lexing, parsing (partial), shunting yard

classes/parser/inputstream.php

@@ -0,0 +1,123 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle 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 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle 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 Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Lexer for qtype_formulas
+ *
+ * @package    qtype_formulas
+ * @copyright  2022 Philipp Imhof
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+namespace qtype_formulas;
+
+/**
+ * Helper class to go through the input.
+ */
+class InputStream {
+    const EOF = '';
+
+    /** @var integer current position in the input */
+    private $position = -1;
+
+    /** @var integer the row (line number) of the current character */
+    private $row = 1;
+
+    /** @var integer the column number of the current character */
+    private $column = 0;
+
+    /** @var integer the length of the input */
+    private $length = 0;
+
+    /** @var array array containing the input's individual characters */
+    private $input = [];
+
+    /**
+     * Constructor
+     *
+     * @param string $str the raw input
+     */
+    public function __construct($str) {
+        // Input can contain unicode characters, so it's better to operate on an array of characters
+        // rather than a string.
+        $this->input = mb_str_split($str);
+        $this->length = count($this->input);
+    }
+
+    /**
+     * Return the next character of the stream, without consuming it. The optional
+     * parameter allows to retrieve characters farther behind, if they exist.
+     *
+     * @param integer $skip skip a certain number of characters
+     * @return string
+     */
+    public function peek($skip = 0) {
+        if ($this->position < $this->length - $skip - 1) {
+            return $this->input[$this->position + $skip + 1];
+        }
+        return self::EOF;
+    }
+
+    /**
+     * Return the next character of the stream and move the position index one step forward.
+     *
+     * @return string
+     */
+    public function read() {
+        $nextchar = $this->peek();
+        if ($nextchar !== self::EOF) {
+            $this->advance($nextchar === "\n");
+        }
+        return $nextchar;
+    }
+
+    /**
+     * Advance the position index by one and keep row/column numbers in sync.
+     *
+     * @param boolean $newline
+     * @return void
+     */
+    private function advance($newline) {
+        $this->position++;
+        $this->column++;
+        if ($newline) {
+            $this->row++;
+            $this->column = 0;
+        }
+    }
+
+    /**
+     * Stop processing the input and indicate the human readable position (row/column) where the error occurred.
+     *
+     * @param string $message error message
+     * @return void
+     * @throws Exception
+     */
+    public function die($message) {
+        throw new \Exception($this->row . ':' . $this->column . ':' . $message);
+    }
+
+    /**
+     * Return the human readable position (row/column) of the current character in the input stream
+     *
+     * @return array position, associative array with keys 'row' and 'column'
+     */
+    public function get_position() {
+        return ['row' => $this->row, 'column' => $this->column];
+    }
+}
+
+