Breadth-first zip

Property-tested and verified to return every possible value exactly one in a strictly increasing sum of indices.

Why?

For whenever you have a multiple iterators and want to cover every possible value from overall smallest to largest.

E.g. for three instances of 0..3, you'd get this:

0 0 0 # sum = 0
0 0 1 # sum = 1
0 1 0
1 0 0
0 0 2 # sum = 2
0 1 1
0 2 0
1 0 1
1 1 0
2 0 0
0 1 2 # sum = 3
0 2 1
1 0 2
1 1 1
1 2 0
2 0 1
2 1 0
0 2 2 # sum = 4
1 1 2
1 2 1
2 0 2
2 1 1
2 2 0
1 2 2 # sum = 5
2 1 2
2 2 1
2 2 2 # sum = 6

Inputs can be any non-empty iterator, even combining different sizes.

lib.rs:

Breadth-first exhaustive zip for repeatable iterators. Behavior matches the following pseudocode specification:

• Initialize a counter i at zero.
• When propmted, pull the first element from each iterator.
  • If any iterator is empty, return None.
• When prompted again, advance only the last iterator.
• Continue to do so until the last iterator terminates or reaches its ith element.
  • When it does so, reset it and pull the next element from the second-to-last iterator.
• Repeat this process until we exhaust the first iterator.
  • When you've done that, increase i and repeat.
• Once i exceeds the longest iterator's length, we're done: return None.