feat: command service

    - process business logic: command evalutaion
    - unit test command evaluation
    - setup repo interface

backend/package.json

@@ -16,6 +16,7 @@
         "@types/express": "^5.0.0",
         "@types/node": "^22.10.7",
         "express": "^4.21.2",
+        "mathjs": "^14.0.1",
         "pino": "^9.6.0",
         "socket.io": "^4.8.1"
     },

backend/src/command-service.spec.ts

@@ -0,0 +1,44 @@
+import CommandService from './command-service';
+import { pino } from 'pino';
+
+const mockRepository = {
+    saveCommand: jest.fn(),
+    getLatest: jest.fn(),
+};
+
+describe('evaluate', () => {
+    describe.each([
+        { command: '', hasError: true },
+        { command: ' ', hasError: true },
+        { command: '1', expected: '1' },
+        { command: '1+', hasError: true },
+        { command: '2 + + 1', hasError: true },
+
+        { command: '1 + 1', expected: '2' },
+        { command: '123 * 23 + 45 - 6', expected: '2868' },
+        { command: '10 * 5 / 2', expected: '25' },
+        { command: '10.5 * 5 / 2.5', expected: '21' },
+
+        { command: '1a2b3c', hasError: true },
+        { command: '1 + 2 = 3', hasError: true },
+        { command: '(123 * 23) + 45 - 6', hasError: true },
+
+        { command: '1 + 1', expected: '2' },
+        { command: '123 * 23 + 45 - 6', expected: '2868' },
+        { command: '10 * 5 / 2', expected: '25' },
+        { command: '10.54 * 7 / 3', expected: '24.593333333333334' },
+        { command: '5 * 7 / 3 * 3', expected: '35' },
+
+    ])('.evaluate($command)', ({ command, hasError, expected }) => {
+        test(`returns ${hasError}`, () => {
+            const clientId = 'whatever';
+            const commandService = new CommandService(pino(), mockRepository);
+            if (hasError) {
+                expect(() => commandService.evaluateAndSave(clientId, command)).toThrow('Invalid command');
+            } else {
+                expect(commandService.evaluateAndSave(clientId, command)).toBe(expected);
+                expect(mockRepository.saveCommand).toHaveBeenCalledWith(clientId, command, expected);
+            }
+        });
+    });
+});

backend/src/command-service.ts

@@ -0,0 +1,67 @@
+import { Logger } from "pino";
+import { evaluate as evaluateMathjs } from 'mathjs';
+import { CommandAndResult } from "./entities/command-result.entity";
+
+interface IRepository {
+    saveCommand(clientId: string, command: string, result: string): void;
+    getLatest(clientId: string, count: number): CommandAndResult[];
+}
+
+const historyCount = 10;
+
+class CommandService {
+    private operators = new Set(['+', '-', '*', '/']);
+    private repository: IRepository;
+    private logger: Logger;
+    constructor(logger: Logger, repository: IRepository) {
+        this.logger = logger;
+        this.repository = repository;
+    }
+
+    // evaluateAndSave evaluates a mathematical expression and saves the result to the repository
+    public evaluateAndSave(clientId: string, expression: string): string {
+        const result = this.evaluate(expression);
+        this.repository.saveCommand(clientId, expression, result);
+        return result;
+    }
+
+    // getHistory returns the latest 10 commands for a client
+    public getHistory(clientId: string): CommandAndResult[] {
+        return this.repository.getLatest(clientId, historyCount);
+    }
+
+    // evaluate evaluate a mathematical expression
+    private evaluate(expression: string): string {
+        if (!this.isValidCommand(expression)) {
+            throw new Error('Invalid command');
+        }
+        return evaluateMathjs(expression).toString();
+    }
+
+    // isValidCommand checks if the command is valid
+    private isValidCommand(s: string): boolean {
+        s = s.replace(/ /g,''); // remove all spaces
+        if (s.length === 0) {
+            return false;
+        }
+        // allowed characters: 0-9, +, -, *, /, .
+        const regex = /^[\d+\-*/.]+$/;
+        if (!regex.test(s)) {
+            return false;
+        }
+        // operators cannot be adjacent to each other
+        for (let i = 0; i < s.length - 1; i++) {
+            if (this.operators.has(s[i]) && this.operators.has(s[i + 1])) {
+                return false;
+            }
+        }
+
+        // operators cannot be at the beginning or end of the string
+        if (this.operators.has(s[0]) || this.operators.has(s[s.length - 1])) {
+            return false;
+        }
+        return true;
+    }
+}
+
+export default CommandService;

backend/src/entities/command-result.entity.ts

@@ -0,0 +1,5 @@
+// CommandAndResult is a round trip of a command and its result
+export type CommandAndResult = {
+    expression: string;
+    result: string;
+}