Segment descriptors represent the structure of nested arrays. For each segment, it stores the length and the starting index in the flat data array.

Example:

    flat array data:  [1, 2, 3, 4, 5, 6, 7, 8]
      (segmentation)   ----  -------  -  ----
      segd  lengths: [2, 3, 1, 2]
            indices: [0, 2, 5, 6]
           elements: 8 

O(1). Construct a new segment descriptor.

O(1). Yield an empty segment descriptor, with no elements or segments.

O(1). Yield a singleton segment descriptor. The single segment covers the given number of elements.

O(n). Convert a length array into a segment descriptor.

The array contains the length of each segment, and we compute the indices from that. Runtime is O(n) in the number of segments.

O(1). Yield the overall number of segments.

O(1). Yield the lengths of the individual segments.

O(1). Yield the segment indices of a segment descriptor.

O(1). Yield the number of data elements.

O(n). Extract a slice of a segment descriptor, avoiding copying where possible.

We can share the segment lengths with the original segment descriptor, but still need to recompute the starting indices of each. Hence runtime is O(n) in the number of segments sliced out.

NOTE: In the new segment descriptor, the starting index of the first segment will be 0.

O(n). Extract a slice of a segment descriptor, copying everything.

In contrast to sliceUSegd, this function copies the array of segment lengths as well as recomputing the starting indices of each.

NOTE: In the new segment descriptor, the starting index of the first segment will be 0.