@heinlein-video/rrule/src/iterinfo/easter.ts

rrule fork. Includes the src/ folder for typescript sourceMaps

export function easter(y: number, offset = 0) {
  const a = y % 19
  const b = Math.floor(y / 100)
  const c = y % 100
  const d = Math.floor(b / 4)
  const e = b % 4
  const f = Math.floor((b + 8) / 25)
  const g = Math.floor((b - f + 1) / 3)
  const h = Math.floor(19 * a + b - d - g + 15) % 30
  const i = Math.floor(c / 4)
  const k = c % 4
  const l = Math.floor(32 + 2 * e + 2 * i - h - k) % 7
  const m = Math.floor((a + 11 * h + 22 * l) / 451)
  const month = Math.floor((h + l - 7 * m + 114) / 31)
  const day = ((h + l - 7 * m + 114) % 31) + 1
  const date = Date.UTC(y, month - 1, day + offset)
  const yearStart = Date.UTC(y, 0, 1)

  return [Math.ceil((date - yearStart) / (1000 * 60 * 60 * 24))]
}