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https://github.com/SerenityOS/serenity
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253a96277e
When working with FixedMemoryStreams, and especially MappedFiles, you may don't want to copy the underlying data when you read from the stream. Pointing into that data is perfectly fine as long as you know the lifetime of it is long enough. This commit adds a couple of methods for reading either a single value, or a span of them, in this way. As noted, for single values you sadly get a raw pointer instead of a reference, but that's the only option right now.
114 lines
3.8 KiB
C++
114 lines
3.8 KiB
C++
/*
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* Copyright (c) 2021, kleines Filmröllchen <filmroellchen@serenityos.org>.
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* Copyright (c) 2023, Sam Atkins <atkinssj@serenityos.org>.
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#pragma once
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#include <AK/Error.h>
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#include <AK/OwnPtr.h>
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#include <AK/Stream.h>
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#include <AK/Vector.h>
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namespace AK {
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/// A stream class that allows for reading/writing on a preallocated memory area
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/// using a single read/write head.
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class FixedMemoryStream : public SeekableStream {
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public:
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explicit FixedMemoryStream(Bytes bytes, bool writing_enabled = true);
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explicit FixedMemoryStream(ReadonlyBytes bytes);
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virtual bool is_eof() const override;
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virtual bool is_open() const override;
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virtual void close() override;
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virtual ErrorOr<void> truncate(size_t) override;
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virtual ErrorOr<Bytes> read_some(Bytes bytes) override;
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virtual ErrorOr<void> read_until_filled(Bytes bytes) override;
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virtual ErrorOr<size_t> seek(i64 offset, SeekMode seek_mode = SeekMode::SetPosition) override;
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virtual ErrorOr<size_t> write_some(ReadonlyBytes bytes) override;
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virtual ErrorOr<void> write_until_depleted(ReadonlyBytes bytes) override;
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size_t offset() const;
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size_t remaining() const;
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/// Read a value, but referring to the stream's underlying data instead of copying it.
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/// Of course, only use this if you know the lifetime of the data will exceed the value's.
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// FIXME: Would be nicer to be able to return T& but Variant (and thus ErrorOr) can't hold references.
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template<typename T>
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requires(Traits<T>::is_trivially_serializable())
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ErrorOr<T*> read_in_place()
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{
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if constexpr (!IsConst<T>) {
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if (!m_writing_enabled)
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return Error::from_string_view_or_print_error_and_return_errno("Tried to obtain a non-const reference from a read-only FixedMemoryStream"sv, EINVAL);
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}
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T* value = reinterpret_cast<T*>(m_bytes.offset_pointer(m_offset));
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TRY(discard(sizeof(T)));
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return value;
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}
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/// Read a span of values, referring to the stream's underlying data instead of copying it.
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/// Of course, only use this if you know the lifetime of the data will exceed the span's.
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template<typename T>
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requires(Traits<T>::is_trivially_serializable())
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ErrorOr<Span<T>> read_in_place(size_t count)
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{
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if constexpr (!IsConst<T>) {
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if (!m_writing_enabled)
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return Error::from_string_view_or_print_error_and_return_errno("Tried to obtain a non-const span from a read-only FixedMemoryStream"sv, EINVAL);
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}
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Span<T> span { m_bytes.offset_pointer(m_offset), count };
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TRY(discard(sizeof(T) * count));
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return span;
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}
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private:
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Bytes m_bytes;
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size_t m_offset { 0 };
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bool m_writing_enabled { true };
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};
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/// A stream class that allows for writing to an automatically allocating memory area
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/// and reading back the written data afterwards.
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class AllocatingMemoryStream final : public Stream {
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public:
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static constexpr size_t CHUNK_SIZE = 4096;
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virtual ErrorOr<Bytes> read_some(Bytes) override;
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virtual ErrorOr<size_t> write_some(ReadonlyBytes) override;
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virtual ErrorOr<void> discard(size_t) override;
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virtual bool is_eof() const override;
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virtual bool is_open() const override;
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virtual void close() override;
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size_t used_buffer_size() const;
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ErrorOr<Optional<size_t>> offset_of(ReadonlyBytes needle) const;
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private:
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// Note: We set the inline buffer capacity to zero to make moving chunks as efficient as possible.
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using Chunk = AK::Detail::ByteBuffer<0>;
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ErrorOr<ReadonlyBytes> next_read_range();
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ErrorOr<Bytes> next_write_range();
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void cleanup_unused_chunks();
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Vector<Chunk> m_chunks;
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size_t m_read_offset = 0;
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size_t m_write_offset = 0;
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};
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}
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#if USING_AK_GLOBALLY
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using AK::AllocatingMemoryStream;
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using AK::FixedMemoryStream;
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#endif
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