<?php declare(strict_types=1);

namespace PhpParser;

use PhpParser\Node\Expr;
use PhpParser\Node\Scalar;

use function array_merge;

/**
 * Evaluates constant expressions.
 *
 * This evaluator is able to evaluate all constant expressions (as defined by PHP), which can be
 * evaluated without further context. If a subexpression is not of this type, a user-provided
 * fallback evaluator is invoked. To support all constant expressions that are also supported by
 * PHP (and not already handled by this class), the fallback evaluator must be able to handle the
 * following node types:
 *
 *  * All Scalar\MagicConst\* nodes.
 *  * Expr\ConstFetch nodes. Only null/false/true are already handled by this class.
 *  * Expr\ClassConstFetch nodes.
 *
 * The fallback evaluator should throw ConstExprEvaluationException for nodes it cannot evaluate.
 *
 * The evaluation is dependent on runtime configuration in two respects: Firstly, floating
 * point to string conversions are affected by the precision ini setting. Secondly, they are also
 * affected by the LC_NUMERIC locale.
 */
class ConstExprEvaluator {
    /** @var callable|null */
    private $fallbackEvaluator;

    /**
     * Create a constant expression evaluator.
     *
     * The provided fallback evaluator is invoked whenever a subexpression cannot be evaluated. See
     * class doc comment for more information.
     *
     * @param callable|null $fallbackEvaluator To call if subexpression cannot be evaluated
     */
    public function __construct(?callable $fallbackEvaluator = null) {
        $this->fallbackEvaluator = $fallbackEvaluator ?? function (Expr $expr) {
            throw new ConstExprEvaluationException(
                "Expression of type {$expr->getType()} cannot be evaluated"
            );
        };
    }

    /**
     * Silently evaluates a constant expression into a PHP value.
     *
     * Thrown Errors, warnings or notices will be converted into a ConstExprEvaluationException.
     * The original source of the exception is available through getPrevious().
     *
     * If some part of the expression cannot be evaluated, the fallback evaluator passed to the
     * constructor will be invoked. By default, if no fallback is provided, an exception of type
     * ConstExprEvaluationException is thrown.
     *
     * See class doc comment for caveats and limitations.
     *
     * @param Expr $expr Constant expression to evaluate
     * @return mixed Result of evaluation
     *
     * @throws ConstExprEvaluationException if the expression cannot be evaluated or an error occurred
     */
    public function evaluateSilently(Expr $expr) {
        set_error_handler(function ($num, $str, $file, $line) {
            throw new \ErrorException($str, 0, $num, $file, $line);
        });

        try {
            return $this->evaluate($expr);
        } catch (\Throwable $e) {
            if (!$e instanceof ConstExprEvaluationException) {
                $e = new ConstExprEvaluationException(
                    "An error occurred during constant expression evaluation", 0, $e);
            }
            throw $e;
        } finally {
            restore_error_handler();
        }
    }

    /**
     * Directly evaluates a constant expression into a PHP value.
     *
     * May generate Error exceptions, warnings or notices. Use evaluateSilently() to convert these
     * into a ConstExprEvaluationException.
     *
     * If some part of the expression cannot be evaluated, the fallback evaluator passed to the
     * constructor will be invoked. By default, if no fallback is provided, an exception of type
     * ConstExprEvaluationException is thrown.
     *
     * See class doc comment for caveats and limitations.
     *
     * @param Expr $expr Constant expression to evaluate
     * @return mixed Result of evaluation
     *
     * @throws ConstExprEvaluationException if the expression cannot be evaluated
     */
    public function evaluateDirectly(Expr $expr) {
        return $this->evaluate($expr);
    }

    /** @return mixed */
    private function evaluate(Expr $expr) {
        if ($expr instanceof Scalar\Int_
            || $expr instanceof Scalar\Float_
            || $expr instanceof Scalar\String_
        ) {
            return $expr->value;
        }

        if ($expr instanceof Expr\Array_) {
            return $this->evaluateArray($expr);
        }

        // Unary operators
        if ($expr instanceof Expr\UnaryPlus) {
            return +$this->evaluate($expr->expr);
        }
        if ($expr instanceof Expr\UnaryMinus) {
            return -$this->evaluate($expr->expr);
        }
        if ($expr instanceof Expr\BooleanNot) {
            return !$this->evaluate($expr->expr);
        }
        if ($expr instanceof Expr\BitwiseNot) {
            return ~$this->evaluate($expr->expr);
        }

        if ($expr instanceof Expr\BinaryOp) {
            return $this->evaluateBinaryOp($expr);
        }

        if ($expr instanceof Expr\Ternary) {
            return $this->evaluateTernary($expr);
        }

        if ($expr instanceof Expr\ArrayDimFetch && null !== $expr->dim) {
            return $this->evaluate($expr->var)[$this->evaluate($expr->dim)];
        }

        if ($expr instanceof Expr\ConstFetch) {
            return $this->evaluateConstFetch($expr);
        }

        return ($this->fallbackEvaluator)($expr);
    }

    private function evaluateArray(Expr\Array_ $expr): array {
        $array = [];
        foreach ($expr->items as $item) {
            if (null !== $item->key) {
                $array[$this->evaluate($item->key)] = $this->evaluate($item->value);
            } elseif ($item->unpack) {
                $array = array_merge($array, $this->evaluate($item->value));
            } else {
                $array[] = $this->evaluate($item->value);
            }
        }
        return $array;
    }

    /** @return mixed */
    private function evaluateTernary(Expr\Ternary $expr) {
        if (null === $expr->if) {
            return $this->evaluate($expr->cond) ?: $this->evaluate($expr->else);
        }

        return $this->evaluate($expr->cond)
            ? $this->evaluate($expr->if)
            : $this->evaluate($expr->else);
    }

    /** @return mixed */
    private function evaluateBinaryOp(Expr\BinaryOp $expr) {
        if ($expr instanceof Expr\BinaryOp\Coalesce
            && $expr->left instanceof Expr\ArrayDimFetch
        ) {
            // This needs to be special cased to respect BP_VAR_IS fetch semantics
            return $this->evaluate($expr->left->var)[$this->evaluate($expr->left->dim)]
                ?? $this->evaluate($expr->right);
        }

        // The evaluate() calls are repeated in each branch, because some of the operators are
        // short-circuiting and evaluating the RHS in advance may be illegal in that case
        $l = $expr->left;
        $r = $expr->right;
        switch ($expr->getOperatorSigil()) {
            case '&':   return $this->evaluate($l) &   $this->evaluate($r);
            case '|':   return $this->evaluate($l) |   $this->evaluate($r);
            case '^':   return $this->evaluate($l) ^   $this->evaluate($r);
            case '&&':  return $this->evaluate($l) &&  $this->evaluate($r);
            case '||':  return $this->evaluate($l) ||  $this->evaluate($r);
            case '??':  return $this->evaluate($l) ??  $this->evaluate($r);
            case '.':   return $this->evaluate($l) .   $this->evaluate($r);
            case '/':   return $this->evaluate($l) /   $this->evaluate($r);
            case '==':  return $this->evaluate($l) ==  $this->evaluate($r);
            case '>':   return $this->evaluate($l) >   $this->evaluate($r);
            case '>=':  return $this->evaluate($l) >=  $this->evaluate($r);
            case '===': return $this->evaluate($l) === $this->evaluate($r);
            case 'and': return $this->evaluate($l) and $this->evaluate($r);
            case 'or':  return $this->evaluate($l) or  $this->evaluate($r);
            case 'xor': return $this->evaluate($l) xor $this->evaluate($r);
            case '-':   return $this->evaluate($l) -   $this->evaluate($r);
            case '%':   return $this->evaluate($l) %   $this->evaluate($r);
            case '*':   return $this->evaluate($l) *   $this->evaluate($r);
            case '!=':  return $this->evaluate($l) !=  $this->evaluate($r);
            case '!==': return $this->evaluate($l) !== $this->evaluate($r);
            case '+':   return $this->evaluate($l) +   $this->evaluate($r);
            case '**':  return $this->evaluate($l) **  $this->evaluate($r);
            case '<<':  return $this->evaluate($l) <<  $this->evaluate($r);
            case '>>':  return $this->evaluate($l) >>  $this->evaluate($r);
            case '<':   return $this->evaluate($l) <   $this->evaluate($r);
            case '<=':  return $this->evaluate($l) <=  $this->evaluate($r);
            case '<=>': return $this->evaluate($l) <=> $this->evaluate($r);
        }

        throw new \Exception('Should not happen');
    }

    /** @return mixed */
    private function evaluateConstFetch(Expr\ConstFetch $expr) {
        $name = $expr->name->toLowerString();
        switch ($name) {
            case 'null': return null;
            case 'false': return false;
            case 'true': return true;
        }

        return ($this->fallbackEvaluator)($expr);
    }
}