fix: math allow negative sign at biginning

backend/README.md

@@ -18,7 +18,9 @@ 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.
+- `negative sign` is only allowed at the beginning of an expression but not after multiplication or division
+    - e.g. `-5*3 + 1` will return `-14`
+    - 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

backend/src/math/evaluate.spec.ts

@@ -29,7 +29,10 @@ describe('evaluate', () => {
         { command: '10.5 * 5 / 2.5', expected: '21' },
         { command: '.5 + 1', expected: '1.5' },
 
-
+        // negative sign at the beginning
+        { command: '-1', expected: '-1' },
+        { command: '-1 + 1', expected: '0' },
+        { command: '-5*3 + 1', expected: '-14' },
         // Zero cases
         { command: '0 * 5', expected: '0' },
         { command: '0 / 1', expected: '0' },

backend/src/math/evaluate.ts

@@ -18,9 +18,8 @@ function isValidCommand(s: string): boolean {
             return false;
         }
     }
-
-    // operators cannot be at the beginning or end of the string
-    if (operators.has(s[0]) || operators.has(s[s.length - 1])) {
+    // operators cannot be at the end of the string
+    if (operators.has(s[s.length - 1])) {
         return false;
     }
     return true;
@@ -50,6 +49,10 @@ const parseExpressionMDs = (s: string): ExpressionMD[] => {
     const expressions: ExpressionMD[] = [];
     let currExp = "";
     let prevOp = true;
+    if (s[0] === '-') {
+        prevOp = false;
+        s = s.slice(1);
+    }
     for (const token of s.split('')) {
         if (token === '+') {
             expressions.push(new ExpressionMD(prevOp, currExp));