fix #168

Author: arnog

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 ### Bug Fixes
 
+- **([#168](https://github.com/cortex-js/compute-engine/issues/168))
+  Absolute Value**: Fixed parsing of nested absolute value expressions that
+  start with a double bar (e.g. `||3-5|-4|`), which previously produced an
+  invalid structure instead of evaluating correctly.
+
 - **([#244](https://github.com/cortex-js/compute-engine/issues/244))
   Serialization**: Fixed LaTeX and ASCIIMath serialization ambiguity for
   negative bases and negated powers. Powers now render `(-2)^2` (instead of

src/compute-engine/latex-syntax/parse.ts

@@ -1405,7 +1405,30 @@ export class _Parser implements Parser {
       this.skipSpace();
       let body = this.parseExpression();
       this.skipSpace();
-      if (!this.matchBoundary()) {
+      const boundary = this._boundaries[this._boundaries.length - 1]?.tokens;
+      const matchedBoundary = this.matchBoundary();
+      const sameTrigger =
+        (typeof def.openTrigger === 'string' &&
+          typeof def.closeTrigger === 'string' &&
+          def.openTrigger === def.closeTrigger) ||
+        (Array.isArray(def.openTrigger) &&
+          Array.isArray(def.closeTrigger) &&
+          def.openTrigger.length === def.closeTrigger.length &&
+          def.openTrigger.every((tok, i) => tok === def.closeTrigger[i]));
+      if (matchedBoundary && isEmptySequence(body) && sameTrigger && boundary) {
+        // If the open/close delimiter are identical and the body is empty,
+        // we may have consumed an inner delimiter (e.g. "||3-5|-4|").
+        // Retry parsing without the boundary and look for the closing delimiter.
+        this.index = bodyStart;
+        this.skipSpace();
+        body = this.parseExpression();
+        this.skipSpace();
+        if (!this.matchAll(boundary)) {
+          this.index = start;
+          if (!this.atEnd) continue;
+          return null;
+        }
+      } else if (!matchedBoundary) {
         // We couldn't parse the body up to the closing delimiter.
         // This could be a case where the boundary of the enclosure is
         // ambiguous, i.e. `|(a+|b|+c)|`. Attempt to parse without the boundary

test/compute-engine/latex-syntax/matchfix.test.ts

@@ -110,6 +110,12 @@ describe('MATCHFIX abs and norm', () => {
       simplify  = |a| + 3
     `));
 
+  test('||3-5|-4|', () => {
+    const expr = engine.parse('||3-5|-4|');
+    expect(expr.isValid).toBe(true);
+    expect(expr.evaluate().toString()).toBe('2');
+  });
+
   test('||a||', () =>
     expect(check('||a||')).toMatchInlineSnapshot(`["Norm", "a"]`));
   test('||a||+|b|', () =>