avutil/timestamp: introduce av_ts_make_time_string2 for better precision

av_ts_make_time_string() used "%.6g" format, but this format was losing
precision even when the timestamp to be printed was not that large. For example
for 3 hours (10800) seconds, only 1 decimal digit was printed, which made this
format inaccurate when it was used in e.g. the silencedetect filter. Other
detection filters printing timestamps had similar issues. Also time base
parameter of the function was *AVRational instead of AVRational.

Resolve these problems by introducing a new function, av_ts_make_time_string2().

We change the used format to "%.*f", use a precision of 6, except when printing
values near 0, in which case we calculate the precision dynamically to aim for
a similar precision in normal form as with %.6g.  No longer using scientific
representation can make parsing the timestamp easier for the users, we can
safely do this because the theoretical maximum of INT64_MAX*INT32_MAX still
fits into the string buffer in normal form.

We somewhat imitate %g by trimming ending zeroes and the potential decimal
point characters. In order not to trim "inf" as well, we assume that the
decimal point string does not contain the letter "f". Note that depending on
printf %f implementation, we might trim "infinity" to "inf".

Thanks for Allan Cady for bringing up this issue.

Signed-off-by: Marton Balint <cus@passwd.hu>

5 files changed, 51 insertions, 1 deletions

diff --git a/doc/APIchanges b/doc/APIchanges
index 7762907255..38fbfcca20 100644
--- a/doc/APIchanges
+++ b/doc/APIchanges
@@ -2,6 +2,11 @@ The last version increases of all libraries were on 2024-03-07
 
 API changes, most recent first:
 
+2024-03-xx - xxxxxxxxxx - lavu 59.7.100 - timestamp.h
+  Add av_ts_make_time_string2() for better timestamp precision, the new
+  function accepts AVRational as time base instead of *AVRational, and is not
+  an inline function like its predecessor.
+
 2024-03-xx - xxxxxxxxxx - lavu 59.6.100 - film_grain_params.h
   Add av_film_grain_params_select().
 
diff --git a/libavutil/Makefile b/libavutil/Makefile
index 008fcfcd9c..6e6fa8d800 100644
--- a/libavutil/Makefile
+++ b/libavutil/Makefile
@@ -174,6 +174,7 @@ OBJS = adler32.o                                                        \
        threadmessage.o                                                  \
        time.o                                                           \
        timecode.o                                                       \
+       timestamp.o                                                      \
        tree.o                                                           \
        twofish.o                                                        \
        utils.o                                                          \
diff --git a/libavutil/timestamp.c b/libavutil/timestamp.c
new file mode 100644
index 0000000000..2a3e3012a4
--- /dev/null
+++ b/libavutil/timestamp.c
@@ -0,0 +1,36 @@
+/*
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "timestamp.h"
+
+char *av_ts_make_time_string2(char *buf, int64_t ts, AVRational tb)
+{
+    if (ts == AV_NOPTS_VALUE) {
+        snprintf(buf, AV_TS_MAX_STRING_SIZE, "NOPTS");
+    } else {
+        double val = av_q2d(tb) * ts;
+        double log = floor(log10(fabs(val)));
+        int precision = (isfinite(log) && log < 0) ? -log + 5 : 6;
+        int last = snprintf(buf, AV_TS_MAX_STRING_SIZE, "%.*f", precision, val);
+        last = FFMIN(last, AV_TS_MAX_STRING_SIZE - 1) - 1;
+        for (; last && buf[last] == '0'; last--);
+        for (; last && buf[last] != 'f' && (buf[last] < '0' || buf[0] > '9'); last--);
+        buf[last + 1] = '\0';
+    }
+    return buf;
+}
diff --git a/libavutil/timestamp.h b/libavutil/timestamp.h
index 2b37781eba..7e6da894df 100644
--- a/libavutil/timestamp.h
+++ b/libavutil/timestamp.h
@@ -62,6 +62,14 @@ static inline char *av_ts_make_string(char *buf, int64_t ts)
  * @param tb the timebase of the timestamp
  * @return the buffer in input
  */
+char *av_ts_make_time_string2(char *buf, int64_t ts, AVRational tb);
+
+/**
+ * Fill the provided buffer with a string containing a timestamp
+ * representation.
+ *
+ * @see av_ts_make_time_string2
+ */
 static inline char *av_ts_make_time_string(char *buf, int64_t ts,
                                            const AVRational *tb)
 {
diff --git a/libavutil/version.h b/libavutil/version.h
index 445102073f..80e6aacd0f 100644
--- a/libavutil/version.h
+++ b/libavutil/version.h
@@ -79,7 +79,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  59
-#define LIBAVUTIL_VERSION_MINOR   6
+#define LIBAVUTIL_VERSION_MINOR   7
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \