Maximum triangle path sum: Difference between revisions

'use strict';

// MAX PATH SUM -----------------------------------------------------------

// A list of lists folded down to a list of just one remaining integer.

// The head function returns that integer from the list.

head(

foldr1(

// The accumulator, zipped with the tail of the

// accumulator, yields pairs of adjacent sums so far.

(ys, xs) => zipWith3(

// Plus greater of two below

(a, b, c) => a + max(b, c),

xs, ys, tail(ys)

),

xss

);

// GENERIC FUNCTIONS ------------------------------------------------------

// Right fold using final element as initial accumulator

// foldr1 :: (a -> a -> a) -> [a] -> a

const foldr1 = (f, xs) =>

xs.length > 0 ? init(xs)

.reduceRight(f, last(xs)) : [];

// head :: [a] -> a

const head = xs => xs.length ? xs[0] : undefined;

// init :: [a] -> [a]

const init = xs => xs.length ? xs.slice(0, -1) : undefined;

// last :: [a] -> a

const last = xs => xs.length ? xs.slice(-1)[0] : undefined;

// max :: Ord a => a -> a -> a

const max = (a, b) => b > a ? b : a;

// minimum :: [a] -> a

const minimum = xs =>

xs.reduce((a, x) => (x < a || a === undefined ? x : a), undefined);

// tail :: [a] -> [a]

const tail = xs => xs.length ? xs.slice(1) : undefined;

// Function of arity 3 mapped over nth items of each of 3 lists

// zipWith3 :: (a -> b -> c -> d) -> [a] -> [b] -> [c] -> [d]

const zipWith3 = (f, xs, ys, zs) =>

Array.from({

length: minimum([xs.length, ys.length, zs.length])

// TEST -------------------------------------------------------------------