code: func fillIndex(json string, c *parseContext) {
  	if len(c.value.Raw) > 0 && !c.calcd {
  		jhdr := *(*reflect.StringHeader)(unsafe.Pointer(&json))
  		rhdr := *(*reflect.StringHeader)(unsafe.Pointer(&(c.value.Raw)))
  		c.value.Index = int(rhdr.Data - jhdr.Data)
  		if c.value.Index < 0 || c.value.Index >= len(json) {
  			c.value.Index = 0
  		}
  	}
  }
docs: // fillIndex finds the position of Raw data and assigns it to the Index field
  // of the resulting value. If the position cannot be found then Index zero is
  // used instead.