<?php
/*
Copyright (c) 2020 Sunil Mohan Adapa <sunil at medhas dot org>
Drop in replacement for native gettext.
This file is part of PHP-gettext.
PHP-gettext is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
PHP-gettext is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with PHP-gettext; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/
/**
* Lexical analyzer for gettext plurals expression. Takes a string to parse
* during construction and returns a single token every time peek() or
* fetch_token() are called. The special string '__END__' is returned if there
* are no more tokens to be read. Spaces are ignored during tokenization.
*/
class PluralsLexer {
private $string;
private $position;
/**
* Constructor
*
* @param string string Contains the value gettext plurals expression to
* analyze.
*/
public function __construct(string $string) {
$this->string = $string;
$this->position = 0;
}
/**
* Return the next token and the length to advance the read position without
* actually advancing the read position. Tokens for operators and variables
* are simple strings containing the operator or variable. If there are no
* more token to provide, the special value ['__END__', 0] is returned. If
* there was an unexpected input an Exception is raised.
*
* @access private
* @throws Exception If there is unexpected input in the provided string.
* @return array The next token and length to advance the current position.
*/
private function _tokenize() {
$buf = $this->string;
// Consume all spaces until the next token
$index = $this->position;
while ($index < strlen($buf) && $buf[$index] == ' ') {
$index++;
}
$this->position = $index;
// Return special token if next of the string is reached.
if (strlen($buf) - $index == 0) {
return ['__END__', 0];
}
// Operators with two characters
$doubles = ['==', '!=', '>=', '<=', '&&', '||'];
$next = substr($buf, $index, 2);
if (in_array($next, $doubles)) {
return [$next, 2];
}
// Operators with single character or variable 'n'.
$singles = [
'n', '(', ')', '?', ':', '+', '-', '*', '/', '%', '!', '>', '<'];
if (in_array($buf[$index], $singles)) {
return [$buf[$index], 1];
}
// Whole number constants, return an integer.
$digits = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'];
$pos = $index;
while ($pos < strlen($buf) && in_array($buf[$pos], $digits)) {
$pos++;
}
if ($pos != $index) {
$length = $pos - $index;
return [(int)substr($buf, $index, $length), $length];
}
// Throw and exception for all other unexpected input in the string.
throw new Exception('Lexical analysis failed');
}
/**
* Return the next token without actually advancing the read position.
* Tokens for operators and variables are simple strings containing the
* operator or variable. If there are no more tokens to provide, the special
* value '__END__' is returned. If there was an unexpected input an
* Exception is raised.
*
* @throws Exception If there is unexpected input in the provided string.
* @return string The next token.
*/
public function peek() {
list($token, $length) = $this->_tokenize();
return $token;
}
/**
* Return the next token after advancing the read position. Tokens for
* operators and variables are simple strings containing the operator or
* variable. If there are no more token to provide, the special value
* '__END__' is returned. If there was an unexpected input an Exception is
* raised.
*
* @throws Exception If there is unexpected input in the provided string.
* @return string The next token.
*/
public function fetch_token() {
list($token, $length) = $this->_tokenize();
$this->position += $length;
return $token;
}
}
/**
* A parsed representation of the gettext plural expression. This is a tree
* containing further expressions depending on how nested the given input is.
* Calling the evaluate() function computes the value of the expression if the
* variable 'n' is set a certain value. This is used to decide which plural
* string translation to use based on the number items at hand.
*/
class PluralsExpression {
private $operator;
private $operands;
const BINARY_OPERATORS = [
'==', '!=', '>=', '<=', '&&', '||', '+', '-', '*', '/', '%', '>', '<'];
const UNARY_OPERATORS = ['!'];
/**
* Constructor
*
* @param string Operator for the expression.
* @param (int|string|PuralsExpression)[] Variable number of operands of the
* expression. One int operand is expected in case the operator is 'const'.
* One string operand with value 'n' is expected in case the operator is
* 'var'. For all other operators, the operands much be objects of type
* PluralExpression. Unary operators expect one operand, binary operators
* expect two operands and trinary operators expect three operands.
*/
public function __construct($operator, ...$operands) {
$this->operator = $operator;
$this->operands = $operands;
}
/**
* Return a parenthesized string representation of the expression for
* debugging purposes.
*
* @return string A string representation of the expression.
*/
public function to_string() {
if ($this->operator == 'const' || $this->operator == 'var') {
return $this->operands[0];
} elseif (in_array($this->operator, self::BINARY_OPERATORS)) {
return sprintf(
"(%s %s %s)", $this->operands[0]->to_string(), $this->operator,
$this->operands[1]->to_string());
} elseif (in_array($this->operator, self::UNARY_OPERATORS)) {
return sprintf(
"(%s %s)", $this->operator, $this->operands[0]->to_string());
} elseif ($this->operator == '?') {
return sprintf(
"(%s ? %s : %s)", $this->operands[0]->to_string(),
$this->operands[1]->to_string(),
$this->operands[2]->to_string());
}
}
/**
* Return the computed value of the expression if the variable 'n' is set to
* a certain value.
*
* @param int The value of the variable n to use when evaluating.
* @throws Exception If the expression has been constructed incorrectly.
* @return int The value of the expression after evaluation.
*/
public function evaluate($n) {
if (!in_array($this->operator, ['const', 'var'])) {
$operand1 = $this->operands[0]->evaluate($n);
}
if (in_array($this->operator, self::BINARY_OPERATORS) ||
$this->operator == '?') {
$operand2 = $this->operands[1]->evaluate($n);
}
if ($this->operator == '?') {
$operand3 = $this->operands[2]->evaluate($n);
}
switch ($this->operator) {
case 'const':
return $this->operands[0];
case 'var':
return $n;
case '!':
return !($operand1);
case '==':
return $operand1 == $operand2;
case '!=':
return $operand1 != $operand2;
case '>=':
return $operand1 >= $operand2;
case '<=':
return $operand1 <= $operand2;
case '>':
return $operand1 > $operand2;
case '<':
return $operand1 < $operand2;
case '&&':
return $operand1 && $operand2;
case '||':
return $operand1 || $operand2;
case '+':
return $operand1 + $operand2;
case '-':
return $operand1 - $operand2;
case '*':
return $operand1 * $operand2;
case '/':
return (int)($operand1 / $operand2);
case '%':
return $operand1 % $operand2;
case '?':
return $operand1 ? $operand2 : $operand3;
default:
throw new Exception('Invalid expression');
}
}
}
/**
* A simple operator-precedence parser for gettext plural expressions. Takes a
* string during construction and returns a PluralsExpression tree when
* parse() is called.
*/
class PluralsParser {
private $lexer;
/*
* Operator precedence. The parsing only happens with minimum precedence of
* 0. However, ':' and ')' exist here to make sure that parsing does not
* proceed beyond them when they are not to be parsed.
*/
private const PREC = [
':' => -1, '?' => 0, '||' => 1, '&&' => 2, '==' => 3, '!=' => 3,
'>' => 4, '<' => 4, '>=' => 4, '<=' => 4, '+' => 5, '-' => 5, '*' => 6,
'/' => 6, '%' => 6, '!' => 7, '__END__' => -1, ')' => -1
];
// List of right associative operators
private const RIGHT_ASSOC = ['?'];
/**
* Constructor
*
* @param string string the plural expression to be parsed.
*/
public function __construct(string $string) {
$this->lexer = new PluralsLexer($string);
}
/**
* Expect a primary next for parsing and return a PluralsExpression or throw
* and exception otherwise. A primary can be the variable 'n', an whole
* number constant, a unary operator expression string with '!', or a
* parenthesis expression.
*
* @throws Exception If the next token is not a primary or if parenthesis
* expression is not closes properly with ')'.
* @return PluralsExpression That is constructed from the parsed primary.
*/
private function _parse_primary() {
$token = $this->lexer->fetch_token();
if ($token === 'n') {
return new PluralsExpression('var', 'n');
} elseif (is_int($token)) {
return new PluralsExpression('const', (int)$token);
} elseif ($token === '!') {
return new PluralsExpression('!', $this->_parse_primary());
} elseif ($token === '(') {
$result = $this->_parse($this->_parse_primary(), 0);
if ($this->lexer->fetch_token() != ')') {
throw new Exception('Mismatched parenthesis');
}
return $result;
}
throw new Exception('Primary expected');
}
/**
* Fetch an operator from the lexical analyzer and test for it. Optionally
* advance the position of the lexical analyzer to next token. Raise
* exception if the token retrieved is not an operator.
*
* @access private
* @param bool peek A flag to indicate whether the position of the lexical
* analyzer should *not* be advanced. If false, the lexical analyzer is
* advanced by one token.
* @throws Exception If the token read is not an operator.
* @return string The operator that has been fetched from the lexical
* analyzer.
*/
private function _parse_operator($peek) {
if ($peek) {
$token = $this->lexer->peek();
} else {
$token = $this->lexer->fetch_token();
}
if ($token !== NULL && !array_key_exists($token, self::PREC)) {
throw new Exception('Operator expected');
}
return $token;
}
/**
* A parsing method suitable for recursion.
*
* @access private
* @param ParserExpression left_side A pre-parsed left-hand side expression
* of the file expression to be constructed. This helps with recursion.
* @param int min_precedence The minimum value of precedence for the
* operators to be considered for parsing. Parsing will stop and current
* expression is returned if an operator of a lower precedence is
* encountered.
* @throws Exception If the input string does not conform to the grammar of
* the gettext plural expression.
* @return ParserExpression A complete expression after parsing.
*/
private function _parse($left_side, $min_precedence) {
$next_token = $this->_parse_operator(true);
while (self::PREC[$next_token] >= $min_precedence) {
$operator = $this->_parse_operator(false);
$right_side = $this->_parse_primary();
$next_token = $this->_parse_operator(true);
/*
* Consume (recursively) into right hand side all expressions of higher
* precedence.
*/
while ((self::PREC[$operator] < self::PREC[$next_token]) ||
((self::PREC[$operator] == self::PREC[$next_token]) &&
in_array($operator, self::RIGHT_ASSOC))) {
$right_side = $this->_parse(
$right_side, self::PREC[$next_token]);
$next_token = $this->_parse_operator(true);
}
if ($operator != '?') {
/*
* Handling for all binary operators. Consume into left hand side all
* expressions of equal precedence.
*/
$left_side = new PluralsExpression($operator, $left_side, $right_side);
} else {
// Special handling for (a ? b : c) expression
$operator = $this->lexer->fetch_token();
if ($operator != ':') {
throw new Exception('Invalid ? expression');
}
$right_side2 = $this->_parse(
$this->_parse_primary(), self::PREC[$operator] + 1);
$next_token = $this->_parse_operator(true);
$left_side = new PluralsExpression(
'?', $left_side, $right_side, $right_side2);
}
}
return $left_side;
}
/**
* A simple implementation of an operator-precedence parser. See:
* https://en.wikipedia.org/wiki/Operator-precedence_parser for an analysis
* of the algorithm.
*
* @throws Exception If the input string does not conform to the grammar of
* the gettext plural expression.
* @return ParserExpression A complete expression after parsing.
*/
public function parse() {
$expression = $this->_parse($this->_parse_primary(), 0);
// Special handling for an extra ')' at the end.
if ($this->lexer->peek() != '__END__') {
throw new Exception('Could not parse completely');
}
return $expression;
}
}
/**
* Provides a class to parse the value of the 'Plural-Forms:' header in the
* gettext translation files. Holds the expression tree and the number of
* plurals after parsing. Parsing happens during construction which takes as
* its only argument the string to parse. Error during parsing are silently
* suppressed and the fallback behavior is used with the value for Germanic
* languages as follows: "nplurals=2; plural=n == 1 ? 0 : 1;".
*/
class PluralHeader {
public $total;
public $expression;
/**
* Constructor
*
* @param string The value of the Plural-Forms: header as seen in .po files.
*/
function __construct($string) {
try {
list($total, $expression) = $this->parse($string);
} catch (Exception $e) {
$string = "nplurals=2; plural=n == 1 ? 0 : 1;";
list($total, $expression) = $this->parse($string);
}
$this->total = $total;
$this->expression = $expression;
}
/**
* Return the number of plural forms and the parsed expression tree.
*
* @access private
* @param string string The value of the Plural-Forms: header.
* @throws Exception If the string could not be parsed.
* @return array The number of plural forms and parsed expression tree.
*/
private function parse($string) {
$regex = "/^\s*nplurals\s*=\s*(\d+)\s*;\s*plural\s*=([^;]+);/i";
if (preg_match($regex, $string, $matches)) {
$total = (int)$matches[1];
$expression_string = $matches[2];
} else {
throw new Exception('Invalid header value');
}
$parser = new PluralsParser($expression_string);
$expression = $parser->parse();
return [$total, $expression];
}
}