    const std = @import("std");
    const assert = std.debug.assert;
    const RingBuffer = std.RingBuffer;

    const types = @import("../types.zig");
    const frame = types.frame;
    const Table = types.compressed_block.Table;
    const LiteralsSection = types.compressed_block.LiteralsSection;
    const SequencesSection = types.compressed_block.SequencesSection;

    const huffman = @import("huffman.zig");
    const readers = @import("../readers.zig");

    const decodeFseTable = @import("fse.zig").decodeFseTable;

    const readInt = std.mem.readIntLittle;

    pub const Error = error{
        BlockSizeOverMaximum,
        MalformedBlockSize,
        ReservedBlock,
        MalformedRleBlock,
        MalformedCompressedBlock,
    };

    pub const DecodeState = struct {
        repeat_offsets: [3]u32,

        offset: StateData(8),
        match: StateData(9),
        literal: StateData(9),

        offset_fse_buffer: []Table.Fse,
        match_fse_buffer: []Table.Fse,
        literal_fse_buffer: []Table.Fse,

        fse_tables_undefined: bool,

        literal_stream_reader: readers.ReverseBitReader,
        literal_stream_index: usize,
        literal_streams: LiteralsSection.Streams,
        literal_header: LiteralsSection.Header,
        huffman_tree: ?LiteralsSection.HuffmanTree,

        literal_written_count: usize,
        written_count: usize = 0,

        fn StateData(comptime max_accuracy_log: comptime_int) type {
            return struct {
                state: State,
                table: Table,
                accuracy_log: u8,

                const State = std.meta.Int(.unsigned, max_accuracy_log);
            };
        }

        pub fn init(
            literal_fse_buffer: []Table.Fse,
            match_fse_buffer: []Table.Fse,
            offset_fse_buffer: []Table.Fse,
        ) DecodeState {
            return DecodeState{
                .repeat_offsets = .{
                    types.compressed_block.start_repeated_offset_1,
                    types.compressed_block.start_repeated_offset_2,
                    types.compressed_block.start_repeated_offset_3,
                },

                .offset = undefined,
                .match = undefined,
                .literal = undefined,

                .literal_fse_buffer = literal_fse_buffer,
                .match_fse_buffer = match_fse_buffer,
                .offset_fse_buffer = offset_fse_buffer,

                .fse_tables_undefined = true,

                .literal_written_count = 0,
                .literal_header = undefined,
                .literal_streams = undefined,
                .literal_stream_reader = undefined,
                .literal_stream_index = undefined,
                .huffman_tree = null,

                .written_count = 0,
            };
        }

        /// Prepare the decoder to decode a compressed block. Loads the literals
        /// stream and Huffman tree from `literals` and reads the FSE tables from
        /// `source`.
        ///
        /// Errors returned:
        ///   - `error.BitStreamHasNoStartBit` if the (reversed) literal bitstream's
        ///     first byte does not have any bits set
        ///   - `error.TreelessLiteralsFirst` `literals` is a treeless literals
        ///     section and the decode state does not have a Huffman tree from a
        ///     previous block
        ///   - `error.RepeatModeFirst` on the first call if one of the sequence FSE
        ///     tables is set to repeat mode
        ///   - `error.MalformedAccuracyLog` if an FSE table has an invalid accuracy
        ///   - `error.MalformedFseTable` if there are errors decoding an FSE table
        ///   - `error.EndOfStream` if `source` ends before all FSE tables are read
        pub fn prepare(
            self: *DecodeState,
            source: anytype,
            literals: LiteralsSection,
            sequences_header: SequencesSection.Header,
        ) !void {
            self.literal_written_count = 0;
            self.literal_header = literals.header;
            self.literal_streams = literals.streams;

            if (literals.huffman_tree) |tree| {
                self.huffman_tree = tree;
            } else if (literals.header.block_type == .treeless and self.huffman_tree == null) {
                return error.TreelessLiteralsFirst;
            }

            switch (literals.header.block_type) {
                .raw, .rle => {},
                .compressed, .treeless => {
                    self.literal_stream_index = 0;
                    switch (literals.streams) {
                        .one => |slice| try self.initLiteralStream(slice),
                        .four => |streams| try self.initLiteralStream(streams[0]),
                    }
                },
            }

            if (sequences_header.sequence_count > 0) {
                try self.updateFseTable(source, .literal, sequences_header.literal_lengths);
                try self.updateFseTable(source, .offset, sequences_header.offsets);
                try self.updateFseTable(source, .match, sequences_header.match_lengths);
                self.fse_tables_undefined = false;
            }
        }

        /// Read initial FSE states for sequence decoding.
        ///
        /// Errors returned:
        ///   - `error.EndOfStream` if `bit_reader` does not contain enough bits.
        pub fn readInitialFseState(self: *DecodeState, bit_reader: *readers.ReverseBitReader) error{EndOfStream}!void {
            self.literal.state = try bit_reader.readBitsNoEof(u9, self.literal.accuracy_log);
            self.offset.state = try bit_reader.readBitsNoEof(u8, self.offset.accuracy_log);
            self.match.state = try bit_reader.readBitsNoEof(u9, self.match.accuracy_log);
        }

        fn updateRepeatOffset(self: *DecodeState, offset: u32) void {
            self.repeat_offsets[2] = self.repeat_offsets[1];
            self.repeat_offsets[1] = self.repeat_offsets[0];
            self.repeat_offsets[0] = offset;
        }

        fn useRepeatOffset(self: *DecodeState, index: usize) u32 {
            if (index == 1)
                std.mem.swap(u32, &self.repeat_offsets[0], &self.repeat_offsets[1])
            else if (index == 2) {
                std.mem.swap(u32, &self.repeat_offsets[0], &self.repeat_offsets[2]);
                std.mem.swap(u32, &self.repeat_offsets[1], &self.repeat_offsets[2]);
            }
            return self.repeat_offsets[0];
        }

        const DataType = enum { offset, match, literal };

        fn updateState(
            self: *DecodeState,
            comptime choice: DataType,
            bit_reader: *readers.ReverseBitReader,
        ) error{ MalformedFseBits, EndOfStream }!void {
            switch (@field(self, @tagName(choice)).table) {
                .rle => {},
                .fse => |table| {
                    const data = table[@field(self, @tagName(choice)).state];
                    const T = @TypeOf(@field(self, @tagName(choice))).State;
                    const bits_summand = try bit_reader.readBitsNoEof(T, data.bits);
                    const next_state = std.math.cast(
                        @TypeOf(@field(self, @tagName(choice))).State,
                        data.baseline + bits_summand,
                    ) orelse return error.MalformedFseBits;
                    @field(self, @tagName(choice)).state = next_state;
                },
            }
        }

        const FseTableError = error{
            MalformedFseTable,
            MalformedAccuracyLog,
            RepeatModeFirst,
            EndOfStream,
        };

        fn updateFseTable(
            self: *DecodeState,
            source: anytype,
            comptime choice: DataType,
            mode: SequencesSection.Header.Mode,
        ) !void {
            const field_name = @tagName(choice);
            switch (mode) {
                .predefined => {
                    @field(self, field_name).accuracy_log =
                        @field(types.compressed_block.default_accuracy_log, field_name);

                    @field(self, field_name).table =
                        @field(types.compressed_block, "predefined_" ++ field_name ++ "_fse_table");
                },
                .rle => {
                    @field(self, field_name).accuracy_log = 0;
                    @field(self, field_name).table = .{ .rle = try source.readByte() };
                },
                .fse => {
                    var bit_reader = readers.bitReader(source);

                    const table_size = try decodeFseTable(
                        &bit_reader,
                        @field(types.compressed_block.table_symbol_count_max, field_name),
                        @field(types.compressed_block.table_accuracy_log_max, field_name),
                        @field(self, field_name ++ "_fse_buffer"),
                    );
                    @field(self, field_name).table = .{
                        .fse = @field(self, field_name ++ "_fse_buffer")[0..table_size],
                    };
                    @field(self, field_name).accuracy_log = std.math.log2_int_ceil(usize, table_size);
                },
                .repeat => if (self.fse_tables_undefined) return error.RepeatModeFirst,
            }
        }

        const Sequence = struct {
            literal_length: u32,
            match_length: u32,
            offset: u32,
        };

        fn nextSequence(
            self: *DecodeState,
            bit_reader: *readers.ReverseBitReader,
        ) error{ InvalidBitStream, EndOfStream }!Sequence {
            const raw_code = self.getCode(.offset);
            const offset_code = std.math.cast(u5, raw_code) orelse {
                return error.InvalidBitStream;
            };
            const offset_value = (@as(u32, 1) << offset_code) + try bit_reader.readBitsNoEof(u32, offset_code);

            const match_code = self.getCode(.match);
            if (match_code >= types.compressed_block.match_length_code_table.len)
                return error.InvalidBitStream;
            const match = types.compressed_block.match_length_code_table[match_code];
            const match_length = match[0] + try bit_reader.readBitsNoEof(u32, match[1]);

            const literal_code = self.getCode(.literal);
            if (literal_code >= types.compressed_block.literals_length_code_table.len)
                return error.InvalidBitStream;
            const literal = types.compressed_block.literals_length_code_table[literal_code];
            const literal_length = literal[0] + try bit_reader.readBitsNoEof(u32, literal[1]);

            const offset = if (offset_value > 3) offset: {
                const offset = offset_value - 3;
                self.updateRepeatOffset(offset);
                break :offset offset;
            } else offset: {
                if (literal_length == 0) {
                    if (offset_value == 3) {
                        const offset = self.repeat_offsets[0] - 1;
                        self.updateRepeatOffset(offset);
                        break :offset offset;
                    }
                    break :offset self.useRepeatOffset(offset_value);
                }
                break :offset self.useRepeatOffset(offset_value - 1);
            };

            if (offset == 0) return error.InvalidBitStream;

            return .{
                .literal_length = literal_length,
                .match_length = match_length,
                .offset = offset,
            };
        }

        fn executeSequenceSlice(
            self: *DecodeState,
            dest: []u8,
            write_pos: usize,
            sequence: Sequence,
        ) (error{MalformedSequence} || DecodeLiteralsError)!void {
            if (sequence.offset > write_pos + sequence.literal_length) return error.MalformedSequence;

            try self.decodeLiteralsSlice(dest[write_pos..], sequence.literal_length);
            const copy_start = write_pos + sequence.literal_length - sequence.offset;
            for (
                dest[write_pos + sequence.literal_length ..][0..sequence.match_length],
                dest[copy_start..][0..sequence.match_length],
            ) |*d, s| d.* = s;
            self.written_count += sequence.match_length;
        }

        fn executeSequenceRingBuffer(
            self: *DecodeState,
            dest: *RingBuffer,
            sequence: Sequence,
        ) (error{MalformedSequence} || DecodeLiteralsError)!void {
            if (sequence.offset > @min(dest.data.len, self.written_count + sequence.literal_length))
                return error.MalformedSequence;

            try self.decodeLiteralsRingBuffer(dest, sequence.literal_length);
            const copy_start = dest.write_index + dest.data.len - sequence.offset;
            const copy_slice = dest.sliceAt(copy_start, sequence.match_length);
            for (copy_slice.first) |b| dest.writeAssumeCapacity(b);
            for (copy_slice.second) |b| dest.writeAssumeCapacity(b);
            self.written_count += sequence.match_length;
        }

        const DecodeSequenceError = error{
            InvalidBitStream,
            EndOfStream,
            MalformedSequence,
            MalformedFseBits,
        } || DecodeLiteralsError;

        /// Decode one sequence from `bit_reader` into `dest`, written starting at
        /// `write_pos` and update FSE states if `last_sequence` is `false`.
        /// `prepare()` must be called for the block before attempting to decode
        /// sequences.
        ///
        /// Errors returned:
        ///   - `error.MalformedSequence` if the decompressed sequence would be
        ///     longer than `sequence_size_limit` or the sequence's offset is too
        ///     large
        ///   - `error.UnexpectedEndOfLiteralStream` if the decoder state's literal
        ///     streams do not contain enough literals for the sequence (this may
        ///     mean the literal stream or the sequence is malformed).
        ///   - `error.InvalidBitStream` if the FSE sequence bitstream is malformed
        ///   - `error.EndOfStream` if `bit_reader` does not contain enough bits
        ///   - `error.DestTooSmall` if `dest` is not large enough to holde the
        ///     decompressed sequence
        pub fn decodeSequenceSlice(
            self: *DecodeState,
            dest: []u8,
            write_pos: usize,
            bit_reader: *readers.ReverseBitReader,
            sequence_size_limit: usize,
            last_sequence: bool,
        ) (error{DestTooSmall} || DecodeSequenceError)!usize {
            const sequence = try self.nextSequence(bit_reader);
            const sequence_length = @as(usize, sequence.literal_length) + sequence.match_length;
            if (sequence_length > sequence_size_limit) return error.MalformedSequence;
            if (sequence_length > dest[write_pos..].len) return error.DestTooSmall;

            try self.executeSequenceSlice(dest, write_pos, sequence);
            if (!last_sequence) {
                try self.updateState(.literal, bit_reader);
                try self.updateState(.match, bit_reader);
                try self.updateState(.offset, bit_reader);
            }
            return sequence_length;
        }

        /// Decode one sequence from `bit_reader` into `dest`; see
        /// `decodeSequenceSlice`.
        pub fn decodeSequenceRingBuffer(
            self: *DecodeState,
            dest: *RingBuffer,
            bit_reader: anytype,
            sequence_size_limit: usize,
            last_sequence: bool,
        ) DecodeSequenceError!usize {
            const sequence = try self.nextSequence(bit_reader);
            const sequence_length = @as(usize, sequence.literal_length) + sequence.match_length;
            if (sequence_length > sequence_size_limit) return error.MalformedSequence;

            try self.executeSequenceRingBuffer(dest, sequence);
            if (!last_sequence) {
                try self.updateState(.literal, bit_reader);
                try self.updateState(.match, bit_reader);
                try self.updateState(.offset, bit_reader);
            }
            return sequence_length;
        }

        fn nextLiteralMultiStream(
            self: *DecodeState,
        ) error{BitStreamHasNoStartBit}!void {
            self.literal_stream_index += 1;
            try self.initLiteralStream(self.literal_streams.four[self.literal_stream_index]);
        }

        fn initLiteralStream(self: *DecodeState, bytes: []const u8) error{BitStreamHasNoStartBit}!void {
            try self.literal_stream_reader.init(bytes);
        }

        fn isLiteralStreamEmpty(self: *DecodeState) bool {
            switch (self.literal_streams) {
                .one => return self.literal_stream_reader.isEmpty(),
                .four => return self.literal_stream_index == 3 and self.literal_stream_reader.isEmpty(),
            }
        }

        const LiteralBitsError = error{
            BitStreamHasNoStartBit,
            UnexpectedEndOfLiteralStream,
        };
        fn readLiteralsBits(
            self: *DecodeState,
            bit_count_to_read: usize,
        ) LiteralBitsError!u16 {
            return self.literal_stream_reader.readBitsNoEof(u16, bit_count_to_read) catch bits: {
                if (self.literal_streams == .four and self.literal_stream_index < 3) {
                    try self.nextLiteralMultiStream();
                    break :bits self.literal_stream_reader.readBitsNoEof(u16, bit_count_to_read) catch
                        return error.UnexpectedEndOfLiteralStream;
                } else {
                    return error.UnexpectedEndOfLiteralStream;
                }
            };
        }

        const DecodeLiteralsError = error{
            MalformedLiteralsLength,
            NotFound,
        } || LiteralBitsError;

        /// Decode `len` bytes of literals into `dest`.
        ///
        /// Errors returned:
        ///   - `error.MalformedLiteralsLength` if the number of literal bytes
        ///     decoded by `self` plus `len` is greater than the regenerated size of
        ///     `literals`
        ///   - `error.UnexpectedEndOfLiteralStream` and `error.NotFound` if there
        ///     are problems decoding Huffman compressed literals
        pub fn decodeLiteralsSlice(
            self: *DecodeState,
            dest: []u8,
            len: usize,
        ) DecodeLiteralsError!void {
            if (self.literal_written_count + len > self.literal_header.regenerated_size)
                return error.MalformedLiteralsLength;

            switch (self.literal_header.block_type) {
                .raw => {
                    const literal_data = self.literal_streams.one[self.literal_written_count..][0..len];
                    @memcpy(dest[0..len], literal_data);
                    self.literal_written_count += len;
                    self.written_count += len;
                },
                .rle => {
                    for (0..len) |i| {
                        dest[i] = self.literal_streams.one[0];
                    }
                    self.literal_written_count += len;
                    self.written_count += len;
                },
                .compressed, .treeless => {
                    // const written_bytes_per_stream = (literals.header.regenerated_size + 3) / 4;

                    const huffman_tree = self.huffman_tree orelse unreachable;
                    const max_bit_count = huffman_tree.max_bit_count;
                    const starting_bit_count = LiteralsSection.HuffmanTree.weightToBitCount(
                        huffman_tree.nodes[huffman_tree.symbol_count_minus_one].weight,
                        max_bit_count,
                    );
                    var bits_read: u4 = 0;
                    var huffman_tree_index: usize = huffman_tree.symbol_count_minus_one;
                    var bit_count_to_read: u4 = starting_bit_count;
                    for (0..len) |i| {
                        var prefix: u16 = 0;
                        while (true) {
                            const new_bits = self.readLiteralsBits(bit_count_to_read) catch |err| {
                                return err;
                            };
                            prefix <<= bit_count_to_read;
                            prefix |= new_bits;
                            bits_read += bit_count_to_read;
                            const result = huffman_tree.query(huffman_tree_index, prefix) catch |err| {
                                return err;
                            };

                            switch (result) {
                                .symbol => |sym| {
                                    dest[i] = sym;
                                    bit_count_to_read = starting_bit_count;
                                    bits_read = 0;
                                    huffman_tree_index = huffman_tree.symbol_count_minus_one;
                                    break;
                                },
                                .index => |index| {
                                    huffman_tree_index = index;
                                    const bit_count = LiteralsSection.HuffmanTree.weightToBitCount(
                                        huffman_tree.nodes[index].weight,
                                        max_bit_count,
                                    );
                                    bit_count_to_read = bit_count - bits_read;
                                },
                            }
                        }
                    }
                    self.literal_written_count += len;
                    self.written_count += len;
                },
            }
        }

        /// Decode literals into `dest`; see `decodeLiteralsSlice()`.
        pub fn decodeLiteralsRingBuffer(
            self: *DecodeState,
            dest: *RingBuffer,
            len: usize,
        ) DecodeLiteralsError!void {
            if (self.literal_written_count + len > self.literal_header.regenerated_size)
                return error.MalformedLiteralsLength;

            switch (self.literal_header.block_type) {
                .raw => {
                    const literals_end = self.literal_written_count + len;
                    const literal_data = self.literal_streams.one[self.literal_written_count..literals_end];
                    dest.writeSliceAssumeCapacity(literal_data);
                    self.literal_written_count += len;
                    self.written_count += len;
                },
                .rle => {
                    for (0..len) |_| {
                        dest.writeAssumeCapacity(self.literal_streams.one[0]);
                    }
                    self.literal_written_count += len;
                    self.written_count += len;
                },
                .compressed, .treeless => {
                    // const written_bytes_per_stream = (literals.header.regenerated_size + 3) / 4;

                    const huffman_tree = self.huffman_tree orelse unreachable;
                    const max_bit_count = huffman_tree.max_bit_count;
                    const starting_bit_count = LiteralsSection.HuffmanTree.weightToBitCount(
                        huffman_tree.nodes[huffman_tree.symbol_count_minus_one].weight,
                        max_bit_count,
                    );
                    var bits_read: u4 = 0;
                    var huffman_tree_index: usize = huffman_tree.symbol_count_minus_one;
                    var bit_count_to_read: u4 = starting_bit_count;
                    for (0..len) |_| {
                        var prefix: u16 = 0;
                        while (true) {
                            const new_bits = try self.readLiteralsBits(bit_count_to_read);
                            prefix <<= bit_count_to_read;
                            prefix |= new_bits;
                            bits_read += bit_count_to_read;
                            const result = try huffman_tree.query(huffman_tree_index, prefix);

                            switch (result) {
                                .symbol => |sym| {
                                    dest.writeAssumeCapacity(sym);
                                    bit_count_to_read = starting_bit_count;
                                    bits_read = 0;
                                    huffman_tree_index = huffman_tree.symbol_count_minus_one;
                                    break;
                                },
                                .index => |index| {
                                    huffman_tree_index = index;
                                    const bit_count = LiteralsSection.HuffmanTree.weightToBitCount(
                                        huffman_tree.nodes[index].weight,
                                        max_bit_count,
                                    );
                                    bit_count_to_read = bit_count - bits_read;
                                },
                            }
                        }
                    }
                    self.literal_written_count += len;
                    self.written_count += len;
                },
            }
        }

        fn getCode(self: *DecodeState, comptime choice: DataType) u32 {
            return switch (@field(self, @tagName(choice)).table) {
                .rle => |value| value,
                .fse => |table| table[@field(self, @tagName(choice)).state].symbol,
            };
        }
    };

    /// Decode a single block from `src` into `dest`. The beginning of `src` must be
    /// the start of the block content (i.e. directly after the block header).
    /// Increments `consumed_count` by the number of bytes read from `src` to decode
    /// the block and returns the decompressed size of the block.
    ///
    /// Errors returned:
    ///
    ///   - `error.BlockSizeOverMaximum` if block's size is larger than 1 << 17 or
    ///     `dest[written_count..].len`
    ///   - `error.MalformedBlockSize` if `src.len` is smaller than the block size
    ///     and the block is a raw or compressed block
    ///   - `error.ReservedBlock` if the block is a reserved block
    ///   - `error.MalformedRleBlock` if the block is an RLE block and `src.len < 1`
    ///   - `error.MalformedCompressedBlock` if there are errors decoding a
    ///     compressed block
    ///   - `error.DestTooSmall` is `dest` is not large enough to hold the
    ///     decompressed block
    pub fn decodeBlock(
        dest: []u8,
        src: []const u8,
        block_header: frame.Zstandard.Block.Header,
        decode_state: *DecodeState,
        consumed_count: *usize,
        block_size_max: usize,
        written_count: usize,
    ) (error{DestTooSmall} || Error)!usize {
        const block_size = block_header.block_size;
        if (block_size_max < block_size) return error.BlockSizeOverMaximum;
        switch (block_header.block_type) {
            .raw => {
                if (src.len < block_size) return error.MalformedBlockSize;
                if (dest[written_count..].len < block_size) return error.DestTooSmall;
                @memcpy(dest[written_count..][0..block_size], src[0..block_size]);
                consumed_count.* += block_size;
                decode_state.written_count += block_size;
                return block_size;
            },
            .rle => {
                if (src.len < 1) return error.MalformedRleBlock;
                if (dest[written_count..].len < block_size) return error.DestTooSmall;
                for (written_count..block_size + written_count) |write_pos| {
                    dest[write_pos] = src[0];
                }
                consumed_count.* += 1;
                decode_state.written_count += block_size;
                return block_size;
            },
            .compressed => {
                if (src.len < block_size) return error.MalformedBlockSize;
                var bytes_read: usize = 0;
                const literals = decodeLiteralsSectionSlice(src[0..block_size], &bytes_read) catch
                    return error.MalformedCompressedBlock;
                var fbs = std.io.fixedBufferStream(src[bytes_read..block_size]);
                const fbs_reader = fbs.reader();
                const sequences_header = decodeSequencesHeader(fbs_reader) catch
                    return error.MalformedCompressedBlock;

                decode_state.prepare(fbs_reader, literals, sequences_header) catch
                    return error.MalformedCompressedBlock;

                bytes_read += fbs.pos;

                var bytes_written: usize = 0;
                {
                    const bit_stream_bytes = src[bytes_read..block_size];
                    var bit_stream: readers.ReverseBitReader = undefined;
                    bit_stream.init(bit_stream_bytes) catch return error.MalformedCompressedBlock;

                    if (sequences_header.sequence_count > 0) {
                        decode_state.readInitialFseState(&bit_stream) catch
                            return error.MalformedCompressedBlock;

                        var sequence_size_limit = block_size_max;
                        for (0..sequences_header.sequence_count) |i| {
                            const write_pos = written_count + bytes_written;
                            const decompressed_size = decode_state.decodeSequenceSlice(
                                dest,
                                write_pos,
                                &bit_stream,
                                sequence_size_limit,
                                i == sequences_header.sequence_count - 1,
                            ) catch |err| switch (err) {
                                error.DestTooSmall => return error.DestTooSmall,
                                else => return error.MalformedCompressedBlock,
                            };
                            bytes_written += decompressed_size;
                            sequence_size_limit -= decompressed_size;
                        }
                    }

                    if (!bit_stream.isEmpty()) {
                        return error.MalformedCompressedBlock;
                    }
                }

                if (decode_state.literal_written_count < literals.header.regenerated_size) {
                    const len = literals.header.regenerated_size - decode_state.literal_written_count;
                    if (len > dest[written_count + bytes_written ..].len) return error.DestTooSmall;
                    decode_state.decodeLiteralsSlice(dest[written_count + bytes_written ..], len) catch
                        return error.MalformedCompressedBlock;
                    bytes_written += len;
                }

                switch (decode_state.literal_header.block_type) {
                    .treeless, .compressed => {
                        if (!decode_state.isLiteralStreamEmpty()) return error.MalformedCompressedBlock;
                    },
                    .raw, .rle => {},
                }

                consumed_count.* += block_size;
                return bytes_written;
            },
            .reserved => return error.ReservedBlock,
        }
    }

    /// Decode a single block from `src` into `dest`; see `decodeBlock()`. Returns
    /// the size of the decompressed block, which can be used with `dest.sliceLast()`
    /// to get the decompressed bytes. `error.BlockSizeOverMaximum` is returned if
    /// the block's compressed or decompressed size is larger than `block_size_max`.
    pub fn decodeBlockRingBuffer(
        dest: *RingBuffer,
        src: []const u8,
        block_header: frame.Zstandard.Block.Header,
        decode_state: *DecodeState,
        consumed_count: *usize,
        block_size_max: usize,
    ) Error!usize {
        const block_size = block_header.block_size;
        if (block_size_max < block_size) return error.BlockSizeOverMaximum;
        switch (block_header.block_type) {
            .raw => {
                if (src.len < block_size) return error.MalformedBlockSize;
                const data = src[0..block_size];
                dest.writeSliceAssumeCapacity(data);
                consumed_count.* += block_size;
                decode_state.written_count += block_size;
                return block_size;
            },
            .rle => {
                if (src.len < 1) return error.MalformedRleBlock;
                for (0..block_size) |_| {
                    dest.writeAssumeCapacity(src[0]);
                }
                consumed_count.* += 1;
                decode_state.written_count += block_size;
                return block_size;
            },
            .compressed => {
                if (src.len < block_size) return error.MalformedBlockSize;
                var bytes_read: usize = 0;
                const literals = decodeLiteralsSectionSlice(src[0..block_size], &bytes_read) catch
                    return error.MalformedCompressedBlock;
                var fbs = std.io.fixedBufferStream(src[bytes_read..block_size]);
                const fbs_reader = fbs.reader();
                const sequences_header = decodeSequencesHeader(fbs_reader) catch
                    return error.MalformedCompressedBlock;

                decode_state.prepare(fbs_reader, literals, sequences_header) catch
                    return error.MalformedCompressedBlock;

                bytes_read += fbs.pos;

                var bytes_written: usize = 0;
                {
                    const bit_stream_bytes = src[bytes_read..block_size];
                    var bit_stream: readers.ReverseBitReader = undefined;
                    bit_stream.init(bit_stream_bytes) catch return error.MalformedCompressedBlock;

                    if (sequences_header.sequence_count > 0) {
                        decode_state.readInitialFseState(&bit_stream) catch
                            return error.MalformedCompressedBlock;

                        var sequence_size_limit = block_size_max;
                        for (0..sequences_header.sequence_count) |i| {
                            const decompressed_size = decode_state.decodeSequenceRingBuffer(
                                dest,
                                &bit_stream,
                                sequence_size_limit,
                                i == sequences_header.sequence_count - 1,
                            ) catch return error.MalformedCompressedBlock;
                            bytes_written += decompressed_size;
                            sequence_size_limit -= decompressed_size;
                        }
                    }

                    if (!bit_stream.isEmpty()) {
                        return error.MalformedCompressedBlock;
                    }
                }

                if (decode_state.literal_written_count < literals.header.regenerated_size) {
                    const len = literals.header.regenerated_size - decode_state.literal_written_count;
                    decode_state.decodeLiteralsRingBuffer(dest, len) catch
                        return error.MalformedCompressedBlock;
                    bytes_written += len;
                }

                switch (decode_state.literal_header.block_type) {
                    .treeless, .compressed => {
                        if (!decode_state.isLiteralStreamEmpty()) return error.MalformedCompressedBlock;
                    },
                    .raw, .rle => {},
                }

                consumed_count.* += block_size;
                if (bytes_written > block_size_max) return error.BlockSizeOverMaximum;
                return bytes_written;
            },
            .reserved => return error.ReservedBlock,
        }
    }

    /// Decode a single block from `source` into `dest`. Literal and sequence data
    /// from the block is copied into `literals_buffer` and `sequence_buffer`, which
    /// must be large enough or `error.LiteralsBufferTooSmall` and
    /// `error.SequenceBufferTooSmall` are returned (the maximum block size is an
    /// upper bound for the size of both buffers). See `decodeBlock`
    /// and `decodeBlockRingBuffer` for function that can decode a block without
    /// these extra copies. `error.EndOfStream` is returned if `source` does not
    /// contain enough bytes.
    pub fn decodeBlockReader(
        dest: *RingBuffer,
        source: anytype,
        block_header: frame.Zstandard.Block.Header,
        decode_state: *DecodeState,
        block_size_max: usize,
        literals_buffer: []u8,
        sequence_buffer: []u8,
    ) !void {
        const block_size = block_header.block_size;
        var block_reader_limited = std.io.limitedReader(source, block_size);
        const block_reader = block_reader_limited.reader();
        if (block_size_max < block_size) return error.BlockSizeOverMaximum;
        switch (block_header.block_type) {
            .raw => {
                if (block_size == 0) return;
                const slice = dest.sliceAt(dest.write_index, block_size);
                try source.readNoEof(slice.first);
                try source.readNoEof(slice.second);
                dest.write_index = dest.mask2(dest.write_index + block_size);
                decode_state.written_count += block_size;
            },
            .rle => {
                const byte = try source.readByte();
                for (0..block_size) |_| {
                    dest.writeAssumeCapacity(byte);
                }
                decode_state.written_count += block_size;
            },
            .compressed => {
                const literals = try decodeLiteralsSection(block_reader, literals_buffer);
                const sequences_header = try decodeSequencesHeader(block_reader);

                try decode_state.prepare(block_reader, literals, sequences_header);

                var bytes_written: usize = 0;
                {
                    const size = try block_reader.readAll(sequence_buffer);
                    var bit_stream: readers.ReverseBitReader = undefined;
                    try bit_stream.init(sequence_buffer[0..size]);

                    if (sequences_header.sequence_count > 0) {
                        if (sequence_buffer.len < block_reader_limited.bytes_left)
                            return error.SequenceBufferTooSmall;

                        decode_state.readInitialFseState(&bit_stream) catch
                            return error.MalformedCompressedBlock;

                        var sequence_size_limit = block_size_max;
                        for (0..sequences_header.sequence_count) |i| {
                            const decompressed_size = decode_state.decodeSequenceRingBuffer(
                                dest,
                                &bit_stream,
                                sequence_size_limit,
                                i == sequences_header.sequence_count - 1,
                            ) catch return error.MalformedCompressedBlock;
                            sequence_size_limit -= decompressed_size;
                            bytes_written += decompressed_size;
                        }
                    }

                    if (!bit_stream.isEmpty()) {
                        return error.MalformedCompressedBlock;
                    }
                }

                if (decode_state.literal_written_count < literals.header.regenerated_size) {
                    const len = literals.header.regenerated_size - decode_state.literal_written_count;
                    decode_state.decodeLiteralsRingBuffer(dest, len) catch
                        return error.MalformedCompressedBlock;
                    bytes_written += len;
                }

                switch (decode_state.literal_header.block_type) {
                    .treeless, .compressed => {
                        if (!decode_state.isLiteralStreamEmpty()) return error.MalformedCompressedBlock;
                    },
                    .raw, .rle => {},
                }

                if (bytes_written > block_size_max) return error.BlockSizeOverMaximum;
                if (block_reader_limited.bytes_left != 0) return error.MalformedCompressedBlock;
                decode_state.literal_written_count = 0;
            },
            .reserved => return error.ReservedBlock,
        }
    }

    /// Decode the header of a block.
    pub fn decodeBlockHeader(src: *const [3]u8) frame.Zstandard.Block.Header {
        const last_block = src[0] & 1 == 1;
        const block_type = @as(frame.Zstandard.Block.Type, @enumFromInt((src[0] & 0b110) >> 1));
        const block_size = ((src[0] & 0b11111000) >> 3) + (@as(u21, src[1]) << 5) + (@as(u21, src[2]) << 13);
        return .{
            .last_block = last_block,
            .block_type = block_type,
            .block_size = block_size,
        };
    }

    /// Decode the header of a block.
    ///
    /// Errors returned:
    ///   - `error.EndOfStream` if `src.len < 3`
    pub fn decodeBlockHeaderSlice(src: []const u8) error{EndOfStream}!frame.Zstandard.Block.Header {
        if (src.len < 3) return error.EndOfStream;
        return decodeBlockHeader(src[0..3]);
    }

    /// Decode a `LiteralsSection` from `src`, incrementing `consumed_count` by the
    /// number of bytes the section uses.
    ///
    /// Errors returned:
    ///   - `error.MalformedLiteralsHeader` if the header is invalid
    ///   - `error.MalformedLiteralsSection` if there are decoding errors
    ///   - `error.MalformedAccuracyLog` if compressed literals have invalid
    ///     accuracy
    ///   - `error.MalformedFseTable` if compressed literals have invalid FSE table
    ///   - `error.MalformedHuffmanTree` if there are errors decoding a Huffamn tree
    ///   - `error.EndOfStream` if there are not enough bytes in `src`
    pub fn decodeLiteralsSectionSlice(
        src: []const u8,
        consumed_count: *usize,
    ) (error{ MalformedLiteralsHeader, MalformedLiteralsSection, EndOfStream } || huffman.Error)!LiteralsSection {
        var bytes_read: usize = 0;
        const header = header: {
            var fbs = std.io.fixedBufferStream(src);
            defer bytes_read = fbs.pos;
            break :header decodeLiteralsHeader(fbs.reader()) catch return error.MalformedLiteralsHeader;
        };
        switch (header.block_type) {
            .raw => {
                if (src.len < bytes_read + header.regenerated_size) return error.MalformedLiteralsSection;
                const stream = src[bytes_read .. bytes_read + header.regenerated_size];
                consumed_count.* += header.regenerated_size + bytes_read;
                return LiteralsSection{
                    .header = header,
                    .huffman_tree = null,
                    .streams = .{ .one = stream },
                };
            },
            .rle => {
                if (src.len < bytes_read + 1) return error.MalformedLiteralsSection;
                const stream = src[bytes_read .. bytes_read + 1];
                consumed_count.* += 1 + bytes_read;
                return LiteralsSection{
                    .header = header,
                    .huffman_tree = null,
                    .streams = .{ .one = stream },
                };
            },
            .compressed, .treeless => {
                const huffman_tree_start = bytes_read;
                const huffman_tree = if (header.block_type == .compressed)
                    try huffman.decodeHuffmanTreeSlice(src[bytes_read..], &bytes_read)
                else
                    null;
                const huffman_tree_size = bytes_read - huffman_tree_start;
                const total_streams_size = std.math.sub(usize, header.compressed_size.?, huffman_tree_size) catch
                    return error.MalformedLiteralsSection;

                if (src.len < bytes_read + total_streams_size) return error.MalformedLiteralsSection;
                const stream_data = src[bytes_read .. bytes_read + total_streams_size];

                const streams = try decodeStreams(header.size_format, stream_data);
                consumed_count.* += bytes_read + total_streams_size;
                return LiteralsSection{
                    .header = header,
                    .huffman_tree = huffman_tree,
                    .streams = streams,
                };
            },
        }
    }

    /// Decode a `LiteralsSection` from `src`, incrementing `consumed_count` by the
    /// number of bytes the section uses. See `decodeLiterasSectionSlice()`.
    pub fn decodeLiteralsSection(
        source: anytype,
        buffer: []u8,
    ) !LiteralsSection {
        const header = try decodeLiteralsHeader(source);
        switch (header.block_type) {
            .raw => {
                try source.readNoEof(buffer[0..header.regenerated_size]);
                return LiteralsSection{
                    .header = header,
                    .huffman_tree = null,
                    .streams = .{ .one = buffer },
                };
            },
            .rle => {
                buffer[0] = try source.readByte();
                return LiteralsSection{
                    .header = header,
                    .huffman_tree = null,
                    .streams = .{ .one = buffer[0..1] },
                };
            },
            .compressed, .treeless => {
                var counting_reader = std.io.countingReader(source);
                const huffman_tree = if (header.block_type == .compressed)
                    try huffman.decodeHuffmanTree(counting_reader.reader(), buffer)
                else
                    null;
                const huffman_tree_size = @as(usize, @intCast(counting_reader.bytes_read));
                const total_streams_size = std.math.sub(usize, header.compressed_size.?, huffman_tree_size) catch
                    return error.MalformedLiteralsSection;

                if (total_streams_size > buffer.len) return error.LiteralsBufferTooSmall;
                try source.readNoEof(buffer[0..total_streams_size]);
                const stream_data = buffer[0..total_streams_size];

                const streams = try decodeStreams(header.size_format, stream_data);
                return LiteralsSection{
                    .header = header,
                    .huffman_tree = huffman_tree,
                    .streams = streams,
                };
            },
        }
    }

    fn decodeStreams(size_format: u2, stream_data: []const u8) !LiteralsSection.Streams {
        if (size_format == 0) {
            return .{ .one = stream_data };
        }

        if (stream_data.len < 6) return error.MalformedLiteralsSection;

        const stream_1_length = @as(usize, readInt(u16, stream_data[0..2]));
        const stream_2_length = @as(usize, readInt(u16, stream_data[2..4]));
        const stream_3_length = @as(usize, readInt(u16, stream_data[4..6]));

        const stream_1_start = 6;
        const stream_2_start = stream_1_start + stream_1_length;
        const stream_3_start = stream_2_start + stream_2_length;
        const stream_4_start = stream_3_start + stream_3_length;

        if (stream_data.len < stream_4_start) return error.MalformedLiteralsSection;

        return .{ .four = .{
            stream_data[stream_1_start .. stream_1_start + stream_1_length],
            stream_data[stream_2_start .. stream_2_start + stream_2_length],
            stream_data[stream_3_start .. stream_3_start + stream_3_length],
            stream_data[stream_4_start..],
        } };
    }

    /// Decode a literals section header.
    ///
    /// Errors returned:
    ///   - `error.EndOfStream` if there are not enough bytes in `source`
    pub fn decodeLiteralsHeader(source: anytype) !LiteralsSection.Header {
        const byte0 = try source.readByte();
        const block_type = @as(LiteralsSection.BlockType, @enumFromInt(byte0 & 0b11));
        const size_format = @as(u2, @intCast((byte0 & 0b1100) >> 2));
        var regenerated_size: u20 = undefined;
        var compressed_size: ?u18 = null;
        switch (block_type) {
            .raw, .rle => {
                switch (size_format) {
                    0, 2 => {
                        regenerated_size = byte0 >> 3;
                    },
                    1 => regenerated_size = (byte0 >> 4) + (@as(u20, try source.readByte()) << 4),
                    3 => regenerated_size = (byte0 >> 4) +
                        (@as(u20, try source.readByte()) << 4) +
                        (@as(u20, try source.readByte()) << 12),
                }
            },
            .compressed, .treeless => {
                const byte1 = try source.readByte();
                const byte2 = try source.readByte();
                switch (size_format) {
                    0, 1 => {
                        regenerated_size = (byte0 >> 4) + ((@as(u20, byte1) & 0b00111111) << 4);
                        compressed_size = ((byte1 & 0b11000000) >> 6) + (@as(u18, byte2) << 2);
                    },
                    2 => {
                        const byte3 = try source.readByte();
                        regenerated_size = (byte0 >> 4) + (@as(u20, byte1) << 4) + ((@as(u20, byte2) & 0b00000011) << 12);
                        compressed_size = ((byte2 & 0b11111100) >> 2) + (@as(u18, byte3) << 6);
                    },
                    3 => {
                        const byte3 = try source.readByte();
                        const byte4 = try source.readByte();
                        regenerated_size = (byte0 >> 4) + (@as(u20, byte1) << 4) + ((@as(u20, byte2) & 0b00111111) << 12);
                        compressed_size = ((byte2 & 0b11000000) >> 6) + (@as(u18, byte3) << 2) + (@as(u18, byte4) << 10);
                    },
                }
            },
        }
        return LiteralsSection.Header{
            .block_type = block_type,
            .size_format = size_format,
            .regenerated_size = regenerated_size,
            .compressed_size = compressed_size,
        };
    }

    /// Decode a sequences section header.
    ///
    /// Errors returned:
    ///   - `error.ReservedBitSet` if the reserved bit is set
    ///   - `error.EndOfStream` if there are not enough bytes in `source`
    pub fn decodeSequencesHeader(
        source: anytype,
    ) !SequencesSection.Header {
        var sequence_count: u24 = undefined;

        const byte0 = try source.readByte();
        if (byte0 == 0) {
            return SequencesSection.Header{
                .sequence_count = 0,
                .offsets = undefined,
                .match_lengths = undefined,
                .literal_lengths = undefined,
            };
        } else if (byte0 < 128) {
            sequence_count = byte0;
        } else if (byte0 < 255) {
            sequence_count = (@as(u24, (byte0 - 128)) << 8) + try source.readByte();
        } else {
            sequence_count = (try source.readByte()) + (@as(u24, try source.readByte()) << 8) + 0x7F00;
        }

        const compression_modes = try source.readByte();

        const matches_mode = @as(SequencesSection.Header.Mode, @enumFromInt((compression_modes & 0b00001100) >> 2));
        const offsets_mode = @as(SequencesSection.Header.Mode, @enumFromInt((compression_modes & 0b00110000) >> 4));
        const literal_mode = @as(SequencesSection.Header.Mode, @enumFromInt((compression_modes & 0b11000000) >> 6));
        if (compression_modes & 0b11 != 0) return error.ReservedBitSet;

        return SequencesSection.Header{
            .sequence_count = sequence_count,
            .offsets = offsets_mode,
            .match_lengths = matches_mode,
            .literal_lengths = literal_mode,
        };
    }