Merge pull request #1405 from nghttp2/huffman

Faster Huffman encoding/decoding

lib/nghttp2_hd_huffman.c

@@ -29,114 +29,7 @@
 #include <stdio.h>
 
 #include "nghttp2_hd.h"
-
-/*
- * Encodes huffman code |sym| into |*dest_ptr|, whose least |rembits|
- * bits are not filled yet.  The |rembits| must be in range [1, 8],
- * inclusive.  At the end of the process, the |*dest_ptr| is updated
- * and points where next output should be placed. The number of
- * unfilled bits in the pointed location is returned.
- */
-static ssize_t huff_encode_sym(nghttp2_bufs *bufs, size_t *avail_ptr,
-                               size_t rembits, const nghttp2_huff_sym *sym) {
-  int rv;
-  size_t nbits = sym->nbits;
-  uint32_t code = sym->code;
-
-  /* We assume that sym->nbits <= 32 */
-  if (rembits > nbits) {
-    nghttp2_bufs_fast_orb_hold(bufs, (uint8_t)(code << (rembits - nbits)));
-    return (ssize_t)(rembits - nbits);
-  }
-
-  if (rembits == nbits) {
-    nghttp2_bufs_fast_orb(bufs, (uint8_t)code);
-    --*avail_ptr;
-    return 8;
-  }
-
-  nghttp2_bufs_fast_orb(bufs, (uint8_t)(code >> (nbits - rembits)));
-  --*avail_ptr;
-
-  nbits -= rembits;
-  if (nbits & 0x7) {
-    /* align code to MSB byte boundary */
-    code <<= 8 - (nbits & 0x7);
-  }
-
-  if (*avail_ptr < (nbits + 7) / 8) {
-    /* slow path */
-    if (nbits > 24) {
-      rv = nghttp2_bufs_addb(bufs, (uint8_t)(code >> 24));
-      if (rv != 0) {
-        return rv;
-      }
-      nbits -= 8;
-    }
-    if (nbits > 16) {
-      rv = nghttp2_bufs_addb(bufs, (uint8_t)(code >> 16));
-      if (rv != 0) {
-        return rv;
-      }
-      nbits -= 8;
-    }
-    if (nbits > 8) {
-      rv = nghttp2_bufs_addb(bufs, (uint8_t)(code >> 8));
-      if (rv != 0) {
-        return rv;
-      }
-      nbits -= 8;
-    }
-    if (nbits == 8) {
-      rv = nghttp2_bufs_addb(bufs, (uint8_t)code);
-      if (rv != 0) {
-        return rv;
-      }
-      *avail_ptr = nghttp2_bufs_cur_avail(bufs);
-      return 8;
-    }
-
-    rv = nghttp2_bufs_addb_hold(bufs, (uint8_t)code);
-    if (rv != 0) {
-      return rv;
-    }
-    *avail_ptr = nghttp2_bufs_cur_avail(bufs);
-    return (ssize_t)(8 - nbits);
-  }
-
-  /* fast path, since most code is less than 8 */
-  if (nbits < 8) {
-    nghttp2_bufs_fast_addb_hold(bufs, (uint8_t)code);
-    *avail_ptr = nghttp2_bufs_cur_avail(bufs);
-    return (ssize_t)(8 - nbits);
-  }
-
-  /* handle longer code path */
-  if (nbits > 24) {
-    nghttp2_bufs_fast_addb(bufs, (uint8_t)(code >> 24));
-    nbits -= 8;
-  }
-
-  if (nbits > 16) {
-    nghttp2_bufs_fast_addb(bufs, (uint8_t)(code >> 16));
-    nbits -= 8;
-  }
-
-  if (nbits > 8) {
-    nghttp2_bufs_fast_addb(bufs, (uint8_t)(code >> 8));
-    nbits -= 8;
-  }
-
-  if (nbits == 8) {
-    nghttp2_bufs_fast_addb(bufs, (uint8_t)code);
-    *avail_ptr = nghttp2_bufs_cur_avail(bufs);
-    return 8;
-  }
-
-  nghttp2_bufs_fast_addb_hold(bufs, (uint8_t)code);
-  *avail_ptr = nghttp2_bufs_cur_avail(bufs);
-  return (ssize_t)(8 - nbits);
-}
+#include "nghttp2_net.h"
 
 size_t nghttp2_hd_huff_encode_count(const uint8_t *src, size_t len) {
   size_t i;
@@ -151,81 +44,97 @@ size_t nghttp2_hd_huff_encode_count(const uint8_t *src, size_t len) {
 
 int nghttp2_hd_huff_encode(nghttp2_bufs *bufs, const uint8_t *src,
                            size_t srclen) {
-  int rv;
-  ssize_t rembits = 8;
-  size_t i;
+  const nghttp2_huff_sym *sym;
+  const uint8_t *end = src + srclen;
+  uint64_t code = 0;
+  uint32_t x;
+  size_t nbits = 0;
   size_t avail;
+  int rv;
 
   avail = nghttp2_bufs_cur_avail(bufs);
 
-  for (i = 0; i < srclen; ++i) {
-    const nghttp2_huff_sym *sym = &huff_sym_table[src[i]];
-    if (rembits == 8) {
-      if (avail) {
-        nghttp2_bufs_fast_addb_hold(bufs, 0);
-      } else {
-        rv = nghttp2_bufs_addb_hold(bufs, 0);
-        if (rv != 0) {
-          return rv;
-        }
-        avail = nghttp2_bufs_cur_avail(bufs);
+  for (; src != end;) {
+    sym = &huff_sym_table[*src++];
+    code |= (uint64_t)sym->code << (32 - nbits);
+    nbits += sym->nbits;
+    if (nbits < 32) {
+      continue;
+    }
+    if (avail >= 4) {
+      x = htonl((uint32_t)(code >> 32));
+      memcpy(bufs->cur->buf.last, &x, 4);
+      bufs->cur->buf.last += 4;
+      avail -= 4;
+      code <<= 32;
+      nbits -= 32;
+      continue;
+    }
+
+    for (; nbits >= 8;) {
+      rv = nghttp2_bufs_addb(bufs, (uint8_t)(code >> 56));
+      if (rv != 0) {
+        return rv;
       }
+      code <<= 8;
+      nbits -= 8;
     }
-    rembits = huff_encode_sym(bufs, &avail, (size_t)rembits, sym);
-    if (rembits < 0) {
-      return (int)rembits;
-    }
+
+    avail = nghttp2_bufs_cur_avail(bufs);
   }
-  /* 256 is special terminal symbol, pad with its prefix */
-  if (rembits < 8) {
-    /* if rembits < 8, we should have at least 1 buffer space
-       available */
-    const nghttp2_huff_sym *sym = &huff_sym_table[256];
-    assert(avail);
-    /* Caution we no longer adjust avail here */
-    nghttp2_bufs_fast_orb(
-        bufs, (uint8_t)(sym->code >> (sym->nbits - (size_t)rembits)));
+
+  for (; nbits >= 8;) {
+    rv = nghttp2_bufs_addb(bufs, (uint8_t)(code >> 56));
+    if (rv != 0) {
+      return rv;
+    }
+    code <<= 8;
+    nbits -= 8;
+  }
+
+  if (nbits) {
+    rv = nghttp2_bufs_addb(
+        bufs, (uint8_t)((uint8_t)(code >> 56) | ((1 << (8 - nbits)) - 1)));
+    if (rv != 0) {
+      return rv;
+    }
   }
 
   return 0;
 }
 
 void nghttp2_hd_huff_decode_context_init(nghttp2_hd_huff_decode_context *ctx) {
-  ctx->state = 0;
-  ctx->accept = 1;
+  ctx->fstate = NGHTTP2_HUFF_ACCEPTED;
 }
 
 ssize_t nghttp2_hd_huff_decode(nghttp2_hd_huff_decode_context *ctx,
                                nghttp2_buf *buf, const uint8_t *src,
                                size_t srclen, int final) {
-  size_t i;
+  const uint8_t *end = src + srclen;
+  nghttp2_huff_decode node = {ctx->fstate, 0};
+  const nghttp2_huff_decode *t = &node;
+  uint8_t c;
 
   /* We use the decoding algorithm described in
      http://graphics.ics.uci.edu/pub/Prefix.pdf */
-  for (i = 0; i < srclen; ++i) {
-    const nghttp2_huff_decode *t;
-
-    t = &huff_decode_table[ctx->state][src[i] >> 4];
-    if (t->flags & NGHTTP2_HUFF_FAIL) {
-      return NGHTTP2_ERR_HEADER_COMP;
-    }
-    if (t->flags & NGHTTP2_HUFF_SYM) {
+  for (; src != end;) {
+    c = *src++;
+    t = &huff_decode_table[t->fstate & 0x1ff][c >> 4];
+    if (t->fstate & NGHTTP2_HUFF_SYM) {
       *buf->last++ = t->sym;
     }
 
-    t = &huff_decode_table[t->state][src[i] & 0xf];
-    if (t->flags & NGHTTP2_HUFF_FAIL) {
-      return NGHTTP2_ERR_HEADER_COMP;
-    }
-    if (t->flags & NGHTTP2_HUFF_SYM) {
+    t = &huff_decode_table[t->fstate & 0x1ff][c & 0xf];
+    if (t->fstate & NGHTTP2_HUFF_SYM) {
       *buf->last++ = t->sym;
     }
-
-    ctx->state = t->state;
-    ctx->accept = (t->flags & NGHTTP2_HUFF_ACCEPTED) != 0;
   }
-  if (final && !ctx->accept) {
+
+  ctx->fstate = t->fstate;
+
+  if (final && !(ctx->fstate & NGHTTP2_HUFF_ACCEPTED)) {
     return NGHTTP2_ERR_HEADER_COMP;
   }
-  return (ssize_t)i;
+
+  return (ssize_t)srclen;
 }

lib/nghttp2_hd_huffman.h

@@ -34,21 +34,20 @@
 typedef enum {
   /* FSA accepts this state as the end of huffman encoding
      sequence. */
-  NGHTTP2_HUFF_ACCEPTED = 1,
+  NGHTTP2_HUFF_ACCEPTED = 1 << 14,
   /* This state emits symbol */
-  NGHTTP2_HUFF_SYM = (1 << 1),
-  /* If state machine reaches this state, decoding fails. */
-  NGHTTP2_HUFF_FAIL = (1 << 2)
+  NGHTTP2_HUFF_SYM = 1 << 15,
 } nghttp2_huff_decode_flag;
 
 typedef struct {
-  /* huffman decoding state, which is actually the node ID of internal
-     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;
+  /* fstate is the current huffman decoding state, which is actually
+     the node ID of internal huffman tree with
+     nghttp2_huff_decode_flag OR-ed.  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.  The node ID 256 is
+     a special node and it is a terminal state that means decoding
+     failed. */
+  uint16_t fstate;
   /* symbol if NGHTTP2_HUFF_SYM flag set */
   uint8_t sym;
 } nghttp2_huff_decode;
@@ -56,12 +55,8 @@ typedef struct {
 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. */
-  uint8_t state;
-  /* nonzero if we can say that the decoding process succeeds at this
-     state */
-  uint8_t accept;
+  /* fstate is the current huffman decoding state. */
+  uint16_t fstate;
 } nghttp2_hd_huff_decode_context;
 
 typedef struct {

mkhufftbl.py

@@ -357,9 +357,8 @@ def _build_transition_table(ctx, node):
 def huffman_tree_build_transition_table(ctx):
     _build_transition_table(ctx, ctx.root)
 
-NGHTTP2_HUFF_ACCEPTED = 1
-NGHTTP2_HUFF_SYM = 1 << 1
-NGHTTP2_HUFF_FAIL = 1 << 2
+NGHTTP2_HUFF_ACCEPTED = 1 << 14
+NGHTTP2_HUFF_SYM = 1 << 15
 
 def _print_transition_table(node):
     if node.term is not None:
@@ -374,8 +373,7 @@ def _print_transition_table(node):
             out = sym
             flags |= NGHTTP2_HUFF_SYM
         if nd is None:
-            id = 0
-            flags |= NGHTTP2_HUFF_FAIL
+            id = 256
         else:
             id = nd.id
             if id is None:
@@ -384,13 +382,32 @@ def _print_transition_table(node):
                 flags |= NGHTTP2_HUFF_ACCEPTED
             elif nd.accept:
                 flags |= NGHTTP2_HUFF_ACCEPTED
-        print '  {{{}, 0x{:02x}, {}}},'.format(id, flags, out)
+        print '  {{0x{:02x}, {}}},'.format(id | flags, out)
     print '},'
     _print_transition_table(node.left)
     _print_transition_table(node.right)
 
 def huffman_tree_print_transition_table(ctx):
     _print_transition_table(ctx.root)
+    print '/* 256 */'
+    print '{'
+    print '  {0x100, 0},'
+    print '  {0x100, 0},'
+    print '  {0x100, 0},'
+    print '  {0x100, 0},'
+    print '  {0x100, 0},'
+    print '  {0x100, 0},'
+    print '  {0x100, 0},'
+    print '  {0x100, 0},'
+    print '  {0x100, 0},'
+    print '  {0x100, 0},'
+    print '  {0x100, 0},'
+    print '  {0x100, 0},'
+    print '  {0x100, 0},'
+    print '  {0x100, 0},'
+    print '  {0x100, 0},'
+    print '  {0x100, 0},'
+    print '},'
 
 if __name__ == '__main__':
     ctx = Context()
@@ -423,9 +440,12 @@ typedef struct {
     print '''\
 const nghttp2_huff_sym huff_sym_table[] = {'''
     for i in range(257):
+        nbits = symbol_tbl[i][0]
+        k = int(symbol_tbl[i][1], 16)
+        k = k << (32 - nbits)
         print '''\
   {{ {}, 0x{}u }}{}\
-'''.format(symbol_tbl[i][0], symbol_tbl[i][1], ',' if i < 256 else '')
+'''.format(symbol_tbl[i][0], hex(k)[2:], ',' if i < 256 else '')
     print '};'
     print ''
 
@@ -433,14 +453,12 @@ const nghttp2_huff_sym huff_sym_table[] = {'''
 enum {{
   NGHTTP2_HUFF_ACCEPTED = {},
   NGHTTP2_HUFF_SYM = {},
-  NGHTTP2_HUFF_FAIL = {},
 }} nghttp2_huff_decode_flag;
-'''.format(NGHTTP2_HUFF_ACCEPTED, NGHTTP2_HUFF_SYM, NGHTTP2_HUFF_FAIL)
+'''.format(NGHTTP2_HUFF_ACCEPTED, NGHTTP2_HUFF_SYM)
 
     print '''\
 typedef struct {
-  uint8_t state;
-  uint8_t flags;
+  uint16_t fstate;
   uint8_t sym;
 } nghttp2_huff_decode;
 '''

tests/main.c

@@ -402,6 +402,7 @@ int main() {
                    test_nghttp2_hd_deflate_hd_vec) ||
       !CU_add_test(pSuite, "hd_decode_length", test_nghttp2_hd_decode_length) ||
       !CU_add_test(pSuite, "hd_huff_encode", test_nghttp2_hd_huff_encode) ||
+      !CU_add_test(pSuite, "hd_huff_decode", test_nghttp2_hd_huff_decode) ||
       !CU_add_test(pSuite, "adjust_local_window_size",
                    test_nghttp2_adjust_local_window_size) ||
       !CU_add_test(pSuite, "check_header_name",

tests/nghttp2_hd_test.c

@@ -1538,3 +1538,32 @@ void test_nghttp2_hd_huff_encode(void) {
 
   nghttp2_bufs_free(&bufs);
 }
+
+void test_nghttp2_hd_huff_decode(void) {
+  const uint8_t e[] = {0x1f, 0xff, 0xff, 0xff, 0xff, 0xff};
+  nghttp2_hd_huff_decode_context ctx;
+  nghttp2_buf outbuf;
+  uint8_t b[256];
+  ssize_t len;
+
+  nghttp2_buf_wrap_init(&outbuf, b, sizeof(b));
+  nghttp2_hd_huff_decode_context_init(&ctx);
+  len = nghttp2_hd_huff_decode(&ctx, &outbuf, e, 1, 1);
+
+  CU_ASSERT(1 == len);
+  CU_ASSERT(0 == memcmp("a", outbuf.pos, 1));
+
+  /* Premature sequence must elicit decoding error */
+  nghttp2_buf_wrap_init(&outbuf, b, sizeof(b));
+  nghttp2_hd_huff_decode_context_init(&ctx);
+  len = nghttp2_hd_huff_decode(&ctx, &outbuf, e, 2, 1);
+
+  CU_ASSERT(NGHTTP2_ERR_HEADER_COMP == len);
+
+  /* Fully decoding EOS is error */
+  nghttp2_buf_wrap_init(&outbuf, b, sizeof(b));
+  nghttp2_hd_huff_decode_context_init(&ctx);
+  len = nghttp2_hd_huff_decode(&ctx, &outbuf, e, 2, 6);
+
+  CU_ASSERT(NGHTTP2_ERR_HEADER_COMP == len);
+}

tests/nghttp2_hd_test.h

@@ -50,5 +50,6 @@ void test_nghttp2_hd_public_api(void);
 void test_nghttp2_hd_deflate_hd_vec(void);
 void test_nghttp2_hd_decode_length(void);
 void test_nghttp2_hd_huff_encode(void);
+void test_nghttp2_hd_huff_decode(void);
 
 #endif /* NGHTTP2_HD_TEST_H */