feat: replace math.js with self developed algorithm

hhow09 · Jan 22, 2025 · 7f2c5bc · 7f2c5bc
1 parent 1cb3f49
commit 7f2c5bc
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Showing 8 changed files with 219 additions and 153 deletions.
diff --git a/backend/README.md b/backend/README.md
@@ -15,3 +15,10 @@ npm run start
 ```bash
 npm run test
 ```
+
+## Limitations on calculation
+- All whitespace is ignored, therefore `1 + 2 3` will consider as `1 + 23`
+- negative sign in expression is not supported: e.g. `5 * -3` will return error.
+- large number will be converted to exponential notation: e.g. `999999999999 * 999999999999` will return `9.99999999998e+23`
+    - default threshold of exponent is `20`
+    - ref: https://mikemcl.github.io/decimal.js/#precision
diff --git a/backend/jest.config.js b/backend/jest.config.js
@@ -8,6 +8,7 @@ module.exports = {
     transform: {
       "^.+.tsx?$": ["ts-jest",{}],
     },
+    testMatch: ['**/*.spec.ts'],
     setupFiles: ["dotenv/config"]
   };
 
diff --git a/backend/package-lock.json b/backend/package-lock.json
diff --git a/backend/package.json b/backend/package.json
@@ -15,9 +15,9 @@
         "@types/body-parser": "^1.19.5",
         "@types/express": "^5.0.0",
         "@types/node": "^22.10.7",
+        "decimal.js": "^10.4.3",
         "dotenv": "^16.4.7",
         "express": "^4.21.2",
-        "mathjs": "^14.0.1",
         "mongodb": "^6.12.0",
         "pino": "^9.6.0",
         "socket.io": "^4.8.1"

diff --git a/backend/src/command-service.ts b/backend/src/command-service.ts
@@ -1,15 +1,14 @@
 import { Logger } from "pino";
-import { evaluate as evaluateMathjs } from 'mathjs';
 import { CommandAndResult } from "./entities/command-result.entity";
 import { IRepository } from "./repositories";
+import { isValidCommand, evaluate } from "./math/evaluate";
 
 export interface ICommandService {
     evaluateAndSave(clientId: string, expression: string): Promise<string>;
     getHistory(clientId: string): Promise<CommandAndResult[]>;
 }
 
 class CommandService implements ICommandService {
-    private operators = new Set(['+', '-', '*', '/']);
     private repository: IRepository;
     private logger: Logger;
     constructor(logger: Logger, repository: IRepository) {
@@ -31,35 +30,10 @@ class CommandService implements ICommandService {
 
     // evaluate evaluate a mathematical expression
     private evaluate(expression: string): string {
-        if (!this.isValidCommand(expression)) {
+        if (!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;
+        return evaluate(expression);
     }
 }
 

diff --git a/backend/src/command-service.spec.ts → backend/src/math/evaluate.spec.ts b/backend/src/command-service.spec.ts → backend/src/math/evaluate.spec.ts
@@ -1,10 +1,4 @@
-import CommandService from './command-service';
-import { pino } from 'pino';
-
-const mockRepository = {
-    saveCommand: jest.fn(),
-    getLatest: jest.fn(),
-};
+import { evaluate } from './evaluate';
 
 type TestCase = {
     command: string;
@@ -13,39 +7,51 @@ type TestCase = {
 };
 
 describe('evaluate', () => {
-    const logger = pino();
     describe.each<TestCase>([
         { command: '', hasError: true },
         { command: ' ', hasError: true },
-        { command: '1', expected: '1' },
         { command: '1+', hasError: true },
+        { command: '1 */ 2', 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 },
 
+        // // does not support negative numbers in expression
+        { command: '5 * -3', hasError: true },
+
+        // // basic
+        { command: '1', expected: '1' },
         { command: '1 + 1', expected: '2' },
         { command: '123 * 23 + 45 - 6', expected: '2868' },
+        { command: '1 + 259 - 68*77', expected: '-4976' },
         { command: '10 * 5 / 2', expected: '25' },
-        { command: '10.54 * 7 / 3', expected: '24.593333333333334' },
+        { command: '10.5 * 5 / 2.5', expected: '21' },
+        { command: '.5 + 1', expected: '1.5' },
+
+
+        // Zero cases
+        { command: '0 * 5', expected: '0' },
+        { command: '0 / 1', expected: '0' },
+        { command: '1 / 0', expected: 'Infinity' },
+        { command: '0 / 0', expected: 'NaN' },
+
+        // decimal default precision is 20.
+        // ref: https://mikemcl.github.io/decimal.js/#precision
+        { command: '10.54 * 7 / 3', expected: '24.593333333333333333' }, 
         { command: '5 * 7 / 3 * 3', expected: '35' },
+        { command: '1 + 2 * 3 / 4', expected: '2.5' },
+
+        // test edge cases
+        { command: '999999999999 * 999999999999', expected: '9.99999999998e+23' },
 
     ])('.evaluate($command)', ({ command, hasError, expected }) => {
-        test(`returns ${hasError}`, async () => {
-            const clientId = 'whatever';
-            const commandService = new CommandService(logger, mockRepository);
+        test(`returns ${hasError}`, () => {
             if (hasError) {
-                await expect(commandService.evaluateAndSave(clientId, command)).rejects.toThrow('Invalid command');
+                expect(() => evaluate(command)).toThrow('Invalid command');
             } else {
-                const res = await commandService.evaluateAndSave(clientId, command);
+                const res = evaluate(command);
                 expect(res).toBe(expected);
-                expect(mockRepository.saveCommand).toHaveBeenCalledWith(clientId, command, expected);
             }
         });
     });