68 lines
1.9 KiB
JavaScript
68 lines
1.9 KiB
JavaScript
(() => {
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'use strict';
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// PASCAL'S TRIANGLE ------------------------------------------------------
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// pascal :: Int -> [[Int]]
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const pascal = n =>
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foldl(a => {
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const xs = a.slice(-1)[0]; // Previous row
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return append(a, [
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zipWith(
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(a, b) => a + b,
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append([0], xs),
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append(xs, [0])
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)
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]);
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}, [
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[1] // Initial seed row
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], enumFromTo(1, n - 1));
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// GENERIC FUNCTIONS ------------------------------------------------------
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// (++) :: [a] -> [a] -> [a]
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const append = (xs, ys) => xs.concat(ys);
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// enumFromTo :: Int -> Int -> [Int]
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const enumFromTo = (m, n) =>
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Array.from({
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length: Math.floor(n - m) + 1
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}, (_, i) => m + i);
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// flip :: (a -> b -> c) -> b -> a -> c
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const flip = f => (a, b) => f(b, a);
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// foldl :: (b -> a -> b) -> b -> [a] -> b
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const foldl = (f, a, xs) => xs.reduce(f, a);
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// foldr (a -> b -> b) -> b -> [a] -> b
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const foldr = (f, a, xs) => xs.reduceRight(flip(f), a);
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// map :: (a -> b) -> [a] -> [b]
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const map = (f, xs) => xs.map(f);
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// min :: Ord a => a -> a -> a
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const min = (a, b) => b < a ? b : a;
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// zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
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const zipWith = (f, xs, ys) =>
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Array.from({
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length: min(xs.length, ys.length)
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}, (_, i) => f(xs[i], ys[i]));
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// TEST -------------------------------------------------------------------
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return foldr((x, a) => {
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const strIndent = a.indent;
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return {
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rows: strIndent + map(n => (' ' + n)
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.slice(-4), x)
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.join('') + '\n' + a.rows,
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indent: strIndent + ' '
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};
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}, {
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rows: '',
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indent: ''
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}, pascal(7))
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.rows;
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})();
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