diff --git a/.rustfmt.toml b/.rustfmt.toml
new file mode 100644
index 0000000..b196eaa
--- /dev/null
+++ b/.rustfmt.toml
@@ -0,0 +1 @@
+tab_spaces = 2
diff --git a/src/main.rs b/src/main.rs
index e7a11a9..c7cd600 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,3 +1,163 @@
+const SECONDS_PER_MIN: i64 = 60;
+
+#[derive(Debug, PartialEq)]
+enum TimerState {
+  Work,
+  Break,
+  Paused,
+}
+
+struct Termato {
+  work_mins: i64,
+  break_mins: i64,
+  state: TimerState,
+  prior_state: TimerState,
+  is_running: bool,
+  duration_in_secs: i64,
+  time_remaining_in_sec: i64,
+}
+
+impl Termato {
+  fn new(work_mins: i64, break_mins: i64) -> Self {
+    Termato {
+      work_mins,
+      break_mins,
+      state: TimerState::Work,
+      prior_state: TimerState::Work,
+      is_running: false,
+      duration_in_secs: work_mins * SECONDS_PER_MIN,
+      time_remaining_in_sec: break_mins * SECONDS_PER_MIN,
+    }
+  }
+
+  fn toggle_mode(&mut self) {
+    let is_on_break = self.state == TimerState::Break
+      || (self.state == TimerState::Paused && self.prior_state == TimerState::Break);
+
+    if is_on_break {
+      self.state = TimerState::Work;
+      self.prior_state = TimerState::Work;
+      self.duration_in_secs = self.work_mins * SECONDS_PER_MIN;
+    } else {
+      self.state = TimerState::Break;
+      self.prior_state = TimerState::Break;
+      self.duration_in_secs = self.break_mins * SECONDS_PER_MIN;
+    }
+
+    self.time_remaining_in_sec = self.duration_in_secs;
+  }
+
+  fn tick(&mut self) {
+    if self.is_running && self.time_remaining_in_sec > 0 {
+      self.time_remaining_in_sec -= 1;
+
+      if self.time_remaining_in_sec == 0 {
+        self.toggle_mode();
+      }
+    }
+  }
+}
+
 fn main() {
-    println!("Hello, world!");
+    let mut termato = Termato::new(1, 1);
+
+    loop {
+        termato.is_running = true;
+
+        termato.tick();
+
+        println!("Time remaining: {}", termato.time_remaining_in_sec);
+        println!("State: {:?}", termato.state);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+
+  #[test]
+  fn termato_when_called_it_should_return_expected_starting_state() {
+    let result = Termato::new(1, 1);
+
+    assert_eq!(result.work_mins, 1);
+    assert_eq!(result.break_mins, 1);
+    assert_eq!(result.state, TimerState::Work);
+    assert_eq!(result.prior_state, TimerState::Work);
+    assert!(!result.is_running);
+    assert_eq!(result.duration_in_secs, 60);
+    assert_eq!(result.time_remaining_in_sec, 60);
+  }
+
+  #[test]
+  fn termato_tick_when_called_and_not_running_it_should_do_nothing() {
+    let mut termato = Termato::new(1, 1);
+
+    termato.tick();
+
+    assert_eq!(termato.time_remaining_in_sec, 60);
+  }
+
+  #[test]
+  fn termato_tick_when_called_and_no_time_remaining_it_should_do_nothing() {
+    let mut termato = Termato::new(1, 1);
+    termato.is_running = true;
+    termato.time_remaining_in_sec = 0;
+
+    termato.tick();
+
+    assert_eq!(termato.time_remaining_in_sec, 0);
+  }
+
+  #[test]
+  fn termato_tick_when_running_and_time_reamining_it_should_reduce_remaining_by_one() {
+    let mut termato = Termato::new(1, 1);
+    termato.is_running = true;
+
+    termato.tick();
+
+    assert_eq!(termato.time_remaining_in_sec, 59);
+  }
+
+  #[test]
+  fn termato_tick_when_running_on_work_and_time_remaining_reaches_zero_it_should_toggle_to_break() {
+    let mut termato = Termato::new(1, 2);
+    termato.is_running = true;
+    termato.time_remaining_in_sec = 1;
+
+    termato.tick();
+
+    assert_eq!(termato.state, TimerState::Break);
+    assert_eq!(termato.prior_state, TimerState::Break);
+    assert_eq!(termato.duration_in_secs, 120);
+    assert_eq!(termato.time_remaining_in_sec, 120);
+  }
+
+  #[test]
+  fn termato_tick_when_running_on_break_and_time_remaining_reaches_zero_it_should_toggle_to_work() {
+    let mut termato = Termato::new(2, 1);
+    termato.state = TimerState::Break;
+    termato.is_running = true;
+    termato.time_remaining_in_sec = 1;
+
+    termato.tick();
+
+    assert_eq!(termato.state, TimerState::Work);
+    assert_eq!(termato.prior_state, TimerState::Work);
+    assert_eq!(termato.duration_in_secs, 120);
+    assert_eq!(termato.time_remaining_in_sec, 120);
+  }
+
+  #[test]
+  fn termato_toggle_mode_when_paused_and_previous_state_is_break_it_should_toggle_to_work() {
+    let mut termato = Termato::new(2, 1);
+    termato.state = TimerState::Paused;
+    termato.prior_state = TimerState::Break;
+
+    termato.toggle_mode();
+
+    assert_eq!(termato.state, TimerState::Work);
+    assert_eq!(termato.prior_state, TimerState::Work);
+    assert_eq!(termato.duration_in_secs, 120);
+    assert_eq!(termato.time_remaining_in_sec, 120);
+  }
 }