diff --git a/back/package.json b/back/package.json
index d4bf6349..1ea43f30 100644
--- a/back/package.json
+++ b/back/package.json
@@ -8,7 +8,8 @@
     "dev": "ts-node-dev --respawn --transpileOnly ./server.ts",
     "prod": "tsc && node ./dist/server.js",
     "test": "ts-node node_modules/jasmine/bin/jasmine --config=jasmine.json",
-    "lint": "node_modules/.bin/eslint src/ . --ext .ts"
+    "lint": "node_modules/.bin/eslint src/ . --ext .ts",
+    "fix": "node_modules/.bin/eslint src/ . --ext .ts"
   },
   "repository": {
     "type": "git",
diff --git a/back/position-test.js b/back/position-test.js
new file mode 100644
index 00000000..01fb5cdf
--- /dev/null
+++ b/back/position-test.js
@@ -0,0 +1,148 @@
+// Constants
+let MIN_DISTANCE = 12;
+let MAX_PER_GROUP = 3;
+let NB_USERS = 10;
+
+// Utils
+let rand = function(min, max) {
+    min = Math.ceil(min);
+    max = Math.floor(max);
+    return Math.floor(Math.random() * (max - min + 1)) + min;
+};
+
+let compareDistances = function(distA, distB) {
+    if (distA.distance < distB.distance) {
+        return -1;
+    }
+    if (distA.distance > distB.distance) {
+        return 1;
+    }
+    return 0;
+};
+
+let computeDistance = function (user1, user2) {
+    return Math.sqrt(Math.pow(user2.X - user1.X, 2) + Math.pow(user2.Y - user1.Y, 2));
+};
+
+// Test Data
+let users = [];
+for(let i = 1; i <= NB_USERS; i++) {
+    let user = {};
+    user.id = rand(0,99999);
+    user.X = rand(0, 40);
+    user.Y = rand(0, 40);
+    users.push(user);
+}
+
+// Compute distance between each user
+let getDistanceOfEachUser = function(users) {
+    let i = 0;
+    let distances = [];
+
+    users.forEach(function(user1, key1) {
+        users.forEach(function(user2, key2) {
+            if(key1 < key2) {
+                let distanceObj = {};
+                distanceObj.distance = computeDistance(user1, user2);
+                distanceObj.first = user1;
+                distanceObj.second = user2;
+
+                distances[i] = distanceObj;
+                i++;
+            }
+        });
+    });
+
+    return distances;
+};
+
+// Organise groups
+let createGroups = function(distances) {
+    let i = 0;
+    let groups = [];
+    let alreadyInAGroup = [];
+
+    for(let j = 0; j < distances.length; j++) {
+        let dist = distances[j];
+
+        if(dist.distance <= MIN_DISTANCE) {
+            if(typeof groups[i] === 'undefined') {
+                groups[i] = [];
+            }
+
+            if(groups[i].indexOf(dist.first) === -1 && typeof alreadyInAGroup[dist.first.id] === 'undefined') {
+                if(groups[i].length > 1) {
+                    // if group is not empty we check current user can be added in the group according to its distance to the others already in it
+                    for(let l = 0; l < groups[i].length; l++) {
+                        let userTotest = groups[i][l];
+                        if(computeDistance(dist.first, userTotest) <= MIN_DISTANCE) {
+                            groups[i].push(dist.first);
+                            alreadyInAGroup[dist.first.id] = true;
+                            break;
+                        }
+                    }
+                } else {
+                    groups[i].push(dist.first);
+                    alreadyInAGroup[dist.first.id] = true;
+                }
+            }
+
+            if(groups[i].length === MAX_PER_GROUP) {
+                i++; // on créé un nouveau groupe
+                if(i > (NB_USERS / MAX_PER_GROUP)) {
+                    console.log('There is no room left for user ID : ' + dist.second.id + ' !');
+                    break;
+                }
+                continue;
+            }
+
+            if(groups[i].indexOf(dist.second) === -1 && typeof alreadyInAGroup[dist.second.id] === 'undefined') {
+                if(groups[i].length > 1) {
+                    // if group is not empty we check current user can be added in the group according to its distance to the others already in it
+                    for(let l = 0; l < groups[i].length; l++) {
+                        let userTotest = groups[i][l];
+                        if(computeDistance(dist.second, userTotest) <= MIN_DISTANCE) {
+                            groups[i].push(dist.second);
+                            alreadyInAGroup[dist.second.id] = true;
+                            break;
+                        }
+                    }
+                } else {
+                    groups[i].push(dist.second);
+                    alreadyInAGroup[dist.second.id] = true;
+                }
+            }
+        }
+    }
+
+    return groups;
+};
+
+let distances = getDistanceOfEachUser(users);
+
+// ordonner par distance pour prioriser l'association en groupe des utilisateurs les plus proches
+distances.sort(compareDistances);
+
+let groups = createGroups(distances);
+
+// Compute distance between each user of a already existing group
+let checkGroupDistance = function(groups) {
+    for(let i = 0; i < groups.length; i++) {
+        let group = groups[i];
+            group.forEach(function(user1, key1) {
+                group.forEach(function(user2, key2) {
+                    if(key1 < key2) {
+                        let distance = computeDistance(user1, user2);
+                        if(distance > MIN_DISTANCE) {
+                            // TODO : message a user1 et user2
+                        }
+                    }
+            });
+        });
+    }
+};
+
+console.log(users);
+console.log(distances);
+console.log(groups);
+
diff --git a/back/src/Controller/IoSocketController.ts b/back/src/Controller/IoSocketController.ts
index 1fc114a4..acd5843f 100644
--- a/back/src/Controller/IoSocketController.ts
+++ b/back/src/Controller/IoSocketController.ts
@@ -87,7 +87,8 @@ export class IoSocketController{
     //Hydrate and manage error
     hydrateMessageReceive(message : string) : MessageUserPosition | Error{
         try {
-            return new MessageUserPosition(message);
+            let data = JSON.parse(message);
+            return new MessageUserPosition(data);
         }catch (err) {
             //TODO log error
             return new Error(err);
diff --git a/back/src/Model/Distance.ts b/back/src/Model/Distance.ts
new file mode 100644
index 00000000..150c144f
--- /dev/null
+++ b/back/src/Model/Distance.ts
@@ -0,0 +1,7 @@
+import {MessageUserPosition} from "../Model/Websocket/MessageUserPosition";
+
+export interface Distance {
+    distance: number,
+    first: MessageUserPosition,
+    second: MessageUserPosition,
+}
\ No newline at end of file
diff --git a/back/src/Model/Group.ts b/back/src/Model/Group.ts
new file mode 100644
index 00000000..795c0e8e
--- /dev/null
+++ b/back/src/Model/Group.ts
@@ -0,0 +1,73 @@
+import {MessageUserPosition} from "./Websocket/MessageUserPosition";
+import { World } from "./World";
+import { UserInterface } from "./UserInterface";
+
+export class Group {
+    static readonly MAX_PER_GROUP = 4;
+
+    private users: UserInterface[];
+    private connectCallback: (user1: string, user2: string) => void;
+    private disconnectCallback: (user1: string, user2: string) => void;
+
+
+    constructor(users: UserInterface[], connectCallback: (user1: string, user2: string) => void, disconnectCallback: (user1: string, user2: string) => void) {
+        this.users = [];
+        this.connectCallback = connectCallback;
+        this.disconnectCallback = disconnectCallback;
+
+        users.forEach((user: UserInterface) => {
+            this.join(user);
+        });
+    }
+
+    getUsers(): UserInterface[] {
+        return this.users;
+    }
+
+    isFull(): boolean {
+        return this.users.length >= Group.MAX_PER_GROUP;
+    }
+
+    join(user: UserInterface): void
+    {
+        // Broadcast on the right event
+        this.users.forEach((groupUser: UserInterface) => {
+            this.connectCallback(user.id, groupUser.id);
+        });
+        this.users.push(user);
+        user.group = this;
+    }
+
+    isPartOfGroup(user: UserInterface): boolean
+    {
+        return this.users.indexOf(user) !== -1;
+    }
+
+    isStillIn(user: UserInterface): boolean
+    {
+        if(!this.isPartOfGroup(user)) {
+            return false;
+        }
+        let stillIn = true;
+        for(let i = 0; i <= this.users.length; i++) {
+            let userInGroup = this.users[i];
+            let distance = World.computeDistance(user, userInGroup);
+            if(distance > World.MIN_DISTANCE) {
+                stillIn = false;
+                break;
+            }
+        }
+        return stillIn;
+    }
+
+    removeFromGroup(users: UserInterface[]): void
+    {
+        for(let i = 0; i < users.length; i++){
+            let user = users[i];
+            const index = this.users.indexOf(user, 0);
+            if (index > -1) {
+                this.users.splice(index, 1);
+            }
+        }
+    }
+}
diff --git a/back/src/Model/UserInterface.ts b/back/src/Model/UserInterface.ts
new file mode 100644
index 00000000..743f8b4d
--- /dev/null
+++ b/back/src/Model/UserInterface.ts
@@ -0,0 +1,8 @@
+import { Group } from "./Group";
+import { PointInterface } from "./Websocket/PointInterface";
+
+export interface UserInterface {
+    id: string,
+    group?: Group,
+    position: PointInterface
+}
\ No newline at end of file
diff --git a/back/src/Model/Websocket/Message.ts b/back/src/Model/Websocket/Message.ts
index 8e6f2c9a..d726968f 100644
--- a/back/src/Model/Websocket/Message.ts
+++ b/back/src/Model/Websocket/Message.ts
@@ -2,8 +2,7 @@ export class Message {
     userId: string;
     roomId: string;
 
-    constructor(message: string) {
-        let data = JSON.parse(message);
+    constructor(data: any) {
         if(!data.userId || !data.roomId){
             throw Error("userId or roomId cannot be null");
         }
diff --git a/back/src/Model/Websocket/MessageUserPosition.ts b/back/src/Model/Websocket/MessageUserPosition.ts
index 117c99cb..a3161b69 100644
--- a/back/src/Model/Websocket/MessageUserPosition.ts
+++ b/back/src/Model/Websocket/MessageUserPosition.ts
@@ -27,9 +27,8 @@ export class Point implements PointInterface{
 export class MessageUserPosition extends Message{
     position: PointInterface;
 
-    constructor(message: string) {
-        super(message);
-        let data = JSON.parse(message);
+    constructor(data: any) {
+        super(data);
         this.position = new Point(data.position.x, data.position.y, data.position.direction);
     }
 
@@ -42,4 +41,4 @@ export class MessageUserPosition extends Message{
                 })
         );
     }
-}
\ No newline at end of file
+}
diff --git a/back/src/Model/World.ts b/back/src/Model/World.ts
new file mode 100644
index 00000000..804a176b
--- /dev/null
+++ b/back/src/Model/World.ts
@@ -0,0 +1,210 @@
+import {MessageUserPosition, Point} from "./Websocket/MessageUserPosition";
+import {PointInterface} from "./Websocket/PointInterface";
+import {Group} from "./Group";
+import {Distance} from "./Distance";
+import {UserInterface} from "./UserInterface";
+
+export class World {
+    static readonly MIN_DISTANCE = 160;
+
+    // Users, sorted by ID
+    private users: Map<string, UserInterface>;
+    private groups: Group[];
+
+    private connectCallback: (user1: string, user2: string) => void;
+    private disconnectCallback: (user1: string, user2: string) => void;
+
+    constructor(connectCallback: (user1: string, user2: string) => void, disconnectCallback: (user1: string, user2: string) => void) 
+    {
+        this.users = new Map<string, UserInterface>();
+        this.groups = [];
+        this.connectCallback = connectCallback;
+        this.disconnectCallback = disconnectCallback;
+    }    
+
+    public join(userPosition: MessageUserPosition): void {
+        this.users.set(userPosition.userId, {
+            id: userPosition.userId,
+            position: userPosition.position
+        });
+    }
+
+    public updatePosition(userPosition: MessageUserPosition): void {
+        let context = this;
+        let user = this.users.get(userPosition.userId);
+        if(typeof user === 'undefined') {
+            return;
+        }
+
+        user.position.x = userPosition.position.x;
+        user.position.y = userPosition.position.y;
+
+        if (typeof user.group === 'undefined') {
+            // If the user is not part of a group:
+            //  should he join a group?
+            let closestUser: UserInterface|null = this.searchClosestAvailableUser(user);
+
+            if (closestUser !== null) {
+                // Is the closest user part of a group?
+                if (typeof closestUser.group === 'undefined') {
+                    let group: Group = new Group([
+                        user,
+                        closestUser
+                    ], this.connectCallback, this.disconnectCallback);
+                } else {
+                    closestUser.group.join(user);
+                }
+            }
+            
+        }
+       // TODO : vérifier qu'ils ne sont pas déja dans un groupe plein 
+    }
+
+    /**
+     * Looks for the closest user that is:
+     * - close enough (distance <= MIN_DISTANCE)
+     * - not in a group OR in a group that is not full
+     */
+    private searchClosestAvailableUser(user: UserInterface): UserInterface|null
+    {
+/*
+        let sortedUsersByDistance: UserInteface[] = Array.from(this.users.values()).sort((user1: UserInteface, user2: UserInteface): number => {
+            let distance1 = World.computeDistance(user, user1);
+            let distance2 = World.computeDistance(user, user2);
+            return distance1 - distance2;
+        });
+
+        // The first element should be the current user (distance 0). Let's remove it.
+        if (sortedUsersByDistance[0] === user) {
+            sortedUsersByDistance.shift();
+        }
+
+        for(let i = 0; i < sortedUsersByDistance.length; i++) {
+            let currentUser = sortedUsersByDistance[i];
+            let distance = World.computeDistance(currentUser, user);
+            if(distance > World.MIN_DISTANCE) {
+                return;
+            }
+        }
+*/
+        let usersToBeGroupedWith: Distance[] = [];
+        let minimumDistanceFound: number = World.MIN_DISTANCE;
+        let matchingUser: UserInterface | null = null;
+        this.users.forEach(function(currentUser, userId) {
+            if(currentUser === user) {
+                return;
+            }
+
+            let distance = World.computeDistance(user, currentUser); // compute distance between peers.
+            
+            if(distance <= minimumDistanceFound) {
+
+                if (typeof currentUser.group === 'undefined' || !currentUser.group.isFull()) {
+                    // We found a user we can bind to.
+                    minimumDistanceFound = distance;
+                    matchingUser = currentUser;
+                    return;
+                }
+            /*
+                if(context.groups.length > 0) {
+                    
+                    context.groups.forEach(group => {
+                        if(group.isPartOfGroup(userPosition)) { // Is the user in a group ?
+                            if(group.isStillIn(userPosition)) { // Is the user leaving the group ? (is the user at more than max distance of each player)
+                                
+                                // Should we split the group? (is each player reachable from the current player?)
+                                // This is needed if
+                                //         A <==> B <==> C <===> D
+                                // becomes A <==> B <=====> C <> D
+                                // If C moves right, the distance between B and C is too great and we must form 2 groups
+
+                            }
+                        } else {
+                            // If the user is in no group
+                            // Is there someone in a group close enough and with room in the group ?
+                        }
+                    });
+
+                } else {
+                    // Aucun groupe n'existe donc je stock les users assez proches de moi
+                    let dist: Distance = {
+                        distance: distance,
+                        first: userPosition,
+                        second: user // TODO: convertir en messageUserPosition
+                    }
+                    usersToBeGroupedWith.push(dist);
+                }
+            */
+            }
+        
+        }, this.users);
+
+        return matchingUser;
+    }
+
+    public static computeDistance(user1: UserInterface, user2: UserInterface): number
+    {
+        return Math.sqrt(Math.pow(user2.position.x - user1.position.x, 2) + Math.pow(user2.position.y - user1.position.y, 2));
+    }
+
+    /*getDistancesBetweenGroupUsers(group: Group): Distance[]
+    {
+        let i = 0;
+        let users = group.getUsers();
+        let distances: Distance[] = [];
+        users.forEach(function(user1, key1) {
+            users.forEach(function(user2, key2) {
+                if(key1 < key2) {
+                    distances[i] = {
+                        distance: World.computeDistance(user1, user2),
+                        first: user1,
+                        second: user2
+                    };
+                    i++;
+                }
+            });
+        });
+        
+        distances.sort(World.compareDistances);
+
+        return distances;
+    }
+
+    filterGroup(distances: Distance[], group: Group): void
+    {
+        let users = group.getUsers();
+        let usersToRemove = false;
+        let groupTmp: MessageUserPosition[] = [];
+        distances.forEach(dist => {
+            if(dist.distance <= World.MIN_DISTANCE) {
+                let users = [dist.first];
+                let usersbis = [dist.second]
+                groupTmp.push(dist.first);
+                groupTmp.push(dist.second);
+            } else {
+                usersToRemove = true;
+            }
+        });
+
+        if(usersToRemove) {
+            // Detecte le ou les users qui se sont fait sortir du groupe
+            let difference = users.filter(x => !groupTmp.includes(x));
+
+            // TODO : Notify users un difference that they have left the group 
+        }
+
+        let newgroup = new Group(groupTmp);
+        this.groups.push(newgroup);
+    }
+
+    private static compareDistances(distA: Distance, distB: Distance): number
+    {
+        if (distA.distance < distB.distance) {
+            return -1;
+        }
+        if (distA.distance > distB.distance) {
+            return 1;
+        }
+        return 0;
+    }*/
+}
\ No newline at end of file
diff --git a/back/tests/MessageTest.ts b/back/tests/MessageTest.ts
index 69d57fce..070ddf57 100644
--- a/back/tests/MessageTest.ts
+++ b/back/tests/MessageTest.ts
@@ -3,27 +3,27 @@ import {Message} from "../src/Model/Websocket/Message";
 
 describe("Message Model", () => {
     it("should find userId and roomId", () => {
-        let message = JSON.stringify({userId: "test1", roomId: "test2"});
+        let message = {userId: "test1", roomId: "test2"};
         let messageObject = new Message(message);
         expect(messageObject.userId).toBe("test1");
         expect(messageObject.roomId).toBe("test2");
     })
 
     it("should expose a toJson method", () => {
-        let message = JSON.stringify({userId: "test1", roomId: "test2"});
+        let message = {userId: "test1", roomId: "test2"};
         let messageObject = new Message(message);
         expect(messageObject.toJson()).toEqual({userId: "test1", roomId: "test2"});
     })
 
     it("should find throw error when no userId", () => {
-        let message = JSON.stringify({roomId: "test2"});
+        let message = {roomId: "test2"};
         expect(() => {
             let messageObject = new Message(message);
         }).toThrow(new Error("userId or roomId cannot be null"));
     })
 
     it("should find throw error when no roomId", () => {
-        let message = JSON.stringify({userId: "test1"});
+        let message = {userId: "test1"};
         expect(() => {
             let messageObject = new Message(message);
         }).toThrow(new Error("userId or roomId cannot be null"));
diff --git a/back/tests/WorldTest.ts b/back/tests/WorldTest.ts
new file mode 100644
index 00000000..1f5affc8
--- /dev/null
+++ b/back/tests/WorldTest.ts
@@ -0,0 +1,103 @@
+import "jasmine";
+import {Message} from "../src/Model/Websocket/Message";
+import {World} from "../src/Model/World";
+import {MessageUserPosition, Point} from "../src/Model/Websocket/MessageUserPosition";
+import { Group } from "../src/Model/Group";
+import {Distance} from "../src/Model//Distance";
+
+describe("World", () => {
+    it("should connect user1 and user2", () => {
+        let connectCalled: boolean = false;
+        let connect = (user1: string, user2: string): void => {
+            connectCalled = true;
+        }
+        let disconnect = (user1: string, user2: string): void => {
+            
+        }
+
+        let world = new World(connect, disconnect);
+
+        world.join(new MessageUserPosition({
+            userId: "foo",
+            roomId: 1,
+            position: new Point(100, 100)
+        }));
+
+        world.join(new MessageUserPosition({
+            userId: "bar",
+            roomId: 1,
+            position: new Point(500, 100)
+        }));
+
+        world.updatePosition(new MessageUserPosition({
+            userId: "bar",
+            roomId: 1,
+            position: new Point(261, 100)
+        }));
+
+        expect(connectCalled).toBe(false);
+
+        world.updatePosition(new MessageUserPosition({
+            userId: "bar",
+            roomId: 1,
+            position: new Point(101, 100)
+        }));
+
+        expect(connectCalled).toBe(true);
+
+        connectCalled = false;
+        world.updatePosition(new MessageUserPosition({
+            userId: "bar",
+            roomId: 1,
+            position: new Point(102, 100)
+        }));
+        expect(connectCalled).toBe(false);
+    });
+    /** 
+    it('Should return the distances between all users', () => {
+        let connectCalled: boolean = false;
+        let connect = (user1: string, user2: string): void => {
+            connectCalled = true;
+        }
+        let disconnect = (user1: string, user2: string): void => {
+            
+        }
+
+        let world = new World(connect, disconnect);
+        let user1 = new MessageUserPosition({
+            userId: "foo",
+            roomId: 1,
+            position: new Point(100, 100)
+        });
+
+        world.join(user1);
+
+        let user2 = new MessageUserPosition({
+            userId: "bar",
+            roomId: 1,
+            position: new Point(500, 100)
+        });
+        world.join(user2);
+
+        let user3 = new MessageUserPosition({
+            userId: "baz",
+            roomId: 1,
+            position: new Point(101, 100)
+        });
+
+        let user4 = new MessageUserPosition({
+            userId: "buz",
+            roomId: 1,
+            position: new Point(105, 100)
+        })
+
+        let group = new Group([user1, user2, user3, user4]);
+
+        let distances = world.getDistancesBetweenGroupUsers(group)
+
+        console.log(distances);
+
+        //expect(distances).toBe([]);
+    })
+    **/
+})
\ No newline at end of file