diff --git a/src/fft.rs b/src/fft.rs
new file mode 100644
index 0000000..3f9fe7f
--- /dev/null
+++ b/src/fft.rs
@@ -0,0 +1,147 @@
+use std::fs;
+use std::str;
+
+
+const BASE_SEQUENCE: [i64; 4] = [0, 1, 0, -1];
+
+struct SequenceIterator {
+    i: usize,
+    j: usize,
+    jmax: usize,
+}
+
+impl SequenceIterator {
+    fn new(digit: usize) -> SequenceIterator {
+        SequenceIterator {
+            i: 0,
+            j: 0,
+            jmax: digit,
+        }
+    }
+}
+
+impl Iterator for SequenceIterator {
+    type Item = i64;
+
+    fn next(&mut self) -> Option<i64> {
+        // advance first, because we wanted to skip the first digit.
+        self.j += 1;
+        if self.j == self.jmax {
+            self.j = 0;
+            self.i += 1;
+            if self.i == BASE_SEQUENCE.len() {
+                self.i = 0;
+            }
+        }
+
+        Some(BASE_SEQUENCE[self.i] as i64)
+    }
+}
+
+#[test]
+fn sequence_check() {
+    let seq1 = SequenceIterator::new(1);
+    assert_eq!(vec![1, 0, -1, 0, 1, 0, -1, 0], seq1.take(8).collect::<Vec<i64>>());
+    let seq2 = SequenceIterator::new(2);
+    assert_eq!(vec![0, 1, 1, 0, 0, -1, -1, 0], seq2.take(8).collect::<Vec<i64>>());
+    let seq4 = SequenceIterator::new(4);
+    assert_eq!(vec![0, 0, 0, 1, 1, 1, 1, 0], seq4.take(8).collect::<Vec<i64>>());
+}
+
+struct NumStr {
+    vals: Vec<u8>,
+}
+
+impl NumStr {
+    fn new(s: &str) -> NumStr {
+        let mut vals = Vec::with_capacity(s.len());
+
+        for c in s.chars() {
+            vals.push(c.to_digit(10).expect("Found non-digit?!") as u8);
+        }
+
+        NumStr{ vals }
+    }
+
+    fn apply_round(&mut self) {
+        let orig = self.vals.clone();
+        let mut i = 1;
+
+        while i <= self.vals.len() {
+            let mulseq = SequenceIterator::new(i);
+            let mut total = 0;
+
+            for (digit, mul) in orig.iter().zip(mulseq) {
+                total += (*digit as i64) * mul;
+            }
+
+            self.vals[i-1] = (total.abs() % 10) as u8;
+
+            i += 1;
+        }
+    }
+
+    fn apply_rounds(&mut self, num: usize) {
+        for _ in 0..num {
+            self.apply_round();
+        }
+    }
+
+    fn to_string(&self, chars: usize) -> String {
+        let mut res = String::new();
+
+        for d in self.vals.iter().take(chars) {
+            let c = match d {
+                0 => '0',
+                1 => '1',
+                2 => '2',
+                3 => '3',
+                4 => '4',
+                5 => '5',
+                6 => '6',
+                7 => '7',
+                8 => '8',
+                9 => '9',
+                _ => panic!("digit not a digit"),
+            };
+            res.push(c);
+        }
+
+        res
+    }
+}
+
+#[test]
+fn basic_apply() {
+    let mut base = NumStr::new("12345678");
+    base.apply_round();
+    assert_eq!("48226158", &base.to_string(8));
+    base.apply_round();
+    assert_eq!("34040438", &base.to_string(8));
+    base.apply_round();
+    assert_eq!("03415518", &base.to_string(8));
+    base.apply_round();
+    assert_eq!("01029498", &base.to_string(8));
+}
+
+#[test]
+fn examples() {
+    let mut example1 = NumStr::new("80871224585914546619083218645595"); 
+    example1.apply_rounds(100);
+    assert_eq!("24176176", &example1.to_string(8));
+    let mut example2 = NumStr::new("19617804207202209144916044189917"); 
+    example2.apply_rounds(100);
+    assert_eq!("73745418", &example2.to_string(8));
+    let mut example3 = NumStr::new("69317163492948606335995924319873"); 
+    example3.apply_rounds(100);
+    assert_eq!("52432133", &example3.to_string(8));
+}
+
+#[test]
+fn day16() {
+    let day16_contents = fs::read("inputs/day16").expect("Couldn't open day14 problem");
+    let day16_str = str::from_utf8(&day16_contents).expect("Couldn't decode day14 problem");
+    let mut day16 = NumStr::new(&day16_str.trim());
+    day16.apply_rounds(100);
+    assert_eq!("63794407", &day16.to_string(8));
+}
\ No newline at end of file
diff --git a/src/main.rs b/src/main.rs
index 3cf2638..a2b7076 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -2,6 +2,8 @@ mod arcade;
 mod args;
 #[cfg(test)]
 mod chemistry;
+#[cfg(test)]
+mod fft;
 mod fuel;
 mod image;
 mod machine;