fsharp-learning/exercism/fsharp/nucleotide-count/NucleotideCount.fs

module NucleotideCount

let isValidNucleotide nucleotide =
    nucleotide = 'A'
    || nucleotide = 'C'
    || nucleotide = 'G'
    || nucleotide = 'T'

let validInput (strand: string) =
    String.collect (fun c -> if isValidNucleotide c then string (c) else "") strand = strand

let nucleotideCounts (strand: string): Option<Map<char, int>> =
    if validInput strand then
        let mutable counts =
            Map.empty.Add('A', 0).Add('C', 0).Add('G', 0).Add('T', 0)

        Seq.iter (fun c ->
            if counts.ContainsKey c then
                let current = counts.Item c
                counts <- counts.Remove(c)
                counts <- counts.Add(c, current + 1)
            else
                counts <- counts.Add(c, 1)) strand
        Some(counts)
    else
        None