diff --git a/xdelta3/xdelta3-compr.h b/xdelta3/xdelta3-compr.h
new file mode 100644
index 00000000..746cd838
--- /dev/null
+++ b/xdelta3/xdelta3-compr.h
@@ -0,0 +1,42 @@
+int gz_detect_func(uint8_t *data, int len)
+{
+  uint8_t flags;
+  if(len < 9) return -1;
+  flags = data[8];
+  switch(flags) {
+    case 2: return 9; /* maximum compr */
+    case 4: return 1; /* minimum compr */
+  }
+  return 6; /* could be 2..8 but 6 is default */
+}
+
+int bz2_detect_func(uint8_t *data, int len)
+{
+  if(len < 4) return -1;
+  return data[3]&0xf;
+}
+
+int xz_detect_func(uint8_t *data, int len)
+{
+  int offs;
+  int dict_size;
+  /* there might be extra headers which need to be skipped */
+  for( offs = 14; offs < 26; offs++) {
+    if(offs+2 >= len) return -1;
+    if(data[offs+0] != 0x21) continue; /* LZMA filter */
+    if(data[offs+1] != 0x01) continue; /* Size of Filter Properties:  1 byte */
+    dict_size = data[offs+2];
+    switch(dict_size) {
+      case 12: return 0;
+      case 16: return 1;
+      case 18: return 2;
+      case 20: return 3; /* could also be 4 */
+      case 22: return 6; /* could also be 5 but 6 is the default so the guess is correct in 99% of cases */
+      case 24: return 7;
+      case 26: return 8;
+      case 28: return 9;
+      default: return -1; /* not guessable */
+    }
+  }
+  return -1;
+}
diff --git a/xdelta3/xdelta3-internal.h b/xdelta3/xdelta3-internal.h
index 35de56bd..5fa3d82b 100644
--- a/xdelta3/xdelta3-internal.h
+++ b/xdelta3/xdelta3-internal.h
@@ -87,6 +87,7 @@ struct _main_file
   const char         *realname;      /* File name or /dev/stdin,
 				      * /dev/stdout, /dev/stderr. */
   const main_extcomp *compressor;    /* External compression struct. */
+  int                 compression_level; /* 0..9 or -1 if not detected */
   int                 flags;         /* RD_FIRST, RD_NONEXTERNAL, ... */
   xoff_t              nread;         /* for input position */
   xoff_t              nwrite;        /* for output position */
diff --git a/xdelta3/xdelta3-main.h b/xdelta3/xdelta3-main.h
index 018a6971..f2572fd6 100644
--- a/xdelta3/xdelta3-main.h
+++ b/xdelta3/xdelta3-main.h
@@ -207,6 +207,8 @@ struct _main_extcomp
   const char    *magic;
   usize_t        magic_size;
   int            flags;
+/* function for detecting compression level of input file */
+  int          (*detect_func)(uint8_t *data, int len);
 };
 
 /* Merge state: */
@@ -287,16 +289,17 @@ static xd3_stream *recode_stream = NULL;
 /* merge_stream is used by merge commands for storing the source encoding */
 static xd3_stream *merge_stream = NULL;
 
+#include "xdelta3-compr.h"
 /* This array of compressor types is compiled even if EXTERNAL_COMPRESSION is
  * false just so the program knows the mapping of IDENT->NAME. */
 static main_extcomp extcomp_types[] =
 {
-  { "bzip2",    "-c",   "bzip2",      "-dc",   "B", "BZh",          3, 0 },
-  { "gzip",     "-c",   "gzip",       "-dc",   "G", "\037\213",     2, 0 },
-  { "compress", "-c",   "uncompress", "-c",    "Z", "\037\235",     2, 0 },
+  { "bzip2",    "-c",   "bzip2",      "-dc",   "B", "BZh",          3, 0, bz2_detect_func },
+  { "gzip",     "-cn",  "gzip",       "-dc",   "G", "\037\213",     2, 0, gz_detect_func },
+  { "compress", "-c",   "uncompress", "-c",    "Z", "\037\235",     2, 0, NULL },
 
   /* Xz is lzma with a magic number http://tukaani.org/xz/format.html */
-  { "xz", "-c", "xz", "-dc", "Y", "\xfd\x37\x7a\x58\x5a\x00", 2, 0 },
+  { "xz", "-c", "xz", "-dc", "Y", "\xfd\x37\x7a\x58\x5a\x00", 6, 0, xz_detect_func },
 };
 
 static int main_input (xd3_cmd cmd, main_file *ifile,
@@ -2453,7 +2456,12 @@ main_secondary_decompress_check (main_file  *file,
 
 	      if (memcmp (check_buf, decomp->magic, decomp->magic_size) == 0)
 		{
+		  int compression_level = -1;
 		  decompressor = decomp;
+		  if(decomp->detect_func) {
+		    compression_level = decomp->detect_func(check_buf, try_read);
+		  }
+		  file->compression_level = compression_level;
 		  break;
 		}
 	    }