avcodec/bitstream: Add second function to create VLCs

When using ff_init_vlc_sparse() to create a VLC, three input tables are
used: A table for lengths, one for codes and one for symbols; the latter
one can be omitted, then a default one will be used. These input tables
will be traversed twice, once to get the long codes (which will be put
into subtables) and once for the small codes. The long codes are then
sorted so that entries that should be in the same subtable are
contiguous.

This commit adds an alternative to ff_init_vlc_sparse():
ff_init_vlc_from_lengths(). It is based upon the observation that if
lengths, codes and symbols tables are permuted (in the same way) so that
the codes are ordered from left to right in the corresponding tree and
if said tree is complete (i.e. every non-leaf node has two children),
the codes can be easily computed from the lengths and are therefore
redundant. This means that if one initializes such a VLC with explicitly
coded lengths, codes and symbols, the codes can be avoided; and even if
one has no explicitly coded symbols, it might still be beneficial to
remove the codes even when one has to add a new symbol table, because
codes are typically longer than symbols so that the latter often fit
into a smaller type, saving space.

Furthermore, given that the codes here are by definition ordered from
left to right, it is unnecessary to sort them again; for the same
reason, one does not have to traverse the input twice. This function
proved to be faster than ff_init_vlc_sparse() whenever it has been
benchmarked.

This function is usable for static tables (they can simply be permuted
once) as well as in scenarios where the tables are naturally ordered
from left to right in the tree; the latter e.g. happens with Smacker,
Theora and several other formats.

In order to make it also usable for (static) tables with incomplete trees,
negative lengths are used to indicate that there is an open end of a
certain length.

Finally, ff_init_vlc_from_lengths() has one downside compared to
ff_init_vlc_sparse(): The latter uses tables that can be reused by
encoders. Of course, one could calculate the needed table at runtime
if one so wishes, but it is nevertheless an obstacle.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

1 files changed, 102 insertions, 36 deletions

diff --git a/libavcodec/bitstream.c b/libavcodec/bitstream.c
index c7a87734e5..4ce49fd51c 100644
--- a/libavcodec/bitstream.c
+++ b/libavcodec/bitstream.c
@@ -132,6 +132,8 @@ static int alloc_table(VLC *vlc, int size, int use_static)
     return index;
 }
 
+#define LOCALBUF_ELEMS 1500 // the maximum currently needed is 1296 by rv34
+
 typedef struct VLCcode {
     uint8_t bits;
     VLC_TYPE symbol;
@@ -140,6 +142,31 @@ typedef struct VLCcode {
     uint32_t code;
 } VLCcode;
 
+static int vlc_common_init(VLC *vlc_arg, int nb_bits, int nb_codes,
+                           VLC **vlc, VLC *localvlc, VLCcode **buf,
+                           int flags)
+{
+    *vlc = vlc_arg;
+    (*vlc)->bits = nb_bits;
+    if (flags & INIT_VLC_USE_NEW_STATIC) {
+        av_assert0(nb_codes <= LOCALBUF_ELEMS);
+        *localvlc = *vlc_arg;
+        *vlc = localvlc;
+        (*vlc)->table_size = 0;
+    } else {
+        (*vlc)->table           = NULL;
+        (*vlc)->table_allocated = 0;
+        (*vlc)->table_size      = 0;
+    }
+    if (nb_codes > LOCALBUF_ELEMS) {
+        *buf = av_malloc_array(nb_codes, sizeof(VLCcode));
+        if (!*buf)
+            return AVERROR(ENOMEM);
+    }
+
+    return 0;
+}
+
 static int compare_vlcspec(const void *a, const void *b)
 {
     const VLCcode *sa = a, *sb = b;
@@ -248,6 +275,27 @@ static int build_table(VLC *vlc, int table_nb_bits, int nb_codes,
     return table_index;
 }
 
+static int vlc_common_end(VLC *vlc, int nb_bits, int nb_codes, VLCcode *codes,
+                          int flags, VLC *vlc_arg, VLCcode localbuf[LOCALBUF_ELEMS])
+{
+    int ret = build_table(vlc, nb_bits, nb_codes, codes, flags);
+
+    if (flags & INIT_VLC_USE_NEW_STATIC) {
+        if(vlc->table_size != vlc->table_allocated)
+            av_log(NULL, AV_LOG_ERROR, "needed %d had %d\n", vlc->table_size, vlc->table_allocated);
+
+        av_assert0(ret >= 0);
+        *vlc_arg = *vlc;
+    } else {
+        if (codes != localbuf)
+            av_free(codes);
+        if (ret < 0) {
+            av_freep(&vlc->table);
+            return ret;
+        }
+    }
+    return 0;
+}
 
 /* Build VLC decoding tables suitable for use with get_vlc().
 
@@ -278,30 +326,14 @@ int ff_init_vlc_sparse(VLC *vlc_arg, int nb_bits, int nb_codes,
                        const void *symbols, int symbols_wrap, int symbols_size,
                        int flags)
 {
-    VLCcode *buf;
+    VLCcode localbuf[LOCALBUF_ELEMS], *buf = localbuf;
     int i, j, ret;
-    VLCcode localbuf[1500]; // the maximum currently needed is 1296 by rv34
     VLC localvlc, *vlc;
 
-    vlc = vlc_arg;
-    vlc->bits = nb_bits;
-    if (flags & INIT_VLC_USE_NEW_STATIC) {
-        av_assert0(nb_codes <= FF_ARRAY_ELEMS(localbuf));
-        localvlc = *vlc_arg;
-        vlc = &localvlc;
-        vlc->table_size = 0;
-    } else {
-        vlc->table           = NULL;
-        vlc->table_allocated = 0;
-        vlc->table_size      = 0;
-    }
-    if (nb_codes > FF_ARRAY_ELEMS(localbuf)) {
-        buf = av_malloc_array(nb_codes, sizeof(VLCcode));
-        if (!buf)
-            return AVERROR(ENOMEM);
-    } else
-        buf = localbuf;
-
+    ret = vlc_common_init(vlc_arg, nb_bits, nb_codes, &vlc, &localvlc,
+                          &buf, flags);
+    if (ret < 0)
+        return ret;
 
     av_assert0(symbols_size <= 2 || !symbols);
     j = 0;
@@ -342,26 +374,60 @@ int ff_init_vlc_sparse(VLC *vlc_arg, int nb_bits, int nb_codes,
     COPY(len && len <= nb_bits);
     nb_codes = j;
 
-    ret = build_table(vlc, nb_bits, nb_codes, buf, flags);
-
-    if (flags & INIT_VLC_USE_NEW_STATIC) {
-        if(vlc->table_size != vlc->table_allocated)
-            av_log(NULL, AV_LOG_ERROR, "needed %d had %d\n", vlc->table_size, vlc->table_allocated);
+    return vlc_common_end(vlc, nb_bits, nb_codes, buf,
+                          flags, vlc_arg, localbuf);
+}
 
-        av_assert0(ret >= 0);
-        *vlc_arg = *vlc;
-    } else {
-        if (buf != localbuf)
-            av_free(buf);
-        if (ret < 0) {
-            av_freep(&vlc->table);
-            return ret;
+int ff_init_vlc_from_lengths(VLC *vlc_arg, int nb_bits, int nb_codes,
+                             const int8_t *lens, int lens_wrap,
+                             const void *symbols, int symbols_wrap, int symbols_size,
+                             int offset, int flags, void *logctx)
+{
+    VLCcode localbuf[LOCALBUF_ELEMS], *buf = localbuf;
+    VLC localvlc, *vlc;
+    uint64_t code;
+    int ret, j, len_max = FFMIN(32, 3 * nb_bits);
+
+    ret = vlc_common_init(vlc_arg, nb_bits, nb_codes, &vlc, &localvlc,
+                          &buf, flags);
+    if (ret < 0)
+        return ret;
+
+    j = code = 0;
+    for (int i = 0; i < nb_codes; i++, lens += lens_wrap) {
+        int len = *lens;
+        if (len > 0) {
+            unsigned sym;
+
+            buf[j].bits = len;
+            if (symbols)
+                GET_DATA(sym, symbols, i, symbols_wrap, symbols_size)
+            else
+                sym = i;
+            buf[j].symbol = sym + offset;
+            buf[j++].code = code;
+        } else if (len <  0) {
+            len = -len;
+        } else
+            continue;
+        if (len > len_max || code & ((1U << (32 - len)) - 1)) {
+            av_log(logctx, AV_LOG_ERROR, "Invalid VLC (length %u)\n", len);
+            goto fail;
+        }
+        code += 1U << (32 - len);
+        if (code > UINT32_MAX + 1ULL) {
+            av_log(logctx, AV_LOG_ERROR, "Overdetermined VLC tree\n");
+            goto fail;
         }
     }
-    return 0;
+    return vlc_common_end(vlc, nb_bits, j, buf,
+                          flags, vlc_arg, localbuf);
+fail:
+    if (buf != localbuf)
+        av_free(buf);
+    return AVERROR_INVALIDDATA;
 }
 
-
 void ff_free_vlc(VLC *vlc)
 {
     av_freep(&vlc->table);