feat:mp3 (#3)

---------

Co-authored-by: intelligence <intelligence@debian.local>
Reviewed-on: #3

macos/Tests/TransuiTests/MP3EncoderTests.swift

@@ -0,0 +1,45 @@
+import XCTest
+
+@testable import Transui
+
+final class MP3EncoderTests: XCTestCase {
+  func testExtractsSupportedPCMDataFromWAV() throws {
+    let samples = Data([0x01, 0x02, 0x03, 0x04])
+    let wav = makeWAV(samples: samples)
+
+    XCTAssertEqual(try MP3Encoder.pcmData(fromWAV: wav), samples)
+  }
+
+  func testRejectsWrongSampleRate() {
+    XCTAssertThrowsError(try MP3Encoder.pcmData(fromWAV: makeWAV(sampleRate: 44_100)))
+  }
+
+  private func makeWAV(sampleRate: UInt32 = 16_000, samples: Data = Data([0, 0])) -> Data {
+    var data = Data("RIFF".utf8)
+    append(UInt32(36 + samples.count), to: &data)
+    data.append(Data("WAVEfmt ".utf8))
+    append(UInt32(16), to: &data)
+    append(UInt16(1), to: &data)
+    append(UInt16(1), to: &data)
+    append(sampleRate, to: &data)
+    append(sampleRate * 2, to: &data)
+    append(UInt16(2), to: &data)
+    append(UInt16(16), to: &data)
+    data.append(Data("data".utf8))
+    append(UInt32(samples.count), to: &data)
+    data.append(samples)
+    return data
+  }
+
+  private func append(_ value: UInt16, to data: inout Data) {
+    data.append(UInt8(value & 0xff))
+    data.append(UInt8(value >> 8))
+  }
+
+  private func append(_ value: UInt32, to data: inout Data) {
+    data.append(UInt8(value & 0xff))
+    data.append(UInt8((value >> 8) & 0xff))
+    data.append(UInt8((value >> 16) & 0xff))
+    data.append(UInt8((value >> 24) & 0xff))
+  }
+}