<?php
namespace DigiDiscount;

defined( 'ABSPATH' ) || exit;

final class Engine {
    public static function allocate( int $amount, array $weights ): array {
        $weights = array_map( static fn( $v ) => max( 0, (int) $v ), $weights );
        $total = array_sum( $weights );
        $amount = max( 0, min( $amount, $total ) );
        $out = array_fill_keys( array_keys( $weights ), 0 );
        if ( ! $total || ! $amount ) {
            return $out;
        }
        $fractions = array();
        foreach ( $weights as $key => $weight ) {
            $exact = $amount * ( $weight / $total );
            $out[ $key ] = (int) floor( $exact );
            $fractions[ $key ] = $exact - $out[ $key ];
        }
        arsort( $fractions, SORT_NUMERIC );
        $left = $amount - array_sum( $out );
        foreach ( $fractions as $key => $fraction ) {
            if ( $left <= 0 ) {
                break;
            }
            if ( $out[ $key ] < $weights[ $key ] ) {
                ++$out[ $key ];
                --$left;
            }
        }
        return $out;
    }

    public static function eligible( array $line, array $rule ): bool {
        $ids = array( (int) $line['product_id'], (int) ( $line['variation_id'] ?? 0 ) );
        $cats = $line['categories'] ?? array();
        if ( ! empty( $line['gift'] ) || ( ! empty( $rule['exclude_sale'] ) && ! empty( $line['sale'] ) ) ) {
            return false;
        }
        foreach ( array( 'products' => $ids, 'categories' => $cats ) as $field => $values ) {
            if ( ! empty( $rule[ 'exclude_' . $field ] ) && array_intersect( $values, $rule[ 'exclude_' . $field ] ) ) {
                return false;
            }
            if ( ! empty( $rule[ $field ] ) && ! array_intersect( $values, $rule[ $field ] ) ) {
                return false;
            }
        }
        return (bool) apply_filters( 'digidiscount_line_eligible', true, $line, $rule );
    }

    public static function candidate( array $lines, array $rule, array $context ): array {
        $empty = array( 'discounts' => array(), 'shipping' => false, 'gifts' => array() );
        if ( empty( $rule['enabled'] ) || ( ! empty( $rule['currency'] ) && $rule['currency'] !== $context['currency'] ) ) {
            return $empty;
        }
        $now = $context['now'] ?? time();
        if ( ( ! empty( $rule['start'] ) && $now < $rule['start'] ) || ( ! empty( $rule['end'] ) && $now >= $rule['end'] ) ) {
            return $empty;
        }
        if ( empty( $rule['allow_coupons'] ) && ! empty( $context['coupons'] ) ) {
            return $empty;
        }
        $eligible = array_filter( $lines, static fn( $line ) => self::eligible( $line, $rule ) );
        $scope = 'cart' === ( $rule['threshold_scope'] ?? 'eligible' ) ? array_filter( $lines, static fn( $line ) => empty( $line['gift'] ) ) : $eligible;
        $total = array_sum( array_column( $scope, 'total' ) );
        $quantity = array_sum( array_column( $scope, 'qty' ) );
        if ( ! $eligible || $total < (int) ( $rule['min_total'] ?? 0 ) || $quantity < ( $rule['min_qty'] ?? 0 ) || ( ! empty( $rule['max_qty'] ) && $quantity > $rule['max_qty'] ) ) {
            return $empty;
        }
        if ( ! apply_filters( 'digidiscount_rule_matches', true, $rule, $lines, $context ) ) {
            return $empty;
        }
        $weights = array_column( $eligible, 'total', 'key' );
        if ( in_array( $rule['type'], array( 'product', 'category', 'cart', 'threshold' ), true ) ) {
            $discounts = self::basic( $eligible, $rule );
            $result = array( 'discounts' => $discounts, 'shipping' => false, 'gifts' => array() );
        } else {
            $result = apply_filters( 'digidiscount_rule_result', $empty, $rule, $eligible, $lines, $context );
        }
        foreach ( $result['discounts'] as $key => $discount ) {
            $result['discounts'][ $key ] = min( max( 0, (int) $discount ), $weights[ $key ] ?? 0 );
        }
        if ( ! empty( $rule['max_discount'] ) && array_sum( $result['discounts'] ) > $rule['max_discount'] ) {
            $result['discounts'] = self::allocate( (int) $rule['max_discount'], $result['discounts'] );
        }
        return $result;
    }

    public static function basic( array $lines, array $rule ): array {
        $weights = array_column( $lines, 'total', 'key' );
        $amount = max( 0, (float) $rule['amount'] );
        $discounts = array();
        if ( 'fixed' === $rule['kind'] ) {
            return self::allocate( (int) round( $amount ), $weights );
        }
        foreach ( $lines as $line ) {
            $discounts[ $line['key'] ] = 'percent' === $rule['kind']
                ? (int) round( $line['total'] * min( 100, $amount ) / 100 )
                : (int) min( $line['total'], round( $amount * $line['qty'] ) );
        }
        return $discounts;
    }

    public static function calculate( array $lines, array $rules, array $context, string $mode = 'best' ): array {
        $result = array( 'discounts' => array(), 'rules' => array(), 'shipping' => false, 'gifts' => array() );
        usort( $rules, static fn( $a, $b ) => ( (int) $a['priority'] <=> (int) $b['priority'] ) ?: strcmp( $a['id'], $b['id'] ) );
        $mode = apply_filters( 'digidiscount_processing_mode', in_array( $mode, array( 'priority', 'best', 'stack' ), true ) ? $mode : 'best', $mode );
        $working = $lines;
        $groups = array();
        foreach ( $rules as $rule ) {
            if ( 'stack' === $mode && ! empty( $rule['group'] ) && isset( $groups[ $rule['group'] ] ) ) {
                continue;
            }
            $candidate = self::candidate( $working, $rule, $context );
            $sum = array_sum( $candidate['discounts'] );
            if ( ! $sum && ! $candidate['shipping'] && ! $candidate['gifts'] ) {
                continue;
            }
            $entry = array( 'id' => $rule['id'], 'name' => $rule['name'], 'amount' => $sum );
            if ( 'stack' === $mode ) {
                if ( ! empty( $result['rules'] ) && empty( $rule['stackable'] ) ) {
                    continue;
                }
                foreach ( $candidate['discounts'] as $key => $discount ) {
                    $result['discounts'][ $key ] = ( $result['discounts'][ $key ] ?? 0 ) + $discount;
                    $working[ $key ]['total'] -= $discount;
                }
                $result['rules'][] = $entry;
                $result['shipping'] = $result['shipping'] || $candidate['shipping'];
                $result['gifts'] = array_merge( $result['gifts'], $candidate['gifts'] );
                $groups[ $rule['group'] ?? '' ] = true;
                if ( empty( $rule['stackable'] ) ) {
                    break;
                }
            } elseif ( empty( $result['rules'] ) || $sum > array_sum( $result['discounts'] ) ) {
                $result = array_merge( $candidate, array( 'rules' => array( $entry ) ) );
                if ( 'priority' === $mode ) {
                    break;
                }
            }
        }
        return $result;
    }
}