Problem Statement

In a two-dimensional world, we begin with any bar-chart (or row of close-packed 'towers', each of unit width), and then it rains, completely filling all convex enclosures in the chart with water.

In the example above, a bar chart representing the values [5, 3, 7, 2, 6, 4, 5, 9, 1, 2] has filled, collecting 14 units of water. Write a function, in your language, from a given array of heights, to the number of water units that can be held in this way, by a corresponding bar chart. Calculate the number of water units that could be collected by bar charts representing each of the following seven series:

See, also:

Let's start with the solution:

cmax: function => [
    m: neg ∞
    map & 'x -> m:<=max @[m x]
]

vmin: $ => [map couple & & => min]

vsub: $ => [map couple & & 'p -> p\0 - p\1]

water: function [a][
    sum vsub vmin reverse cmax reverse a cmax a a
]

loop [
    [1, 5, 3, 7, 2],
    [5, 3, 7, 2, 6, 4, 5, 9, 1, 2],
    [2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1],
    [5, 5, 5, 5],
    [5, 6, 7, 8],
    [8, 7, 7, 6],
    [6, 7, 10, 7, 6]
] 'a -> print [a "->" water a]