<?php

class Char{
	protected static $letters = array(
		"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m",
		"n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"
	);
	
	protected static $zhNums = array(
		"零", "一", "二", "三", "四", "五", "六", "七", "八", "九", "十",
	);
	
	protected static $legal = array("UPPER", "LOWER");


	public static function numToLetter($num, $begain = 0, $case = "UPPER"){
		if(!in_array($case, self::$legal))
			trigger_error('param 3 must be one of "UPPER", "LOWER"');
		
		$num = (int)$num;
		$begain = (int)$begain;
		
		if(is_int($num) && is_int($begain) && $case){
			if($begain < 0) $begain = 0;
			$num = $num - $begain;
			
			if($num < 0) $num = 0;
			else if($num > 25) $num = 25;
			
			
			$letter = self::$letters[$num];
			if($case === "UPPER")
				$letter = strtoupper($letter);
			
			return $letter;
		}
		
		return $num;
	}
	
	public static function letterToNum($letter, $begain = 0){
		$letter = strtolower($letter);
		$rs = array_keys(self::$letters, $letter, true);
		
		if($rs)
			$position = current($rs) + $begain;
		else
			$position = null;
		
		return $position;
	}
	
	public static function numToZh($num){
		$len = count(self::$zhNums) - 1;
		$num = (int)$num;
		
		if($num < 0) $num = 0;
		if($num > $len) return null;
			
		return self::$zhNums[$num];
	}
}