Reduce huffman decoding table

Previously we have uint16_t as state member variable in
nghttp2_huff_decode structure to express -1 as failure.  This is
because we have 256 valid states.  However, we can express failed
state using flags member variable and make state uint8_t.  This commit
does this and as a result the size of decoding table is reduced.

lib/nghttp2_hd_huffman.c

@@ -179,10 +179,8 @@ ssize_t nghttp2_hd_huff_decode(nghttp2_hd_huff_decode_context *ctx,
     for(j = 0; j < 2; ++j) {
       const nghttp2_huff_decode *t;
 
-      assert(ctx->state >= 0);
-
       t = &huff_decode_table[ctx->state][in];
-      if(t->state == -1) {
+      if(t->flags & NGHTTP2_HUFF_FAIL) {
         return NGHTTP2_ERR_HEADER_COMP;
       }
       if(t->flags & NGHTTP2_HUFF_SYM) {

lib/nghttp2_hd_huffman.h

@@ -36,13 +36,17 @@ typedef enum {
      sequence. */
   NGHTTP2_HUFF_ACCEPTED = 1,
   /* This state emits symbol */
-  NGHTTP2_HUFF_SYM = (1 << 1)
+  NGHTTP2_HUFF_SYM = (1 << 1),
+  /* If state machine reaches this state, decoding fails. */
+  NGHTTP2_HUFF_FAIL = (1 << 2),
 } nghttp2_huff_decode_flag;
 
 typedef struct {
   /* huffman decoding state, which is actually the node ID of internal
-     huffman tree */
-  int16_t state;
+     huffman tree.  We have 257 leaf nodes, but they are identical to
+     root node other than emitting a symbol, so we have 256 internal
+     nodes [1..255], inclusive. */
+  uint8_t state;
   /* bitwise OR of zero or more of the nghttp2_huff_decode_flag */
   uint8_t flags;
   /* symbol if NGHTTP2_HUFF_SYM flag set */
@@ -54,7 +58,7 @@ typedef nghttp2_huff_decode huff_decode_table_type[16];
 typedef struct {
   /* Current huffman decoding state. We stripped leaf nodes, so the
      value range is [0..255], inclusive. */
-  int16_t state;
+  uint8_t state;
   /* nonzero if we can say that the decoding process succeeds at this
      state */
   uint8_t accept;

lib/nghttp2_hd_huffman_data.c

@@ -3624,7 +3624,7 @@ const nghttp2_huff_decode huff_decode_table[][16] = {
     {0, 0x03, 253},
     {0, 0x03, 254},
     {0, 0x03, 255},
-    {-1, 0x00, 0},
+    {0, 0x04, 0},
     {189, 0x00, 0},
     {191, 0x00, 0},
     {192, 0x00, 0},
@@ -3772,8 +3772,8 @@ const nghttp2_huff_decode huff_decode_table[][16] = {
     {14, 0x03, 254},
     {1, 0x02, 255},
     {14, 0x03, 255},
-    {-1, 0x00, 0},
-    {-1, 0x00, 0},
+    {0, 0x04, 0},
+    {0, 0x04, 0},
     {0, 0x03, 0},
     {0, 0x03, 1},
     {0, 0x03, 2},
@@ -3840,10 +3840,10 @@ const nghttp2_huff_decode huff_decode_table[][16] = {
   },
   /* 187 */
   {
-    {-1, 0x00, 0},
-    {-1, 0x00, 0},
-    {-1, 0x00, 0},
-    {-1, 0x00, 0},
+    {0, 0x04, 0},
+    {0, 0x04, 0},
+    {0, 0x04, 0},
+    {0, 0x04, 0},
     {1, 0x02, 0},
     {14, 0x03, 0},
     {1, 0x02, 1},
@@ -3859,14 +3859,14 @@ const nghttp2_huff_decode huff_decode_table[][16] = {
   },
   /* 188 */
   {
-    {-1, 0x00, 0},
-    {-1, 0x00, 0},
-    {-1, 0x00, 0},
-    {-1, 0x00, 0},
-    {-1, 0x00, 0},
-    {-1, 0x00, 0},
-    {-1, 0x00, 0},
-    {-1, 0x00, 0},
+    {0, 0x04, 0},
+    {0, 0x04, 0},
+    {0, 0x04, 0},
+    {0, 0x04, 0},
+    {0, 0x04, 0},
+    {0, 0x04, 0},
+    {0, 0x04, 0},
+    {0, 0x04, 0},
     {2, 0x02, 0},
     {6, 0x02, 0},
     {15, 0x02, 0},

mkhufftbl.py

@@ -84,6 +84,7 @@ def dfs(node, root):
 
 NGHTTP2_HUFF_ACCEPTED = 1
 NGHTTP2_HUFF_SYM = 1 << 1
+NGHTTP2_HUFF_FAIL = 1 << 2
 
 def dfs_print(node):
     if node.term is not None:
@@ -100,7 +101,8 @@ def dfs_print(node):
             out = syms[0]
             flags |= NGHTTP2_HUFF_SYM
         if nd is None:
-            id = -1
+            id = 0
+            flags |= NGHTTP2_HUFF_FAIL
         else:
             id = nd.id
             if id is None:
@@ -159,13 +161,14 @@ print ''
 print '''\
 enum {{
   NGHTTP2_HUFF_ACCEPTED = {},
-  NGHTTP2_HUFF_SYM = {}
+  NGHTTP2_HUFF_SYM = {},
+  NGHTTP2_HUFF_FAIL = {},
 }} nghttp2_huff_decode_flag;
-'''.format(NGHTTP2_HUFF_ACCEPTED, NGHTTP2_HUFF_SYM)
+'''.format(NGHTTP2_HUFF_ACCEPTED, NGHTTP2_HUFF_SYM, NGHTTP2_HUFF_FAIL)
 
 print '''\
 typedef struct {
-  int16_t state;
+  uint8_t state;
   uint8_t flags;
   uint8_t sym;
 } nghttp2_huff_decode;