<?php
/**
 * Transparent forecasting math.
 *
 * @package ProfitEngine
 */

declare( strict_types=1 );

namespace ProfitEngine\Forecasting;

defined( 'ABSPATH' ) || exit;

/**
 * Pure, WordPress-free forecasting formulas.
 *
 * Every recommendation the plugin makes is derived from these functions, and the
 * inputs are surfaced to the merchant so the suggestion is never a black box.
 *
 *   avg_daily_sales = units_sold / window_days
 *   days_remaining  = current_stock / avg_daily_sales
 *   safety_stock    = configured, or avg_daily_sales * safety_days (auto)
 *   reorder_point   = manual, or (avg_daily_sales * lead_time) + safety_stock
 *   suggested_qty   = max(0, avg_daily_sales * (lead_time + coverage_days)
 *                       + safety_stock − current_stock − qty_on_open_pos)
 *                     rounded up to the supplier MOQ
 */
class VelocityCalculator {

	/**
	 * Average daily sales over a window.
	 *
	 * @param float $units_sold  Units sold in the window (net of refunds).
	 * @param int   $window_days Window length in days.
	 */
	public function avg_daily( float $units_sold, int $window_days ): float {
		if ( $window_days <= 0 ) {
			return 0.0;
		}
		return $units_sold / $window_days;
	}

	/**
	 * Days of stock remaining at the current sales rate.
	 *
	 * @param float $current_stock Current stock.
	 * @param float $avg_daily     Average daily sales.
	 * @return float Infinite (PHP_FLOAT_MAX) when there is no demand.
	 */
	public function days_remaining( float $current_stock, float $avg_daily ): float {
		if ( $avg_daily <= 0.0 ) {
			return PHP_FLOAT_MAX;
		}
		return $current_stock / $avg_daily;
	}

	/**
	 * Resolve the safety stock (configured value wins; else auto from velocity).
	 *
	 * @param int   $configured  Configured safety stock (0 = auto).
	 * @param float $avg_daily   Average daily sales.
	 * @param int   $safety_days Days of demand to hold as a buffer.
	 */
	public function safety_stock( int $configured, float $avg_daily, int $safety_days ): float {
		if ( $configured > 0 ) {
			return (float) $configured;
		}
		return $avg_daily * $safety_days;
	}

	/**
	 * Reorder point (manual override wins).
	 *
	 * @param int|null $manual       Manual reorder point, or null for auto.
	 * @param float    $avg_daily    Average daily sales.
	 * @param int      $lead_time    Supplier lead time in days.
	 * @param float    $safety_stock Safety stock.
	 */
	public function reorder_point( ?int $manual, float $avg_daily, int $lead_time, float $safety_stock ): float {
		if ( null !== $manual ) {
			return (float) $manual;
		}
		return ( $avg_daily * $lead_time ) + $safety_stock;
	}

	/**
	 * Suggested reorder quantity, net of stock already on hand and on order,
	 * rounded up to the supplier's minimum order quantity.
	 *
	 * @param float $avg_daily      Average daily sales.
	 * @param int   $lead_time      Lead time in days.
	 * @param int   $coverage_days  Days of forward cover to buy.
	 * @param float $safety_stock   Safety stock.
	 * @param float $current_stock  Current stock.
	 * @param float $on_open_pos    Quantity already on open purchase orders.
	 * @param int   $moq            Supplier minimum order quantity.
	 */
	public function suggested_qty(
		float $avg_daily,
		int $lead_time,
		int $coverage_days,
		float $safety_stock,
		float $current_stock,
		float $on_open_pos,
		int $moq
	): int {
		$target = ( $avg_daily * ( $lead_time + $coverage_days ) ) + $safety_stock;
		$need   = $target - $current_stock - $on_open_pos;

		if ( $need <= 0 ) {
			return 0;
		}

		$qty = (int) ceil( $need );

		if ( $moq > 0 && $qty < $moq ) {
			$qty = $moq;
		} elseif ( $moq > 0 ) {
			// Round up to a whole multiple of the MOQ.
			$qty = (int) ( ceil( $qty / $moq ) * $moq );
		}

		return $qty;
	}

	/**
	 * Classify urgency.
	 *
	 * @param float $avg_daily      Average daily sales.
	 * @param float $current_stock  Current stock.
	 * @param float $days_remaining Days remaining.
	 * @param int   $lead_time      Lead time in days.
	 * @param int   $coverage_days  Coverage window in days.
	 * @param float $reorder_point  Reorder point.
	 * @return string 'critical' | 'warning' | 'recommended' | 'ok'
	 */
	public function severity(
		float $avg_daily,
		float $current_stock,
		float $days_remaining,
		int $lead_time,
		int $coverage_days,
		float $reorder_point
	): string {
		// No demand: nothing to reorder, regardless of stock level.
		if ( $avg_daily <= 0.0 ) {
			return 'ok';
		}

		if ( $days_remaining < $lead_time ) {
			return 'critical';
		}
		if ( $current_stock <= $reorder_point ) {
			return 'warning';
		}
		if ( $days_remaining < ( $lead_time + $coverage_days ) ) {
			return 'recommended';
		}
		return 'ok';
	}
}