C code to index large text library and find similar

I need a mini-app (Compiled C on Linux) that groups similar sentences together. I have 100,000 sentences (say in a PostgresSQL DB, Unicode text). It must perform VERY fast - by indexing each root-word to a 16bit integer (which would reduce its memory foot print), then re-creating a new data structure with sentence delimeters and sentence length. Group into buckets of similar sentence length. Then iterate through doing word-by-word comparisons (16bit comparisons). Two algos are acceptable:- 1. Simple - Take a source sentence and iterate through XORing word by word (irrespective of word order or word frequency). If there are more than x words outstanding - then it is NOT a similar sentence. X in this case would be 25% of the number of total words. We leave such large gap so that we don't need to worry about word roots. From the smaller data set - we then proceed to do a classic levenstechn comparison - but with an upper bound of x deviation - meaning after it detects more than say 10% deviation - it exists that comparison. Here it is a character by character comparison. The app should communicate with a folder of .gz files that contain the text and it could use a text boundary to distinguish each sentence. The output would need to be a new text file that sorts every sentence into groups of similarity - separated by a text boundary. I need something in 36 hours. A mediocre algorithm is fine.