From 381890892ba81038b4a47cd179bd9fa4a5ba3073 Mon Sep 17 00:00:00 2001
From: Camden Dixie O'Brien <cobrien@witekio.com>
Date: Thu, 2 Oct 2025 15:33:05 +0100
Subject: [PATCH] Implement step-wise animation locked to time series

---
 animation.c | 38 +++++++++++++++++++++++++++++++-------
 animation.h |  8 +++++---
 main.c      |  4 +++-
 3 files changed, 39 insertions(+), 11 deletions(-)

diff --git a/animation.c b/animation.c
index 87fb6f9..4340554 100644
--- a/animation.c
+++ b/animation.c
@@ -44,19 +44,43 @@ void animation_cleanup(animation_state_t *state)
 
 void animation_start(animation_state_t *state, const model_t *model)
 {
+	state->current_keyframe = model->animation.keyframe_count - 1;
+	state->next_keyframe = 0;
+
 	state->current = model->bones;
+	state->next = model->animation.keyframes;
+
+	state->duration_us = model->animation.times_us[0];
+	state->elapsed_us = 0;
 }
 
-void animation_update(animation_state_t *state, const model_t *model)
+void animation_update(
+    animation_state_t *state, const model_t *model, uint32_t dt_us)
 {
-	const size_t offset = state->keyframe * model->bone_count;
-	state->current = model->animation.keyframes + offset;
+	state->elapsed_us += dt_us;
+	if (state->elapsed_us >= state->duration_us) { // Move to next keyframe
+		state->elapsed_us -= state->duration_us;
+
+		state->current_keyframe = state->next_keyframe;
+		const uint32_t keyframe_count = model->animation.keyframe_count;
+		if (state->current_keyframe + 1 < keyframe_count) {
+			state->next_keyframe = state->current_keyframe + 1;
+			state->duration_us
+			    = model->animation.times_us[state->next_keyframe]
+			    - model->animation.times_us[state->current_keyframe];
+		} else {
+			// Handle loop-around
+			state->next_keyframe = 0;
+			state->duration_us
+			    = model->animation.times_us[state->next_keyframe];
+		}
+
+		state->current = state->next;
+		const uint32_t offset = state->next_keyframe * model->bone_count;
+		state->next = model->animation.keyframes + offset;
+	}
 
 	compute_bone_matrices(
 	    state->current, model->bone_count, model->bone_parents,
 	    model->inverse_bind_matrices, state->bone_matrices);
-
-	++state->keyframe;
-	if (state->keyframe >= model->animation.keyframe_count)
-		state->keyframe = 0;
 }
diff --git a/animation.h b/animation.h
index 7bbd104..f33f02b 100644
--- a/animation.h
+++ b/animation.h
@@ -5,14 +5,16 @@
 
 typedef struct {
 	mat4_t *bone_matrices;
-	bone_t *current;
-	unsigned keyframe;
+	const bone_t *current, *next;
+	uint32_t current_keyframe, next_keyframe;
+	uint32_t duration_us, elapsed_us;
 } animation_state_t;
 
 animation_state_t animation_init(const model_t *model);
 void animation_cleanup(animation_state_t *state);
 
 void animation_start(animation_state_t *state, const model_t *model);
-void animation_update(animation_state_t *state, const model_t *model);
+void animation_update(
+    animation_state_t *state, const model_t *model, uint32_t dt_us);
 
 #endif
diff --git a/main.c b/main.c
index 3edbfce..790ea99 100644
--- a/main.c
+++ b/main.c
@@ -10,6 +10,8 @@
 #define W 800
 #define H 600
 
+#define DT (1'000'000 / 60) // Assume 60 Hz for now
+
 static void
 key_callback(GLFWwindow *window, int key, int scancode, int action, int mods)
 {
@@ -79,7 +81,7 @@ int main(int argc, char *argv[])
 		glfwPollEvents();
 
 		mvp.model = mat4_mul(rot, mvp.model);
-		animation_update(&anim, &model);
+		animation_update(&anim, &model, DT);
 		gpu_render(&gpu, &mvp, anim.bone_matrices);
 	}