TempoConverter: ↑ performance (cache envelopes)

If multiple sequential events have simple events with the same timing
structure we often repeat the same efforts of creating tempo envelopes
by cutting the original envelope. We can reduce the work load by using
a cache.

mutwo/core_converters/tempos.py

@@ -133,6 +133,7 @@ def __init__(
         self._apply_converter_on_events_tempo_envelope = (
             apply_converter_on_events_tempo_envelope
         )
+        self._start_and_end_to_tempo_converter_dict = {}
 
     # ###################################################################### #
     #                          static methods                                #
@@ -166,6 +167,21 @@ def _tempo_envelope_to_beat_length_in_seconds_envelope(
     #                         private methods                                #
     # ###################################################################### #
 
+    def _start_and_end_to_tempo_converter(self, start, end):
+        key = (start.duration, end.duration)
+        try:
+            t = self._start_and_end_to_tempo_converter_dict[key]
+        except KeyError:
+            t = self._start_and_end_to_tempo_converter_dict[key] = TempoConverter(
+                self._tempo_envelope.cut_out(
+                    start,
+                    end,
+                    mutate=False,
+                ),
+                apply_converter_on_events_tempo_envelope=False,
+            )
+        return t
+
     def _convert_simple_event(
         self,
         simple_event: core_events.SimpleEvent,
@@ -186,14 +202,14 @@ def _convert_event(
         depth: int = 0,
     ) -> core_events.abc.ComplexEvent[core_events.abc.Event]:
         if self._apply_converter_on_events_tempo_envelope:
-            event_to_convert.tempo_envelope = TempoConverter(
-                self._tempo_envelope.cut_out(
-                    absolute_entry_delay,
-                    absolute_entry_delay + event_to_convert.duration,
-                    mutate=False,
-                ),
-                apply_converter_on_events_tempo_envelope=False,
-            ).convert(event_to_convert.tempo_envelope)
+            start, end = (
+                absolute_entry_delay,
+                absolute_entry_delay + event_to_convert.duration,
+            )
+            local_tempo_converter = self._start_and_end_to_tempo_converter(start, end)
+            event_to_convert.tempo_envelope = local_tempo_converter(
+                event_to_convert.tempo_envelope
+            )
         return super()._convert_event(event_to_convert, absolute_entry_delay, depth)
 
     # ###################################################################### #