Description

Description:

New $round function + precision argument for $trunc function

Engineering Ticket Description:

Inclusion of an $int or $round operator, which can reliably group numbers which are semantically the same is thus desired.

Example:
In attempting to perform bucketing in an aggregation pipeline, where the buckets are defined as "each Xth of range Y", these functions are needed.

bucket = int( (x - min) / (max - min) * bucket_count )
// This relies on the int() function removing any real part of the number.

// At a semantic level, the following is the same:
bucket = (x - min - mod(x - min, (max - min) * bucket_count) / ((max - min) / bucket_count) 

Since this is all floating point math it is subject to errors such as 1.000000001 vs 0.999999999, and therefor less accurate.

Scope of changes

• Update $trunc with new syntax, noting original syntax is still valid
• Create $round  docs and link appropriately
• Crosslink between $trunc and $round for users on one page looking for functionality of the other
• update 4.2 release notes