commit 842e3bf1b0d01f7306d0e7555ae891474a541f55
Author: Maxim Vershinin <im@vmn.su>
Date:   Tue Jul 28 16:25:11 2026 +0500

    Initial commit. Basic featues are implemented

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..bee8a64
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+__pycache__
diff --git a/__init__.py b/__init__.py
new file mode 100644
index 0000000..2f17147
--- /dev/null
+++ b/__init__.py
@@ -0,0 +1,3 @@
+"""
+Audio splitter package – split an audio file using a tracklist.
+"""
diff --git a/constants.py b/constants.py
new file mode 100644
index 0000000..f608d35
--- /dev/null
+++ b/constants.py
@@ -0,0 +1,20 @@
+"""Global constants and default values."""
+
+# Mapping from user‑friendly format names to FFmpeg format identifiers,
+# file extensions, and whether the container is audio‑only.
+FORMAT_INFO = {
+    'mp3':      {'ffmpeg': 'mp3',      'ext': '.mp3',  'audio_only': True},
+    'm4a':      {'ffmpeg': 'mp4',      'ext': '.m4a',  'audio_only': True},
+    'mp4':      {'ffmpeg': 'mp4',      'ext': '.mp4',  'audio_only': False},
+    'mkv':      {'ffmpeg': 'matroska', 'ext': '.mkv',  'audio_only': False},
+    'matroska': {'ffmpeg': 'matroska', 'ext': '.mkv',  'audio_only': False},
+    'ogg':      {'ffmpeg': 'ogg',      'ext': '.ogg',  'audio_only': True},
+    'opus':     {'ffmpeg': 'ogg',      'ext': '.opus', 'audio_only': True},
+    'flac':     {'ffmpeg': 'flac',     'ext': '.flac', 'audio_only': True},
+    'wav':      {'ffmpeg': 'wav',      'ext': '.wav',  'audio_only': True},
+    'aac':      {'ffmpeg': 'adts',     'ext': '.aac',  'audio_only': True},
+}
+
+# Default characters to replace when --replace-bad-chars is enabled.
+# Includes common punctuation, single quote, and a trailing space.
+DEFAULT_BAD_CHARS = r',!@#№$;:%^&?*(){}[]\/<>+=~`\' '
diff --git a/core.py b/core.py
new file mode 100644
index 0000000..9839ed3
--- /dev/null
+++ b/core.py
@@ -0,0 +1,130 @@
+"""Core logic: orchestrates the splitting process."""
+
+import os
+import subprocess
+
+from .constants import FORMAT_INFO
+from .ffmpeg import get_audio_duration, get_stream_info, get_metadata, build_ffmpeg_command
+from .formats import determine_output_format, validate_format_compatibility
+from .timestamp import parse_track_timestamps, resolve_end_times
+from .filename import build_filename
+from .metadata import build_metadata_dict
+from .utils import format_time
+
+
+def split_audio(input_file, output_directory, tracks, args):
+    """
+    Main orchestration function: split the audio file into tracks.
+
+    Args:
+        input_file: Path to the input media file.
+        output_directory: Directory where output files will be saved.
+        tracks: List of dicts, each containing parsed fields.
+        args: Parsed command‑line arguments (namespace).
+
+    Raises:
+        RuntimeError: If no audio stream is found.
+        ValueError: If timestamp parsing or format compatibility fails.
+    """
+    # --------------------------------------------------------------------------
+    # 1. Setup
+    # --------------------------------------------------------------------------
+    os.makedirs(output_directory, exist_ok=True)
+
+    total_duration = get_audio_duration(input_file)
+    stream_info = get_stream_info(input_file)
+
+    print(f"Detected streams: audio={stream_info['has_audio']}, "
+          f"video={stream_info['has_video']}, subs={stream_info['has_subtitle']}")
+    print(f"Audio codec: {stream_info.get('audio_codec', 'unknown')}")
+
+    if not stream_info['has_audio']:
+        raise RuntimeError("No audio stream found in input file.")
+
+    # --------------------------------------------------------------------------
+    # 2. Format decision
+    # --------------------------------------------------------------------------
+    output_format = determine_output_format(stream_info, args.format, args.transcode_to)
+    print(f"Output container: {output_format}")
+
+    validate_format_compatibility(output_format, stream_info,
+                                  args.drop_video, args.drop_subs)
+
+    # Determine file extension.
+    extension_info = FORMAT_INFO.get(output_format, {})
+    extension = extension_info.get('ext', '.mkv')
+
+    # --------------------------------------------------------------------------
+    # 3. Parse timestamps
+    # --------------------------------------------------------------------------
+    track_times = parse_track_timestamps(tracks)
+    resolved_times = resolve_end_times(track_times, total_duration)
+
+    # --------------------------------------------------------------------------
+    # 4. Fetch original metadata (for fallbacks)
+    # --------------------------------------------------------------------------
+    input_metadata = get_metadata(input_file)
+    original_album = input_metadata.get('album')
+    original_title = input_metadata.get('title')
+    original_comments = input_metadata.get('comments', [])
+
+    if original_album:
+        print(f"Original album: '{original_album}'")
+    if original_title:
+        print(f"Original title: '{original_title}'")
+    if original_comments:
+        print(f"Found {len(original_comments)} comment(s) in input file.")
+        for idx, comment in original_comments:
+            print(f"  Stream {idx}: '{comment}'")
+
+    # --------------------------------------------------------------------------
+    # 5. Process each track
+    # --------------------------------------------------------------------------
+    for idx, track in enumerate(tracks, start=1):
+        start_seconds, end_seconds = resolved_times[idx - 1]
+        duration_seconds = end_seconds - start_seconds
+
+        if duration_seconds <= 0:
+            print(f"Warning: Track {idx} has zero or negative duration, skipping.")
+            continue
+
+        track_name = track.get('tn', 'Unknown')
+
+        # ----------------------------------------------------------------------
+        # 5a. Build filename
+        # ----------------------------------------------------------------------
+        clean_filename = build_filename(track, idx, extension, args)
+        output_path = os.path.join(output_directory, clean_filename)
+
+        # ----------------------------------------------------------------------
+        # 5b. Handle existing files
+        # ----------------------------------------------------------------------
+        if args.skip_existing and os.path.exists(output_path):
+            print(f"Skipping track {idx}: {output_path} already exists.")
+            continue
+
+        # ----------------------------------------------------------------------
+        # 5c. Build metadata
+        # ----------------------------------------------------------------------
+        metadata = build_metadata_dict(track, idx, input_metadata, args)
+
+        # ----------------------------------------------------------------------
+        # 5d. Build and execute FFmpeg command
+        # ----------------------------------------------------------------------
+        cmd = build_ffmpeg_command(
+            input_file, start_seconds, duration_seconds, output_path,
+            stream_info, output_format, args.transcode_to,
+            args.drop_video, args.drop_subs,
+            metadata=metadata
+        )
+
+        print(f"Extracting track {idx}: {track_name} "
+              f"({format_time(start_seconds)} - {format_time(end_seconds)})")
+
+        result = subprocess.run(cmd, capture_output=True, text=True, check=False)
+
+        if result.returncode != 0:
+            print(f"ERROR extracting track {idx}:")
+            print(result.stderr)
+        else:
+            print(f"  -> Saved to: {output_path}")
diff --git a/ffmpeg.py b/ffmpeg.py
new file mode 100644
index 0000000..653decc
--- /dev/null
+++ b/ffmpeg.py
@@ -0,0 +1,223 @@
+"""FFmpeg / FFprobe interactions and command building."""
+
+import subprocess
+import json
+
+from .constants import FORMAT_INFO
+from .utils import format_time
+
+
+def get_audio_duration(input_file: str) -> float:
+    """
+    Retrieve the total duration (in seconds) of the input file using ffprobe.
+
+    Args:
+        input_file: Path to the media file.
+
+    Returns:
+        Duration in seconds as a float.
+    """
+    cmd = [
+        'ffprobe', '-v', 'error',
+        '-show_entries', 'format=duration',
+        '-of', 'default=noprint_wrappers=1:nokey=1',
+        input_file
+    ]
+    result = subprocess.run(cmd, capture_output=True, text=True, check=False)
+    return float(result.stdout.strip())
+
+
+def has_stream_type(input_file: str, stream_type: str) -> bool:
+    """
+    Check whether the input file contains a stream of the given type.
+
+    Args:
+        input_file: Path to the media file.
+        stream_type: 'a' for audio, 'v' for video, 's' for subtitle.
+
+    Returns:
+        True if at least one such stream exists, False otherwise.
+    """
+    cmd = [
+        'ffprobe', '-v', 'error',
+        '-select_streams', stream_type,
+        '-show_entries', 'stream=codec_type',
+        '-of', 'default=noprint_wrappers=1:nokey=1',
+        input_file
+    ]
+    result = subprocess.run(cmd, capture_output=True, text=True, check=False)
+    return bool(result.stdout.strip())
+
+
+def get_audio_codec(input_file: str) -> str:
+    """
+    Return the codec name of the first audio stream.
+
+    Args:
+        input_file: Path to the media file.
+
+    Returns:
+        Codec name as a lowercase string, or None if no audio stream exists.
+    """
+    cmd = [
+        'ffprobe', '-v', 'error',
+        '-select_streams', 'a:0',
+        '-show_entries', 'stream=codec_name',
+        '-of', 'default=noprint_wrappers=1:nokey=1',
+        input_file
+    ]
+    result = subprocess.run(cmd, capture_output=True, text=True, check=False)
+    codec = result.stdout.strip().lower()
+    return codec if codec else None
+
+
+def get_stream_info(input_file: str):
+    """
+    Collect information about the streams present in the input file.
+
+    Args:
+        input_file: Path to the media file.
+
+    Returns:
+        A dictionary with keys: has_audio, has_video, has_subtitle, audio_codec.
+    """
+    return {
+        'has_audio': has_stream_type(input_file, 'a'),
+        'has_video': has_stream_type(input_file, 'v'),
+        'has_subtitle': has_stream_type(input_file, 's'),
+        'audio_codec': get_audio_codec(input_file)
+    }
+
+def build_ffmpeg_command(input_file, start_seconds, duration_seconds, output_path,
+                         stream_info, format_opt, transcode_audio,
+                         drop_video, drop_subs, metadata=None):
+    """
+    Construct the FFmpeg command line as a list of arguments.
+
+    Args:
+        input_file: Path to the input media file.
+        start_seconds: Start time for the segment (in seconds).
+        duration_seconds: Duration of the segment (in seconds).
+        output_path: Destination path for the output file.
+        stream_info: Dictionary from get_stream_info().
+        format_opt: Output container format (e.g., 'mp3').
+        transcode_audio: Audio codec to transcode to (or None).
+        drop_video: True to remove video streams.
+        drop_subs: True to remove subtitle streams.
+        metadata: Optional dict of metadata key/value pairs to write.
+
+    Returns:
+        A list of command‑line arguments suitable for subprocess.run().
+    """
+    cmd = [
+        'ffmpeg',
+        '-i', input_file,
+        '-ss', format_time(start_seconds),
+        '-t', format_time(duration_seconds)
+    ]
+
+    # -------------------- Clear all original metadata --------------------
+    cmd.append('-map_metadata')
+    cmd.append('-1')
+
+    # -------------------- Apply custom metadata --------------------
+    if metadata:
+        for key, value in metadata.items():
+            if value is not None and value != '':
+                cmd.extend(['-metadata', f"{key}={value}"])
+
+    # -------------------- Stream mapping --------------------
+    # Map the streams we want to keep.
+    if drop_video and drop_subs:
+        cmd.extend(['-map', '0:a:0'])
+    elif drop_video:
+        cmd.extend(['-map', '0:a:0', '-map', '0:s?'])
+    elif drop_subs:
+        cmd.extend(['-map', '0:a:0', '-map', '0:v:0'])
+    else:
+        cmd.extend(['-map', '0'])
+
+    # -------------------- Audio codec --------------------
+    if transcode_audio:
+        cmd.extend(['-c:a', transcode_audio])
+        if transcode_audio in ('libmp3lame', 'mp3'):
+            cmd.extend(['-b:a', '192k'])
+        elif transcode_audio in ('libopus', 'opus'):
+            cmd.extend(['-b:a', '128k'])
+    else:
+        cmd.extend(['-c:a', 'copy'])
+
+    # -------------------- Video codec --------------------
+    if not drop_video and stream_info['has_video']:
+        cmd.extend(['-c:v', 'copy'])
+    else:
+        cmd.append('-vn')
+
+    # -------------------- Subtitle codec --------------------
+    if not drop_subs and stream_info['has_subtitle']:
+        cmd.extend(['-c:s', 'copy'])
+    else:
+        cmd.append('-sn')
+
+    # -------------------- Output format --------------------
+    if format_opt:
+        ffmpeg_format = FORMAT_INFO.get(format_opt, {}).get('ffmpeg', format_opt)
+        cmd.extend(['-f', ffmpeg_format])
+
+    # Overwrite output if it already exists.
+    cmd.extend(['-y', output_path])
+
+    return cmd
+
+def get_metadata(input_file: str) -> dict:
+    """
+    Retrieve metadata from the input file using ffprobe with JSON output.
+    Returns a dict with:
+        - album: merged from all sources (last wins)
+        - title: merged from all sources (last wins)
+        - comments: list of (stream_index, comment) tuples
+    """
+    cmd = [
+        'ffprobe', '-v', 'quiet',
+        '-print_format', 'json',
+        '-show_entries', 'format_tags:stream_tags',
+        input_file
+    ]
+    result = subprocess.run(cmd, capture_output=True, text=True, check=False)
+
+    if result.returncode != 0:
+        return {'album': None, 'title': None, 'comments': []}
+
+    try:
+        data = json.loads(result.stdout)
+        # Collect album and title (merged, last wins).
+        album = None
+        title = None
+        comments = []  # list of (stream_index, comment)
+
+        # Format tags.
+        fmt_tags = data.get('format', {}).get('tags', {})
+        album = fmt_tags.get('album') or album
+        title = fmt_tags.get('title') or title
+        # Format does not have a stream index; we'll treat it as -1 if needed.
+
+        # Stream tags.
+        for idx, stream in enumerate(data.get('streams', [])):
+            stream_tags = stream.get('tags', {})
+            # Album and title: update if present.
+            if 'album' in stream_tags:
+                album = stream_tags['album']
+            if 'title' in stream_tags:
+                title = stream_tags['title']
+            # Comment: collect all occurrences.
+            if 'comment' in stream_tags:
+                comments.append((idx, stream_tags['comment']))
+
+        return {
+            'album': album,
+            'title': title,
+            'comments': comments
+        }
+    except (json.JSONDecodeError, KeyError):
+        return {'album': None, 'title': None, 'comments': []}
+
diff --git a/filename.py b/filename.py
new file mode 100644
index 0000000..741be7b
--- /dev/null
+++ b/filename.py
@@ -0,0 +1,53 @@
+"""Output filename generation."""
+
+import os
+import re
+
+from .utils import apply_replacement, cleanup_good_chars, apply_template
+
+
+def build_filename(track, idx, extension, args):
+    """
+    Build the complete output filename.
+
+    Args:
+        track: Dict of parsed track data.
+        idx: Track index (1‑based).
+        extension: File extension with leading dot (e.g., '.mp3').
+        args: Parsed command‑line arguments.
+
+    Returns:
+        Sanitized filename.
+    """
+    # Prepare placeholder values.
+    placeholders = {
+        'tn': track.get('tn', ''),
+        'an': track.get('an', ''),
+        'al': track.get('al', ''),
+        'date': track.get('date', ''),
+        'ext': extension.lstrip('.'),   # e.g., 'mp3'
+        'num': f"{idx:02d}"
+    }
+
+    # Apply the template.
+    raw_filename = apply_template(args.output_template, placeholders)
+
+    # If --number-tracks is used, prepend the track number.
+    if args.number_tracks:
+        raw_filename = f"{idx:02d} - {raw_filename}"
+
+    # Apply character replacement if requested.
+    if args.replace_bad_chars:
+        clean_filename = apply_replacement(raw_filename,
+                                           args.bad_chars,
+                                           args.replacement_char)
+        clean_filename = cleanup_good_chars(clean_filename, args.replacement_char)
+    else:
+        clean_filename = raw_filename
+        # Warn about unsafe characters.
+        forbidden = set(r'<>:"/\\|?*')
+        if any(c in forbidden for c in clean_filename):
+            print(f"Warning: Track {idx} filename contains unsafe characters "
+                  f"({clean_filename}) – may cause filesystem errors.")
+
+    return clean_filename
diff --git a/formats.py b/formats.py
new file mode 100644
index 0000000..26c0d3d
--- /dev/null
+++ b/formats.py
@@ -0,0 +1,55 @@
+"""Container format decision and validation."""
+
+from .constants import FORMAT_INFO
+
+
+def determine_output_format(stream_info, user_format, transcode_audio):
+    """
+    Decide which container format to use.
+
+    Args:
+        stream_info: Dict from get_stream_info().
+        user_format: User‑requested format (or None).
+        transcode_audio: Audio codec to transcode to (or None).
+
+    Returns:
+        A format name that exists in FORMAT_INFO.
+    """
+    if user_format:
+        return user_format
+
+    # If video or subtitles exist, use MKV (which supports everything).
+    if stream_info['has_video'] or stream_info['has_subtitle']:
+        return 'matroska'
+
+    # Audio‑only: choose based on the current audio codec.
+    audio_codec = stream_info.get('audio_codec', '')
+    if audio_codec == 'mp3':
+        return 'mp3'
+    else:
+        return 'mp4'  # .m4a
+
+
+def validate_format_compatibility(format_name, stream_info, drop_video, drop_subs):
+    """
+    Ensure the chosen container can accommodate the streams we intend to keep.
+
+    Raises:
+        ValueError: If the format is incompatible with the intended streams.
+    """
+    info = FORMAT_INFO.get(format_name)
+    if not info:
+        print(f"Warning: Unknown format '{format_name}'. Proceeding, but may fail.")
+        return
+
+    if info['audio_only']:
+        if stream_info['has_video'] and not drop_video:
+            raise ValueError(
+                f"Format '{format_name}' does not support video streams. "
+                "Please use --drop-video or choose a container that supports video."
+            )
+        if stream_info['has_subtitle'] and not drop_subs:
+            raise ValueError(
+                f"Format '{format_name}' does not support subtitle streams. "
+                "Please use --drop-subs or choose a container that supports subtitles."
+            )
diff --git a/main.py b/main.py
new file mode 100644
index 0000000..257cdb6
--- /dev/null
+++ b/main.py
@@ -0,0 +1,204 @@
+"""Command‑line interface and entry point."""
+
+import os
+import sys
+import subprocess
+import argparse
+
+from .constants import DEFAULT_BAD_CHARS
+from .tracklist import read_tracklist, parse_format
+from .core import split_audio
+
+
+def main():
+    """Parse arguments and start the splitting process."""
+    parser = argparse.ArgumentParser(
+        description="Split an audio file into tracks using a tracklist.",
+        epilog="Tracklist format: mm:ss track_name - author_name"
+    )
+
+    # Positional arguments.
+    parser.add_argument('input_file', help='Input audio/video file')
+    parser.add_argument('tracklist_file', help='Tracklist file')
+
+    # Optional arguments.
+    parser.add_argument(
+        '--output-dir', '-o',
+        help='Output directory for split tracks (default: <input_basename>_splits)'
+    )
+    parser.add_argument(
+        '--format',
+        help='Output container format (e.g., mp3, m4a, mkv, mp4, ogg, opus)'
+    )
+    parser.add_argument(
+        '--transcode-to',
+        help='Re-encode audio to this codec (e.g., libmp3lame, aac, libopus)'
+    )
+    parser.add_argument(
+        '--drop-video',
+        action='store_true',
+        help='Remove video streams from output'
+    )
+    parser.add_argument(
+        '--drop-subs',
+        action='store_true',
+        help='Remove subtitle streams from output'
+    )
+    parser.add_argument(
+        '--number-tracks',
+        action='store_true',
+        help='Prepend track number to output filenames (convenience; use %%num in template for full control)'
+    )
+    parser.add_argument(
+        '--replace-bad-chars',
+        action='store_true',
+        help='Replace problematic characters in filenames (default: off)'
+    )
+    parser.add_argument(
+        '--replacement-char',
+        default='_',
+        help='Character used as replacement (default: "_")'
+    )
+    parser.add_argument(
+        '--bad-chars',
+        default=DEFAULT_BAD_CHARS,
+        help='String of characters to replace (default includes space and single quote)'
+    )
+    parser.add_argument(
+        '--skip-existing',
+        action='store_true',
+        help='Skip extraction if output file already exists (default: overwrite)'
+    )
+    parser.add_argument(
+        '--tracklist-format',
+        default='%ts %tn - %an',
+        help='Format of each line in the tracklist using placeholders: '
+             '%%ts (timestamp), %%tn (track name), %%an (author), %%al (album), '
+             '%%date (date/year), %%ext (file extension). '
+             'Default: "%%ts %%tn - %%an"'
+    )
+    parser.add_argument(
+        '--output-template',
+        default='%an-%tn.%ext',
+        help='Template for output filenames using placeholders: '
+             '%%tn (track name), %%an (author), %%al (album), '
+             '%%date (date/year), %%ext (file extension), %%num (track number). '
+             'Default: "%%an-%%tn.%%ext"'
+    )
+    parser.add_argument(
+        '--dry-run',
+        action='store_true',
+        help='Parse and display the tracklist without splitting any files'
+    )
+
+    # ------------------------------------------------------------
+    # Metadata options (new)
+    # ------------------------------------------------------------
+    parser.add_argument(
+        '--album',
+        help='Set album name in output metadata (overrides parsed %%al and original album)'
+    )
+    parser.add_argument(
+        '--comment',
+        help='Explicit comment text (overrides all other comment settings)'
+    )
+    parser.add_argument(
+        '--no-comment',
+        action='store_true',
+        help='Explicitly ignore any comment (no comment written)'
+    )
+    parser.add_argument(
+        '--comment-stream',
+        type=int,
+        default=None,
+        help='Select comment from a specific stream index (0‑based). Default: first stream with a comment.'
+    )
+    parser.add_argument(
+        '--merge-comments',
+        action='store_true',
+        help='Merge all comments from all streams into one (separated by --comment-separator)'
+    )
+    parser.add_argument(
+        '--comment-separator',
+        default='; ',
+        help='Separator used when merging comments (default: "; ")'
+    )
+
+    args = parser.parse_args()
+
+    # --------------------------------------------------------------------------
+    # Input validation
+    # --------------------------------------------------------------------------
+    if not os.path.exists(args.input_file):
+        print(f"Error: Input file not found: {args.input_file}")
+        sys.exit(1)
+
+    if not os.path.exists(args.tracklist_file):
+        print(f"Error: Tracklist file not found: {args.tracklist_file}")
+        sys.exit(1)
+
+    # Ensure the replacement character is a single character.
+    if len(args.replacement_char) != 1:
+        print("Error: --replacement-char must be a single character.")
+        sys.exit(1)
+
+    # Check that FFmpeg is installed.
+    try:
+        subprocess.run(['ffmpeg', '-version'], capture_output=True, check=True)
+    except (subprocess.CalledProcessError, FileNotFoundError):
+        print("Error: FFmpeg not found. Please install FFmpeg first.")
+        print("  - https://ffmpeg.org/download.html")
+        sys.exit(1)
+
+    # Parse the tracklist using the user‑provided format.
+    try:
+        tokens = parse_format(args.tracklist_format)
+    except ValueError as error:
+        print(f"Error in --tracklist-format: {error}")
+        sys.exit(1)
+
+    # Read and parse the tracklist file.
+    tracks = read_tracklist(args.tracklist_file, tokens)
+    if not tracks:
+        print("Error: No valid tracks found in tracklist file.")
+        sys.exit(1)
+
+    print(f"Found {len(tracks)} tracks.")
+
+    # Dry‑run mode: display parsed data and exit.
+    if args.dry_run:
+        print("\nParsed tracklist:")
+        print("-" * 60)
+        if tracks:
+            fields = sorted(tracks[0].keys())
+            header = " | ".join(f"{f:>10}" for f in fields)
+            print(header)
+            print("-" * len(header))
+            for idx, track in enumerate(tracks, 1):
+                values = [f"{track.get(f, ''):>10}" for f in fields]
+                print(f"{idx:3d} | " + " | ".join(values))
+        print("-" * 60)
+        print("Dry‑run complete. No files were created.")
+        sys.exit(0)
+
+    # Determine the output directory.
+    if args.output_dir:
+        output_dir = args.output_dir
+        print(f"Using custom output directory: {output_dir}")
+    else:
+        base_name = os.path.splitext(os.path.basename(args.input_file))[0]
+        output_dir = base_name + "_splits"
+        print(f"Using default output directory: {output_dir}")
+
+    # Run the splitter.
+    try:
+        split_audio(args.input_file, output_dir, tracks, args)
+    except (RuntimeError, ValueError) as error:
+        print(f"Error: {error}")
+        sys.exit(1)
+
+    print("Done!")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/metadata.py b/metadata.py
new file mode 100644
index 0000000..2da7fe6
--- /dev/null
+++ b/metadata.py
@@ -0,0 +1,99 @@
+"""Metadata selection and building."""
+
+
+def select_album_value(track, input_metadata, args):
+    """
+    Determine the album value according to precedence.
+
+    Precedence: --album > %al > original album > original title > None.
+
+    Args:
+        track: Dict of parsed track data.
+        input_metadata: Dict from get_metadata().
+        args: Parsed command‑line arguments.
+
+    Returns:
+        Album string or None.
+    """
+    original_album = input_metadata.get('album')
+    original_title = input_metadata.get('title')
+
+    if args.album:
+        return args.album
+    elif track.get('al'):
+        return track.get('al')
+    elif original_album:
+        return original_album
+    elif original_title:
+        return original_title
+    else:
+        return None
+
+
+def select_comment_value(input_metadata, args):
+    """
+    Determine the comment value according to precedence.
+
+    Precedence: --no-comment > --comment > --merge-comments >
+    --comment-stream > first comment from file.
+
+    Args:
+        input_metadata: Dict from get_metadata().
+        args: Parsed command‑line arguments.
+
+    Returns:
+        Comment string or None.
+    """
+    original_comments = input_metadata.get('comments', [])
+
+    if args.no_comment:
+        return None
+    elif args.comment:
+        return args.comment
+    elif args.merge_comments and original_comments:
+        separator = args.comment_separator if args.comment_separator else '; '
+        comments_text = separator.join(comment for _, comment in original_comments)
+        return comments_text if comments_text else None
+    elif args.comment_stream is not None:
+        if 0 <= args.comment_stream < len(original_comments):
+            return original_comments[args.comment_stream][1]
+        else:
+            print(f"Warning: Stream {args.comment_stream} not found in comments list. "
+                  f"Using first comment (if any).")
+            if original_comments:
+                return original_comments[0][1]
+            return None
+    elif original_comments:
+        # Default: use the first comment.
+        return original_comments[0][1]
+    else:
+        return None
+
+
+def build_metadata_dict(track, idx, input_metadata, args):
+    """
+    Assemble the final metadata dictionary for FFmpeg.
+
+    Args:
+        track: Dict of parsed track data.
+        idx: Track index (1‑based).
+        input_metadata: Dict from get_metadata().
+        args: Parsed command‑line arguments.
+
+    Returns:
+        Dict with metadata fields (empty values removed).
+    """
+    album_value = select_album_value(track, input_metadata, args)
+    comment_value = select_comment_value(input_metadata, args)
+
+    metadata = {
+        'title': track.get('tn', ''),
+        'artist': track.get('an', ''),
+        'album': album_value,
+        'date': track.get('date', ''),
+        'track': f"{idx:02d}",
+        'comment': comment_value
+    }
+
+    # Remove empty fields.
+    return {k: v for k, v in metadata.items() if v}
diff --git a/timestamp.py b/timestamp.py
new file mode 100644
index 0000000..aaab1c5
--- /dev/null
+++ b/timestamp.py
@@ -0,0 +1,76 @@
+"""Timestamp parsing and resolution."""
+
+from .utils import parse_timestamp
+
+
+def parse_track_timestamps(tracks):
+    """
+    Parse timestamps from each track.
+
+    Args:
+        tracks: List of dicts from tracklist parsing.
+
+    Returns:
+        List of (start_seconds, end_seconds_or_None) for each track.
+
+    Raises:
+        ValueError: If timestamp is invalid or range is malformed.
+    """
+    track_times = []
+    for idx, track in enumerate(tracks, 1):
+        ts_str = track.get('ts', '').strip()
+        if not ts_str:
+            raise ValueError(f"Track {idx}: timestamp is empty")
+
+        # Detect range: contains a dash.
+        if '-' in ts_str:
+            parts = ts_str.split('-', 1)
+            start_str = parts[0].strip()
+            end_str = parts[1].strip()
+            if not start_str or not end_str:
+                raise ValueError(f"Track {idx}: invalid range format '{ts_str}'. Expected 'start-end'.")
+            try:
+                start_sec = parse_timestamp(start_str)
+                end_sec = parse_timestamp(end_str)
+            except ValueError as e:
+                raise ValueError(f"Track {idx}: invalid timestamp in range '{ts_str}': {e}")
+            if end_sec <= start_sec:
+                raise ValueError(f"Track {idx}: end time ({end_str}) must be after start time ({start_str})")
+            track_times.append((start_sec, end_sec))
+        else:
+            # Start-only: we will compute end later.
+            try:
+                start_sec = parse_timestamp(ts_str)
+            except ValueError as e:
+                raise ValueError(f"Track {idx}: invalid timestamp '{ts_str}': {e}")
+            track_times.append((start_sec, None))
+
+    return track_times
+
+
+def resolve_end_times(track_times, total_duration):
+    """
+    Resolve None ends to next start or total duration.
+
+    Args:
+        track_times: List of (start_sec, end_sec_or_None).
+        total_duration: Total duration of the input file.
+
+    Returns:
+        List of (start_sec, end_sec) with all ends resolved.
+
+    Raises:
+        ValueError: If end time is not after start time.
+    """
+    resolved = []
+    for idx, (start_sec, end_sec) in enumerate(track_times):
+        if end_sec is None:
+            if idx + 1 < len(track_times):
+                next_start = track_times[idx + 1][0]
+                end_sec = next_start
+            else:
+                end_sec = int(total_duration)
+            if end_sec <= start_sec:
+                raise ValueError(f"Track {idx+1}: end time ({end_sec}) is not after start ({start_sec})")
+        resolved.append((start_sec, end_sec))
+    return resolved
diff --git a/tracklist.py b/tracklist.py
new file mode 100644
index 0000000..cc6811e
--- /dev/null
+++ b/tracklist.py
@@ -0,0 +1,155 @@
+"""Tracklist file parsing with flexible format support."""
+
+import re
+
+
+def parse_format(format_str: str):
+    """
+    Convert a format string (e.g., "%ts %tn - %an") into a list of tokens.
+
+    Each token is a dict with keys:
+        'type': 'literal' or 'placeholder'
+        'value': the literal text or the placeholder name (e.g., 'tn')
+
+    Placeholders are identified by a '%' followed by alphabetic characters.
+    A literal '%%' is escaped to a single '%'.
+
+    Args:
+        format_str: The user‑provided format string.
+
+    Returns:
+        List of token dicts.
+
+    Raises:
+        ValueError: If the format is invalid (e.g., unknown placeholder).
+    """
+    # Known placeholders.
+    valid_placeholders = {'ts', 'tn', 'an', 'al', 'date', 'ext'}
+
+    tokens = []
+    i = 0
+    while i < len(format_str):
+        ch = format_str[i]
+        if ch == '%':
+            # Check for escaped '%%'.
+            if i + 1 < len(format_str) and format_str[i + 1] == '%':
+                tokens.append({'type': 'literal', 'value': '%'})
+                i += 2
+                continue
+            # Must be a placeholder.
+            # Match '%' followed by letters.
+            match = re.match(r'%([a-zA-Z]+)', format_str[i:])
+            if not match:
+                raise ValueError(f"Invalid placeholder at position {i}: '{format_str[i:]}'")
+            placeholder = match.group(1)
+            if placeholder not in valid_placeholders:
+                raise ValueError(f"Unknown placeholder '%{placeholder}'. "
+                                 f"Allowed: {', '.join(valid_placeholders)}")
+            tokens.append({'type': 'placeholder', 'value': placeholder})
+            i += len(match.group(0))
+        else:
+            # Literal character.
+            # Collect consecutive non‑'%' characters.
+            j = i
+            while j < len(format_str) and format_str[j] != '%':
+                j += 1
+            tokens.append({'type': 'literal', 'value': format_str[i:j]})
+            i = j
+    return tokens
+
+
+def parse_line(line: str, tokens):
+    """
+    Parse a single line of the tracklist using the token list.
+
+    Returns a dict mapping placeholder names to extracted strings.
+    If a placeholder is not found, its value is None.
+
+    Args:
+        line: The raw line from the tracklist file.
+        tokens: The token list from parse_format().
+
+    Returns:
+        A dict with keys for each placeholder present in the format.
+
+    Raises:
+        ValueError: If the line cannot be parsed according to the format.
+    """
+    # Strip leading/trailing whitespace but preserve internal spaces.
+    line = line.strip()
+    if not line:
+        raise ValueError("Empty line")
+
+    result = {}
+    pos = 0
+    # We need to match the tokens in order.
+    for token in tokens:
+        if token['type'] == 'literal':
+            literal = token['value']
+            if not line.startswith(literal, pos):
+                raise ValueError(f"Expected literal '{literal}' at position {pos}, got '{line[pos:]}'")
+            pos += len(literal)
+        else:  # placeholder
+            placeholder = token['value']
+            # If this is the last token, capture the rest of the line.
+            if token is tokens[-1]:
+                result[placeholder] = line[pos:].strip() or None
+                pos = len(line)
+            else:
+                # Find the next literal to use as a delimiter.
+                # We need to look ahead to the next literal token.
+                next_literal = None
+                for next_token in tokens[tokens.index(token) + 1:]:
+                    if next_token['type'] == 'literal':
+                        next_literal = next_token['value']
+                        break
+                if next_literal is None:
+                    # If no more literals, capture the rest.
+                    result[placeholder] = line[pos:].strip() or None
+                    pos = len(line)
+                else:
+                    # Find the occurrence of the next literal in the line starting from pos.
+                    next_pos = line.find(next_literal, pos)
+                    if next_pos == -1:
+                        raise ValueError(f"Could not find literal '{next_literal}' after placeholder '{placeholder}'")
+                    result[placeholder] = line[pos:next_pos].strip() or None
+                    pos = next_pos
+    return result
+
+
+def read_tracklist(filepath: str, tokens):
+    """
+    Read and parse the tracklist file using the provided token list.
+
+    Each line is parsed into a dict of fields. The timestamp field ('ts')
+    is required; if missing, an error is raised.
+
+    Args:
+        filepath: Path to the tracklist file.
+        tokens: Token list from parse_format().
+
+    Returns:
+        A list of dicts, one per track.
+
+    Raises:
+        ValueError: If a line cannot be parsed or the timestamp is missing.
+    """
+    tracks = []
+    with open(filepath, 'r', encoding='utf-8') as file:
+        for line_num, raw_line in enumerate(file, 1):
+            line = raw_line.strip()
+            if not line:
+                continue
+            try:
+                parsed = parse_line(line, tokens)
+            except ValueError as error:
+                print(f"Warning: Skipping line {line_num}: {error}")
+                continue
+
+            # Ensure timestamp is present.
+            if 'ts' not in parsed or parsed['ts'] is None:
+                raise ValueError(f"Line {line_num}: timestamp (%ts) is missing or empty")
+
+            tracks.append(parsed)
+
+    return tracks
diff --git a/utils.py b/utils.py
new file mode 100644
index 0000000..1cd097d
--- /dev/null
+++ b/utils.py
@@ -0,0 +1,171 @@
+"""General‑purpose helper functions."""
+
+import re
+
+
+def parse_timestamp(timestamp: str) -> int:
+    """
+    Convert a timestamp in 'mm:ss' or 'HH:MM:SS' format to total seconds.
+
+    Args:
+        timestamp: String in the format 'mm:ss' or 'HH:MM:SS'.
+
+    Returns:
+        Total number of seconds.
+
+    Raises:
+        ValueError: If the format is unrecognized.
+    """
+    parts = timestamp.strip().split(':')
+    if len(parts) == 2:
+        minutes, seconds = map(int, parts)
+        return minutes * 60 + seconds
+    elif len(parts) == 3:
+        hours, minutes, seconds = map(int, parts)
+        return hours * 3600 + minutes * 60 + seconds
+    else:
+        raise ValueError(f"Invalid timestamp format: {timestamp}")
+
+
+def format_time(seconds: int) -> str:
+    """
+    Convert seconds to 'HH:MM:SS' format for FFmpeg.
+
+    Args:
+        seconds: Total number of seconds.
+
+    Returns:
+        Time string in 'HH:MM:SS' format.
+    """
+    hours = seconds // 3600
+    minutes = (seconds % 3600) // 60
+    secs = seconds % 60
+    return f"{hours:02d}:{minutes:02d}:{secs:02d}"
+
+
+def sanitize_filename(name: str) -> str:
+    """
+    Remove characters that are problematic on common filesystems.
+
+    This is a fallback sanitizer; the main replacement is handled by
+    apply_replacement() when --replace-bad-chars is used.
+
+    Args:
+        name: Original filename candidate.
+
+    Returns:
+        Sanitized string with dangerous characters replaced by '_'.
+    """
+    # Replace characters that are forbidden on Windows/Linux/macOS.
+    return re.sub(r'[<>:"/\\|?*]', '_', name).strip()
+
+
+def apply_replacement(name: str, bad_chars: str, replacement_char: str) -> str:
+    """
+    Replace every occurrence of any character in bad_chars with replacement_char.
+
+    Args:
+        name: The original string.
+        bad_chars: String containing all characters to replace.
+        replacement_char: The character to insert.
+
+    Returns:
+        The transformed string.
+    """
+    if not bad_chars:
+        return name
+    # Build a translation table mapping each bad character to the replacement.
+    translation_table = str.maketrans({c: replacement_char for c in set(bad_chars)})
+    return name.translate(translation_table)
+
+
+def cleanup_good_chars(name: str, good_char: str) -> str:
+    """
+    Clean up the filename after replacement to avoid awkward sequences.
+
+    Steps performed:
+        1. Strip leading/trailing good_char characters.
+        2. Collapse consecutive good_char characters into a single one.
+        3. Remove good_char characters that are adjacent to a single non‑good_char
+           (i.e., pattern good_char + X + good_char becomes just X),
+           and repeat this step until no more such patterns exist.
+        4. After each iteration, re‑apply stripping and collapsing.
+
+    This makes filenames more human‑readable, e.g.:
+        "author_-_song" -> "author-song"
+        "hello__world" -> "hello_world"
+        "_hello_" -> "hello"
+
+    Args:
+        name: The string to clean (after replacement).
+        good_char: The character used as replacement.
+
+    Returns:
+        The cleaned string.
+    """
+    # Escape the good_char for regex usage.
+    escaped = re.escape(good_char)
+
+    # Helper to collapse consecutive good chars and strip edges.
+    def collapse_and_strip(s: str) -> str:
+        # Collapse multiple consecutive good chars into one.
+        s = re.sub(rf'{escaped}+', good_char, s)
+        # Remove leading/trailing good chars.
+        s = s.strip(good_char)
+        return s
+
+    # Apply stripping and collapsing initially.
+    name = collapse_and_strip(name)
+
+    # Remove patterns: good_char + X + good_char (where X is any char != good_char).
+    # Repeat until no further changes.
+    while True:
+        # Replace pattern: good_char (non-good-char) good_char -> just the non-good-char.
+        # The negative lookahead ensures X is not the good_char.
+        pattern = rf'{escaped}([^{escaped}]){escaped}'
+        new_name = re.sub(pattern, r'\1', name)
+        if new_name == name:
+            break
+        name = new_name
+        # Re‑apply stripping and collapsing after removal.
+        name = collapse_and_strip(name)
+
+    return name
+
+def apply_template(template: str, placeholders: dict) -> str:
+    """
+    Replace placeholders in a template string with values from a dict.
+
+    Placeholders are of the form %key (e.g., %tn, %an).
+    Use %% to escape a literal percent sign.
+
+    Args:
+        template: The template string.
+        placeholders: Dict mapping placeholder names to values.
+
+    Returns:
+        The rendered string.
+    """
+    import re
+    result = []
+    i = 0
+    while i < len(template):
+        ch = template[i]
+        if ch == '%':
+            if i + 1 < len(template) and template[i + 1] == '%':
+                result.append('%')
+                i += 2
+                continue
+            match = re.match(r'%([a-zA-Z]+)', template[i:])
+            if match:
+                key = match.group(1)
+                value = placeholders.get(key, '')
+                result.append(value)
+                i += len(match.group(0))
+            else:
+                result.append(ch)
+                i += 1
+        else:
+            result.append(ch)
+            i += 1
+    return ''.join(result)