pcm/Resampler: add virtual method Flush()

Wired to Filter::Flush().

Closes #153

src/pcm/GlueResampler.cxx

@@ -81,3 +81,9 @@ GluePcmResampler::Resample(ConstBuffer<void> src)
 
 	return resampler->Resample(src);
 }
+
+ConstBuffer<void>
+GluePcmResampler::Flush()
+{
+	return resampler->Flush();
+}

src/pcm/GlueResampler.hxx

@@ -61,6 +61,8 @@ public:
 	void Reset() noexcept;
 
 	ConstBuffer<void> Resample(ConstBuffer<void> src);
+
+	ConstBuffer<void> Flush();
 };
 
 #endif

src/pcm/PcmConvert.cxx

@@ -152,3 +152,22 @@ PcmConvert::Convert(ConstBuffer<void> buffer)
 
 	return buffer;
 }
+
+ConstBuffer<void>
+PcmConvert::Flush()
+{
+	if (enable_resampler) {
+		auto buffer = resampler.Flush();
+		if (!buffer.IsNull()) {
+			if (enable_format)
+				buffer = format_converter.Convert(buffer);
+
+			if (enable_channels)
+				buffer = channels_converter.Convert(buffer);
+
+			return buffer;
+		}
+	}
+
+	return nullptr;
+}

src/pcm/PcmConvert.hxx

@@ -81,6 +81,12 @@ public:
 	 * @return the destination buffer
 	 */
 	ConstBuffer<void> Convert(ConstBuffer<void> src);
+
+	/**
+	 * Flush pending data and return it.  This should be called
+	 * repepatedly until it returns nullptr.
+	 */
+	ConstBuffer<void> Flush();
 };
 
 void

src/pcm/Resampler.hxx

@@ -70,6 +70,14 @@ public:
 	 * filter_close() or filter_filter())
 	 */
 	virtual ConstBuffer<void> Resample(ConstBuffer<void> src) = 0;
+
+	/**
+	 * Flush pending data and return it.  This should be called
+	 * repepatedly until it returns nullptr.
+	 */
+	virtual ConstBuffer<void> Flush() {
+		return nullptr;
+	}
 };
 
 #endif

src/pcm/SoxrResampler.cxx

@@ -160,3 +160,24 @@ SoxrPcmResampler::Resample(ConstBuffer<void> src)
 
 	return { output_buffer, o_done * frame_size };
 }
+
+ConstBuffer<void>
+SoxrPcmResampler::Flush()
+{
+	const size_t frame_size = channels * sizeof(float);
+	const size_t o_frames = 1024;
+
+	float *output_buffer = (float *)buffer.Get(o_frames * frame_size);
+
+	size_t o_done;
+	soxr_error_t e = soxr_process(soxr, nullptr, 0, nullptr,
+				      output_buffer, o_frames, &o_done);
+	if (e != nullptr)
+		throw FormatRuntimeError("soxr error: %s", e);
+
+	if (o_done == 0)
+		/* flush complete */
+		output_buffer = nullptr;
+
+	return { output_buffer, o_done * frame_size };
+}

src/pcm/SoxrResampler.hxx

@@ -42,6 +42,7 @@ public:
 	AudioFormat Open(AudioFormat &af, unsigned new_sample_rate) override;
 	void Close() noexcept override;
 	ConstBuffer<void> Resample(ConstBuffer<void> src) override;
+	ConstBuffer<void> Flush() override;
 };
 
 void