srt.go

package astisub

import (
	"bufio"
	"fmt"
	"io"
	"strconv"
	"strings"
	"time"
)

// Constants
const (
	srtTimeBoundariesSeparator = " --> "
)

// Vars
var (
	bytesSRTTimeBoundariesSeparator = []byte(srtTimeBoundariesSeparator)
)

// parseDurationSRT parses an .srt duration
func parseDurationSRT(i string) (d time.Duration, err error) {
	for _, s := range []string{",", "."} {
		if d, err = parseDuration(i, s, 3); err == nil {
			return
		}
	}
	return
}

// ReadFromSRT parses an .srt content
func ReadFromSRT(i io.Reader) (o *Subtitles, err error) {
	// Init
	o = NewSubtitles()
	var scanner = bufio.NewScanner(i)

	// Scan
	var line string
	var lineNum int
	var s = &Item{}
	for scanner.Scan() {
		// Fetch line
		line = strings.TrimSpace(scanner.Text())
		lineNum++

		// Remove BOM header
		if lineNum == 1 {
			line = strings.TrimPrefix(line, string(BytesBOM))
		}

		// Line contains time boundaries
		if strings.Contains(line, srtTimeBoundariesSeparator) {
			// Remove last item of previous subtitle since it should be the index.
			// If the last line is empty then the item is missing an index.
			var index string
			if len(s.Lines) != 0 {
				index = s.Lines[len(s.Lines)-1].String()
				if index != "" {
					s.Lines = s.Lines[:len(s.Lines)-1]
				}
			}

			// Remove trailing empty lines
			if len(s.Lines) > 0 {
				for i := len(s.Lines) - 1; i >= 0; i-- {
					if len(s.Lines[i].Items) > 0 {
						for j := len(s.Lines[i].Items) - 1; j >= 0; j-- {
							if len(s.Lines[i].Items[j].Text) == 0 {
								s.Lines[i].Items = s.Lines[i].Items[:j]
							} else {
								break
							}
						}
						if len(s.Lines[i].Items) == 0 {
							s.Lines = s.Lines[:i]
						}

					}
				}
			}

			// Init subtitle
			s = &Item{}

			// Fetch Index
			if index != "" {
				s.Index, _ = strconv.Atoi(index)
			}

			// Extract time boundaries
			s1 := strings.Split(line, srtTimeBoundariesSeparator)
			if l := len(s1); l < 2 {
				err = fmt.Errorf("astisub: line %d: time boundaries has only %d element(s)", lineNum, l)
				return
			}
			// We do this to eliminate extra stuff like positions which are not documented anywhere
			s2 := strings.Split(s1[1], " ")

			// Parse time boundaries
			if s.StartAt, err = parseDurationSRT(s1[0]); err != nil {
				err = fmt.Errorf("astisub: line %d: parsing srt duration %s failed: %w", lineNum, s1[0], err)
				return
			}
			if s.EndAt, err = parseDurationSRT(s2[0]); err != nil {
				err = fmt.Errorf("astisub: line %d: parsing srt duration %s failed: %w", lineNum, s2[0], err)
				return
			}

			// Append subtitle
			o.Items = append(o.Items, s)
		} else {
			// Add text
			s.Lines = append(s.Lines, Line{Items: []LineItem{{Text: strings.TrimSpace(line)}}})
		}
	}
	return
}

// formatDurationSRT formats an .srt duration
func formatDurationSRT(i time.Duration) string {
	return formatDuration(i, ",", 3)
}

// WriteToSRT writes subtitles in .srt format
func (s Subtitles) WriteToSRT(o io.Writer) (err error) {
	// Do not write anything if no subtitles
	if len(s.Items) == 0 {
		err = ErrNoSubtitlesToWrite
		return
	}

	// Add BOM header
	var c []byte
	c = append(c, BytesBOM...)

	// Loop through subtitles
	for k, v := range s.Items {
		// Add time boundaries
		c = append(c, []byte(strconv.Itoa(k+1))...)
		c = append(c, bytesLineSeparator...)
		c = append(c, []byte(formatDurationSRT(v.StartAt))...)
		c = append(c, bytesSRTTimeBoundariesSeparator...)
		c = append(c, []byte(formatDurationSRT(v.EndAt))...)
		c = append(c, bytesLineSeparator...)

		// Loop through lines
		for _, l := range v.Lines {
			c = append(c, []byte(l.String())...)
			c = append(c, bytesLineSeparator...)
		}

		// Add new line
		c = append(c, bytesLineSeparator...)
	}

	// Remove last new line
	c = c[:len(c)-1]

	// Write
	if _, err = o.Write(c); err != nil {
		err = fmt.Errorf("astisub: writing failed: %w", err)
		return
	}
	return
}