/**
 * "Nice numbers" axis tick generator (Heckbert 1990) — produces clean,
 * human-friendly tick values (0 / 50 / 100 / 150 rather than 0 / 37 / 74 /
 * 111, an artifact of slicing frac*max into equal fractions) for any linear
 * axis, from just the data's max value.
 */
function niceNumber(range, round) {
  const exponent = Math.floor(Math.log10(range))
  const fraction = range / 10 ** exponent
  let niceFraction
  if (round) {
    if (fraction < 1.5) niceFraction = 1
    else if (fraction < 3) niceFraction = 2
    else if (fraction < 7) niceFraction = 5
    else niceFraction = 10
  } else {
    if (fraction <= 1) niceFraction = 1
    else if (fraction <= 2) niceFraction = 2
    else if (fraction <= 5) niceFraction = 5
    else niceFraction = 10
  }
  return niceFraction * 10 ** exponent
}

/**
 * @param {number} maxValue - the largest value the axis must cover
 * @param {number} tickCount - approximate desired number of ticks
 * @returns {{ ticks: number[], niceMax: number }}
 */
export function niceTicks(maxValue, tickCount = 5) {
  if (!(maxValue > 0)) return { ticks: [0], niceMax: 1 }
  const niceRange = niceNumber(maxValue, false)
  const niceStep = niceNumber(niceRange / (tickCount - 1), true)
  const niceMax = Math.ceil(maxValue / niceStep) * niceStep
  const ticks = []
  for (let v = 0; v <= niceMax + niceStep / 2; v += niceStep) {
    ticks.push(Math.round(v * 1e6) / 1e6)
  }
  return { ticks, niceMax }
}