+static bool
+about_equal (float a, float b)
+{
+ /* A film of F seconds at f FPS will be Ff frames;
+ Consider some delta FPS d, so if we run the same
+ film at (f + d) FPS it will last F(f + d) seconds.
+
+ Hence the difference in length over the length of the film will
+ be F(f + d) - Ff frames
+ = Ff + Fd - Ff frames
+ = Fd frames
+ = Fd/f seconds
+
+ So if we accept a difference of 1 frame, ie 1/f seconds, we can
+ say that
+
+ 1/f = Fd/f
+ ie 1 = Fd
+ ie d = 1/F
+
+ So for a 3hr film, ie F = 3 * 60 * 60 = 10800, the acceptable
+ FPS error is 1/F ~= 0.0001 ~= 10-e4
+ */
+
+ return (fabs (a - b) < 1e-4);
+}
+
+class FrameRateCandidate
+{
+public:
+ FrameRateCandidate (float source_, int dcp_)
+ : source (source_)
+ , dcp (dcp_)
+ {}
+
+ float source;
+ int dcp;
+};
+
+int
+best_dcp_frame_rate (float source_fps)
+{
+ list<int> const allowed_dcp_frame_rates = Config::instance()->allowed_dcp_frame_rates ();
+
+ /* Work out what rates we could manage, including those achieved by using skip / repeat. */
+ list<FrameRateCandidate> candidates;
+
+ /* Start with the ones without skip / repeat so they will get matched in preference to skipped/repeated ones */
+ for (list<int>::const_iterator i = allowed_dcp_frame_rates.begin(); i != allowed_dcp_frame_rates.end(); ++i) {
+ candidates.push_back (FrameRateCandidate (*i, *i));
+ }
+
+ /* Then the skip/repeat ones */
+ for (list<int>::const_iterator i = allowed_dcp_frame_rates.begin(); i != allowed_dcp_frame_rates.end(); ++i) {
+ candidates.push_back (FrameRateCandidate (float (*i) / 2, *i));
+ candidates.push_back (FrameRateCandidate (float (*i) * 2, *i));
+ }
+
+ /* Pick the best one, bailing early if we hit an exact match */
+ float error = numeric_limits<float>::max ();
+ boost::optional<FrameRateCandidate> best;
+ list<FrameRateCandidate>::iterator i = candidates.begin();
+ while (i != candidates.end()) {
+
+ if (about_equal (i->source, source_fps)) {
+ best = *i;
+ break;
+ }
+
+ float const e = fabs (i->source - source_fps);
+ if (e < error) {
+ error = e;
+ best = *i;
+ }
+
+ ++i;
+ }
+
+ assert (best);
+ return best->dcp;
+}
+