Port Baystruments PR from tgstation

code/modules/instruments/_instrument_key.dm

@@ -0,0 +1,33 @@
+#define KEY_TWELTH (1/12)
+
+/**
+  * Instrument key datums contain everything needed to know how to play a specific
+  * note of an instrument.*
+  */
+/datum/instrument_key
+	/// The numerical key of what this is, from 1 to 127 on a standard piano keyboard.
+	var/key
+	/// The actual sample file that will be loaded when playing.
+	var/sample
+	/// The frequency to play the sample to get our desired note.
+	var/frequency
+	/// Deviation up/down from the pivot point that uses its sample. Used to calculate frequency.
+	var/deviation
+
+/datum/instrument_key/New(sample, key, deviation, frequency)
+	src.sample = sample
+	src.key = key
+	src.deviation = deviation
+	src.frequency = frequency
+	if(!frequency && deviation)
+		calculate()
+
+/**
+  * Calculates and stores our deviation.
+  */
+/datum/instrument_key/proc/calculate()
+	if(!deviation)
+		CRASH("Invalid calculate call: No deviation or sample in instrument_key")
+	frequency = 2 ** (KEY_TWELTH * deviation)
+
+#undef KEY_TWELTH