import type { MapScaleUnits } from './units';

/** Round numbers a reader can actually divide by. */
const NICE_STEPS = [1, 2, 5];

/**
 * Picks the largest "nice" distance that still fits inside `maxWidthPx`.
 *
 * A bar labelled 1.37 km is useless for estimating anything; the whole point is
 * that the reader can halve it or triple it by eye.
 */
export function niceScaleDistance(
  metersPerPixel: number,
  maxWidthPx: number,
  units: MapScaleUnits
): { distance: number; widthPx: number; label: string } | null {
  if (!Number.isFinite(metersPerPixel) || metersPerPixel <= 0) return null;

  const unitMeters = units === 'imperial' ? 0.3048 : 1;
  const maxUnits = (metersPerPixel * maxWidthPx) / unitMeters;
  if (maxUnits <= 0) return null;

  const magnitude = Math.floor(Math.log10(maxUnits));

  let best = NICE_STEPS[0] * 10 ** magnitude;

  for (let exponent = magnitude; exponent <= magnitude + 1; exponent += 1) {
    NICE_STEPS.forEach(step => {
      const candidate = step * 10 ** exponent;
      if (candidate <= maxUnits && candidate > best) best = candidate;
    });
  }

  const widthPx = (best * unitMeters) / metersPerPixel;

  const label =
    units === 'imperial'
      ? best >= 5280
        ? `${(best / 5280).toFixed(best / 5280 < 10 ? 1 : 0)} mi`
        : `${Math.round(best)} ft`
      : best >= 1000
        ? `${(best / 1000).toFixed(best / 1000 < 10 ? 1 : 0)} km`
        : `${Math.round(best)} m`;

  return { distance: best, widthPx, label };
}