[FFmpeg-devel] [PATCH v2] avutil/eval: add function to track variable use

[Gyan]

As suggested by Michael in the review for the vf_scale patch, new eval 
function tracks count of all variables in a parsed
expression in an array.

Thanks,
Gyan
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From 6d4d8ab2b582991ccb787d18d14be31256a785aa Mon Sep 17 00:00:00 2001
From: Gyan Doshi <ffmpeg at gyani.pro>
Date: Sat, 2 Nov 2019 20:16:42 +0530
Subject: [PATCH v2] 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.
---
 doc/APIchanges      |  3 +++
 libavutil/eval.c    | 16 ++++++++++++++++
 libavutil/eval.h    | 10 ++++++++++
 libavutil/version.h |  4 ++--
 4 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/doc/APIchanges b/doc/APIchanges
index 48168f82e6..f39eda7afc 100644
--- a/doc/APIchanges
+++ b/doc/APIchanges
@@ -15,6 +15,9 @@ libavutil:     2017-10-21
 
 API changes, most recent first:
 
+2019-11-16 - xxxxxxxxxx - lavu 56.36.100 - eval API
+  Add av_expr_count_vars().
+
 2019-10-14 - f3746d31f9 - lavu 56.35.101 - opt.h
   Add AV_OPT_FLAG_RUNTIME_PARAM.
 
diff --git a/libavutil/eval.c b/libavutil/eval.c
index 48832979e2..62d2ae938b 100644
--- a/libavutil/eval.c
+++ b/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 };
diff --git a/libavutil/eval.h b/libavutil/eval.h
index dacd22b96e..9bdb10cca2 100644
--- a/libavutil/eval.h
+++ b/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().
  */
diff --git a/libavutil/version.h b/libavutil/version.h
index 27d663baf1..af3abf7265 100644
--- a/libavutil/version.h
+++ b/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, \
-- 
2.24.0