feat: add interface to retrieve metadata from block decompressor

include/dwarfs/block_compressor.h

@@ -25,6 +25,7 @@
 #include <cstdio>
 #include <functional>
 #include <memory>
+#include <optional>
 #include <span>
 #include <stdexcept>
 #include <string>
@@ -128,6 +129,8 @@ class block_decompressor {
 
   compression_type type() const { return impl_->type(); }
 
+  std::optional<std::string> metadata() const { return impl_->metadata(); }
+
   static std::vector<uint8_t>
   decompress(compression_type type, const uint8_t* data, size_t size) {
     std::vector<uint8_t> target;
@@ -142,6 +145,7 @@ class block_decompressor {
 
     virtual bool decompress_frame(size_t frame_size) = 0;
     virtual size_t uncompressed_size() const = 0;
+    virtual std::optional<std::string> metadata() const = 0;
 
     virtual compression_type type() const = 0;
   };

src/dwarfs/compression/brotli.cpp

@@ -127,6 +127,8 @@ class brotli_block_decompressor final : public block_decompressor::impl {
 
   compression_type type() const override { return compression_type::BROTLI; }
 
+  std::optional<std::string> metadata() const override { return std::nullopt; }
+
   bool decompress_frame(size_t frame_size) override {
     size_t pos = decompressed_.size();
 

src/dwarfs/compression/flac.cpp

@@ -425,6 +425,21 @@ class flac_block_decompressor final : public block_decompressor::impl {
 
   compression_type type() const override { return compression_type::FLAC; }
 
+  std::optional<std::string> metadata() const override {
+    auto const flags = header_.flags().value();
+    folly::dynamic meta = folly::dynamic::object
+        // clang-format off
+      ("endianness",         flags & kFlagBigEndian ? "big" : "little")
+      ("signedness",         flags & kFlagSigned ? "signed" : "unsigned")
+      ("padding",            flags & kFlagLsbPadding ? "lsb" : "msb")
+      ("bytes_per_sample",   (flags & kBytesPerSampleMask) + 1)
+      ("bits_per_sample",    header_.bits_per_sample().value())
+      ("number_of_channels", header_.num_channels().value())
+      ; // clang-format on
+
+    return folly::toJson(meta);
+  }
+
   bool decompress_frame(size_t frame_size) override {
     size_t pos = decompressed_.size();
 

src/dwarfs/compression/lz4.cpp

@@ -125,6 +125,8 @@ class lz4_block_decompressor final : public block_decompressor::impl {
 
   compression_type type() const override { return compression_type::LZ4; }
 
+  std::optional<std::string> metadata() const override { return std::nullopt; }
+
   bool decompress_frame(size_t) override {
     if (!error_.empty()) {
       DWARFS_THROW(runtime_error, error_);

src/dwarfs/compression/lzma.cpp

@@ -224,6 +224,8 @@ class lzma_block_decompressor final : public block_decompressor::impl {
 
   compression_type type() const override { return compression_type::LZMA; }
 
+  std::optional<std::string> metadata() const override { return std::nullopt; }
+
   bool decompress_frame(size_t frame_size) override {
     if (!error_.empty()) {
       DWARFS_THROW(runtime_error, error_);

src/dwarfs/compression/null.cpp

@@ -81,6 +81,8 @@ class null_block_decompressor final : public block_decompressor::impl {
 
   compression_type type() const override { return compression_type::NONE; }
 
+  std::optional<std::string> metadata() const override { return std::nullopt; }
+
   bool decompress_frame(size_t frame_size) override {
     if (decompressed_.size() + frame_size > uncompressed_size_) {
       frame_size = uncompressed_size_ - decompressed_.size();

src/dwarfs/compression/zstd.cpp

@@ -145,6 +145,8 @@ class zstd_block_decompressor final : public block_decompressor::impl {
 
   compression_type type() const override { return compression_type::ZSTD; }
 
+  std::optional<std::string> metadata() const override { return std::nullopt; }
+
   bool decompress_frame(size_t /*frame_size*/) override {
     if (!error_.empty()) {
       DWARFS_THROW(runtime_error, error_);