Truncate delimited NSString without removing delimiters - ios

Question

I have some data in an NSString, separated by colons:
#"John:Doe:1970:Male:Dodge:Durango"
I need to limit the total length of this string to 100 characters. But I also need to ensure the correct number of colons are present.
What would be a reasonable to way to truncate the string but also add the extra colons so I can parse it into the correct number of fields on the other side?
For example, if my limit was 18, you would end up with something like this:
#"John:Doe:1970:Ma::"
Here's an updated version of my own latest pass at this. Uses #blinkenlights algorithm:
+ (NSUInteger)occurrencesOfSubstring:(NSString *)substring inString:(NSString *)string {
// http://stackoverflow.com/a/5310084/878969
return [string length] - [[string stringByReplacingOccurrencesOfString:substring withString:#""] length] / [substring length];
}
+ (NSString *)truncateString:(NSString *)string toLength:(NSUInteger)length butKeepDelmiter:(NSString *)delimiter {
if (string.length <= length)
return string;
NSAssert(delimiter.length == 1, #"Expected delimiter to be a string containing a single character");
int numDelimitersInOriginal = [[self class] occurrencesOfSubstring:delimiter inString:string];
NSMutableString *truncatedString = [[string substringToIndex:length] mutableCopy];
int numDelimitersInTruncated = [[self class] occurrencesOfSubstring:delimiter inString:truncatedString];
int numDelimitersToAdd = numDelimitersInOriginal - numDelimitersInTruncated;
int index = length - 1;
while (numDelimitersToAdd > 0) { // edge case not handled here
NSRange nextRange = NSMakeRange(index, 1);
index -= 1;
NSString *rangeSubstring = [truncatedString substringWithRange:nextRange];
if ([rangeSubstring isEqualToString:delimiter])
continue;
[truncatedString replaceCharactersInRange:nextRange withString:delimiter];
numDelimitersToAdd -= 1;
}
return truncatedString;
}
Note that I don't think this solution handles the edge case from CRD where the number of delimiters is less than the limit.
The reason I need the correct number of colons is the code on the server will split on colon and expect to get 5 strings back.
You can assume the components of the colon separated string do not themselves contain colons.

Answer 1

Your current algorithm will not produce the correct result when one or more of the characters among the last colonsToAdd is a colon.
You can use this approach instead:
Cut the string at 100 characters, and store the characters in an NSMutableString
Count the number of colons, and subtract that number from the number that you need
Starting at the back of the string, replace non-colon characters with colons until you have the right number of colons.

Answer 2

I tend towards #dasblinkenlight, it's just an algorithm after all, but here's some code. Few modern shorthands - used an old compiler. ARC assumed. Won't claim it's efficient, or beautiful, but it does work and handles edge cases (repeated colons, too many fields for limit):
- (NSString *)abbreviate:(NSString *)input limit:(NSUInteger)limit
{
NSMutableArray *fields = [[input componentsSeparatedByString:#":"] mutableCopy];
NSUInteger colonCount = fields.count - 1;
if (colonCount >= limit)
return [#"" stringByPaddingToLength:limit withString:#":" startingAtIndex:0];
NSUInteger nonColonsRemaining = limit - colonCount;
for (NSUInteger ix = 0; ix <= colonCount; ix++)
{
if (nonColonsRemaining > 0)
{
NSString *fieldValue = [fields objectAtIndex:ix];
NSUInteger fieldLength = fieldValue.length;
if (fieldLength <= nonColonsRemaining)
nonColonsRemaining -= fieldLength;
else
{
[fields replaceObjectAtIndex:ix withObject:[fieldValue substringToIndex:nonColonsRemaining]];
nonColonsRemaining = 0;
}
}
else
[fields replaceObjectAtIndex:ix withObject:#""];
}
return [fields componentsJoinedByString:#":"];
}