Thanks! I enjoyed reading your blog posts about building your code search engine. One minor point of clarification, we do not use a positional ngram index, which as you note blows up the index size. Instead, we use the covering sparse ngrams to produce candidate documents and then search the content.

An early version of Blackbird experimented with trigrams plus a bitmask of the next character, but it didn't work well because it wasn't selective enough. This is mentioned in the blog post:

We tried a number of strategies to fix this like adding follow masks, which use bitmasks for the character following the trigram (basically halfway to quad grams), but they saturate too quickly to be useful.