Question

I have a function that extracts longest common prefix of strings rounded on the last whole word. For example, if I have list:

BRIGGS & STRATTON 290400 
BRIGGS & STRATTON 290700 
BRIGGS & STRATTON 294400 
BRIGGS & STRATTON 294700 

and I call longestPrefix(array); it will return BRIGGS & STRATTON.

I am trying to chunk list of strings into sublists, grouped by the nonempty common prefix, but I can’t get any my thought to work.

Edit:

Sample list is generated in that way that for two successive elements, my function either returns empty or nonempty string:

ALLIS-CHALMERS 816 
ALLIS-CHALMERS 818 
ALLIS-CHALMERS 917 AND 919 
ARIENS GT-17 
ARIENS GT-18 AND GT-19 
BOLENS 1600 
BOLENS 1700 
BOLENS 1900 HT-18 
BOLENS HT-20 AND HT-23 
BOLENS QT-16 
BRIGGS & STRATTON 290400 
BRIGGS & STRATTON 290700 

Answer 1

The value returned by longestPrefix(array) could potentially be different for each non-empty subset of your set. Additionally, each string may belong to multiple “groups”, depending on what other items are there in a group: for example, if the original list contained some BRIGGS & SON items, the BRIGGS & STRATTON items could have been grouped with BRIGGS & items as well.

If you redefine the requirement as grouping by the longest common prefix, you could compute pairwise common prefixes, get the longest one, and group by it, like this:

void AddLongest(IDictionary<string,string> dict, string s, string p) {
    string current;
    if (!dict.TryGetValue(s, out current) || p.Length > current.Length) {
        dict[s] = p;
    }
}

var longestPrefix = new Dictionary<string,string>();
for (int i = 0 ; i != myStrings.Length ; i++) {
    for (int j = i+1 ; j != myStrings.Length ; j++) {
        var common = FindLongestPrefix(new[] {myStrings[i], myStrings[j]});
        AddLongest(longestPrefix, myStrings[i], common);
        AddLongest(longestPrefix, myStrings[j], common);
    }
}
// Now you can use LINQ to group by the longest common prefix:
var groups = myStrings.GroupBy(s => longestPrefix[s]);