I 开发者_如何学Pythonhave a method that converts an int
to a base60 string
(using 0-9, a-z, and A-Z chars), but can't work out how to convert it back again. Here is my method for converting base10 to base60:
public static function toBase60(value:Number):String
{
var targetBase:uint = 60;
value = value.toString().split('.')[0];
var digits:Array = new Array();
while (value > 0)
{
digits.push(baseChars[value % targetBase]);
value = Math.floor(value / targetBase);
}
var myResult:String = digits.reverse().join('');
return myResult;
}
Works well. But how do I get the base60 string back into a base10 int? I happen to be using ActionScript 3, but really, examples in any programming language, generic explanations or sudo code would be great.
total = 0;
for each digit (front to back)
total = total * 60 + digit
One way of doing this could be:
public static function fromBase60(value:String):Number {
var result:Number = 0;
var targetBase:uint = 60;
var digitValue:int = 0;
for(var i:int = 0, j:int = value.length - 1; j >= 0; i++,j--) {
digitValue = reverseMap[value.charAt(j)];
result += Math.pow(targetBase,i) * digitValue;
}
return result;
}
Looks like you have an array that maps numbers (digits) to characters, so you could build a reverse map upfront and make the lookup easier. With some code like this:
// add this code to your class
private static var reverseMap:Object = {};
private static function buildReverseMap():void {
var len:int = baseChars.length;
for(var i:int = 0; i < len; i++) {
reverseMap[baseChars[i]] = i;
}
}
// initialize the reverse map
{
buildReverseMap();
}
Edit
Alternative implementation, based on algorithm posted by Tom Sirgedas. This avoids calling Math.pow, though I doubt you'll note much difference in practice (performance-wise):
public static function fromBase60(value:String):Number {
var result:Number = 0;
var targetBase:uint = 60;
var digitValue:int = 0;
var len:int = value.length;
for(var i:int = 0; i < len; i++) {
digitValue = reverseMap[value.charAt(i)];
result = result * targetBase + digitValue;
}
return result;
}
Assuming you have a function int digit60to10(char digit)
that converts [0-9a-zA-Z] into the equivalent decimal value for a single digit, you can do this:
int decimalValue = 0;
foreach digit in digits (most to least significant):
decimalValue *= 60;
decimalValue += digit60to10(digit);
This could give you some problems. But that is not Base 60, it's base 62
Edit Since the above is not a viewed as a valid answer, here is how I converted base 62 <=> 10 in PHP, though there are many methods to do so. http://ken-soft.com/?p=544
I also, explained why I posted that as an answer in the below comment (though I agree it was way too brief) :) sorry.
Edit why is this getting voted down? What I said is true!
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