LibVideo: Read Matroska lazily so that large files can start quickly

The Demuxer class was changed to return errors for more functions so
that all of the underlying reading can be done lazily. Other than that,
the demuxer interface is unchanged, and only the underlying reader was
modified.

The MatroskaDocument class is no more, and MatroskaReader's getter
functions replace it. Every MatroskaReader getter beyond the Segment
element's position is parsed lazily from the file as needed. This means
that all getter functions can return DecoderErrors which must be
handled by callers.

Userland/Libraries/LibVideo/Containers/Matroska/Reader.h

@@ -7,17 +7,101 @@
 
 #pragma once
 
+#include <AK/Concepts.h>
 #include <AK/Debug.h>
 #include <AK/IntegralMath.h>
 #include <AK/NonnullOwnPtrVector.h>
 #include <AK/Optional.h>
 #include <AK/OwnPtr.h>
+#include <LibCore/MappedFile.h>
 #include <LibVideo/DecoderError.h>
 
 #include "Document.h"
 
 namespace Video::Matroska {
 
+class SampleIterator;
+class Streamer;
+
+class Reader {
+public:
+    typedef Function<DecoderErrorOr<IterationDecision>(TrackEntry const&)> TrackEntryCallback;
+
+    static DecoderErrorOr<Reader> from_file(StringView path);
+    static DecoderErrorOr<Reader> from_data(ReadonlyBytes data);
+
+    EBMLHeader const& header() const { return m_header.value(); }
+
+    DecoderErrorOr<SegmentInformation> segment_information();
+
+    DecoderErrorOr<void> for_each_track(TrackEntryCallback);
+    DecoderErrorOr<void> for_each_track_of_type(TrackEntry::TrackType, TrackEntryCallback);
+    DecoderErrorOr<TrackEntry> track_for_track_number(u64);
+    DecoderErrorOr<size_t> track_count();
+
+    DecoderErrorOr<SampleIterator> create_sample_iterator(u64 track_number);
+    DecoderErrorOr<void> seek_to_random_access_point(SampleIterator&, Time);
+
+private:
+    Reader(ReadonlyBytes data)
+        : m_data(data)
+    {
+    }
+
+    DecoderErrorOr<void> parse_initial_data();
+
+    DecoderErrorOr<Optional<size_t>> find_first_top_level_element_with_id([[maybe_unused]] StringView element_name, u32 element_id);
+
+    DecoderErrorOr<void> ensure_tracks_are_parsed();
+    DecoderErrorOr<void> parse_tracks(Streamer&);
+
+    RefPtr<Core::MappedFile> m_mapped_file;
+    ReadonlyBytes m_data;
+
+    Optional<EBMLHeader> m_header;
+
+    size_t m_segment_contents_position { 0 };
+    size_t m_segment_contents_size { 0 };
+
+    HashMap<u32, size_t> m_seek_entries;
+    size_t m_last_top_level_element_position { 0 };
+
+    Optional<SegmentInformation> m_segment_information;
+
+    OrderedHashMap<u64, TrackEntry> m_tracks;
+};
+
+class SampleIterator {
+public:
+    DecoderErrorOr<Block> next_block();
+    Cluster const& current_cluster() { return *m_current_cluster; }
+
+private:
+    friend class Reader;
+
+    SampleIterator(RefPtr<Core::MappedFile> file, ReadonlyBytes data, u64 track_id, size_t position, u64 timestamp_scale)
+        : m_file(move(file))
+        , m_data(data)
+        , m_track_id(track_id)
+        , m_position(position)
+        , m_timestamp_scale(timestamp_scale)
+    {
+    }
+
+    DecoderErrorOr<void> set_position(size_t position);
+
+    RefPtr<Core::MappedFile> m_file;
+    ReadonlyBytes m_data;
+    u64 m_track_id;
+
+    // Must always point to an element ID or the end of the stream.
+    size_t m_position { 0 };
+
+    u64 m_timestamp_scale { 0 };
+
+    Optional<Cluster> m_current_cluster;
+};
+
 class Streamer {
 public:
     Streamer(ReadonlyBytes data)
@@ -54,12 +138,15 @@ public:
 
     ErrorOr<void> read_unknown_element();
 
-    ErrorOr<void> drop_octets(size_t num_octets);
+    ErrorOr<ReadonlyBytes> read_raw_octets(size_t num_octets);
+
+    size_t position() const { return m_position; }
+    size_t remaining() const { return m_data.size() - position(); }
 
     bool at_end() const { return remaining() == 0; }
     bool has_octet() const { return remaining() >= 1; }
 
-    size_t remaining() const { return m_data.size() - m_position; }
+    ErrorOr<void> seek_to_position(size_t position);
 
 private:
     ReadonlyBytes m_data;
@@ -67,20 +154,4 @@ private:
     Vector<size_t> m_octets_read { 0 };
 };
 
-class Reader {
-public:
-    Reader(ReadonlyBytes data)
-        : m_streamer(data)
-    {
-    }
-
-    static DecoderErrorOr<NonnullOwnPtr<MatroskaDocument>> parse_matroska_from_file(StringView path);
-    static DecoderErrorOr<NonnullOwnPtr<MatroskaDocument>> parse_matroska_from_data(ReadonlyBytes data);
-
-    DecoderErrorOr<NonnullOwnPtr<MatroskaDocument>> parse();
-
-private:
-    Streamer m_streamer;
-};
-
 }