The health bar

Problem

In Pixelia, a potion never fills the health bar past its top: whatever is left over is lost. The hero starts with a full bar, and every turn either takes a hit or drinks a potion.

Write a function that takes top (an integer, 1 or more) and changes, a list of integers (negative is a hit, positive is a potion; it may be empty). Apply the changes in order: add each one to the health and, if it goes over top, set it back to top in that same step. Return the index of the change that brings health to 0 or less, or -1 if that never happens. Indexes start at 0, and whatever comes after does not matter. With 10 and [-4, 8, -7, -3], health goes 6, then 14 which stays at 10, then 3, then 0. Reaching 0 already counts as falling: return 3. Without capping at every step, it would have ended at 4 and survived.

Examples

• The example

  10, [-4, 8, -7, -3] → 3

• Survives

  20, [-5, -5, 3, -10] → -1

• Drops below 0, then heals

  5, [-2, -4, 6] → 1

• A potion with a full bar

  8, [5, -8] → 1

• Two wasted potions

  10, [-3, 5, 5, -9, -2] → 4

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

You start with this

def fatal_hit(top, changes):
    pass

function fatalHit(top, changes) {
}