Display which users a choice conflicts with on the finalisation screen

Previously, if a conflicting choice was made, the two offending rows
would turn BRIGHT RED, but no other feedback was visible -- since the
other row would frequently be scrolled off the screen, this is not
exactly the most helpful way the application could tell the user that
something is wrong ("you did something wrong, but I'm not going to tell
you what it is").

Now, the name of the user(s) with which a choice conflicts is/are
displayed on the right-hand side of the table. That space is reserved
even if there is currently no conflict, since having the table jump
around underneath you if you choose a conflicting project is even worse
than the existing behaviour!

See #14.

src/components/choice_editor.js

@@ -19,7 +19,7 @@ along with this program. If not, see <https://www.gnu.org/licenses/>.
 */
 
 
-import React, {Component} from 'react';
+import React, {Component, Fragment} from 'react';
 import { connect } from 'react-redux';
 import {DropdownButton, MenuItem} from 'react-bootstrap';
 import ClassNames from 'classnames';
@@ -75,30 +75,37 @@ class ChoiceEditor extends Component {
         this.props.onClick(studentID, {type, id: parseInt(id, 10)});
     }
 
-
+    // Produces an object mapping user IDs to lists of conflicting
+    // users. If a user's choice does not conflict, it will not be
+    // present in the keys of the returned object.
     invalidChoices() {
-        return Object.keys(this.props.students).filter(userID => {
+        return Object.keys(this.props.students).reduce((obj, userID) => {
             const userOption = this.props.choices[userID];
             if (!userOption) {
-                return false;
+                return obj;
             }
             if (userOption.type === "user") {
-                return false;
+                return obj;
             }
-            return Object.keys(this.props.students).some(otherUserID => {
+            const conflictingUsers = Object.keys(this.props.students).map(otherUserID => {
                 if (otherUserID === userID) {
-                    return false;
+                    return null;
                 }
                 const otherUserOption = this.props.choices[otherUserID];
                 if (!otherUserOption) {
-                    return false;
+                    return null;
                 }
                 if (otherUserOption.type === "user") {
-                    return false;
+                    return null;
                 }
-                return userOption.id === otherUserOption.id;
-            });
-        });
+                if (userOption.id !== otherUserOption.id) {
+                    return null;
+                }
+                return otherUserID;
+            }).filter(x => x != null);
+            obj[userID] = conflictingUsers;
+            return obj;
+        }, {});
     }
 
     renderSaveButtons(submitDisabled, saveDisabled) {
@@ -182,14 +189,15 @@ class ChoiceEditor extends Component {
             const [id, userAll] = kv;
             const user = userAll.data;
             const projectIDs = this.getUserChoices(user);
+            const conflicts = invalidUsers[id] || [];
 
             return (
-                <div className={ClassNames("row", "striped", {"invalid": invalidUsers.includes(id)})} key={id}>
-                    <div className="col-xs-3">{user.name}</div>
+                <div className={ClassNames("row", "striped", {"invalid": conflicts.length})} key={id}>
+                    <div className="col-xs-3 col-lg-2">{user.name}</div>
                     {this.props.showPriority && (
                         <div className="col-xs-1">{user.priority}</div>
                     )}
-                    <div className="col-xs-8">
+                    <div className="col-xs-5 col-lg-6">
                         <ol>
                             {projectIDs.map((projectID, i) => {
                                 const title = projectID != null ? this.getProjectTitle(projectID) : "(No choice)";
@@ -216,16 +224,25 @@ class ChoiceEditor extends Component {
                         </ol>
                        {showButtons && this.renderDropdowns(id)}
                     </div>
+                    <div className="col-xs-3">
+                        {
+                            conflicts.length ?
+                                <Fragment>Choice conflicts with {conflicts.map(uid =>
+                                    this.props.users[uid].data.name
+                                ).join(", ")}</Fragment>
+                                : null
+                        }
+                    </div>
                 </div>
             );
         });
     }
 
     render() {
         const showButtons = Boolean(this.props.choices);
-        const invalidUsers = showButtons? this.invalidChoices(): [];
+        const invalidUsers = showButtons? this.invalidChoices(): {};
         return (
-            <div className="container">            
+            <div className="container">
                 <div className="row">
                     <div className="col-xs-3">Student</div>
                     {this.props.showPriority && (
@@ -236,8 +253,8 @@ class ChoiceEditor extends Component {
                 {this.renderStudentChoices(invalidUsers)}
 
                 {showButtons && this.renderSaveButtons(
-                    invalidUsers.length || Object.keys(this.props.choices).length < Object.keys(this.props.students).length,
-                    Boolean(invalidUsers.length)
+                    Object.keys(invalidUsers).length || Object.keys(this.props.choices).length < Object.keys(this.props.students).length,
+                    Boolean(Object.keys(invalidUsers).length)
                 )}
             </div>
         );