1 files changed, 59 insertions, 0 deletions
diff --git a/lib/zstd.cpp b/lib/zstd.cpp
new file mode 100644
index 0000000..d2c27bc
--- /dev/null
+++ b/lib/zstd.cpp
@@ -0,0 +1,59 @@
+#include "compressionctx.h"
+#include <cassert>
+#include <zstd.h>
+
+using namespace embed;
+
+ZSTD_DDict *dictPtr = nullptr;
+
+ZstdCompressionCtx::ZstdCompressionCtx(const std::span<const uint8_t> &dictionary)
+{
+    if(!dictionary.empty()) {
+        dictPtr = ZSTD_createDDict(dictionary.data(), dictionary.size());
+    }
+}
+
+ZstdCompressionCtx::~ZstdCompressionCtx()
+{
+    if(dictPtr != nullptr) {
+        ZSTD_freeDDict(dictPtr);
+    }
+}
+
+[[nodiscard]]
+std::vector<uint8_t> ZstdCompressionCtx::decompress(const std::span<const uint8_t> &entry) const
+{
+    /* Read the content size from the frame header. For simplicity we require
+     * that it is always present. By default, zstd will write the content size
+     * in the header when it is known. If you can't guarantee that the frame
+     * content size is always written into the header, either use streaming
+     * decompression, or ZSTD_decompressBound().
+     */
+    unsigned long long const rSize = ZSTD_getFrameContentSize(entry.data(), entry.size());
+    assert(rSize != ZSTD_CONTENTSIZE_ERROR); //, "%s: not compressed by zstd!", fname);
+    assert(rSize != ZSTD_CONTENTSIZE_UNKNOWN); //, "%s: original size unknown!", fname);
+    std::vector<uint8_t> rBuff(rSize);
+
+    /* Check that the dictionary ID matches.
+     * If a non-zstd dictionary is used, then both will be zero.
+     * By default zstd always writes the dictionary ID into the frame.
+     * Zstd will check if there is a dictionary ID mismatch as well.
+     */
+    unsigned const expectedDictID = ZSTD_getDictID_fromDDict(dictPtr);
+    unsigned const actualDictID = ZSTD_getDictID_fromFrame(entry.data(), entry.size());
+    assert(actualDictID == expectedDictID); //"DictID mismatch: expected %u got %u",
+
+    /* Decompress using the dictionary.
+     * If you need to control the decompression parameters, then use the
+     * advanced API: ZSTD_DCtx_setParameter(), ZSTD_DCtx_refDDict(), and
+     * ZSTD_decompressDCtx().
+     */
+    ZSTD_DCtx *const dctx = ZSTD_createDCtx();
+    assert(dctx != NULL); //, "ZSTD_createDCtx() failed!");
+    size_t const dSize = ZSTD_decompress_usingDDict(dctx, rBuff.data(), rSize, entry.data(), entry.size(), dictPtr);
+    /* When zstd knows the content size, it will error if it doesn't match. */
+    assert(dSize == rSize); //, "Impossible because zstd will check this condition!");
+
+    ZSTD_freeDCtx(dctx);
+    return rBuff;
+}