One obvious optimization that would half the size of the data and also solve the gaps problem is to keep every border between two states once and not twice (for

One obvious optimization that would half the size of the data and also solve the gaps problem is to keep every border between two states once and not twice (for each polygon). This would require some processing of the geometry to find the intersection points, but assuming that in the original data, the border between two states is the same exact line, it shouldn't be hard.