add file2 impl for zstd (lightly tested and not integrated yet)

src/fragment_store/file2_zstd.hpp

@@ -0,0 +1,51 @@
+#pragma once
+
+#include <solanaceae/file/file2.hpp>
+
+#include <memory>
+
+#include <zstd.h>
+
+// zstd compression wrapper over another file
+// WARNING: only supports sequential writes
+struct File2ZSTDW : public File2I {
+	File2I& _real_file;
+
+	// TODO: hide this detail?
+	std::unique_ptr<ZSTD_CCtx, decltype(&ZSTD_freeCCtx)> _cctx{ZSTD_createCCtx(), &ZSTD_freeCCtx};
+
+	File2ZSTDW(File2I& real);
+	virtual ~File2ZSTDW(void) {}
+
+	bool isGood(void) override;
+
+	// for simplicity and potential future seekability each write is its own frame
+	bool write(const ByteSpan data, int64_t pos = -1) override;
+	std::variant<ByteSpan, std::vector<uint8_t>> read(uint64_t size, int64_t pos = -1) override;
+};
+
+// zstd decompression wrapper over another file
+// WARNING: only supports sequential reads
+// TODO: add seeking support (use frames)
+struct File2ZSTDR : public File2I {
+	File2I& _real_file;
+
+	// TODO: hide this detail?
+	std::unique_ptr<ZSTD_DCtx, decltype(&ZSTD_freeDCtx)> _dctx{ZSTD_createDCtx(), &ZSTD_freeDCtx};
+	std::vector<uint8_t> _in_buffer;
+	std::vector<uint8_t> _out_buffer;
+	std::vector<uint8_t> _decompressed_buffer; // retains decompressed unread data between read() calls
+	ZSTD_inBuffer _z_input{nullptr, 0, 0};
+
+	File2ZSTDR(File2I& real);
+	virtual ~File2ZSTDR(void) {}
+
+	bool isGood(void) override;
+
+	bool write(const ByteSpan data, int64_t pos = -1) override;
+	std::variant<ByteSpan, std::vector<uint8_t>> read(uint64_t size, int64_t pos = -1) override;
+
+	private:
+		bool feedInput(std::variant<ByteSpan, std::vector<uint8_t>>&& read_buff);
+};
+