Project Euler – Problem 89 Solution


The rules for writing Roman numerals allow for many ways of writing each number (see FAQ:Roman Numerals). However, there is always a “best” way of writing a particular number.

For example, the following represent all of the legitimate ways of writing the number sixteen:







The last example being considered the most efficient, as it uses the least number of numerals.

The 11K text file, roman.txt (right click and ‘Save Link/Target As…’), contains one thousand numbers written in valid, but not necessarily minimal, Roman numerals; that is, they are arranged in descending units and obey the subtractive pair rule (see FAQ for the definitive rules for this problem).

Find the number of characters saved by writing each of these in their minimal form.

Note: You can assume that all the Roman numerals in the file contain no more than four consecutive identical units.


open System.IO

// define the set of available roman numerals
let romanNumerals = [(['I'],1I);(['I';'V'],4I);(['V'],5I);(['I';'X'],9I);

// define function to get the numeric value of a roman numeral
let getNumericValue (numeral:char) =
    snd (romanNumerals |> Seq.filter (fun (l, v) -> l.Length = 1 && l.[0] = numeral) |> Seq.head)

// define function to convert a roman numerals to its corresponding numeric value
let toNumeric (numerals:string) =
    let rec toNumericRec ((head::tail):char list) (num:bigint) =
        match head, tail with
        | _, [] -> num + getNumericValue head
        | _, hd::tl when getNumericValue head >= getNumericValue hd ->
            toNumericRec tail (num + getNumericValue head)
        | _, hd::[] -> // subtractive pair as last number i.e. XIX - 19
            num + getNumericValue hd - getNumericValue head
        | _, hd::tl -> // subtractive pair not as last number XCV - 95
            toNumericRec tl (num + getNumericValue hd - getNumericValue head)

    toNumericRec (numerals.ToCharArray() |> Array.toList) 0I

// converts an integer value to corresponding minimal roman numerals form
let toMinimalRomanNumerals (value:bigint) =
    let rec toMinimalRomanNumeralsRec (value:bigint) (list:char list) =
        if value = 0I then list
            let (list', value') =
                |> List.sortBy (fun (l, v) -> -v)
                |> Seq.filter (fun (l, v) -> v <= value)
                |> Seq.head
            toMinimalRomanNumeralsRec (value-value') (list@list')

    toMinimalRomanNumeralsRec value []

let original = File.ReadAllLines(@"C:\TEMP\roman.txt")
let minimal = original |> toNumeric |> toMinimalRomanNumerals

let answer =
    let originalLen = original |> Array.sumBy (fun str -> str.Length)
    let minimalLen = minimal |> Array.sumBy (fun str -> str.Length)
    originalLen - minimalLen

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