#define BUTTERFLY(cur, next, prev0, prev1, met0, met1) \
{ \
  /* First state in this set ------------------------------------ */ \
  /* Calculate metrics from the two previous states, use the old
     metric from the previous states plus the "transition-metric" */ \
  const _VITMETRTYPE rFiStAccMetricPrev0 = \
    pOldTrelMetric[prev0] + METRICSET(i)[met0]; \
  const _VITMETRTYPE  rFiStAccMetricPrev1 = \
    pOldTrelMetric[prev1] + METRICSET(i)[met1]; \
  \
  /* Take path with smallest metric */ \
  if (rFiStAccMetricPrev0 < rFiStAccMetricPrev1) \
  { \
    /* Save minimum metric for this state and store decision */ \
    pCurTrelMetric[cur] = rFiStAccMetricPrev0; \
    matdecDecisions[i][cur] = 0; \
  } \
  else \
  { \
    /* Save minimum metric for this state and store decision */ \
    pCurTrelMetric[cur] = rFiStAccMetricPrev1; \
    matdecDecisions[i][cur] = 1; \
  } \
  \
  /* Second state in this set ----------------------------------- */ \
  /* The only difference is that we swapped the matric sets */ \
  const _VITMETRTYPE rSecStAccMetricPrev0 = \
    pOldTrelMetric[prev0] + METRICSET(i)[met1]; \
  const _VITMETRTYPE  rSecStAccMetricPrev1 = \
    pOldTrelMetric[prev1] + METRICSET(i)[met0]; \
  \
  /* Take path with smallest metric */ \
  if (rSecStAccMetricPrev0 < rSecStAccMetricPrev1) \
  { \
    /* Save minimum metric for this state and store decision */ \
    pCurTrelMetric[next] = rSecStAccMetricPrev0; \
    matdecDecisions[i][next] = 0; \
  } \
  else \
  { \
    /* Save minimum metric for this state and store decision */ \
    pCurTrelMetric[next] = rSecStAccMetricPrev1; \
    matdecDecisions[i][next] = 1; \
  } \
}
#endif