From 9e1fbffc37779c8f491d3aa8e319ab233e5ab0da Mon Sep 17 00:00:00 2001
From: wm4 <wm4@nowhere>
Date: Sun, 5 Nov 2017 16:36:18 +0100
Subject: demux_mkv: rewrite packet reading to avoid 1 memcpy()

This directly reads individual mkv sub-packets (block laces) into a
dedicated AVBufferRefs, which can be directly used for creating packets
without a additional copy of the packet data. This also means we switch
parsing of block header fields and lacing metadata to read directly from
the stream, instead of a memory buffer.

This could have been much easier if libavcodec didn't require padding
the packet data with zero bytes. We could just have each packet
reference a slice of the block data. But as it is, the only way to get
padding without a copy is to read the laces into individually allocated
(and padded) memory block, which required a larger rewrite.

This probably makes recovering from broken mkv files slightly worse if
the transport is unseekable. We just read, and then check if we've
overread. But I think that shouldn't be a real concern.

No actual measureable performance change. Potential for some
regressions, as this is quite intrusive, and touches weird obscure shit
like mkv lacing. Still keeping it because I like how it removes some
redundant EBML parsing functions.
---
 demux/ebml.c | 63 +++++++++++++++---------------------------------------------
 1 file changed, 16 insertions(+), 47 deletions(-)

(limited to 'demux/ebml.c')

diff --git a/demux/ebml.c b/demux/ebml.c
index e05f7245b0..d2a8629136 100644
--- a/demux/ebml.c
+++ b/demux/ebml.c
@@ -74,82 +74,51 @@ uint32_t ebml_read_id(stream_t *s)
 }
 
 /*
- * Read a variable length unsigned int.
+ * Read: element content length.
  */
-uint64_t ebml_read_vlen_uint(bstr *buffer)
+uint64_t ebml_read_length(stream_t *s)
 {
     int i, j, num_ffs = 0, len_mask = 0x80;
-    uint64_t num;
-
-    if (buffer->len == 0)
-        return EBML_UINT_INVALID;
+    uint64_t len;
 
-    for (i = 0, num = buffer->start[0]; i < 8 && !(num & len_mask); i++)
+    for (i = 0, len = stream_read_char(s); i < 8 && !(len & len_mask); i++)
         len_mask >>= 1;
     if (i >= 8)
         return EBML_UINT_INVALID;
     j = i + 1;
-    if ((int) (num &= (len_mask - 1)) == len_mask - 1)
+    if ((int) (len &= (len_mask - 1)) == len_mask - 1)
         num_ffs++;
-    if (j > buffer->len)
-        return EBML_UINT_INVALID;
-    for (int n = 0; n < i; n++) {
-        num = (num << 8) | buffer->start[n + 1];
-        if ((num & 0xFF) == 0xFF)
+    while (i--) {
+        len = (len << 8) | stream_read_char(s);
+        if ((len & 0xFF) == 0xFF)
             num_ffs++;
     }
     if (j == num_ffs)
         return EBML_UINT_INVALID;
-    buffer->start += j;
-    buffer->len -= j;
-    return num;
+    if (len >= 1ULL<<63)   // Can happen if stream_read_char returns EOF
+        return EBML_UINT_INVALID;
+    return len;
 }
 
+
 /*
  * Read a variable length signed int.
  */
-int64_t ebml_read_vlen_int(bstr *buffer)
+int64_t ebml_read_signed_length(stream_t *s)
 {
     uint64_t unum;
     int l;
 
     /* read as unsigned number first */
-    size_t len = buffer->len;
-    unum = ebml_read_vlen_uint(buffer);
+    uint64_t offset = stream_tell(s);
+    unum = ebml_read_length(s);
     if (unum == EBML_UINT_INVALID)
         return EBML_INT_INVALID;
-    l = len - buffer->len;
+    l = stream_tell(s) - offset;
 
     return unum - ((1LL << ((7 * l) - 1)) - 1);
 }
 
-/*
- * Read: element content length.
- */
-uint64_t ebml_read_length(stream_t *s)
-{
-    int i, j, num_ffs = 0, len_mask = 0x80;
-    uint64_t len;
-
-    for (i = 0, len = stream_read_char(s); i < 8 && !(len & len_mask); i++)
-        len_mask >>= 1;
-    if (i >= 8)
-        return EBML_UINT_INVALID;
-    j = i + 1;
-    if ((int) (len &= (len_mask - 1)) == len_mask - 1)
-        num_ffs++;
-    while (i--) {
-        len = (len << 8) | stream_read_char(s);
-        if ((len & 0xFF) == 0xFF)
-            num_ffs++;
-    }
-    if (j == num_ffs)
-        return EBML_UINT_INVALID;
-    if (len >= 1ULL<<63)   // Can happen if stream_read_char returns EOF
-        return EBML_UINT_INVALID;
-    return len;
-}
-
 /*
  * Read the next element as an unsigned int.
  */
-- 
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