feat(audio): add audio fade in/out and fix delta time naming

src/audio/audio_fade.cpp

@@ -0,0 +1,62 @@
+#include "Cubed/audio/audio_fade.hpp"
+
+#include <algorithm>
+#include <cmath>
+
+namespace Cubed {
+AudioFade::AudioFade(AudioSource* source, float fade_in, float fade_out)
+    : m_source(source), m_in_duration(fade_in), m_out_duration(fade_out) {
+    if (!m_source) {
+        m_active = false;
+        return;
+    }
+    m_start_gain = 0.0f;
+    m_end_gain = m_source->volume();
+    m_source->set_volume(0.0f);
+}
+AudioFade::~AudioFade() {}
+void AudioFade::update() {
+    if (!m_active || !m_source) {
+        return;
+    }
+
+    if (m_fade_in) {
+        float t =
+            std::clamp(m_source->current_time() / m_in_duration, 0.0f, 1.0f);
+
+        // Ease Out Sine
+        t = std::sin(t * std::numbers::pi_v<float> * 0.5f);
+
+        float gain = std::lerp(m_start_gain, m_end_gain, t);
+        m_source->set_volume(gain);
+        if (t >= 1.0f) {
+            m_source->set_volume(m_end_gain);
+            m_fade_in = false;
+            m_start_gain = m_end_gain;
+            m_end_gain = 0.0f;
+        }
+    } else {
+        float start_time =
+            std::max(0.0f, m_source->duration() - m_out_duration);
+
+        if (m_source->current_time() < start_time) {
+            return;
+        }
+
+        float elapsed = m_source->current_time() - start_time;
+
+        float t = std::clamp(elapsed / m_out_duration, 0.0f, 1.0f);
+
+        // Smoothstep
+        t = t * t * (3.0f - 2.0f * t);
+
+        float gain = std::lerp(m_start_gain, m_end_gain, t);
+        m_source->set_volume(gain);
+
+        if (t >= 1.0f) {
+            m_source->set_volume(m_end_gain);
+            m_active = false;
+        }
+    }
+}
+} // namespace Cubed