输入输出的数据如代码注释

fft

// arm cfft, 复数
    // float[256] as complex[128] => float[256] as complex[128]
    // out: 
    //      DC      0,64s R
    //      conj    1~63, 65~127
    for (int i = 0; i < 128; i++) {
        buffer[i * 2] = xn[i].real;
        buffer[i * 2 + 1] = xn[i].imaginary;
    }
    arm_cfft_f32(&arm_cfft_sR_f32_len128, (float*)buffer, 0, 1);   // forward FFT

    // arm rfft, 实部
    // float[128] => float[128] as complex[64]
    // out: 
    //      DC      0s R and I
    //      complex 1~63
    for (int i = 0; i < 128; i++) {
        buffer[i] = xn[i].real;
    }
    arm_rfft_fast_f32(&arm_rfft_fast_sR_f32_len128, (float*)buffer, (float*)buffer_out, 0);

ifft

// arm cfft, 复数
    // float[256] as complex[128] => float[256] as complex[128]
    // in: 
    //      DC      0,64s R
    //      conj    1~63, 65~127
    for (int i = 0; i < 128; i++) {
        buffer[i * 2] = xk[i].real;
        buffer[i * 2 + 1] = xk[i].imaginary;
    }
    arm_cfft_f32(&arm_cfft_sR_f32_len128, (float*)buffer, 1, 1);   // forward FFT

    // arm rfft, 实部
    // float[128] => float[128] as complex[64]
    // in: 
    //      DC      0s R and I
    //      complex 1~63
    for (int i = 0; i < 128; i++) {
        buffer[i * 2] = xk[i].real;
        buffer[i * 2 + 1] = xk[i].imaginary;
    }
    buffer[1] = xk[64].real;
    arm_rfft_fast_f32(&arm_rfft_fast_sR_f32_len128, (float*)buffer, (float*)buffer_out, 1);

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