LibCore: Introduce BigEndianInputBitStream

BigEndianInputBitStream is the Core::Stream API's bitwise input stream
for big endian input data. The functionality and bitwise read API is
almost unchanged from AK::BitStream, except that this bit stream only
supports big endian operations.

As the behavior for mixing big endian and little endian reads on
AK::BitStream is unknown (and untested), it was never done anyways. So
this was a good opportunity to split up big endian and little endian
reading.

Another API improvement from AK::BitStream is the ability to specify
the return type of the bit read function. Always needing to static_cast
the result of BitStream::read_bits_big_endian into the desired type is
adding a lot of avoidable noise to the users (primarily FlacLoader).

AK/BitStream.h

@@ -12,6 +12,7 @@
 
 namespace AK {
 
+// Obsoleted by LibCore/{Big, Little}EndianInputBitStream.
 class InputBitStream final : public InputStream {
 public:
     explicit InputBitStream(InputStream& stream)

Userland/Libraries/LibCore/InputBitStream.h

@@ -0,0 +1,138 @@
+/*
+ * Copyright (c) 2021, kleines Filmröllchen <filmroellchen@serenityos.org>.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/ByteBuffer.h>
+#include <AK/Concepts.h>
+#include <AK/Error.h>
+#include <AK/NonnullOwnPtr.h>
+#include <AK/NonnullRefPtr.h>
+#include <AK/OwnPtr.h>
+#include <AK/Span.h>
+#include <AK/StdLibExtraDetails.h>
+#include <AK/Types.h>
+#include <LibCore/Stream.h>
+
+namespace Core::Stream {
+
+/// A stream wrapper class that allows you to read arbitrary amounts of bits
+/// in big-endian order from another stream.
+/// Note that this stream does not own its underlying stream, it merely takes a reference.
+class BigEndianInputBitStream : public Stream {
+public:
+    static ErrorOr<NonnullOwnPtr<BigEndianInputBitStream>> construct(Stream& stream)
+    {
+        return adopt_nonnull_own_or_enomem<BigEndianInputBitStream>(new BigEndianInputBitStream(stream));
+    }
+
+    // ^Stream
+    virtual bool is_readable() const override { return m_stream.is_readable(); }
+    virtual ErrorOr<size_t> read(Bytes bytes) override
+    {
+        if (m_current_byte.has_value() && is_aligned_to_byte_boundary()) {
+            bytes[0] = m_current_byte.release_value();
+            return m_stream.read(bytes.slice(1));
+        }
+        align_to_byte_boundary();
+        return m_stream.read(bytes);
+    }
+    virtual bool is_writable() const override { return m_stream.is_writable(); }
+    virtual ErrorOr<size_t> write(ReadonlyBytes bytes) override { return m_stream.write(bytes); }
+    virtual bool write_or_error(ReadonlyBytes bytes) override { return m_stream.write_or_error(bytes); }
+    virtual bool is_eof() const override { return m_stream.is_eof() && !m_current_byte.has_value(); }
+    virtual bool is_open() const override { return m_stream.is_open(); }
+    virtual void close() override
+    {
+        m_stream.close();
+        align_to_byte_boundary();
+    }
+
+    ErrorOr<bool> read_bit()
+    {
+        return read_bits<bool>(1);
+    }
+    /// Depending on the number of bits to read, the return type can be chosen appropriately.
+    /// This avoids a bunch of static_cast<>'s for the user.
+    // TODO: Support u128, u256 etc. as well: The concepts would be quite complex.
+    template<Unsigned T = u64>
+    ErrorOr<T> read_bits(size_t count)
+    {
+        if constexpr (IsSame<bool, T>) {
+            VERIFY(count == 1);
+        }
+        T result = 0;
+
+        size_t nread = 0;
+        while (nread < count) {
+            if (m_current_byte.has_value()) {
+                if constexpr (!IsSame<bool, T> && !IsSame<u8, T>) {
+                    // read as many bytes as possible directly
+                    if (((count - nread) >= 8) && is_aligned_to_byte_boundary()) {
+                        // shift existing data over
+                        result <<= 8;
+                        result |= m_current_byte.value();
+                        nread += 8;
+                        m_current_byte.clear();
+                    } else {
+                        const auto bit = (m_current_byte.value() >> (7 - m_bit_offset)) & 1;
+                        result <<= 1;
+                        result |= bit;
+                        ++nread;
+                        if (m_bit_offset++ == 7)
+                            m_current_byte.clear();
+                    }
+                } else {
+                    // Always take this branch for booleans or u8: there's no purpose in reading more than a single bit
+                    const auto bit = (m_current_byte.value() >> (7 - m_bit_offset)) & 1;
+                    if constexpr (IsSame<bool, T>)
+                        result = bit;
+                    else {
+                        result <<= 1;
+                        result |= bit;
+                    }
+                    ++nread;
+                    if (m_bit_offset++ == 7)
+                        m_current_byte.clear();
+                }
+            } else {
+                // FIXME: This returns Optional so TRY is not useable
+                auto temp_buffer = ByteBuffer::create_uninitialized(1);
+                if (!temp_buffer.has_value())
+                    return Error::from_string_literal("Couldn't allocate temporary byte buffer"sv);
+
+                TRY(m_stream.read(temp_buffer->bytes()));
+                m_current_byte = (*temp_buffer)[0];
+                m_bit_offset = 0;
+            }
+        }
+
+        return result;
+    }
+
+    /// Discards any sub-byte stream positioning the input stream may be keeping track of.
+    /// Non-bitwise reads will implicitly call this.
+    void align_to_byte_boundary()
+    {
+        m_current_byte.clear();
+        m_bit_offset = 0;
+    }
+
+    /// Whether we are (accidentally or intentionally) at a byte boundary right now.
+    ALWAYS_INLINE bool is_aligned_to_byte_boundary() const { return m_bit_offset == 0; }
+
+private:
+    BigEndianInputBitStream(Stream& stream)
+        : m_stream(stream)
+    {
+    }
+
+    Optional<u8> m_current_byte;
+    size_t m_bit_offset { 0 };
+    Stream& m_stream;
+};
+
+}