25 lines
667 B
JavaScript
25 lines
667 B
JavaScript
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const R = require('ramda')
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// Reference: https://en.wikipedia.org/wiki/Luhn_algorithm
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module.exports = val => {
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val = val.replace(/[-\s]/g, '')
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return val.match(/^[0-9-\s]+$/) && luhnCheck(val)
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}
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const luhnCheck =
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R.compose(
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R.equals(0)
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, R.modulo(R.__, 10)
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, R.sum
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, R.map(n => n > 9 ? n - 9 : n) // Subtract 9 from those digits greater than 9
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, R.addIndex(R.map)((n, i) => i % 2 === 0 ? n : n * 2) // Double the value of every second digit
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, R.map(ch => Number(ch))
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, R.reverse)
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/*
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Luhn check in haskell:
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luhn = (0 ==) . (`mod` 10) . sum . map (uncurry (+) . (`divMod` 10)) .
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zipWith (*) (cycle [1,2]) . map digitToInt . reverse
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*/
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