2 files changed, 25 insertions, 7 deletions

diff --git a/timer/include/timer.h b/timer/include/timer.h
index 6dc87d9..75ae367 100644
--- a/timer/include/timer.h
+++ b/timer/include/timer.h
@@ -21,10 +21,10 @@ typedef uint64_t time_delta;
 
 /// A high resolution timer.
 typedef struct Timer {
-  time_point start_time;   // The instant the timer was last started.
-  time_point last_tick;    // The instant the timer was last ticked.
-  time_delta running_time; // Time elapsed since the timer was last started.
-  time_delta delta_time;   // Time elapsed since the last tick.
+  time_point start_time;   ///< The instant the timer was last started.
+  time_point last_tick;    ///< The instant the timer was last ticked.
+  time_delta running_time; ///< Time elapsed since the timer was last started.
+  time_delta delta_time;   ///< Time elapsed since the last tick.
 } Timer;
 
 /// Construct a new timer.
@@ -38,6 +38,15 @@ void timer_start(Timer*);
 /// Update the timer's running and delta times.
 void timer_tick(Timer*);
 
+/// Advance the timer.
+///
+/// This is used for simulations that wish control over time. It should not be
+/// used together with timer_tick().
+///
+/// The input time is relative to the timer's start time. Successive calls to
+/// this function must use increasing values of time.
+void timer_advance(Timer* timer, time_delta t);
+
 /// Get the current time.
 time_point time_now(void);
 
diff --git a/timer/src/timer.c b/timer/src/timer.c
index d886f59..c934077 100644
--- a/timer/src/timer.c
+++ b/timer/src/timer.c
@@ -37,11 +37,20 @@ void timer_start(Timer* timer) {
   timer->delta_time   = 0;
 }
 
+static void timer_update(Timer* timer, time_point this_tick) {
+  timer->running_time = time_diff(timer->start_time, this_tick);
+  timer->delta_time   = time_diff(timer->last_tick, this_tick);
+  timer->last_tick    = this_tick;
+}
+
 void timer_tick(Timer* timer) {
   const time_point this_tick = time_now();
-  timer->running_time        = time_diff(timer->start_time, this_tick);
-  timer->delta_time          = time_diff(timer->last_tick, this_tick);
-  timer->last_tick           = this_tick;
+  timer_update(timer, this_tick);
+}
+
+void timer_advance(Timer* timer, time_delta t) {
+  const time_point this_tick = time_add(timer->start_time, t);
+  timer_update(timer, this_tick);
 }
 
 time_point time_now(void) {