[Deflate] Check that 16 is not the first code length symbol

This symbol in the dynamic Huffman block encodes copy of the previous code length. If this symbol is the first, then there is nothing to copy. Previously, this could cause a crash.
2026-06-23 14:56:41 +00:00 · 2026-02-28 01:26:43 +08:00
parent 3bfb4a1fa2
commit ccf97c995b
1 changed files with 4 additions and 3 deletions
@@ -120,9 +120,10 @@ public class Deflate: DecompressionAlgorithm {
                            // It is a raw code length.
                            codeLengths[n] = symbol
                            n += 1
-                        } else if symbol == 16 {
-                            // Copy previous code length 3 to 6 times.
-                            // Next two bits show how many times we need to copy.
+                        } else if symbol == 16 && n > codeLengths.startIndex {
+                            // Copy previous code length 3 to 6 times. Next two bits show how many times we need to copy.
+                            // This symbol cannot be the first symbol encoding code lengths, since there is nothing to
+                            // copy at this point.
                            guard bitReader.bitsLeft >= 2
                                else { throw DeflateError.symbolNotFound }
                            let copyCount = bitReader.int(fromBits: 2) + 3