exercism/powershell/nucleotide-count/NucleotideCount.ps1

Function Get-NucleotideCount() {
    <#
    .SYNOPSIS
    Given a single stranded DNA string, compute how many times each nucleotide occurs in the string.
    
    .DESCRIPTION
    The genetic language of every living thing on the planet is DNA.
    DNA is a large molecule that is built from an extremely long sequence of individual elements called nucleotides.
    4 types exist in DNA and these differ only slightly and can be represented as the following symbols: 'A' for adenine, 'C' for cytosine, 'G' for guanine, and 'T' thymine.

    The function counts the occurances of A, C, G and T in the supplied strand. It then outputs in the format:

    A:0, C:0, G:0, T:0
    
    .PARAMETER Strand
    The DNA strand to count
    
    .EXAMPLE
    Get-NucleotideCount -Strand "ACGTAGCTT"
    
    Retuns: A:2 C:2 G:2 T:3
    #>
    [CmdletBinding()]
    Param(
        [string]$Strand
    )

    if ($Strand -notmatch "^[ACGT]*$") {
        Throw "Invalid DNA Strand"
    }

    $counts = @{"A" = 0; "C" = 0; "G" = 0; "T" = 0}
    foreach ($char in $Strand.ToCharArray()) { $counts[$char.ToString()] += 1 }

    return "A:$($counts['A']) C:$($counts['C']) G:$($counts['G']) T:$($counts['T'])"
}