import type { TrendDirection } from "../models/types.js";

/**
 * Calculate arithmetic mean of values.
 * Returns 0 for empty array.
 */
export function mean(values: number[]): number {
  if (values.length === 0) return 0;
  const sum = values.reduce((acc, val) => acc + val, 0);
  return sum / values.length;
}

/**
 * Calculate population standard deviation.
 * Returns 0 for empty or single-element array.
 */
export function standardDeviation(values: number[]): number {
  if (values.length <= 1) return 0;

  const avg = mean(values);
  const squaredDiffs = values.map((val) => Math.pow(val - avg, 2));
  const variance = mean(squaredDiffs);

  return Math.sqrt(variance);
}

/**
 * Determine trend direction based on percentage change.
 * Returns "up" if change > threshold, "down" if change < -threshold, "stable" otherwise.
 * Default threshold is 5%.
 */
export function trendDirection(
  current: number,
  previous: number,
  threshold: number = 5
): TrendDirection {
  const change = percentChange(current, previous);

  if (change > threshold) return "up";
  if (change < -threshold) return "down";
  return "stable";
}

/**
 * Calculate percentage change between current and previous values.
 * Returns 0 if previous is 0.
 */
export function percentChange(current: number, previous: number): number {
  if (previous === 0) return 0;
  return ((current - previous) / previous) * 100;
}

/**
 * Calculate how many standard deviations a value is from the mean.
 * Returns 0 if standard deviation is 0.
 */
export function deviationsFromMean(value: number, values: number[]): number {
  const avg = mean(values);
  const stdDev = standardDeviation(values);

  if (stdDev === 0) return 0;
  return (value - avg) / stdDev;
}