reduce educational decoder to single frame decompression

diff --git a/doc/educational_decoder/zstd_decompress.c b/doc/educational_decoder/zstd_decompress.c
index 7c8d8114d4016c84892d739eabe9c9dc495af6ca..93c346312377dac003943fe250386909803d934e 100644 (file)
--- a/doc/educational_decoder/zstd_decompress.c
+++ b/doc/educational_decoder/zstd_decompress.c
@@ -28,6 +28,7 @@ size_t ZSTD_decompress_with_dict(void *const dst, const size_t dst_len,
 /// Get the decompressed size of an input stream so memory can be allocated in
 /// advance
 /// Returns -1 if the size can't be determined
+/// Assumes decompression of a single frame
 size_t ZSTD_get_decompressed_size(const void *const src, const size_t src_len);
 
 /******* UTILITY MACROS AND TYPES *********************************************/
@@ -396,9 +397,9 @@ size_t ZSTD_decompress_with_dict(void *const dst, const size_t dst_len,
     // Multiple frames can be appended into a single file or stream. A frame is
     // totally independent, has a defined beginning and end, and a set of
     // parameters which tells the decoder how to decompress it."
-    while (IO_istream_len(&in) > 0) {
-        decode_frame(&out, &in, &parsed_dict);
-    }
+
+    /* this decoder assumes decompression of a single frame */
+    decode_frame(&out, &in, &parsed_dict);
 
     free_dictionary(&parsed_dict);
 
@@ -424,30 +425,6 @@ static void decompress_data(frame_context_t *const ctx, ostream_t *const out,
 static void decode_frame(ostream_t *const out, istream_t *const in,
                          const dictionary_t *const dict) {
     const u32 magic_number = IO_read_bits(in, 32);
-
-    // Skippable frame
-    //
-    // "Magic_Number
-    //
-    // 4 Bytes, little-endian format. Value : 0x184D2A5?, which means any value
-    // from 0x184D2A50 to 0x184D2A5F. All 16 values are valid to identify a
-    // skippable frame."
-    if ((magic_number & ~0xFU) == 0x184D2A50U) {
-        // "Skippable frames allow the insertion of user-defined data into a
-        // flow of concatenated frames. Its design is pretty straightforward,
-        // with the sole objective to allow the decoder to quickly skip over
-        // user-defined data and continue decoding.
-        //
-        // Skippable frames defined in this specification are compatible with
-        // LZ4 ones."
-        const size_t frame_size = IO_read_bits(in, 32);
-
-        // skip over frame
-        IO_advance_input(in, frame_size);
-
-        return;
-    }
-
     // Zstandard frame
     //
     // "Magic_Number
@@ -460,8 +437,8 @@ static void decode_frame(ostream_t *const out, istream_t *const in,
         return;
     }
 
-    // not a real frame
-    ERROR("Invalid magic number");
+    // not a real frame or a skippable frame
+    ERROR("Tried to decode non-ZSTD frame");
 }
 
 /// Decode a frame that contains compressed data.  Not all frames do as there
@@ -1420,28 +1397,17 @@ static void execute_sequences(frame_context_t *const ctx, ostream_t *const out,
 /******* END SEQUENCE EXECUTION ***********************************************/
 
 /******* OUTPUT SIZE COUNTING *************************************************/
-static void traverse_frame(const frame_header_t *const header, istream_t *const in);
-
 /// Get the decompressed size of an input stream so memory can be allocated in
 /// advance.
-/// This is more complex than the implementation in the reference
-/// implementation, as this API allows for the decompression of multiple
-/// concatenated frames.
+/// This implementation assumes `src` points to a single ZSTD-compressed frame
 size_t ZSTD_get_decompressed_size(const void *src, const size_t src_len) {
     istream_t in = IO_make_istream(src, src_len);
-    size_t dst_size = 0;
 
-    // Each frame header only gives us the size of its frame, so iterate over
-    // all
-    // frames
-    while (IO_istream_len(&in) > 0) {
+    // get decompressed size from ZSTD frame header
+    {
         const u32 magic_number = IO_read_bits(&in, 32);
 
-        if ((magic_number & ~0xFU) == 0x184D2A50U) {
-            // skippable frame, this has no impact on output size
-            const size_t frame_size = IO_read_bits(&in, 32);
-            IO_advance_input(&in, frame_size);
-        } else if (magic_number == 0xFD2FB528U) {
+        if (magic_number == 0xFD2FB528U) {
             // ZSTD frame
             frame_header_t header;
             parse_frame_header(&header, &in);
@@ -1451,54 +1417,13 @@ size_t ZSTD_get_decompressed_size(const void *src, const size_t src_len) {
                 return -1;
             }
 
-            dst_size += header.frame_content_size;
-
-            // Consume the input from the frame to reach the start of the next
-            traverse_frame(&header, &in);
+            return header.frame_content_size;
         } else {
-            // not a real frame
-            ERROR("Invalid magic number");
+            // not a real frame or skippable frame
+            ERROR("ZSTD frame magic number did not match");
         }
     }
-
-    return dst_size;
 }
-
-/// Iterate over each block in a frame to find the end of it, to get to the
-/// start of the next frame
-static void traverse_frame(const frame_header_t *const header, istream_t *const in) {
-    int last_block = 0;
-
-    do {
-        // Parse the block header
-        last_block = IO_read_bits(in, 1);
-        const int block_type = IO_read_bits(in, 2);
-        const size_t block_len = IO_read_bits(in, 21);
-
-        switch (block_type) {
-        case 0: // Raw block, block_len bytes
-            IO_advance_input(in, block_len);
-            break;
-        case 1: // RLE block, 1 byte
-            IO_advance_input(in, 1);
-            break;
-        case 2: // Compressed block, compressed size is block_len
-            IO_advance_input(in, block_len);
-            break;
-        case 3:
-            // Reserved block type
-            CORRUPTION();
-            break;
-        default:
-            IMPOSSIBLE();
-        }
-    } while (!last_block);
-
-    if (header->content_checksum_flag) {
-        IO_advance_input(in, 4);
-    }
-}
-
 /******* END OUTPUT SIZE COUNTING *********************************************/
 
 /******* DICTIONARY PARSING ***************************************************/
@@ -2355,4 +2280,3 @@ static void FSE_copy_dtable(FSE_dtable *const dst, const FSE_dtable *const src)
     memcpy(dst->new_state_base, src->new_state_base, size * sizeof(u16));
 }
 /******* END FSE PRIMITIVES ***************************************************/
-