avutil/eval: add function to track variable use

1)Some filters allow cross-referenced expressions e.g. x=y+10. In
such cases, filters evaluate expressions multiple times for
successful evaluation of all expressions. If the expression for one or
more variables contains a RNG, the result may vary across evaluation
leading to inconsistent values across the cross-referenced expressions.

2)A related case is circular expressions e.g. x=y+10 and y=x+10 which
cannot be succesfully resolved.

3)Certain filter variables may only be applicable in specific eval modes
and lead to a failure of evaluation in other modes e.g. pts is only
relevant for frame eval mode.

At present, there is no reliable means to identify these occurrences and
thus the error messages provided are broad or inaccurate. The helper
function introduced - av_expr_count_vars - allows developers to identify
the use and count of variables in expressions and thus tailor the error
message, allow for a graceful fallback and/or decide evaluation order.

libavutil/eval.c

@@ -735,6 +735,22 @@ end:
     return ret;
 }
 
+int av_expr_count_vars(AVExpr *e, unsigned *counter, int size)
+{
+    int i;
+
+    if (!e || !counter || !size)
+        return AVERROR(EINVAL);
+
+    for (i = 0; e->type != e_const && i < 3 && e->param[i]; i++)
+        av_expr_count_vars(e->param[i], counter, size);
+
+    if (e->type == e_const && e->a.const_index < size)
+        counter[e->a.const_index]++;
+
+    return 0;
+}
+
 double av_expr_eval(AVExpr *e, const double *const_values, void *opaque)
 {
     Parser p = { 0 };

libavutil/eval.h

@@ -86,6 +86,16 @@ int av_expr_parse(AVExpr **expr, const char *s,
  */
 double av_expr_eval(AVExpr *e, const double *const_values, void *opaque);
 
+/**
+ * Track the presence of variables and their number of occurrences in a parsed expression
+ *
+ * @param counter a zero-initialized array where the count of each variable will be stored
+ * @param size size of array
+ * @return 0 on success, a negative value indicates that no expression or array was passed
+ * or size was zero
+ */
+int av_expr_count_vars(AVExpr *e, unsigned *counter, int size);
+
 /**
  * Free a parsed expression previously created with av_expr_parse().
  */

libavutil/version.h

@@ -79,8 +79,8 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  56
-#define LIBAVUTIL_VERSION_MINOR  35
-#define LIBAVUTIL_VERSION_MICRO 101
+#define LIBAVUTIL_VERSION_MINOR  36
+#define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \
                                                LIBAVUTIL_VERSION_MINOR, \