Water Kata

water.php

<?php declare(strict_types = 1);

function volume (array $levels) {
    return array_sum(
        zipWith(
            'subtract',
            zipWith(
                'min',
                scanl('max', $levels),
                scanr('max', $levels)
            ),
            $levels
        )
    );
}

// tests
assert(volume([1, 5, 3, 7, 2]) == 2);
assert(volume([5, 3, 7, 2, 6, 4, 5, 9, 1, 2] ) === 14);
assert(volume([2, 6, 3, 5, 2, 8, 1, 4, 2, 2, 5, 3, 5, 7, 4, 1] ) === 35);
assert(volume([5, 5, 5, 5]) ===  0);
assert(volume([5, 6, 7, 8]) ===  0);
assert(volume([8, 7, 7, 6]) ===  0);
assert(volume([6, 7, 10, 7, 6]) ===  0);


// missing functions in php core
function subtract($x, $y) { return $x - $y; }
function scanl(callable $f, array $l) : array {
    $f2 = function($carry, $item) use ($f)  {
        return array_merge($carry, [$f(last($carry, $item), $item)]);
    };
    return array_reduce($l, $f2, []);
}
function scanr(callable $f, array $l) : array {
    return array_reverse(scanl($f, array_reverse($l)));
}
function head(array $l) {
    if(count($l)) return $l[0];
    throw new InvalidArgumentException("Empty list");
}
function tail(array $l) {
    if(count($l)) return array_slice($l, 1);
    throw new InvalidArgumentException("Empty list");
}
function last(array $l, $default) {
    return
        count($l)
            ? array_slice($l, -1)[0]
            : $default;
}
function zipWith(callable $f, array $left, array $right) : array {
    return
        empty($left) || empty($right)
            ? []
            : array_merge(
                [$f(head($left), head($right))],
                zipWith($f, tail($left), tail($right))
            );
}

assert(head([1, 5, 3, 7, 2]) === 1);
assert(tail([1, 5, 3, 7, 2]) === [5, 3, 7, 2]);
assert(tail([1]) === []);
assert(last([], 3) === 3);
assert(last([1, 5, 3, 7, 2], 3) === 2);
assert(zipWith('max', [1,2,3], [3,2,1]) === [3,2,3]);
assert(scanl('max', [1, 5, 3, 7, 2]) === [1, 5, 5, 7, 7]);
assert(scanr('max', [1, 5, 3, 7, 2]) === [7, 7, 7, 7, 2]);

http://stackoverflow.com/questions/24414700/amazon-water-collected-between-towers

afaik tail([]) should throw an exception

thanks, fixed