Candy Crush Reduction

A one-row candy board is given as a string s of lowercase letters, one letter per candy. A run is a maximal block of equal adjacent letters (in "abbbca" the runs are a, bbb, c, a).

The board resolves in passes:

1. In each pass, every run of length 2 or more is crushed at the same time. A run is always crushed whole; you cannot crush just part of it, and you have no choice about what to crush.
2. The remaining candies then slide together. This can put equal letters next to each other that were not neighbours before; those new runs are crushed in the next pass, not the current one.
3. Passes repeat until a pass finds no run of length 2 or more.

Return the number of candies left when the board stops changing.

s can be long, and a board can need many passes (each pass might remove only a couple of candies), so re-scanning the whole string on every pass may be too slow.