Function that returns affinity between texts?

Question

consider I have a

string1 = "hello hi goodmorning evening [...]"

and I have some minor keywords

compare1 = "hello evening"
compare2 = "hello hi"

I need a function that returns the affinity between the text and keywords. Example:

function(string1,compare1);  // returns: 4
function(string1,compare2);  // returns: 5 (more relevant)

Please note 5 and 4 are just for example.

You could say - write a function that counts occurrences - but for this example this would not work because both got 2 occurrences, but compare1 is less relevant because "hello evening" isn't exactly found in string1 (the 2 words hello and evening are more distant than hello hi)

are there any known-algorithm to do this?

ADD1:

algos like Edit Distance in this case would NOT work. Because string1 is a complete text (like 300-400 words) and the comparing strings are max 4-5 word.

Answer 1

I think there is a pretty good and complete answer to this question here http://answers.google.com/answers/threadview?id=337832

Sorry its on google answers!