ToddB ToddB - 3 months ago 11
iOS Question

Base 62 conversion in Objective-C

I spent much too much time trying to find an implementation for base 62 conversion for Objective-C. I am sure this is a terrible example and there must be an elegant, super-efficient way to do this, but this works, please edit or answer to improve it! But I wanted to help people searching for this to have something that will work. There doesn't appear to be anything specific to be found for an Objective-C implementation.

@implementation Base62Converter

+(int)decode:(NSString*)string
{
int num = 0;
NSString * alphabet = @"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

for (int i = 0, len = [string length]; i < len; i++)
{
NSRange range = [alphabet rangeOfString:[string substringWithRange:NSMakeRange(i,1)]];
num = num * 62 + range.location;
}

return num;
}

+(NSString*)encode:(int)num
{
NSString * alphabet = @"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
NSMutableString * precursor = [NSMutableString stringWithCapacity:3];

while (num > 0)
{
[precursor appendString:[alphabet substringWithRange:NSMakeRange( num % 62, 1 )]];
num /= 62;
}

// http://stackoverflow.com/questions/6720191/reverse-nsstring-text
NSMutableString *reversedString = [NSMutableString stringWithCapacity:[precursor length]];

[precursor enumerateSubstringsInRange:NSMakeRange(0,[precursor length])
options:(NSStringEnumerationReverse |NSStringEnumerationByComposedCharacterSequences)
usingBlock:^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
[reversedString appendString:substring];
}];
return reversedString;
}

@end

Answer

Your code is fine. If anything, make it more generic. Here is a recursive version for any base (same code):

#import <Foundation/Foundation.h>

@interface BaseConversion : NSObject
+(NSString*) formatNumber:(NSUInteger)n toBase:(NSUInteger)base;
+(NSString*) formatNumber:(NSUInteger)n usingAlphabet:(NSString*)alphabet;
@end

@implementation BaseConversion

// Uses the alphabet length as base.
+(NSString*) formatNumber:(NSUInteger)n usingAlphabet:(NSString*)alphabet
{
    NSUInteger base = [alphabet length];
    if (n<base){
        // direct conversion
        NSRange range = NSMakeRange(n, 1);
        return [alphabet substringWithRange:range];
    } else {
        return [NSString stringWithFormat:@"%@%@",

                // Get the number minus the last digit and do a recursive call.
                // Note that division between integer drops the decimals, eg: 769/10 = 76
                [self formatNumber:n/base usingAlphabet:alphabet],

                // Get the last digit and perform direct conversion with the result.
                [alphabet substringWithRange:NSMakeRange(n%base, 1)]];
    }
}

+(NSString*) formatNumber:(NSUInteger)n toBase:(NSUInteger)base 
{
    NSString *alphabet = @"0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"; // 62 digits
    NSAssert([alphabet length]>=base,@"Not enough characters. Use base %ld or lower.",(unsigned long)[alphabet length]);
    return [self formatNumber:n usingAlphabet:[alphabet substringWithRange:NSMakeRange (0, base)]];
}

@end

int main(int argc, char *argv[]) {
    @autoreleasepool {
        NSLog(@"%@",[BaseConversion formatNumber:3735928559 toBase:16]); // deadbeef
        return EXIT_SUCCESS;
    }
}

A Swift 3 version: https://gist.github.com/j4n0/056475333d0ddfe963ac5dc44fa53bf2

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