Added FFT code

src/signal_processing/fft.hpp

@@ -0,0 +1,121 @@
+#pragma once
+
+#include <stdio.h>
+#include <cstdint>
+#include <vector>
+
+#include <math.h>
+#include <cstring> // memset
+#include <fftw3.h>
+
+#include "utility/buffer.hpp"
+
+class FFT_Visualizer {
+	public:
+		FFT_Visualizer(
+			size_t width,
+			size_t height,
+
+			// Advanced options you probably shouldn't touch
+			double HZ_MIN = 20,
+			double HZ_MAX = 20000,
+			uint32_t DFT_TOTAL_SIZE = 1 << 15,
+			// #define conf_spectrum_dft_size 2 (between 1 and 5, inclusive)
+			uint32_t DFT_NONZERO_SIZE = (2048 * (2*2 + 4)),
+			double GAIN = 10
+		);
+
+		~FFT_Visualizer();
+
+		void update(
+			const Buffer& buf
+		);
+
+		size_t compute_buffer_output_size() const;
+
+		const std::vector<size_t>& get_output() {
+			return output;
+		};
+
+	private:
+
+		///
+		// Visualizer parameters
+		///
+
+		// How many bars this visualizer will generate
+		const size_t width;
+		// Resolution of this visualizer's bars.
+		const size_t height;
+		// Leftmost frequency in spectrum
+		const double HZ_MIN;
+		// Rightmost frequency in spectrum
+		const double HZ_MAX;
+		// Not sure what this does
+		const uint32_t DFT_TOTAL_SIZE;
+		// Not sure what this does, either
+		const uint32_t DFT_NONZERO_SIZE;
+		// Visualization spectrum gain
+		// tune if bars are too small
+		const double GAIN;
+		// Not sure what this does
+		const double DYNAMIC_RANGE;
+
+
+		///
+		// FFTW resources
+		///
+
+		fftw_plan plan;
+		// How many output values fftw will give us
+		size_t fftw_results;
+		// Input array. This is filled with buffer data
+		// that has been filtered through a window.
+		double* fftw_input;
+		// FFT output array.
+		fftw_complex *fftw_output;
+
+		///
+		// Intermediate values
+		///
+
+		// The magnitudes of the complex values fftw returns.
+		std::vector<double> freq_magnitudes;
+		// An array of frequency bins, scaled logarithmically.
+		std::vector<double> logspace;
+		// Nearly-complete output.
+		// This is necessary because some frequency bins
+		// may be empty, and need to be interpolated
+		std::vector<
+			std::pair<
+				size_t,
+				double
+			>
+		> partial_output;
+
+
+		// Output vector, with empty bins interpolated.
+		// The maximum possible value of an element in this
+		// vector is `height`.
+		std::vector<size_t> output;
+
+
+		///
+		// Helper methods
+		// See definition for docs.
+		///
+
+		void GenLogspace();
+		double Bin2Hz(size_t bin) const;
+
+		void ApplyWindow(
+			double *output,
+			const int16_t *input,
+			ssize_t samples
+		) const;
+
+		double Interpolate(
+			size_t x,
+			size_t h_idx
+		);
+};