Add un-zigzag table.

diff --git a/driver.c b/driver.c
index 08b684a0715162f058a12fe920c7e9c4fb90fd62..dd1b1ae8d8796840a51b7dbd7c4d8119b25fbad9 100644 (file)
--- a/driver.c
+++ b/driver.c
@@ -5,6 +5,7 @@
 #include "choice.h"
 #include "dehuff.h"
 #include "input.h"
+#include "zigzag.h"
 
 struct jpeg_image {
        unsigned precision;
@@ -121,7 +122,7 @@ void read_scan(struct byte_source* source, struct jpeg_image* image, huffman_tab
                                        possibly_refill(&bits, s);
 
                                        i += r;
-                                       zz[i] = extend(read_bits(&bits, s), s);
+                                       zz[unzigzag[i]] = extend(read_bits(&bits, s), s);
                                }
                                
                                for (unsigned i = 0; i < 63; ++i) {

diff --git a/zigzag.h b/zigzag.h
new file mode 100644 (file)
index 0000000..3fc1377
--- /dev/null
+++ b/zigzag.h
@@ -0,0 +1,26 @@
+#ifndef _ZIGZAG_H
+#define _ZIGZAG_H
+
+#include "idct.h"
+
+// Table for transforming from zig-zag order to natural order.
+// We use the same trick as libjpeg here; there are 16 extra entries
+// after the end of the table, since the run-length decoder could
+// potentially cause entries indices >= 64 to be decoded.
+static const unsigned unzigzag[DCTSIZE2 + 16] = {
+       // Regular entries.
+        0,  1,  8, 16,  9,  2,  3, 10,
+       17, 24, 32, 25, 18, 11,  4,  5,
+       12, 19, 26, 33, 40, 48, 41, 34,
+       27, 20, 13,  6,  7, 14, 21, 28,
+       35, 42, 49, 56, 57, 50, 43, 36,
+       29, 22, 15, 23, 30, 37, 44, 51,
+       58, 59, 52, 45, 38, 31, 39, 46,
+       53, 60, 61, 54, 47, 55, 62, 63,
+
+       // Extra padding entries; should never be referenced in well-formed data.
+       63, 63, 63, 63, 63, 63, 63, 63,
+       63, 63, 63, 63, 63, 63, 63, 63,
+};
+
+#endif /* !defined(_ZIGZAG_H) */