# Search Implementation

Tachi's search implementation uses MongoDB's $text index. This breaks a query into words
and compares each of them to the provided text fields.

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## __textScore

For our code, we mutate the documents we want to return with a special field: `__textScore`.

This field declares how 'close' the provided query was to the $text fields in this document.

This is sometimes exposed in the API for sorting reasons.

!!! bug
	MongoDB's $text matching algorithm isn't great for fuzzy matches - It doesn't
	like song titles like 'A', as it thinks 'a' is an article, and doesn't match it properly as
	a word.

!!! info
	Why not regex for fuzzy matches?
	
	Regex has performance issues on larger datasets and we
	want to avoid it. Most regexes cannot use indexes, and therefore invoke a COLLSCAN, which
	we want to avoid.

## User Searching

Searching users, on the other hand, has to use regex-based
searching.

The `$text` method attempts to break things up based on their
words, but that doesn't help with usernames, as they are all
too frequently `XxX_One_Long_Str1ng_xXx`.

Instead, we use a case insensitive regex - similar to SQL's
`LIKE`.

This means we do not have a `__textScore` property for this
search to sort on. Instead, we just constrict returns to
around 15, and have the user whittle their search down
better.