<?php

declare(strict_types=1);

namespace App\Engine\Cfnpp\Expression;

/**
 * Expression that can be evaluated.
 *
 * @author Adam Pippin <hello@adampippin.ca>
 */
class Expression
{
	/**
	 * List of Tokens in the expression.
	 * @var Token[]
	 */
	protected $tokens;

	/**
	 * Evaluation stack.
	 * @var mixed[]
	 */
	protected $stack;

	/**
	 * Variables that can be referenced in the expression.
	 * @var array<string,mixed>
	 */
	protected $variables;

	/**
	 * Create a new expression.
	 *
	 * @param Token[] $tokens
	 */
	public function __construct(array $tokens)
	{
		$this->tokens = $tokens;
	}

	/**
	 * Find all referenced variables so we can do proper ordering of variable
	 * block evaluate.
	 *
	 * @return string[]
	 */
	public function getReferencedVariables(): array
	{
		$variables = [];
		foreach ($this->tokens as $token)
		{
			if ($token instanceof TokenVariable)
			{
				$variables[] = $token->getName();
			}
		}
		return array_values(array_unique($variables));
	}

	/**
	 * Evaluate the tokens contained in this expression.
	 *
	 * @param  array<string,mixed> $variables variables that can be referenced
	 * @return mixed               if expression evaluated down to a single value a scalar, otherwise an array
	 */
	public function evaluate(array $variables = [])
	{
		$this->variables = $variables;
		$this->stack = [];
		$tokens = $this->tokens;

		while (sizeof($tokens))
		{
			$token = array_shift($tokens);
			assert(isset($token));

			$token_class = get_class($token);
			$token_name = substr($token_class, strrpos($token_class, '\\') + 1);
			$func = 'evaluate'.$token_name;
			if (method_exists($this, $func))
			{
				$this->{$func}($token);
			}
			else
			{
				throw new \Exception('Unhandled expression token type: '.basename(get_class($token)));
			}
		}

		if (sizeof($this->stack) == 1)
		{
			return end($this->stack);
		}

		return $this->stack;
	}

	/**
	 * Evaluate and mutate the stack given a numeric literal.
	 *
	 * @param  TokenNumericLiteral $token
	 * @return void
	 */
	protected function evaluateTokenNumericLiteral(TokenNumericLiteral $token): void
	{
		$this->push($token->getValue());
	}

	/**
	 * Evaluate and mutate the stack given a string literal.
	 *
	 * @param  TokenStringLiteral $token
	 * @return void
	 */
	protected function evaluateTokenStringLiteral(TokenStringLiteral $token): void
	{
		$this->push($token->getValue());
	}

	/**
	 * Evaluate and mutate the stack given a comparison operator.
	 *
	 * $this->pop() == $this->pop() is valid.
	 * @suppress PhanPluginDuplicateExpressionBinaryOp
	 * @param  TokenOperator $token
	 * @return void
	 */
	protected function evaluateTokenOperator(TokenOperator $token): void
	{
		switch ($token->getOperator())
		{
			case 'eq':
				$this->push($this->pop() == $this->pop());
				break;
			case 'neq':
				$this->push($this->pop() != $this->pop());
				break;
			case 'gt':
				$this->push($this->pop(1) > $this->pop());
				break;
			case 'gte':
				$this->push($this->pop(1) >= $this->pop());
				break;
			case 'lt':
				$this->push($this->pop(1) < $this->pop());
				break;
			case 'lte':
				$this->push($this->pop(1) <= $this->pop());
				break;
			case 'and':
				$var1 = $this->pop();
				$var2 = $this->pop();
				$this->push($var1 && $var2);
				break;
			case 'or':
				$var1 = $this->pop();
				$var2 = $this->pop();
				$this->push($var1 || $var2);
				break;
			default:
				throw new \Exception('Unhandled comparison operator: '.$token->getOperator());
		}
	}

	/**
	 * Evaluate and mutate the stack given a function token.
	 *
	 * @param  TokenFunction $token
	 * @return void
	 */
	protected function evaluateTokenFunction(TokenFunction $token): void
	{
		switch ($token->getFunction())
		{
			case 'concat':
				$this->push($this->pop(1).$this->pop());
				break;
			case 'concat*':
				while (sizeof($this->stack) > 1)
				{
					$this->push($this->pop(1).$this->pop());
				}
				break;
			default:
				throw new \Exception('Unhandled function: '.$token->getFunction());
		}
	}

	/**
	 * Evaluate and mutate the stack given a variable reference.
	 *
	 * @param  TokenVariable $token
	 * @return void
	 */
	protected function evaluateTokenVariable(TokenVariable $token)
	{
		$name = $token->getName();
		if (!isset($this->variables[$name]))
		{
			throw new \Exception('Undefined variable: '.$name);
		}
		$this->push($this->variables[$name]);
	}

	/**
	 * Pop an item off the stack.
	 *
	 * @param  int   $offset offset from the end of the stack to pop from
	 * @return mixed
	 */
	protected function pop(int $offset = 0)
	{
		if (sizeof($this->stack) < $offset + 1)
		{
			throw new \Exception('Expression stack underflow!');
		}
		return array_splice($this->stack, -1 * ($offset + 1), 1)[0];
	}

	/**
	 * Push an item onto the end of the stack.
	 *
	 * @param  mixed $value
	 * @return void
	 */
	protected function push($value): void
	{
		array_push($this->stack, $value);
	}
}