Add frequency in equal temperament (#24)

Author: dpsanders

src/MusicTheory.jl

@@ -27,4 +27,5 @@ include("scales.jl")
 include("chords.jl")
 include("triads.jl")
 include("notes.jl")
+include("equal_temperament.jl")
 end

src/equal_temperament.jl

@@ -0,0 +1,13 @@
+abstract type Temperament end
+
+struct EqualTemperament
+    base_pitch::Pitch
+    base_frequency::Float64
+end
+
+
+function frequency(temperament::EqualTemperament, pitch::Pitch)
+    semitone_distance = semitone(pitch) - semitone(temperament.base_pitch)
+    return 2.0^(semitone_distance // 12) * temperament.base_frequency
+end
+

test/equal_temperament.jl

@@ -0,0 +1,5 @@
+@testest "Equal temperament" begin
+    temperament = EqualTemperament(A[4], 440)
+
+    @test frequency(temperament, C[4]) == 261.6255653005986
+end