A function to retrieve the most frequent word pair in a string

word_pair_counts.coffee

#
# Write a function `wordPair` that for a
# given a string, returns the most frequent word pair (word, space, word)
# E.g., 'I want to be a part of it New York, New York' -> 'New York'
#

wordPair = (str) ->
    getWordPairCounts = (str) ->
        words = str.replace(/[^a-zA-Z\s]/g, '').split ' '
        pairCounts = {}
        words.forEach (word, index) ->
            return if index is words.length - 1
            pair = "#{word} #{words[index+1]}"
            pairCounts[pair] = (pairCounts[pair] or 0) + 1
        pairCounts
    maxPair = (pairCounts) ->    
        frequentPair = null
        pairFrequency = 0
        for pair, frequency of pairCounts
            if frequency > pairFrequency
                frequentPair = pair
                pairFrequency = frequency
        frequentPair
    pairCounts = getWordPairCounts str
    maxPair pairCounts

#str = 'I want to be a part of it New York, New York'
#expect(maxPair(str)).toEqual 'New York'