Add asert to check input length is 2^n

- Add a regression test to make sure non-power-of-two FFTs are not
  allowed for the time being

src/lib.rs

@@ -41,7 +41,7 @@ pub fn fft_with_opts(
     opts: &Options,
     planner: &mut Planner,
 ) {
-    assert_eq!(reals.len(), imags.len());
+    assert!(reals.len() == imags.len() && reals.len().is_power_of_two());
     let n: usize = reals.len().ilog2() as usize;
 
     let twiddles_re = &mut planner.twiddles_re;
@@ -89,6 +89,19 @@ mod tests {
 
     use super::*;
 
+    #[should_panic]
+    #[test]
+    fn non_power_of_two_fft() {
+        let num_points = 5;
+
+        // this test will actually always fail at this stage
+        let mut planner = Planner::new(num_points);
+
+        let mut reals = vec![0.0; num_points];
+        let mut imags = vec![0.0; num_points];
+        fft(&mut reals, &mut imags, &mut planner);
+    }
+
     #[test]
     fn fft_correctness() {
         let range = Range { start: 4, end: 17 };

src/planner.rs

@@ -5,6 +5,13 @@ pub struct Planner {
     pub twiddles_im: Vec<f64>,
 }
 
+// TODO(saveliy yusufov): Add a parameter to `new` that will take into consideration whether we do inverse FFT (IFFT)
+// In this case, the twiddle factors should be pre-computed as follows:
+//
+//                  FFT Twiddle Factor: e^{i2π*k/N}
+//                  IFFT Twiddle Factor: e^{-i2π*k/N}
+//
+// source: https://dsp.stackexchange.com/q/73367
 impl Planner {
     /// Create a `Planner` for an FFT of size `num_points`
     ///