This MIGHT work? We'll see

src/anim.c

@@ -30,22 +30,38 @@ static inline size_t stop_index(hexagram_anim *anim, double offset) {
 
 int hexagram_anim_step(hexagram_anim *anim, cairo_t *bg, cairo_t *fg) {
     struct timeval tv;
-    double offset, interval;
+    double offset = 0.0, interval, progress;
     size_t i;
 
-    if (anim->current >= anim->count) {
-        goto done;
-    }
-
     gettimeofday(&anim->now, NULL);
 
     timersub(&anim->now, &anim->start, &tv);
 
+    /* Determine the interval between initialisation and now */
     interval = tv_seconds(&tv);
 
-done:
+    /* Determine current animation stop index and time offset */
-    return 0;
+    for (i=0; i<anim->count; i++) {
+        double duration = anim->stops[i].duration;
+
+        if (duration <= 0) {
+            continue;
+        }
+
+        if (interval < offset + duration) {
+            break;
+        }
+
+        offset += duration;
+    }
+
+    if (i == anim->count) {
+        /* We are apparently at the end. */
+        return 0;
+    }
+
+    /* Determine current progress through current stop */
+    progress = (interval - offset) / anim->stops[i].duration;
 
-next:
     return 1;
 }