune
// offset within Text, including the position just after the final rune.
type Region struct {
	Text   string
	Cursor int
}

// Line follows Python Cthulhu's Line: regions are concatenated exactly as
// supplied and one region may own the visible cursor. A focus of -1 hides it.
type Line struct {
	Regions []Region
	Focus   int
}

func (line Line) Frame() (Frame, error) {
	if line.Focus < -1 || line.Focus >= len(line.Regions) {
		return Frame{}, errors.New("invalid focused braille region")
	}
	var text strings.Builder
	cursor := -1
	position := 0
	for index, region := range line.Regions {
		length := utf8.RuneCountInString(region.Text)
		if index == line.Focus {
			if region.Cursor < 0 || region.Cursor > length {
				return Frame{}, errors.New("invalid braille region cursor")
			}
			cursor = position + region.Cursor
		}
		text.WriteString(region.Text)
		position += length
	}
	return Frame{Text: text.String(), Cursor: cursor}, nil
}