Awesome
map-filter-reduce
A functional query engine, that operates over streams of js objects, and can be optimized via database indexes.
example
count the total number of mentions in ssb.
var mfr = require('map-filter-reduce')
pull(
ssbc.createLogStream(),
mfr([
//get all posts.
{$filter: { value: { content: { type: 'post' } } }},
//extract the mentions
{$map: {mentions: ['value', 'content', 'mentions', 'length']}},
//sum all the lengths
{$reduce: {$sum: true}}
])
pull.log()
)
This example uses a full scan, so it looks at every object in the database. But if we have indexes, that can be made much more efficient, see flumeview-query
processing streams
map-filter-reduce
provides three functional stream processing
operators. usually, generally, filter should be used first,
then map then reduce, but operators can be combined in any order,
and used any number of times.
filter
a filter removes items from an incoming stream.
basic values: strings, numbers, booleans
strings, numbers and booleans match exactly.
{$filter: 'okay'}
matches all strings that equal "okay"
$prefix
$prefix
operator matches strings that start with a given string.
{$filter: {$prefix: 'o'}}
this will match "okay"
"ok"
"oh no"
, etc.
$gt, $lt, $gte, $lte
match values that are greater than, less than, greater than or equal, or less than or equal. A greater operator and a lesser operator may be combined into one comparison.
{$filter: {$gte: 0, $lt: 1}}
will filter numbers from 0 up to but not including 1.
$eq, $ne, $not, $truthy, $is, $in, $type
basic operations:
{$eq: value}
test input is equal to value{$ne: value}
test input is not equal to value{$truthy: true}
test if input converts to true.{$not: true}
test if input converts to false.{$type: true}
returns thetypeof
the input. (useful in $map, but not $filter){$is: 'object'|'string'|'number'|'boolean'|'undefined'}
test if input is particular type, use instead of$type
in filters.{$in: [...]}
check if input is one of a set. input must be a primitive, not an object.
objects
an object is used to filter objects that have the same properties. an object is combined with other operators that apply to the keys.
{$filter: {name: {$prefix: 'bob'}, age: {$gt: 10}}}
select "bob", "bobby", and "bobalobadingdog", if their age is greater than 10.
arrays
arrays are used treated somewhat like strings, in that you can also
use the $prefix
and $gt, $lt, $gte, $lte
operators.
arrays are like objects, but their keys are compared in order from left to right.
{$filter: {$prefix: ['okay']}}
map
If the incoming stream has fields you are not interested in, or fields that you'd like to remap, you can map those objects.
lets say, we have a nested object incoming, but we just want to pluck off the a nested name field.
{$map: {name: ['value', 'name']}}
if the input was {key: k, value: {foo: blah, name: 'bob'}}
the output would just be {name: 'bob'}
.
key: strings, numbers
strings and numbers are used to get keys off the input.
{$map: 'value'}
would return the value property of the input, and drop the rest. use numbers (integers) if the input will be an array.
keys: objects
for a {} object, each value is considered a rule, and applied to the value of the incoming data. a value with the transformations applied will be returned.
the keys in the object do not need to be the same as in the input, since the values will map to which keys to operate on from the input.
{$map: {foo: 'bar', bar: 'foo'}}
transform the input stream into a stream with foo
and bar
keys switched,
and any other keys dropped.
path: arrays
arrays are used to lookup deep into an object. they are analogous to useing chaining dots in javascript.
{$map: ['value', 'name']}
get the value property, and then get the name property off that.
wildcard
to get everything use true
{$map: true}
say, we have an object with many keys, and we want to reduce that to just
timestamp
and then put everything under value
{$map: {timestamp: 'timestamp', value: true}}
reduce
reduce is used to aggregate many values into a representative value.
count
, sum
, min
, max
, are all reduces.
$count
count how many items are in the input
{$reduce: {$count: true}}
$sum
sum elements in the input. input elements must be numbers. sums can be applied to paths, or strings to get the values to sum.
add up all the values for value.age
{$reduce: {$sum: ['value', 'age']}}
$min, $max
get the minimum or maximum for a value.
{$reduce: {$min: ['value', 'age']}}
$collect
get all values an put them into an array.
get all values for foo
{$reduce: {$collect: 'foo'}}
this may produce a large value if there are many items in the input stream.
objects
to apply multiple reduce functions, put them in an object with key names.
{$reduce: {
youngest: {$min: ['value','age']},
oldest: {$max: ['value', 'age']}
}}
groups
To group, use the other reduce functions
along side path expressions, as used under $map
In the following query,
{$reduce: {
country: 'country',
population: {$count: true}
}}
count items per country. Will give a stream of items each with
a {country, population}
fields.
This would return a object, with the shape {<country>:{<city>: <population>}}
.
Note that, like in $map
arrays can be used to drill deep into
an object.
{$reduce: {name: ['value', 'name'], posts: {$count: true}}}
TODO: group by time ranges (day, month, week, year, etc)
$sort
you can also sort the output of a some paths:
{$sort: [['foo', 'bar'], ['baz']]}
the output is ordered by the value at the path foo.bar
, then if two items are equal
at that value, then they are ordered by baz
.
License
MIT