Package locmap provides a concurrency-safe data structure that maps keys to value locations. A key is 128 bits and is specified using two uint64s (keyA, keyB). A value location is specified using a blockID, offset, and length triplet. Each mapping is assigned a timestamp and the greatest timestamp wins. The timestamp usually has some number of the lowest bits in use for state information such as active and inactive entries. For example, the lowest bit might be used as 0 = active, 1 = deletion marker so that deletion events are retained for some time period before being completely removed with Discard. Exactly how many bits are used and what they're used for is outside the scope of the mapping itself. This implementation essentially uses a tree structure of slices of key to location assignments. When a slice fills up, an additional slice is created and half the data is moved to the new slice and the tree structure grows. If a slice empties, it is merged with its pair in the tree structure and the tree shrinks. The tree is balanced by high bits of the key, and locations are distributed in the slices by the low bits. There is also a modified form of the data structure called GroupLocMap that expands the primary key of the map to two 128 bit keys and offers a GetGroup method which retrieves all matching items for the first key.