ODS writer

Added ODS writer
Refactored XLSX writer to abstract some pieces into an abstract multi-sheets writer
Created an abstract style helper
Moved shared components around

src/Spout/Writer/Common/Helper/CellHelper.php

@@ -0,0 +1,71 @@
+<?php
+
+namespace Box\Spout\Writer\Common\Helper;
+
+/**
+ * Class CellHelper
+ * This class provides helper functions when working with cells
+ *
+ * @package Box\Spout\Writer\Common\Helper
+ */
+class CellHelper
+{
+    /**
+     * Returns the cell index (base 26) associated to the base 10 column index.
+     * Excel uses A to Z letters for column indexing, where A is the 1st column,
+     * Z is the 26th and AA is the 27th.
+     * The mapping is zero based, so that 0 maps to A, B maps to 1, Z to 25 and AA to 26.
+     *
+     * @param int $columnIndex The Excel column index (0, 42, ...)
+     * @return string The associated cell index ('A', 'BC', ...)
+     */
+    public static function getCellIndexFromColumnIndex($columnIndex)
+    {
+        $cellIndex = '';
+        $capitalAAsciiValue = ord('A');
+
+        do {
+            $modulus = $columnIndex % 26;
+            $cellIndex = chr($capitalAAsciiValue + $modulus) . $cellIndex;
+
+            // substracting 1 because it's zero-based
+            $columnIndex = intval($columnIndex / 26) - 1;
+        } while ($columnIndex >= 0);
+
+        return $cellIndex;
+    }
+
+    /**
+     * @param $value
+     * @return bool Whether the given value is a non empty string
+     */
+    public static function isNonEmptyString($value)
+    {
+        return (gettype($value) === 'string' && $value !== '');
+    }
+
+    /**
+     * Returns whether the given value is numeric.
+     * A numeric value is from type "integer" or "double" ("float" is not returned by gettype).
+     *
+     * @param $value
+     * @return bool Whether the given value is numeric
+     */
+    public static function isNumeric($value)
+    {
+        $valueType = gettype($value);
+        return ($valueType === 'integer' || $valueType === 'double');
+    }
+
+    /**
+     * Returns whether the given value is boolean.
+     * "true"/"false" and 0/1 are not booleans.
+     *
+     * @param $value
+     * @return bool Whether the given value is boolean
+     */
+    public static function isBoolean($value)
+    {
+        return gettype($value) === 'boolean';
+    }
+}