Return Distinct Elements in a Field of a MongoDB Collection

I’ve been working with MongoDB, and I noticed that querying the unique values in a field of a given collection follows a non-idiomatic pattern. That is to say, it does not look like,

db.collection.find( { "Field": { $unique: 1 } } )

Rather, it looks like this,

db.runCommand( { distinct: <collection>, key: <field> } )

And this returns a document with a "values" and "stats" field, and hopefully an "ok":1 field. This is the documentation.

Using Node and Mongo to Collect Data from Reddit

I am taking a course on MongoDB development with Node.js from Mongo University. In the second week we covered a thing that I thought was very interesting. They walked you through how to grab the JSON data out of a Reddit page. Reddit apparently offers its data up as a JSON if you pass it a .json path. Here is the coffeescript that produces the code provided in the development course.

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Blum Mental Hash with Node and Angular

In this post I’ll walk through building a simple Blum Mental Hash application using Node and Angular. I’ll write my Node and Angular code using CoffeeScript, and I’ll build my HTML templates with Jade.

Writing the Application in CoffeeScript

I really enjoy CoffeeScript. I get that real programmers don’t use CoffeeScript, but I don’t care. Javascript makes me crazy. This is my CoffeeScript code–I require a bunch of stuff: express, http, jade. Then I define an express object named app and set some settings on it. Then I tell the root path, '/', to use the './static' subdirectory on the server. Then I tell any GET requests to '/' to return HTML that’s been rendered by jade using my index.jade template.

#!/usr/bin/env node

express = require "express"
http = require "http"
jade = require "jade"

app = express()

app.set( 'views', './static' )
app.set( 'view engine', 'jade' )
app.engine( 'jade', jade.__express )

app.use '/', express.static('./static')

app.get '/', ( req, res ) ->
    res.render( 'index.jade' )

server = app.listen 8000, ->
    host = server.address().address
    port = server.address().port
    console.log 'Example at http://%s%s', host, port

We can compile this guy at the command line by calling,

coffee -c

This will create an app.js file in your working directory.

The Controller

The file sits in the subdirectory ./static/js/. The first items, mapping and perm, should ideally be picked by the user. In this example I’ve hard coded them for convenience. The convert() function brings everything together; it combines mapping and perm in such a way as to hash a string of capital letters. From there myapp.controller can compute the hash for the user.

#!/usr/bin/env node
# ./static/js/

mapping =
    A: 4,   B: 9,   C: 4
    D: 5,   E: 8,   F: 5
    G: 7,   H: 1,   I: 8
    J: 3,   K: 6,   L: 8
    M: 9,   N: 0,   O: 9
    P: 0,   Q: 6,   R: 1
    S: 9,   T: 6,   U: 4
    V: 7,   W: 5,   X: 0
    Y: 9,   Z: 3

f = ( x, mapping ) ->
    Maps a string `x` to an array of digits
    a = []
    for i in x
        a.push( mapping[i] )
    return a

perm = [ 8, 2, 5, 4, 9, 0, 3, 7, 1, 6 ]

permute = ( i, perm ) ->
    Returns the next value in the permutation
    idx = perm.indexOf( i )
    idx = ( idx + 1 ) % 10
    return perm[ idx ]

g = ( a, perm ) ->
    Hashes a list of digits into a string of digits
    b = [ permute( ( a[0] + a[ a.length-1 ] ) % 10, perm ) ]
    for i in [1...a.length]
        b.push( permute( ( b[i-1] + a[i] ) % 10, perm ) )
    return b.join("")

convert = ( x ) ->
    Convert a string of characters to a hashed string of digits
    a = f( x, mapping )
    return g( a, perm )

myapp = angular.module( "myapp", [] )

myapp.controller "controller", ($scope) ->
    $scope.plain = ""
    $scope.b = ""
    $scope.update = ( plain ) ->
        $scope.b = convert( plain )

This CoffeeScript file should also be compiled into a Javascript file.

The View

The index.jade file lives in ./static. This prompts a user for a password, and then returns a hash of that password below. That’s it.

doctype html
    h1 Manuel Blum's Mental Hash (BMH)
      input(name="Password: ",type="text",ng-model="plain")
      button(ng-click="update(plain)") Convert
      p Hash: {{ b }}

We can serve the page by calling

node app.js

from the command line, and then navigating to localhost:5000.