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, 48 insertions, 0 deletions

diff --git a/libavcodec/vlc.h b/libavcodec/vlc.h
index 22d3e33485..50a1834b84 100644
--- a/libavcodec/vlc.h
+++ b/libavcodec/vlc.h
@@ -49,6 +49,41 @@ int ff_init_vlc_sparse(VLC *vlc, int nb_bits, int nb_codes,
                        const void *codes, int codes_wrap, int codes_size,
                        const void *symbols, int symbols_wrap, int symbols_size,
                        int flags);
+
+/**
+ * Build VLC decoding tables suitable for use with get_vlc2()
+ *
+ * This function takes lengths and symbols and calculates the codes from them.
+ * For this the input lengths and symbols have to be sorted according to "left
+ * nodes in the corresponding tree first".
+ *
+ * @param[in,out] vlc      The VLC to be initialized; table and table_allocated
+ *                         must have been set when initializing a static VLC,
+ *                         otherwise this will be treated as uninitialized.
+ * @param[in] nb_bits      The number of bits to use for the VLC table;
+ *                         higher values take up more memory and cache, but
+ *                         allow to read codes with fewer reads.
+ * @param[in] nb_codes     The number of provided length and (if supplied) symbol
+ *                         entries.
+ * @param[in] lens         The lengths of the codes. Entries > 0 correspond to
+ *                         valid codes; entries == 0 will be skipped and entries
+ *                         with len < 0 indicate that the tree is incomplete and
+ *                         has an open end of length -len at this position.
+ * @param[in] lens_wrap    Stride (in bytes) of the lengths.
+ * @param[in] symbols      The symbols, i.e. what is returned from get_vlc2()
+ *                         when the corresponding code is encountered.
+ *                         May be NULL, then 0, 1, 2, 3, 4,... will be used.
+ * @param[in] symbols_wrap Stride (in bytes) of the symbols.
+ * @param[in] symbols_size Size of the symbols. 1 and 2 are supported.
+ * @param[in] offset       An offset to apply to all the valid symbols.
+ * @param[in] flags        A combination of the INIT_VLC_* flags; notice that
+ *                         INIT_VLC_INPUT_LE is pointless and ignored.
+ */
+int ff_init_vlc_from_lengths(VLC *vlc, 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);
+
 void ff_free_vlc(VLC *vlc);
 
 /* If INIT_VLC_INPUT_LE is set, the LSB bit of the codes used to
@@ -87,4 +122,17 @@ void ff_free_vlc(VLC *vlc);
 #define INIT_LE_VLC_STATIC(vlc, bits, a, b, c, d, e, f, g, static_size) \
     INIT_LE_VLC_SPARSE_STATIC(vlc, bits, a, b, c, d, e, f, g, NULL, 0, 0, static_size)
 
+#define INIT_VLC_STATIC_FROM_LENGTHS(vlc, bits, nb_codes, lens, len_wrap,  \
+                                     symbols, symbols_wrap, symbols_size,  \
+                                     offset, flags, static_size)           \
+    do {                                                                   \
+        static VLC_TYPE table[static_size][2];                             \
+        (vlc)->table           = table;                                    \
+        (vlc)->table_allocated = static_size;                              \
+        ff_init_vlc_from_lengths(vlc, bits, nb_codes, lens, len_wrap,      \
+                                 symbols, symbols_wrap, symbols_size,      \
+                                 offset, flags | INIT_VLC_USE_NEW_STATIC,  \
+                                 NULL);                                    \
+    } while (0)
+
 #endif /* AVCODEC_VLC_H */