rewrite prs functions for clarity; implement staged compression

src/Compression.hh

@@ -4,14 +4,69 @@
 
 #include <string>
 
+#include "Text.hh"
 
 
+
+// Use this class if you need to compress from multiple input buffers, or need
+// to compress multiple chunks and don't want to copy their contents
+// unnecessarily. (For most common use cases, use prs_compress (below) instead.)
+class PRSCompressor {
+public:
+  // To use this class, instantiate it, then call .add() one or more times, then
+  // call .close() and use the returned string as the compressed result.
+  PRSCompressor();
+  ~PRSCompressor() = default;
+
+  // Adds more input data to be compressed, which logically comes after all
+  // previous data provided via add() calls. Cannot be called after close() is
+  // called.
+  void add(const void* data, size_t size);
+  void add(const std::string& data);
+
+  // Ends compression and returns the complete compressed result. It's OK to
+  // std::move() from the returned string reference.
+  std::string& close();
+
+  // Returns the total number of bytes passed to add() calls so far.
+  inline size_t input_size() const {
+    return this->input_bytes;
+  }
+
+private:
+  void add_byte(uint8_t v);
+  void advance();
+  void write_control(bool z);
+  void flush_control();
+
+  bool closed;
+
+  size_t control_byte_offset;
+  uint16_t pending_control_bits;
+
+  size_t input_bytes;
+  parray<uint8_t, 0x100> forward_log;
+  size_t compression_offset;
+  parray<uint8_t, 0x2000> reverse_log;
+
+  StringWriter output;
+};
+
+// Compresses data from a single input buffer using PRS and returns the
+// compressed result. This is a shortcut for constructing a PRSCompressor,
+// calling .add() once, and calling .close().
 std::string prs_compress(const void* vdata, size_t size);
 std::string prs_compress(const std::string& data);
 
+// Decompresses PRS-compressed data.
 std::string prs_decompress(const void* data, size_t size, size_t max_output_size = 0);
 std::string prs_decompress(const std::string& data, size_t max_output_size = 0);
+
+// Returns the decompressed size of PRS-compressed data, without actually
+// decompressing it.
+size_t prs_decompress_size(const void* data, size_t size, size_t max_output_size = 0);
 size_t prs_decompress_size(const std::string& data, size_t max_output_size = 0);
 
+// Decompresses and compresses data using the BC0 algorithm.
 std::string bc0_decompress(const std::string& data);
 std::string bc0_compress(const std::string& data);