video-accessibility/backend/app/lib/vtt.py

import re
from dataclasses import dataclass


@dataclass
class VTTCue:
    start_time: float  # seconds
    end_time: float    # seconds
    text: str
    identifier: str | None = None


class VTTParser:
    """Parser and builder for WebVTT files"""

    @staticmethod
    def parse(vtt_content: str) -> list[VTTCue]:
        """Parse VTT content into a list of cues"""
        lines = vtt_content.strip().split('\n')
        cues = []

        i = 0
        while i < len(lines):
            line = lines[i].strip()

            # Skip WEBVTT header, empty lines, and NOTE lines
            if line == "WEBVTT" or line == "" or line.startswith("NOTE"):
                i += 1
                continue

            # Check if this line is a cue identifier (optional)
            identifier = None
            if " --> " not in line and i + 1 < len(lines) and " --> " in lines[i + 1]:
                identifier = line
                i += 1
                line = lines[i].strip()

            # Parse timing line
            if " --> " in line:
                timing_match = re.match(r'([\d:.,]+)\s+-->\s+([\d:.,]+)', line)
                if timing_match:
                    start_time = VTTParser._parse_timestamp(timing_match.group(1))
                    end_time = VTTParser._parse_timestamp(timing_match.group(2))

                    # Collect text lines until empty line or next cue
                    i += 1
                    text_lines = []
                    while i < len(lines) and lines[i].strip() != "":
                        text_lines.append(lines[i].strip())
                        i += 1

                    if text_lines:
                        cues.append(VTTCue(
                            start_time=start_time,
                            end_time=end_time,
                            text="\n".join(text_lines),
                            identifier=identifier
                        ))
            else:
                i += 1

        return cues

    @staticmethod
    def build(cues: list[VTTCue]) -> str:
        """Build VTT content from a list of cues"""
        lines = ["WEBVTT", ""]

        for cue in cues:
            # Add identifier if present
            if cue.identifier:
                lines.append(cue.identifier)

            # Add timing line
            start_timestamp = VTTParser._format_timestamp(cue.start_time)
            end_timestamp = VTTParser._format_timestamp(cue.end_time)
            lines.append(f"{start_timestamp} --> {end_timestamp}")

            # Add text (can be multi-line)
            lines.append(cue.text)
            lines.append("")  # Empty line between cues

        return "\n".join(lines)

    @staticmethod
    def _parse_timestamp(timestamp: str) -> float:
        """Convert VTT timestamp (HH:MM:SS.mmm or MM:SS.mmm) to seconds"""
        # Clean up timestamp (handle both . and , as decimal separator)
        timestamp = timestamp.replace(',', '.')

        # Split by colon
        parts = timestamp.split(':')

        if len(parts) == 3:  # HH:MM:SS.mmm
            hours, minutes, seconds = parts
        elif len(parts) == 2:  # MM:SS.mmm
            hours, minutes, seconds = "0", parts[0], parts[1]
        else:
            raise ValueError(f"Invalid timestamp format: {timestamp}")

        # Parse seconds and decimal part
        sec_parts = seconds.split('.')
        whole_seconds = int(sec_parts[0])
        decimal_part = int(sec_parts[1]) if len(sec_parts) > 1 else 0

        # Convert to total seconds
        total_seconds = (
            int(hours) * 3600 +
            int(minutes) * 60 +
            whole_seconds +
            decimal_part / 1000.0
        )

        return total_seconds

    @staticmethod
    def _format_timestamp(seconds: float) -> str:
        """Convert seconds to VTT timestamp format (HH:MM:SS.mmm)"""
        hours = int(seconds // 3600)
        minutes = int((seconds % 3600) // 60)
        secs = seconds % 60

        whole_secs = int(secs)
        milliseconds = int((secs - whole_secs) * 1000)

        return f"{hours:02d}:{minutes:02d}:{whole_secs:02d}.{milliseconds:03d}"


class VTTEditor:
    """Utility class for editing VTT content while preserving timing"""

    @staticmethod
    def translate_preserving_timing(
        vtt_content: str,
        translated_texts: list[str]
    ) -> str:
        """Replace text in VTT cues while preserving all timing information"""
        cues = VTTParser.parse(vtt_content)

        if len(translated_texts) != len(cues):
            raise ValueError(
                f"Text count mismatch: {len(translated_texts)} texts for {len(cues)} cues"
            )

        # Update cue texts
        for i, translated_text in enumerate(translated_texts):
            cues[i].text = translated_text

        return VTTParser.build(cues)

    @staticmethod
    def update_cue_text(vtt_content: str, cue_index: int, new_text: str) -> str:
        """Update text for a specific cue by index"""
        cues = VTTParser.parse(vtt_content)

        if cue_index < 0 or cue_index >= len(cues):
            raise ValueError(f"Invalid cue index: {cue_index}")

        cues[cue_index].text = new_text
        return VTTParser.build(cues)

    @staticmethod
    def validate_vtt(vtt_content: str) -> tuple[bool, list[str]]:
        """Validate VTT content and return errors if any"""
        errors = []

        if not vtt_content.strip().startswith("WEBVTT"):
            errors.append("VTT must start with 'WEBVTT'")

        try:
            cues = VTTParser.parse(vtt_content)

            # Check timing consistency
            for i, cue in enumerate(cues):
                if cue.start_time >= cue.end_time:
                    errors.append(f"Cue {i + 1}: Start time must be before end time")

                if i > 0 and cue.start_time < cues[i - 1].end_time:
                    errors.append(f"Cue {i + 1}: Overlapping with previous cue")

                if not cue.text.strip():
                    errors.append(f"Cue {i + 1}: Empty text content")

        except Exception as e:
            errors.append(f"Parse error: {str(e)}")

        return len(errors) == 0, errors

    @staticmethod
    def get_cue_count(vtt_content: str) -> int:
        """Get the number of cues in VTT content"""
        try:
            cues = VTTParser.parse(vtt_content)
            return len(cues)
        except Exception:
            return 0

    @staticmethod
    def get_total_duration(vtt_content: str) -> float:
        """Get total duration of VTT content in seconds"""
        try:
            cues = VTTParser.parse(vtt_content)
            if not cues:
                return 0.0
            return max(cue.end_time for cue in cues)
        except Exception:
            return 0.0

    @staticmethod
    def validate_translation_timing(source_vtt: str, translated_vtt: str) -> tuple[bool, list[str]]:
        """Verify that translated VTT has identical timestamps to the source VTT"""
        errors = []
        try:
            source_cues = VTTParser.parse(source_vtt)
            translated_cues = VTTParser.parse(translated_vtt)

            if len(source_cues) != len(translated_cues):
                errors.append(
                    f"Cue count mismatch: source has {len(source_cues)}, "
                    f"translation has {len(translated_cues)}"
                )
                return False, errors

            for i, (src, tgt) in enumerate(zip(source_cues, translated_cues)):
                if abs(src.start_time - tgt.start_time) > 0.001:
                    errors.append(
                        f"Cue {i + 1}: start time changed "
                        f"({src.start_time:.3f}s -> {tgt.start_time:.3f}s)"
                    )
                if abs(src.end_time - tgt.end_time) > 0.001:
                    errors.append(
                        f"Cue {i + 1}: end time changed "
                        f"({src.end_time:.3f}s -> {tgt.end_time:.3f}s)"
                    )
        except Exception as e:
            errors.append(f"Validation error: {str(e)}")

        return len(errors) == 0, errors

    @staticmethod
    def adjust_timing_offset(vtt_content: str, offset_seconds: float) -> str:
        """
        Adjust all VTT cue timings by a fixed offset
        Positive offset moves captions later, negative moves them earlier
        """
        cues = VTTParser.parse(vtt_content)

        for cue in cues:
            cue.start_time = max(0.0, cue.start_time + offset_seconds)
            cue.end_time = max(cue.start_time + 0.5, cue.end_time + offset_seconds)

        return VTTParser.build(cues)