go-ethereum/core/state/snapshot/iteration.md

How the fast iterator works

Consider the following example, where we have 6 iterators, sorted from left to right in ascending order.

Our 'primary' A iterator is on the left, containing the elements [0,1,8]

 A  B  C  D  E  F

 0  1  2  4  7  9
 1  2  9  -  14 13
 8  8  -     15 15
 -  -        -  16
                 -

When we call Next on the primary iterator, we get (ignoring the future keys)

A  B  C  D  E  F

1  1  2  4  7  9

We detect that we now got an equality between our element and the next element. And we need to continue Nexting on the next element

1  2  2  4  7  9

And move on:

A  B  C  D  E  F

1  2  9  4  7  9

Now we broke out of the equality, but we need to re-sort the element C

A  B  D  E  F  C

1  2  4  7  9  9

And after shifting it rightwards, we check equality again, and find C == F, and thus call Next on C

A  B  D  E  F  C

1  2  4  7  9  -

At this point, C was exhausted, and is removed

A  B  D  E  F

1  2  4  7  9

And we're done with this step.