streams: Implement BitStreamReader/Writer classes.

Golomb-Rice coding, as specified in BIP 158, involves operations on
individual bits. These classes will be used to implement the
encoding/decoding operations.

src/streams.h

@@ -510,12 +510,105 @@ public:
     }
 };
 
+template <typename IStream>
+class BitStreamReader
+{
+private:
+    IStream& m_istream;
 
+    /// Buffered byte read in from the input stream. A new byte is read into the
+    /// buffer when m_offset reaches 8.
+    uint8_t m_buffer{0};
 
+    /// Number of high order bits in m_buffer already returned by previous
+    /// Read() calls. The next bit to be returned is at this offset from the
+    /// most significant bit position.
+    int m_offset{8};
 
+public:
+    explicit BitStreamReader(IStream& istream) : m_istream(istream) {}
 
+    /** Read the specified number of bits from the stream. The data is returned
+     * in the nbits least signficant bits of a 64-bit uint.
+     */
+    uint64_t Read(int nbits) {
+        if (nbits < 0 || nbits > 64) {
+            throw std::out_of_range("nbits must be between 0 and 64");
+        }
 
+        uint64_t data = 0;
+        while (nbits > 0) {
+            if (m_offset == 8) {
+                m_istream >> m_buffer;
+                m_offset = 0;
+            }
 
+            int bits = std::min(8 - m_offset, nbits);
+            data <<= bits;
+            data |= static_cast<uint8_t>(m_buffer << m_offset) >> (8 - bits);
+            m_offset += bits;
+            nbits -= bits;
+        }
+        return data;
+    }
+};
+
+template <typename OStream>
+class BitStreamWriter
+{
+private:
+    OStream& m_ostream;
+
+    /// Buffered byte waiting to be written to the output stream. The byte is
+    /// written buffer when m_offset reaches 8 or Flush() is called.
+    uint8_t m_buffer{0};
+
+    /// Number of high order bits in m_buffer already written by previous
+    /// Write() calls and not yet flushed to the stream. The next bit to be
+    /// written to is at this offset from the most significant bit position.
+    int m_offset{0};
+
+public:
+    explicit BitStreamWriter(OStream& ostream) : m_ostream(ostream) {}
+
+    ~BitStreamWriter()
+    {
+        Flush();
+    }
+
+    /** Write the nbits least significant bits of a 64-bit int to the output
+     * stream. Data is buffered until it completes an octet.
+     */
+    void Write(uint64_t data, int nbits) {
+        if (nbits < 0 || nbits > 64) {
+            throw std::out_of_range("nbits must be between 0 and 64");
+        }
+
+        while (nbits > 0) {
+            int bits = std::min(8 - m_offset, nbits);
+            m_buffer |= (data << (64 - nbits)) >> (64 - 8 + m_offset);
+            m_offset += bits;
+            nbits -= bits;
+
+            if (m_offset == 8) {
+                Flush();
+            }
+        }
+    }
+
+    /** Flush any unwritten bits to the output stream, padding with 0's to the
+     * next byte boundary.
+     */
+    void Flush() {
+        if (m_offset == 0) {
+            return;
+        }
+
+        m_ostream << m_buffer;
+        m_buffer = 0;
+        m_offset = 0;
+    }
+};