Begin implementing performance layer

src/performance.clj

@@ -0,0 +1,32 @@
+(ns performance)
+
+;; A performance is a data structure that describes the act
+;; of performing a piece of music. It is a vector of events,
+;; where each event describes the playing of a single note
+
+(def simple-performance
+  [{:time 0              ;; the start time in seconds from the beginning of the performance
+    :pitch 27            ;; the note's absolute pitch (see music/abs-pitch)
+    :duration 3/8        ;; duration of the note in seconds
+    :instrument "Cello"  ;; instrument playing the note
+    :volume 50           ;; volume at which to play the note
+    :params []}])        ;; an array of doubles -- not clear what this is for yet
+
+;; A context represents the context in which a musical value is performed.
+;; It is passed around during the interpretation of a musical value to represent
+;; changing tempo, key signature, tempo, etc.
+(def my-context
+  {:time 0              ;; time at which the performance starts in seconds
+   :player {:?? :??}    ;; not sure what this is
+   :instrument "Cello"  ;; instrument currently playing (?)
+   :duration-t 2        ;; the duration in seconds of a whole note
+   :pitch-offset 0      ;; a pitch offset -- number of semitones to transpose notes up or down
+   :volume 50           ;; volume of performance
+   :key [:C :major]})   ;; current key signature
+
+(defn metro
+  "Generates a duration-t value (the duration in seconds of a whole note)
+  from the number of beats per minute and the duration of a beat (1/4 for
+  quarter notes, 1/2 for half notes, etc.)"
+  [bpm beat-duration]
+  (/ 60 (* bpm beat-duration)))