// Round a modelled estimate to a rough magnitude for display.
//
// A model output should never imply single-account precision: "~942.001" reads as a census
// when it is a language-weighted guess. But whole millions ("~3M") throw away a digit the
// model does resolve, and are inconsistent with the "~940K" national figures. So estimates
// are shown to a consistent TWO significant figures with an M/K suffix: a single-digit
// magnitude keeps one decimal ("2.8M", "6.1K", "3.0M"), ten or more shows none ("12M",
// "62K", "940K"). Two is the honest ceiling for a number we label "least certain"; three
// ("2.98M") would overclaim precision we have not measured. A period decimal (not a comma)
// because the abbreviated M/K form reads internationally, where "2,8M" invites a "28M" misread.
//
// Small communities (< 1000) keep their real number — at that scale the digits are not false
// precision and the count deserves to be seen.
//
// Returns the bare figure without a leading "~"; callers add the tilde where they want it,
// so a styled tilde element or an "around"/"roughly" lead-in stays in the caller's hands.
export function roughEstimate(n: number, locale = 'nl-NL'): string {
  if (!Number.isFinite(n) || n <= 0) return '';
  if (n < 1000) return n.toLocaleString(locale);
  const digits = Math.floor(Math.log10(n));
  const factor = Math.pow(10, Math.max(0, digits - 1)); // round to two significant figures
  const r = Math.round(n / factor) * factor;
  const unit = r >= 1_000_000 ? 1_000_000 : 1000;
  const suffix = r >= 1_000_000 ? 'M' : 'K';
  const v = r / unit;
  const s = v < 10 ? v.toFixed(1) : String(Math.round(v));
  return `${s}${suffix}`;
}