const NOTE_NAMES = ['C', 'C#', 'D', 'D#', 'E', 'F', 'F#', 'G', 'G#', 'A', 'A#', 'B'];
const A4_FREQUENCY = 440;
const A4_MIDI_NOTE = 69;

export function noteToFrequency(noteName: string): number | null {
  const match = noteName.match(/^([A-G]#?)(-?\d+)$/i);
  if (!match) return null;

  const [, note, octave] = match;
  const noteIndex = NOTE_NAMES.indexOf(note.toUpperCase());
  if (noteIndex === -1) return null;

  const octaveNum = parseInt(octave, 10);
  const midiNote = (octaveNum + 1) * 12 + noteIndex;
  const semitonesDiff = midiNote - A4_MIDI_NOTE;

  return A4_FREQUENCY * Math.pow(2, semitonesDiff / 12);
}

export function parseFrequencyInput(input: string): number | null {
  const trimmed = input.trim();

  const asNumber = parseFloat(trimmed);
  if (!isNaN(asNumber) && asNumber > 0 && asNumber < 20000) {
    return asNumber;
  }

  return noteToFrequency(trimmed);
}

export function formatFrequency(frequency: number): string {
  return frequency.toFixed(2);
}