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)

5 files changed, 161 insertions, 0 deletions

diff --git a/libavcodec/Makefile b/libavcodec/Makefile
index 8a68c1b929..a0b23a794e 100644
--- a/libavcodec/Makefile
+++ b/libavcodec/Makefile
@@ -1090,6 +1090,7 @@ TESTPROGS-$(CONFIG_GOLOMB)                += golomb
 TESTPROGS-$(CONFIG_IDCTDSP)               += dct
 TESTPROGS-$(CONFIG_IIRFILTER)             += iirfilter
 TESTPROGS-$(HAVE_MMX)                     += motion
+TESTPROGS-$(CONFIG_MPEGVIDEO)             += mpeg12framerate
 TESTPROGS-$(CONFIG_RANGECODER)            += rangecoder
 TESTPROGS-$(CONFIG_SNOW_ENCODER)          += snowenc
 
diff --git a/libavcodec/mpeg12.h b/libavcodec/mpeg12.h
index f551504b8c..1ec99f17e1 100644
--- a/libavcodec/mpeg12.h
+++ b/libavcodec/mpeg12.h
@@ -73,4 +73,8 @@ void ff_mpeg1_encode_mb(MpegEncContext *s, int16_t block[8][64],
 void ff_mpeg1_encode_init(MpegEncContext *s);
 void ff_mpeg1_encode_slice_header(MpegEncContext *s);
 
+void ff_mpeg12_find_best_frame_rate(AVRational frame_rate,
+                                    int *code, int *ext_n, int *ext_d,
+                                    int nonstandard);
+
 #endif /* AVCODEC_MPEG12_H */
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;
+    }
+}
diff --git a/libavcodec/tests/mpeg12framerate.c b/libavcodec/tests/mpeg12framerate.c
new file mode 100644
index 0000000000..595bdb278a
--- /dev/null
+++ b/libavcodec/tests/mpeg12framerate.c
@@ -0,0 +1,87 @@
+/*
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "libavcodec/mpeg12.h"
+#include "libavcodec/mpeg12data.h"
+
+int main(void)
+{
+    int i;
+
+#define TEST_MATCH(frame_rate, code, ext_n, ext_d) do { \
+        AVRational fr = frame_rate; \
+        int c, n, d; \
+        ff_mpeg12_find_best_frame_rate(fr, &c, &n, &d, 0); \
+        if (c != code || n != ext_n || d != ext_d) { \
+            av_log(NULL, AV_LOG_ERROR, "Failed to match %d/%d: " \
+                   "code = %d, ext_n = %d, ext_d = %d.\n", \
+                   fr.num, fr.den, c, n, d); \
+            return 1; \
+        } \
+    } while (0)
+#define TEST_EXACT(frn, frd) do { \
+        AVRational fr = (AVRational) { frn, frd }; \
+        int c, n, d; \
+        ff_mpeg12_find_best_frame_rate(fr, &c, &n, &d, 0); \
+        if (av_cmp_q(fr, av_mul_q(ff_mpeg12_frame_rate_tab[c], \
+                                  (AVRational) { n + 1, d + 1 })) != 0) { \
+            av_log(NULL, AV_LOG_ERROR, "Failed to find exact %d/%d: " \
+                   "code = %d, ext_n = %d, ext_d = %d.\n", \
+                   fr.num, fr.den, c, n, d); \
+            return 1; \
+        } \
+    } while (0)
+
+    // Framerates in the table must be chosen exactly.
+    for (i = 1; i <= 8; i++)
+        TEST_MATCH(ff_mpeg12_frame_rate_tab[i], i, 0, 0);
+
+    // As should the same ones with small perturbations.
+    // (1/1000 used here to be smaller than half the difference
+    // between 24 and 24000/1001.)
+    for (i = 1; i <= 8; i++) {
+        TEST_MATCH(av_sub_q(ff_mpeg12_frame_rate_tab[i],
+                            (AVRational) { 1, 1000 }), i, 0, 0);
+        TEST_MATCH(av_add_q(ff_mpeg12_frame_rate_tab[i],
+                            (AVRational) { 1, 1000 }), i, 0, 0);
+    }
+
+    // Exactly constructable framerates should be exact.  Note that some
+    // values can be made in multiple ways (e.g. 12 = 24 / 2 == 60 / 5),
+    // and there is no reason to favour any particular choice.
+    TEST_EXACT(     1,    1);
+    TEST_EXACT(     2,    1);
+    TEST_EXACT(    12,    1);
+    TEST_EXACT( 15000, 1001);
+    TEST_EXACT(    15,    1);
+    TEST_EXACT(   120,    1);
+    TEST_EXACT(120000, 1001);
+    TEST_EXACT(   200,    1);
+    TEST_EXACT(   240,    1);
+
+    // Values higher than 240 (the highest representable, as 60 * 4 / 1)
+    // should be mapped to 240.
+    for (i = 240; i < 1000; i += 10)
+        TEST_MATCH(((AVRational) { i, 1 }), 8, 3, 0);
+    // Values lower than 24000/32032 (the lowest representable, as
+    // 24000/1001 * 1 / 32) should be mapped to 24000/32032.
+    for (i = 74; i > 0; i--)
+        TEST_MATCH(((AVRational) { i, 100 }), 1, 0, 31);
+
+    return 0;
+}
diff --git a/tests/fate/libavcodec.mak b/tests/fate/libavcodec.mak
index 27d631d16c..a2d5d9fe33 100644
--- a/tests/fate/libavcodec.mak
+++ b/tests/fate/libavcodec.mak
@@ -45,6 +45,11 @@ FATE_LIBAVCODEC-$(CONFIG_IIRFILTER) += fate-iirfilter
 fate-iirfilter: libavcodec/tests/iirfilter$(EXESUF)
 fate-iirfilter: CMD = run libavcodec/tests/iirfilter
 
+FATE_LIBAVCODEC-$(CONFIG_MPEGVIDEO) += fate-mpeg12framerate
+fate-mpeg12framerate: libavcodec/tests/mpeg12framerate$(EXESUF)
+fate-mpeg12framerate: CMD = run libavcodec/tests/mpeg12framerate
+fate-mpeg12framerate: REF = /dev/null
+
 FATE_LIBAVCODEC-yes += fate-libavcodec-options
 fate-libavcodec-options: libavcodec/tests/options$(EXESUF)
 fate-libavcodec-options: CMD = run libavcodec/tests/options