perfect-postcode/frontend/src/lib/format.ts

interface ValueFormat {
  prefix?: string;
  suffix?: string;
  /** Show full integer (no k/M abbreviation) */
  raw?: boolean;
}

export function formatValue(value: number, fmt?: ValueFormat): string {
  const p = fmt?.prefix ?? '';
  const s = fmt?.suffix ?? '';
  if (fmt?.raw) return `${p}${Math.round(value)}${s}`;
  if (Math.abs(value) >= 1_000_000) return `${p}${(value / 1_000_000).toFixed(1)}M${s}`;
  if (Math.abs(value) >= 1_000) return `${p}${(value / 1_000).toFixed(1)}k${s}`;
  if (Number.isInteger(value)) return `${p}${value.toLocaleString()}${s}`;
  return `${p}${value.toFixed(1)}${s}`;
}

export function formatFilterValue(value: number, raw?: boolean): string {
  if (raw) return Math.round(value).toString();
  if (Math.abs(value) >= 1_000_000) return `${(value / 1_000_000).toFixed(1)}M`;
  if (Math.abs(value) >= 1_000) return `${(value / 1_000).toFixed(1)}k`;
  if (Number.isInteger(value)) return value.toString();
  return value.toFixed(2);
}

export function formatDuration(d: string): string {
  if (d === 'F') return 'Freehold';
  if (d === 'L') return 'Leasehold';
  return d;
}

const MONTH_NAMES = [
  'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
  'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec',
];

export function formatTransactionDate(fractionalYear: number): string {
  const year = Math.floor(fractionalYear);
  const monthIndex = Math.min(Math.round((fractionalYear - year) * 12), 11);
  return `${MONTH_NAMES[monthIndex]} ${year}`;
}

export function formatAge(value: number, approximate = true): string {
  if (value >= 1000) return approximate ? `~${Math.round(value)}` : `${Math.round(value)}`;
  return Math.round(value).toString();
}

// Format number with optional decimals, used in PropertyCard
export function formatNumber(value: number | undefined, decimals = 0): string {
  if (value === undefined) return '';
  return decimals > 0 ? value.toFixed(decimals) : Math.round(value).toLocaleString();
}

export function formatRelativeTime(isoDate: string): string {
  const now = Date.now();
  const then = new Date(isoDate).getTime();
  const diffMs = now - then;
  const diffSec = Math.floor(diffMs / 1000);
  if (diffSec < 60) return 'just now';
  const diffMin = Math.floor(diffSec / 60);
  if (diffMin < 60) return `${diffMin}m ago`;
  const diffHr = Math.floor(diffMin / 60);
  if (diffHr < 24) return `${diffHr}h ago`;
  const diffDay = Math.floor(diffHr / 24);
  if (diffDay < 30) return `${diffDay}d ago`;
  return new Date(isoDate).toLocaleDateString();
}

// Percentile-based scale: maps between percentile space (0–100) and absolute values
// using the histogram's CDF. Each percentile step = 1% of data.
export interface PercentileScale {
  toValue: (percentile: number) => number;
  toPercentile: (value: number) => number;
}

export function buildPercentileScale(hist: {
  min: number;
  max: number;
  p1: number;
  p99: number;
  counts: number[];
}): PercentileScale {
  const n = hist.counts.length;
  const total = hist.counts.reduce((a, b) => a + b, 0);

  if (n === 0 || total === 0) {
    const range = hist.max - hist.min || 1;
    return {
      toValue: (p) => hist.min + (p / 100) * range,
      toPercentile: (v) => ((v - hist.min) / range) * 100,
    };
  }

  // Bin boundaries: [min, p1, ..middle edges.., p99, max]
  const boundaries: number[] = [];
  if (n === 1) {
    boundaries.push(hist.min, hist.max);
  } else {
    boundaries.push(hist.min, hist.p1);
    if (n > 2) {
      const middleWidth = (hist.p99 - hist.p1) / (n - 2);
      for (let i = 1; i < n - 1; i++) {
        boundaries.push(hist.p1 + i * middleWidth);
      }
    }
    boundaries.push(hist.max);
  }

  // Cumulative fraction: cumFrac[0]=0, cumFrac[n]=1
  const cumFrac: number[] = [0];
  for (let i = 0; i < n; i++) {
    cumFrac.push(cumFrac[i] + hist.counts[i] / total);
  }
  cumFrac[n] = 1; // ensure exact 1.0

  return {
    toValue(percentile: number): number {
      const target = Math.max(0, Math.min(1, percentile / 100));
      if (target <= 0) return boundaries[0];
      if (target >= 1) return boundaries[n];
      let i = 0;
      for (; i < n - 1; i++) {
        if (cumFrac[i + 1] > target) break;
      }
      const binFrac = cumFrac[i + 1] - cumFrac[i];
      const t = binFrac > 0 ? (target - cumFrac[i]) / binFrac : 0;
      return boundaries[i] + t * (boundaries[i + 1] - boundaries[i]);
    },

    toPercentile(value: number): number {
      if (value <= boundaries[0]) return 0;
      if (value >= boundaries[n]) return 100;
      let i = 0;
      for (; i < n - 1; i++) {
        if (boundaries[i + 1] > value) break;
      }
      const binWidth = boundaries[i + 1] - boundaries[i];
      const t = binWidth > 0 ? (value - boundaries[i]) / binWidth : 0;
      return (cumFrac[i] + t * (cumFrac[i + 1] - cumFrac[i])) * 100;
    },
  };
}

// Calculate weighted mean from histogram with outlier bins.
// Bin 0 = [min, p1), bins 1..n-2 = [p1, p99) evenly, bin n-1 = [p99, max].
export function calculateHistogramMean(histogram: {
  min: number;
  max: number;
  p1: number;
  p99: number;
  counts: number[];
}): number | undefined {
  const n = histogram.counts.length;
  if (n === 0) return undefined;
  const totalCount = histogram.counts.reduce((a, b) => a + b, 0);
  if (totalCount === 0) return undefined;

  const { min, max, p1, p99 } = histogram;
  const middleBins = Math.max(n - 2, 0);
  const middleWidth = middleBins > 0 && p99 > p1 ? (p99 - p1) / middleBins : 0;

  let weightedSum = 0;
  for (let i = 0; i < n; i++) {
    let center: number;
    if (i === 0) center = (min + p1) / 2;
    else if (i === n - 1) center = (p99 + max) / 2;
    else center = p1 + (i - 0.5) * middleWidth;
    weightedSum += center * histogram.counts[i];
  }
  return weightedSum / totalCount;
}