avfilter/vf_fieldhint: add pattern mode of filtering

doc/filters.texi

@@ -12318,7 +12318,9 @@ case of @code{b} it will use only bottom field.
 If line starts with @code{#} or @code{;} that line is skipped.
 
 @item mode
-Can be item @code{absolute} or @code{relative}. Default is @code{absolute}.
+Can be item @code{absolute} or @code{relative} or @code{pattern}. Default is @code{absolute}.
+The @code{pattern} mode is same as @code{relative} mode, except at last entry of file if there
+are more frames to process than @code{hint} file is seek back to start.
 @end table
 
 Example of first several lines of @code{hint} file for @code{relative} mode:

libavfilter/vf_fieldhint.c

@@ -27,6 +27,13 @@
 #include "internal.h"
 #include "video.h"
 
+enum HintModes {
+    ABSOLUTE_HINT,
+    RELATIVE_HINT,
+    PATTERN_HINT,
+    NB_HINTS,
+};
+
 typedef struct FieldHintContext {
     const AVClass *class;
 
@@ -48,9 +55,10 @@ typedef struct FieldHintContext {
 
 static const AVOption fieldhint_options[] = {
     { "hint", "set hint file", OFFSET(hint_file_str), AV_OPT_TYPE_STRING, {.str=NULL}, 0, 0, FLAGS },
-    { "mode", "set hint mode", OFFSET(mode), AV_OPT_TYPE_INT, {.i64=0}, 0, 1, FLAGS, "mode" },
-    {   "absolute", 0, 0, AV_OPT_TYPE_CONST, {.i64=0}, 0, 0, FLAGS, "mode" },
-    {   "relative", 0, 0, AV_OPT_TYPE_CONST, {.i64=1}, 0, 0, FLAGS, "mode" },
+    { "mode", "set hint mode", OFFSET(mode), AV_OPT_TYPE_INT, {.i64=0}, 0, NB_HINTS-1, FLAGS, "mode" },
+    {   "absolute", 0, 0, AV_OPT_TYPE_CONST, {.i64=ABSOLUTE_HINT}, 0, 0, FLAGS, "mode" },
+    {   "relative", 0, 0, AV_OPT_TYPE_CONST, {.i64=RELATIVE_HINT}, 0, 0, FLAGS, "mode" },
+    {   "pattern",  0, 0, AV_OPT_TYPE_CONST, {.i64=PATTERN_HINT},  0, 0, FLAGS, "mode" },
     { NULL }
 };
 
@@ -141,22 +149,30 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
                 return AVERROR_INVALIDDATA;
             }
             switch (s->mode) {
-            case 0:
+            case ABSOLUTE_HINT:
                 if (tf > outlink->frame_count_in + 1 || tf < FFMAX(0, outlink->frame_count_in - 1) ||
                     bf > outlink->frame_count_in + 1 || bf < FFMAX(0, outlink->frame_count_in - 1)) {
                     av_log(ctx, AV_LOG_ERROR, "Out of range frames %"PRId64" and/or %"PRId64" on line %"PRId64" for %"PRId64". input frame.\n", tf, bf, s->line, inlink->frame_count_out);
                     return AVERROR_INVALIDDATA;
                 }
                 break;
-            case 1:
+            case PATTERN_HINT:
+            case RELATIVE_HINT:
                 if (tf > 1 || tf < -1 ||
                     bf > 1 || bf < -1) {
                     av_log(ctx, AV_LOG_ERROR, "Out of range %"PRId64" and/or %"PRId64" on line %"PRId64" for %"PRId64". input frame.\n", tf, bf, s->line, inlink->frame_count_out);
                     return AVERROR_INVALIDDATA;
                 }
+                break;
+            default:
+                return AVERROR_BUG;
             };
             break;
         } else {
+            if (s->mode == PATTERN_HINT) {
+                fseek(s->hint, 0, SEEK_SET);
+                continue;
+            }
             av_log(ctx, AV_LOG_ERROR, "Missing entry for %"PRId64". input frame.\n", inlink->frame_count_out);
             return AVERROR_INVALIDDATA;
         }
@@ -168,11 +184,12 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
     av_frame_copy_props(out, s->frame[1]);
 
     switch (s->mode) {
-    case 0:
+    case ABSOLUTE_HINT:
         top    = s->frame[tf - outlink->frame_count_in + 1];
         bottom = s->frame[bf - outlink->frame_count_in + 1];
         break;
-    case 1:
+    case PATTERN_HINT:
+    case RELATIVE_HINT:
         top    = s->frame[1 + tf];
         bottom = s->frame[1 + bf];
         break;