avfilter/f_sendcmd: implement expr flag

Make possible to parse expressions and store results as arguments
for target filters.

doc/filters.texi

@@ -23666,6 +23666,29 @@ The command is sent when the current frame timestamp leaves the
 specified interval. In other words, the command is sent when the
 previous frame timestamp was in the given interval, and the
 current is not.
+
+@item expr
+The command @var{ARG} is interpreted as expression and result of
+expression is passed as @var{ARG}.
+
+The expression is evaluated through the eval API and can contain the following
+constants:
+
+@table @option
+@item POS
+Original position in the file of the frame, or undefined if undefined
+for the current frame.
+
+@item PTS
+The presentation timestamp in input.
+
+@item N
+The count of the input frame for video or audio, starting from 0.
+
+@item T
+The time in seconds of the current frame.
+@end table
+
 @end table
 
 If @var{FLAGS} is not specified, a default value of @code{[enter]} is

libavfilter/f_sendcmd.c

@@ -25,6 +25,7 @@
 
 #include "libavutil/avstring.h"
 #include "libavutil/bprint.h"
+#include "libavutil/eval.h"
 #include "libavutil/file.h"
 #include "libavutil/opt.h"
 #include "libavutil/parseutils.h"
@@ -35,10 +36,27 @@
 
 #define COMMAND_FLAG_ENTER 1
 #define COMMAND_FLAG_LEAVE 2
+#define COMMAND_FLAG_EXPR  4
+
+static const char *const var_names[] = {
+    "N",     /* frame number */
+    "T",     /* frame time in seconds */
+    "POS",   /* original position in the file of the frame */
+    "PTS",   /* frame pts */
+    NULL
+};
+
+enum var_name {
+    VAR_N,
+    VAR_T,
+    VAR_POS,
+    VAR_PTS,
+    VAR_VARS_NB
+};
 
 static inline char *make_command_flags_str(AVBPrint *pbuf, int flags)
 {
-    static const char * const flag_strings[] = { "enter", "leave" };
+    static const char * const flag_strings[] = { "enter", "leave", "expr" };
     int i, is_first = 1;
 
     av_bprint_init(pbuf, 0, AV_BPRINT_SIZE_AUTOMATIC);
@@ -129,6 +147,7 @@ static int parse_command(Command *cmd, int cmd_count, int interval_count,
 
             if      (!strncmp(*buf, "enter", strlen("enter"))) cmd->flags |= COMMAND_FLAG_ENTER;
             else if (!strncmp(*buf, "leave", strlen("leave"))) cmd->flags |= COMMAND_FLAG_LEAVE;
+            else if (!strncmp(*buf, "expr",  strlen("expr")))  cmd->flags |= COMMAND_FLAG_EXPR;
             else {
                 char flag_buf[64];
                 av_strlcpy(flag_buf, *buf, sizeof(flag_buf));
@@ -450,6 +469,9 @@ static av_cold void uninit(AVFilterContext *ctx)
     av_freep(&s->intervals);
 }
 
+#define TS2D(ts) ((ts) == AV_NOPTS_VALUE ? NAN : (double)(ts))
+#define TS2T(ts, tb) ((ts) == AV_NOPTS_VALUE ? NAN : (double)(ts)*av_q2d(tb))
+
 static int filter_frame(AVFilterLink *inlink, AVFrame *ref)
 {
     AVFilterContext *ctx = inlink->dst;
@@ -476,6 +498,8 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *ref)
             flags += COMMAND_FLAG_LEAVE;
             interval->enabled = 0;
         }
+        if (interval->enabled)
+            flags += COMMAND_FLAG_EXPR;
 
         if (flags) {
             AVBPrint pbuf;
@@ -487,19 +511,43 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *ref)
 
             for (j = 0; flags && j < interval->nb_commands; j++) {
                 Command *cmd = &interval->commands[j];
+                char *cmd_arg = cmd->arg;
                 char buf[1024];
 
                 if (cmd->flags & flags) {
+                    if (cmd->flags & COMMAND_FLAG_EXPR) {
+                        double var_values[VAR_VARS_NB], res;
+
+                        var_values[VAR_N]   = inlink->frame_count_in;
+                        var_values[VAR_POS] = ref->pkt_pos == -1 ? NAN : ref->pkt_pos;
+                        var_values[VAR_PTS] = TS2D(ref->pts);
+                        var_values[VAR_T]   = TS2T(ref->pts, inlink->time_base);
+
+                        if ((ret = av_expr_parse_and_eval(&res, cmd->arg, var_names, var_values,
+                                                          NULL, NULL, NULL, NULL, NULL, 0, NULL)) < 0) {
+                            av_log(ctx, AV_LOG_ERROR, "Invalid expression '%s' for command argument.\n", cmd->arg);
+                            av_frame_free(&ref);
+                            return AVERROR(EINVAL);
+                        }
+
+                        cmd_arg = av_asprintf("%g", res);
+                        if (!cmd_arg) {
+                            av_frame_free(&ref);
+                            return AVERROR(ENOMEM);
+                        }
+                    }
                     av_log(ctx, AV_LOG_VERBOSE,
                            "Processing command #%d target:%s command:%s arg:%s\n",
-                           cmd->index, cmd->target, cmd->command, cmd->arg);
+                           cmd->index, cmd->target, cmd->command, cmd_arg);
                     ret = avfilter_graph_send_command(inlink->graph,
-                                                      cmd->target, cmd->command, cmd->arg,
+                                                      cmd->target, cmd->command, cmd_arg,
                                                       buf, sizeof(buf),
                                                       AVFILTER_CMD_FLAG_ONE);
                     av_log(ctx, AV_LOG_VERBOSE,
                            "Command reply for command #%d: ret:%s res:%s\n",
                            cmd->index, av_err2str(ret), buf);
+                    if (cmd->flags & COMMAND_FLAG_EXPR)
+                        av_freep(&cmd_arg);
                 }
             }
         }