mpeg12: Add a common function to find the best frame rate

This will be used later in both the mpeg2_metadata filter and the
VAAPI MPEG-2 encoder.  Also adds a unit test.

(cherry picked from commit b5859e0b04bdbe12c97cb12ac10a45d51d2d73c9)

1 files changed, 64 insertions, 0 deletions

diff --git a/libavcodec/mpeg12framerate.c b/libavcodec/mpeg12framerate.c
index 094cd180a5..ab3d351173 100644
--- a/libavcodec/mpeg12framerate.c
+++ b/libavcodec/mpeg12framerate.c
@@ -18,6 +18,9 @@
 
 #include "libavutil/rational.h"
 
+#include "mpeg12.h"
+#include "mpeg12data.h"
+
 const AVRational ff_mpeg12_frame_rate_tab[16] = {
     {    0,    0},
     {24000, 1001},
@@ -37,3 +40,64 @@ const AVRational ff_mpeg12_frame_rate_tab[16] = {
     {   15,    1},
     {    0,    0},
 };
+
+void ff_mpeg12_find_best_frame_rate(AVRational frame_rate,
+                                    int *code, int *ext_n, int *ext_d,
+                                    int nonstandard)
+{
+    int mpeg2 = ext_n && ext_d;
+    int max_code = nonstandard ? 12 : 8;
+    int c, n, d, best_c, best_n, best_d;
+    AVRational best_error = { INT_MAX, 1 };
+
+    // Default to NTSC if the inputs make no sense.
+    best_c = 4;
+    best_n = best_d = 1;
+
+    for (c = 1; c <= max_code; c++) {
+        if (av_cmp_q(frame_rate, ff_mpeg12_frame_rate_tab[c]) == 0) {
+            best_c = c;
+            goto found;
+        }
+    }
+
+    for (c = 1; c <= max_code; c++) {
+        for (n = 1; n <= (mpeg2 ? 4 : 1); n++) {
+            for (d = 1; d <= (mpeg2 ? 32 : 1); d++) {
+                AVRational test, error;
+                int cmp;
+
+                test = av_mul_q(ff_mpeg12_frame_rate_tab[c],
+                                (AVRational) { n, d });
+
+                cmp = av_cmp_q(test, frame_rate);
+                if (cmp == 0) {
+                    best_c = c;
+                    best_n = n;
+                    best_d = d;
+                    goto found;
+                }
+
+                if (cmp < 0)
+                    error = av_div_q(frame_rate, test);
+                else
+                    error = av_div_q(test, frame_rate);
+
+                cmp = av_cmp_q(error, best_error);
+                if (cmp < 0 || (cmp == 0 && n == 1 && d == 1)) {
+                    best_c = c;
+                    best_n = n;
+                    best_d = d;
+                    best_error = error;
+                }
+            }
+        }
+    }
+
+found:
+    *code = best_c;
+    if (mpeg2) {
+        *ext_n = best_n - 1;
+        *ext_d = best_d - 1;
+    }
+}