Tide gauge zigzag

Check that a tide gauge is really oscillating the way a healthy float should.

A healthy gauge alternates: it rises, then falls, then rises, and so on. Take each pair of adjacent readings and look at the step between them:

• If the two readings differ by tolerance or less (in either direction), the step is flat and is ignored completely; it neither counts as a rise or a fall nor breaks anything.
• Otherwise the step is a rise (the later reading is higher) or a fall (it is lower).

Considering only the rises and falls, in order, they must strictly alternate. A flat step does not reset anything: after a rise, a flat step, then another rise, the gauge has risen twice in a row. The very first rise or fall may be of either kind.

Input: readings, a list of integers, and tolerance, a non-negative integer.

Output: the index of the first reading that ends a step of the same kind as the previous non-flat step (the later reading of the offending pair), or -1 if the readings alternate correctly throughout.