<?php

namespace ChargeForms\Includes;

if ( ! defined( 'ABSPATH' ) ) { exit; }

/** Small, deterministic arithmetic evaluator for Formula fields. Never uses eval(). */
class Formula_Calculator {
	public static function evaluate( $expression, array $values = [] ) {
		$expression = trim( (string) $expression );
		if ( '' === $expression || strlen( $expression ) > 500 ) { return null; }
		$expression = preg_replace_callback( '/\{([a-z][a-z0-9_-]{0,63})\}/i', static function ( $m ) use ( $values ) {
			$value = $values[ $m[1] ] ?? 0;
			return is_numeric( $value ) ? (string) $value : '0';
		}, $expression );
		if ( preg_match( '/[^0-9+\-*\/().\s]/', $expression ) ) { return null; }
		$tokens = preg_split( '/(\d+(?:\.\d+)?|[+\-*\/()])/', $expression, -1, PREG_SPLIT_DELIM_CAPTURE | PREG_SPLIT_NO_EMPTY );
		$output = []; $ops = []; $precedence = [ '+' => 1, '-' => 1, '*' => 2, '/' => 2 ];
		foreach ( $tokens as $token ) {
			$token = trim( $token ); if ( '' === $token ) { continue; }
			if ( is_numeric( $token ) ) { $output[] = (float) $token; continue; }
			if ( '(' === $token ) { $ops[] = $token; continue; }
			if ( ')' === $token ) { while ( $ops && '(' !== end( $ops ) ) { $output[] = array_pop( $ops ); } if ( ! $ops ) { return null; } array_pop( $ops ); continue; }
			if ( ! isset( $precedence[ $token ] ) ) { return null; }
			while ( $ops && '(' !== end( $ops ) && $precedence[ end( $ops ) ] >= $precedence[ $token ] ) { $output[] = array_pop( $ops ); }
			$ops[] = $token;
		}
		while ( $ops ) { if ( '(' === end( $ops ) ) { return null; } $output[] = array_pop( $ops ); }
		$stack = [];
		foreach ( $output as $token ) {
			if ( is_numeric( $token ) ) { $stack[] = (float) $token; continue; }
			if ( count( $stack ) < 2 ) { return null; }
			$b = array_pop( $stack ); $a = array_pop( $stack );
			if ( '/' === $token && 0.0 === $b ) { return null; }
			$stack[] = '+' === $token ? $a + $b : ( '-' === $token ? $a - $b : ( '*' === $token ? $a * $b : $a / $b ) );
		}
		return 1 === count( $stack ) ? round( (float) $stack[0], 2 ) : null;
	}
}