Detection of invalid sheet name

Based on Excel requirements:
 - it should not be blank
 - it should not exceed 31 characters
 - it should not contain these characters: \ / ? * [ or ]
 - it should be unique

src/Spout/Writer/Exception/InvalidSheetNameException.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace Box\Spout\Writer\Exception;
+
+/**
+ * Class InvalidSheetNameException
+ *
+ * @package Box\Spout\Writer\Exception
+ */
+class InvalidSheetNameException extends WriterException
+{
+}

src/Spout/Writer/XLSX/Sheet.php

@@ -2,9 +2,11 @@
 
 namespace Box\Spout\Writer\XLSX;
 
+use Box\Spout\Writer\Exception\InvalidSheetNameException;
+
 /**
  * Class Sheet
- * Represents a worksheet within a XLSX file
+ * External representation of a worksheet within a XLSX file
  *
  * @package Box\Spout\Writer\XLSX
  */
@@ -12,6 +14,15 @@ class Sheet
 {
     const DEFAULT_SHEET_NAME_PREFIX = 'Sheet';
 
+    /** Sheet name should not exceed 31 characters */
+    const MAX_LENGTH_SHEET_NAME = 31;
+
+    /** @var array Invalid characters that cannot be contained in the sheet name */
+    private static $INVALID_CHARACTERS_IN_SHEET_NAME = ['\\', '/', '?', '*', '[', ']'];
+
+    /** @var array Associative array [SHEET_INDEX] => [SHEET_NAME] keeping track of sheets' name to enforce uniqueness */
+    protected static $SHEETS_NAME_USED = [];
+
     /** @var int Index of the sheet, based on order of creation (zero-based) */
     protected $index;
 
@@ -24,7 +35,7 @@ class Sheet
     public function __construct($sheetIndex)
     {
         $this->index = $sheetIndex;
-        $this->name = self::DEFAULT_SHEET_NAME_PREFIX . ($sheetIndex + 1);
+        $this->setName(self::DEFAULT_SHEET_NAME_PREFIX . ($sheetIndex + 1));
     }
 
     /**
@@ -44,12 +55,116 @@ class Sheet
     }
 
     /**
+     * Sets the name of the sheet. Note that Excel has some restrictions on the name:
+     *  - it should not be blank
+     *  - it should not exceed 31 characters
+     *  - it should not contain these characters: \ / ? * [ or ]
+     *  - it should be unique
+     *
      * @param string $name Name of the sheet
      * @return Sheet
+     * @throws \Box\Spout\Writer\Exception\InvalidSheetNameException If the sheet's name is invalid.
      */
     public function setName($name)
     {
+        if (!$this->isNameValid($name)) {
+            $errorMessage = "The sheet's name is invalid. It did not meet at least one of these requirements:\n";
+            $errorMessage .= " - It should not be blank\n";
+            $errorMessage .= " - It should not exceed 31 characters\n";
+            $errorMessage .= " - It should not contain these characters: \\ / ? * [ or ]\n";
+            $errorMessage .= " - It should be unique";
+            throw new InvalidSheetNameException($errorMessage);
+        }
+
         $this->name = $name;
+        self::$SHEETS_NAME_USED[$this->index] = $name;
+
         return $this;
     }
+
+    /**
+     * Returns whether the given sheet's name is valid.
+     * @see Sheet::setName for validity rules.
+     *
+     * @param string $name
+     * @return bool TRUE if the name is valid, FALSE otherwise.
+     */
+    protected function isNameValid($name)
+    {
+        if (!is_string($name)) {
+            return false;
+        }
+
+        $nameLength = $this->getStringLength($name);
+        $hasValidLength = ($nameLength > 0 && $nameLength <= self::MAX_LENGTH_SHEET_NAME);
+        $containsInvalidCharacters = $this->doesContainInvalidCharacters($name);
+        $isNameUnique = $this->isNameUnique($name);
+
+        return ($hasValidLength && !$containsInvalidCharacters && $isNameUnique);
+    }
+
+    /**
+     * Returns the length of the given string.
+     * It uses the multi-bytes function is available.
+     * @see strlen
+     * @see mb_strlen
+     *
+     * @param string $string
+     * @return int
+     */
+    protected function getStringLength($string)
+    {
+        return extension_loaded('mbstring') ? mb_strlen($string) : strlen($string);
+    }
+
+    /**
+     * Returns the position of the given character/substring in the given string.
+     * It uses the multi-bytes function is available.
+     * @see strpos
+     * @see mb_strpos
+     *
+     * @param string $string Haystack
+     * @param string $char Needle
+     * @return int Index of the char in the string if found (started at 0) or -1 if not found
+     */
+    protected function getCharPosition($string, $char)
+    {
+        $position = extension_loaded('mbstring') ? mb_strpos($string, $char) : strpos($string, $char);
+        return ($position !== false) ? $position : -1;
+    }
+
+    /**
+     * Returns whether the given name contains at least one invalid character.
+     * @see Sheet::$INVALID_CHARACTERS_IN_SHEET_NAME for the full list.
+     *
+     * @param string $name
+     * @return bool TRUE if the name contains invalid characters, FALSE otherwise.
+     */
+    protected function doesContainInvalidCharacters($name)
+    {
+        foreach (self::$INVALID_CHARACTERS_IN_SHEET_NAME as $invalidCharacter) {
+            if ($this->getCharPosition($name, $invalidCharacter) !== -1) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    /**
+     * Returns whether the given name is unique.
+     *
+     * @param string $name
+     * @return bool TRUE if the name is unique, FALSE otherwise.
+     */
+    protected function isNameUnique($name)
+    {
+        foreach (self::$SHEETS_NAME_USED as $sheetIndex => $sheetName) {
+            if ($sheetIndex !== $this->index && $sheetName === $name) {
+                return false;
+            }
+        }
+
+        return true;
+    }
 }