EPI 5.8 – Spreadsheet Column Encoding

Problem:

Write a function that converts excel column IDs to corresponding integers with “A” corresponding to 1 and so on.

Solution:

We will modify the solution in EPI 5.6 slightly so that we have:

indexOf :: Eq a => a -> [a] -> Int
indexOf item xs
    | length firstMatched == 0 = -1
    | otherwise = fst (head firstMatched)
    where
        matchesItem x = snd x /= item
        firstMatched = dropWhile matchesItem 
                        (zip [0 .. (length xs) - 1] xs)
digitIndices :: [Char] -> [Int]
digitIndices digits = [ indexOf (chr i) digits | i <- [0 .. 255]]
spreadsheet_column_encoding :: [Char] -> [Int]
spreadsheet_column_encoding xs = 
        sum [ digit_to_value digit pos | (pos, digit) <- dig_positions xs]
    where
        reverse_indexes xs = [(length xs) - 1, (length xs) - 2 .. 0]
        dig_positions xs = zip  (reverse_indexes xs) xs
        symbols = ['A' .. 'Z']
        ourIndexes = digitIndices symbols
        digit_to_value digit pos = (1 + (ourIndexes !! (ord digit))) * 
                                    floor (26 ** fromIntegral pos)