The Weir-Keeper's Tally

The weir-keeper of bitwick reads the mill-pond at dawn and writes down how far it moved overnight: a positive number for a rise, a negative number for a fall, zero for a still night. One number per day, in order, all season long.

The Waterworks Guild settles by the stretch. A stretch is any run of consecutive days, and it is worth the sum of the readings on those days. The Guild names one mark, and pays for every stretch worth exactly that.

Write tallySpans(levels, target). levels is a list of integers, possibly empty, possibly negative. target is an integer. Return an integer: the number of non-empty contiguous spans of levels whose readings sum to exactly target.

Spans are counted by position, so two spans that begin or end on different days are different spans even when their readings are identical. If levels is empty, return 0.