# What sort of Math do I need for a plagiarism detection algorithm?

#### Jamin2112

I'm trying to figure out the theory behind a simple plagiarism detection algorithm I'm making. The essence is that it's a function

f:S×S→[0, 1]

where S is the set of all strings and [0, 1] is the plagiarism quotient with 0 being no plagiarism and 1 being a completely copied string (that is, f(s1,s2)=1 if s1=s2).

This is analogous a some function that maps two real-valued vectors to the unit interval, something like norm(v1, v2). Only problem here is that the strings can be of different lengths. I guess my algorithm will have to, in that case, take the smaller string and check to see if the larger string contains any substrings suspicious of being plagiarized by the smaller one.

Any ideas?

#### Jamin2112

Ok, I just started to make the function like I described above. It's recursive and I still need the Math stuff figured out. Suggestions welcome.

Code:
double plgrsm_qtnt(std::string s1, std::string s2) {
if (s1.length() == s2.length()) {
// math/stat stuff goes here ...

// ....
} else {
// Use s1 to be the smaller string if isn't already:
if (s1.length() > s2.length())
swap(s1, s2);
// Check s1 against all substrings of s2 and return the
double max_qtnt(0); // since plgrsm_qtnt fallse in [0, 1]
int s1_len(s1.length()), s2_len(s2.length());
for (int i(0), j(s1_len - 1); j != s2_len; ++i, ++j) {
std::string s2_sbstr = s2.substring(i, j);
double this_qtnt = plgrsm_qtnt(s1, s2_sbstr);
if (this_qtnt > max_qtnt)
max_qtnt = this_qtnt;
}
return max_qtnt;
}
}

Mentor

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving