duck

        // start & end indices in routedata
        float iLength = 0;
        int startPointIndex = 0;
        int endPointIndex = 0;
        float midPointI = 0;
        while (iLength < routeLength * i)
        {
            Vector3 pos = routeData[startPointIndex];
            Vector3 npos = routeData[(startPointIndex + 1) % routeData.Count];
            float addLength = (pos - npos).magnitude;
            float prevLength = iLength;
            iLength += addLength;
            if (iLength >= routeLength * i)
            {
                endPointIndex = (startPointIndex + 1) % routeData.Count;
                // t1
                midPointI = (i - (prevLength / routeLength)) / (addLength / routeLength);
            }
            else
            {
                startPointIndex++;
            }
        }