Help to choose algorithm for computing difference between 2 texts?
algorithm shouldn't do deep semantic analyzis, only kind of word counting, word vectorization, substring search


For such tasks, test if built-in Elastic Search / SOLR models work well. These products have multiple models that can be tuned with configuration. With these models, you provide a document and find other documents that are "similar".

Some of the available algorithms are : 


Cosine Similarity 
Okapi_BM25
DFR
LM Jelinek Mercer 


Docs : 

https://www.elastic.co/guide/en/elasticsearch/reference/current/index-modules-similarity.html

https://lucene.apache.org/solr/guide/6_6/morelikethis.html