lavu: add av_rescale_delta()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/mathematics.c

@@ -142,3 +142,26 @@ int64_t av_compare_mod(uint64_t a, uint64_t b, uint64_t mod){
         c-= mod;
     return c;
 }
+
+int64_t av_rescale_delta(AVRational in_tb, int64_t in_ts,  AVRational fs_tb, int duration, int64_t *last, AVRational out_tb){
+    int64_t a, b, this;
+
+    av_assert0(in_ts != AV_NOPTS_VALUE);
+    av_assert0(duration >= 0);
+
+    if (*last == AV_NOPTS_VALUE || !duration || in_tb.num*(int64_t)out_tb.den <= out_tb.num*(int64_t)in_tb.den) {
+simple_round:
+        *last = av_rescale_q(in_ts, in_tb, fs_tb) + duration;
+        return av_rescale_q(in_ts, in_tb, out_tb);
+    }
+
+    a =  av_rescale_q_rnd(2*in_ts-1, in_tb, fs_tb, AV_ROUND_DOWN)   >>1;
+    b = (av_rescale_q_rnd(2*in_ts+1, in_tb, fs_tb, AV_ROUND_UP  )+1)>>1;
+    if (*last < 2*a - b || *last > 2*b - a)
+        goto simple_round;
+
+    this = av_clip64(*last, a, b);
+    *last = this + duration;
+
+    return av_rescale_q(this, fs_tb, out_tb);
+}

libavutil/mathematics.h

@@ -122,6 +122,11 @@ int av_compare_ts(int64_t ts_a, AVRational tb_a, int64_t ts_b, AVRational tb_b);
  */
 int64_t av_compare_mod(uint64_t a, uint64_t b, uint64_t mod);
 
+/**
+ * Rescale a timestamp while preserving known durations.
+ */
+int64_t av_rescale_delta(AVRational in_tb, int64_t in_ts,  AVRational fs_tb, int duration, int64_t *last, AVRational out_tb);
+
 /**
  * @}
  */