The Ember-Shrines of the Isenhal Barrens

Commission of the Lampwardens' Guild, Bitwick chapter

The Isenhal Barrens are a frozen waste where no fire can be lit. Any courier crossing them carries a single ember-lantern, which holds a fixed amount of warmth. The road is marked off in stretches. Walking a stretch drains a set amount of warmth from the lantern. Along the way stand old ember-shrines, where a lantern is rekindled to full.

The Guild wants to know, before a courier sets out, exactly where along a planned route the lantern would fail.

The contract

Write firstColdStretch(cap, stretches).

• cap is a non-negative integer: the lantern's full warmth. The lantern starts the journey full.
• stretches is a list of non-negative integers, the route in walking order.
• A value of 0 is an ember-shrine. Reaching it sets the lantern's warmth to exactly cap.
• Any positive value is a stretch of road costing that much warmth.

The courier takes the entries in order. A stretch can be walked only if its cost is less than or equal to the warmth currently in the lantern. Walking it subtracts the cost from that warmth. A lantern left at exactly 0 warmth has not failed.

Return the 0-based index of the first stretch whose cost is greater than the lantern's current warmth. If the courier reaches the end of the list without that happening, return -1. That includes an empty route.