From 3d0de3f951ec2da7a2d41c6958d0d3583a7832a0 Mon Sep 17 00:00:00 2001
From: HampusM <hampus@hampusmat.com>
Date: Tue, 24 Oct 2023 21:44:56 +0200
Subject: feat(engine): add delta time

---
 engine/src/lib.rs | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/engine/src/lib.rs b/engine/src/lib.rs
index ce953f8..bec1e30 100644
--- a/engine/src/lib.rs
+++ b/engine/src/lib.rs
@@ -1,6 +1,7 @@
 #![deny(clippy::all, clippy::pedantic)]
 
 use std::collections::BTreeMap;
+use std::time::{Duration, Instant};
 
 use glfw::{Window, WindowBuilder};
 
@@ -29,6 +30,7 @@ pub struct Engine
     objects: BTreeMap<ObjectId, Object>,
     window: Window,
     camera: Camera,
+    delta_time: Duration,
 }
 
 impl Engine
@@ -63,6 +65,7 @@ impl Engine
             window,
             objects: BTreeMap::new(),
             camera: Camera::new(),
+            delta_time: Duration::ZERO,
         })
     }
 
@@ -95,7 +98,11 @@ impl Engine
     /// Will return `Err` if updating the window fails.
     pub fn start(&mut self, mut func: impl FnMut(&mut Self)) -> Result<(), Error>
     {
+        let mut prev_frame_start: Option<Instant> = None;
+
         while !self.window.should_close() {
+            self.update_delta_time(&mut prev_frame_start);
+
             func(self);
 
             let window_size = self.window.size().map_err(Error::GetWindowSizeFailed)?;
@@ -118,6 +125,25 @@ impl Engine
     {
         &mut self.camera
     }
+
+    /// Returns the current delta time. Will be 0 if not called from the function
+    /// passed to [`start`].
+    #[must_use]
+    pub fn delta_time(&self) -> &Duration
+    {
+        &self.delta_time
+    }
+
+    fn update_delta_time(&mut self, prev_frame_start: &mut Option<Instant>)
+    {
+        let frame_start_time = Instant::now();
+
+        if let Some(last_frame_start) = prev_frame_start {
+            self.delta_time = frame_start_time.duration_since(*last_frame_start);
+        }
+
+        *prev_frame_start = Some(frame_start_time);
+    }
 }
 
 /// Engine Error
-- 
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