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diff --git a/src/audio/fft.h b/src/audio/fft.h
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+++ b/src/audio/fft.h
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+// Fast Fourier Transform (FFT) implementation
+// Radix-2 Cooley-Tukey algorithm for power-of-2 sizes
+// This implementation matches the JavaScript version in tools/spectral_editor/dct.js
+
+#ifndef AUDIO_FFT_H_
+#define AUDIO_FFT_H_
+
+#include <cstddef>
+
+// Forward FFT: Time domain → Frequency domain
+// Input: real[] (length N), imag[] (length N, can be zeros)
+// Output: real[] and imag[] contain complex frequency bins
+// N must be a power of 2
+void fft_forward(float* real, float* imag, size_t N);
+
+// Inverse FFT: Frequency domain → Time domain
+// Input: real[] (length N), imag[] (length N)
+// Output: real[] and imag[] contain complex time samples (scaled by 1/N)
+// N must be a power of 2
+void fft_inverse(float* real, float* imag, size_t N);
+
+// DCT Type-II via FFT (matches existing dct_512 signature)
+// Input: input[] (length N)
+// Output: output[] (length N) containing DCT-II coefficients
+// N must be a power of 2
+void dct_fft(const float* input, float* output, size_t N);
+
+// IDCT (Inverse DCT Type-II) via FFT (matches existing idct_512 signature)
+// Input: input[] (length N)
+// Output: output[] (length N) containing time-domain samples
+// N must be a power of 2
+void idct_fft(const float* input, float* output, size_t N);
+
+#endif  /* AUDIO_FFT_H_ */