Add IFFT

Benchmarks (Intel Core i5-6300U @ 4x 3GHz, 16GB RAM):
```
dft         time:   [49.610 ms 50.141 ms 50.861 ms]
idft        time:   [49.531 ms 49.938 ms 50.412 ms]
fft         time:   [848.62 us 853.68 us 859.08 us]
ifft        time:   [849.98 us 852.73 us 856.13 us]
```

README.md

@@ -4,3 +4,22 @@ Fast Fourier Transform implementation in Rust.
 
 https://en.wikipedia.org/wiki/Fast_Fourier_transform & [DFT](https://en.wikipedia.org/wiki/Discrete_Fourier_transform)
 
+## Usage
+```rust
+let values: Vec<f64> = vec![0.2, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8];
+
+// compute the FFT (Fast Fourier Transform)
+let fft_res = fft(&values);
+
+// compute the IFFT (Inverse Fast Fourier Transform)
+let ifft_res = ifft(&fft_res);
+
+
+// Also, available directly (and slower) DFT & IDFT:
+
+// compute the DFT (Discrete Fourier Transform)
+let dft_res = dft(&values);
+
+// compute the IDFT (Inverse Discrete Fourier Transform)
+let idft_res = idft(&dft_res);
+```

benches/bench_fft.rs

@@ -12,10 +12,13 @@ fn criterion_benchmark(c: &mut Criterion) {
         .sample_iter(rand::distributions::Standard)
         .take(1024)
         .collect();
+    let f = fft_rs::fft(&values);
 
     c.bench_function("dft", |b| b.iter(|| fft_rs::dft(&values)));
+    c.bench_function("idft", |b| b.iter(|| fft_rs::idft(&f)));
 
     c.bench_function("fft", |b| b.iter(|| fft_rs::fft(&values)));
+    c.bench_function("ifft", |b| b.iter(|| fft_rs::ifft(&f)));
 }
 
 criterion_group!(benches, criterion_benchmark);

src/lib.rs

@@ -3,15 +3,23 @@ use std::f64::consts::PI;
 
 // fft computes the Fast Fourier Transform
 pub fn fft(x: &Vec<f64>) -> Vec<Complex64> {
+    let mut x_compl: Vec<Complex64> = vec![Complex::new(0_f64, 0_f64); x.len()];
+    for i in 0..x.len() {
+        x_compl[i] = Complex::new(x[i], 0_f64);
+    }
+    fft_compl(x_compl)
+}
+
+fn fft_compl(x: Vec<Complex64>) -> Vec<Complex64> {
     let N = x.len();
     if N % 2 > 0 {
         panic!("not a power of 2");
     } else if N <= 2 {
-        return dft(x);
+        return dft_compl(x);
     }
 
-    let mut x_even: Vec<f64> = Vec::new();
-    let mut x_odd: Vec<f64> = Vec::new();
+    let mut x_even: Vec<Complex64> = Vec::new();
+    let mut x_odd: Vec<Complex64> = Vec::new();
     for i in 0..x.len() {
         if i % 2 == 0 {
             x_even.push(x[i]);
@@ -19,8 +27,8 @@ pub fn fft(x: &Vec) -> Vec {
             x_odd.push(x[i]);
         }
     }
-    let mut x_even_cmplx = fft(&x_even);
-    let mut x_odd_cmplx = fft(&x_odd);
+    let mut x_even_cmplx = fft_compl(x_even);
+    let mut x_odd_cmplx = fft_compl(x_odd);
 
     let mut w = Complex::new(0_f64, 2_f64 * PI / N as f64);
     let mut f_k: Vec<Complex64> = Vec::new();
@@ -44,18 +52,46 @@ pub fn fft(x: &Vec) -> Vec {
     r
 }
 
+// ifft computes the Inverse Fast Fourier Transform
+pub fn ifft(x: &Vec<Complex64>) -> Vec<f64> {
+    // use the IFFT method of computing conjugates, then FFT, then conjugate again, and then divide
+    // by N
+    let mut x_conj: Vec<Complex64> = Vec::new();
+    for i in 0..x.len() {
+        x_conj.push(x[i].conj());
+    }
+
+    let x_res = fft_compl(x_conj);
+
+    let mut r: Vec<Complex64> = Vec::new();
+    for i in 0..x_res.len() {
+        r.push(x_res[i].conj());
+    }
+
+    let n = x.len() as f64;
+    let mut rr: Vec<f64> = Vec::new();
+    for i in 0..r.len() {
+        r[i] = r[i] / Complex::new(n, 0_f64);
+        rr.push(r[i].re);
+    }
+    rr
+}
+
 // dft computes the Discrete Fourier Transform
 pub fn dft(x: &Vec<f64>) -> Vec<Complex64> {
     let mut x_compl: Vec<Complex64> = vec![Complex::new(0_f64, 0_f64); x.len()];
     for i in 0..x.len() {
         x_compl[i] = Complex::new(x[i], 0_f64);
     }
+    dft_compl(x_compl)
+}
 
+fn dft_compl(x: Vec<Complex64>) -> Vec<Complex64> {
     let mut w = Complex::new(0_f64, -2_f64 * PI / x.len() as f64);
 
-    // f_k = SUM{n=0, N-1} f_n * e^(-j2pi*k*n)/N
+    // f_k (dft_matrix) = SUM{n=0, N-1} f_n * e^(-j2pi*k*n)/N
     // https://en.wikipedia.org/wiki/Discrete_Fourier_transform
-    let mut f: Vec<Vec<Complex64>> = Vec::new();
+    let mut dft_matrix: Vec<Vec<Complex64>> = Vec::new();
     for i in 0..x.len() {
         let mut f_k: Vec<Complex64> = Vec::new();
         for j in 0..x.len() {
@@ -64,9 +100,9 @@ pub fn dft(x: &Vec) -> Vec {
             let fe = (w * i_compl * j_compl).exp();
             f_k.push(fe);
         }
-        f.push(f_k.clone());
+        dft_matrix.push(f_k.clone());
     }
-    let r = mul_mv(f, x_compl);
+    let r = mul_mv(dft_matrix, x);
     r
 }
 
@@ -74,7 +110,7 @@ pub fn dft(x: &Vec) -> Vec {
 pub fn idft(x: &Vec<Complex64>) -> Vec<f64> {
     let mut w = Complex::new(0_f64, 2_f64 * PI / x.len() as f64);
 
-    // f_k = (SUM{n=0, N-1} f_n * e^(j2pi*k*n)/N)/N
+    // f_k (dft_matrix) = (SUM{n=0, N-1} f_n * e^(j2pi*k*n)/N)/N
     let mut dft_matrix: Vec<Vec<Complex64>> = Vec::new();
     for i in 0..x.len() {
         let mut f_k: Vec<Complex64> = Vec::new();
@@ -143,6 +179,8 @@ fn mul_vv_el(a: Vec, b: Vec) -> Vec {
 #[cfg(test)]
 mod tests {
     use super::*;
+    extern crate rand;
+    use rand::Rng;
 
     #[test]
     fn test_dft_simple_values() {
@@ -180,5 +218,39 @@ mod tests {
         assert_eq!(format!("{:.2}", r[2]), "-0.30-0.40i");
         assert_eq!(format!("{:.2}", r[3]), "-0.30-0.17i");
         assert_eq!(format!("{:.2}", r[4]), "-0.30+0.00i");
+
+        // expect result similar to initial values
+        let o = ifft(&r);
+        println!("{:?}", o);
+        assert_eq!(format!("{:.1}", o[0]), "0.2");
+        assert_eq!(format!("{:.1}", o[1]), "0.2");
+        assert_eq!(format!("{:.1}", o[2]), "0.3");
+        assert_eq!(format!("{:.1}", o[3]), "0.4");
+        assert_eq!(format!("{:.1}", o[4]), "0.5");
+        assert_eq!(format!("{:.1}", o[5]), "0.6");
+        assert_eq!(format!("{:.1}", o[6]), "0.7");
+        assert_eq!(format!("{:.1}", o[7]), "0.8");
+    }
+
+    #[test]
+    fn test_dft_random_values() {
+        let values: Vec<f64> = rand::thread_rng()
+            .sample_iter(rand::distributions::Standard)
+            .take(1024)
+            .collect();
+        let r = dft(&values);
+        println!("{:?}", r.len());
+        let o = idft(&r);
+    }
+
+    #[test]
+    fn test_fft_random_values() {
+        let values: Vec<f64> = rand::thread_rng()
+            .sample_iter(rand::distributions::Standard)
+            .take(1024)
+            .collect();
+        let r = fft(&values);
+        println!("{:?}", r.len());
+        let o = ifft(&r);
     }
 }