const nucleotides = ['A', 'C', 'G', 'T'] as const
type Nucleotide = typeof nucleotides[number]
type NucleotideCounts = Record<Nucleotide, number>

function isNucleotide(char: string): char is Nucleotide {
  return (nucleotides as readonly string[]).includes(char)
}

export const nucleotideCounts = (nucleotide: string): NucleotideCounts =>
  nucleotide.split("").map(char => 
     (!isNucleotide(char))
      ? (() => { throw new Error('Invalid nucleotide in strand')})()
      : char
  ).reduce((result, char) => ({
    ...result, [char]: result[char] + 1
  }), {
      A: 0,
      C: 0,
      G: 0,
      T: 0,
    })