Pendulum crossings

Problem

Valeria hung a pendulum in the lab with a sensor that reads its position every tenth of a second: negative left of center, positive right of it, and 0 right at the center. She wants to know how many times it swung from one side to the other.

Write a function that takes the list of readings (integers or decimals; it may be empty) and returns how many crossings there were, as an integer. A crossing is a reading on the opposite side from the last reading that wasn't 0. A 0 belongs to no side: it isn't a crossing and doesn't change sides. So [2, 0, -3] has one crossing and [2, 0, 3] has none. The first reading that isn't 0 isn't a crossing either, since there's nothing to compare it with.

With [3, -2, -1, 4] there are two crossings: 3 to -2 and -1 to 4.

Examples

• Crosses on every reading

  [3, -2, 4, -1] → 3

• Always on the same side

  [1, 2, 3] → 0

• A zero between different sides

  [2, 0, -2] → 1

• A zero between the same side

  [2, 0, 3] → 0

Besides these, the challenge has hidden tests that are revealed when you submit your solution.

You start with this

def crossings(readings):
    pass

function crossings(readings) {
}