From 0538594bc22d3d1be6114f6a0416f00bfd4f049a Mon Sep 17 00:00:00 2001
From: Arthur Neuman <arthurbneuman@gmail.com>
Date: Thu, 13 Nov 2025 14:17:54 -0500
Subject: [PATCH] Support d/dx syntax

---
 .../dictionary/definitions-arithmetic.ts      | 113 ++++++++++++++++++
 test/compute-engine/calculus.test.ts          |  22 ++++
 2 files changed, 135 insertions(+)

diff --git a/src/compute-engine/latex-syntax/dictionary/definitions-arithmetic.ts b/src/compute-engine/latex-syntax/dictionary/definitions-arithmetic.ts
index 9e1cc477..a41af2ad 100755
--- a/src/compute-engine/latex-syntax/dictionary/definitions-arithmetic.ts
+++ b/src/compute-engine/latex-syntax/dictionary/definitions-arithmetic.ts
@@ -424,6 +424,119 @@ function parseFraction(parser: Parser): Expression | null {
     return ['PartialDerivative', fn, ...vars, degree === null ? 1 : degree];
   }
 
+  // Handle ordinary (Leibniz) derivative notation: \frac{d}{dx} f
+  // Accept forms like: `\frac{d}{dx} f`, `\frac{\mathrm{d}}{dx} f`,
+  // and grouped tokens like `\frac{d}{dx}` where denom is a Sequence ['d', 'x']
+  // or Multiply where one operand is a differential operator.
+  try {
+    const symNumer = symbol(numer);
+    const isDifferentialNumer =
+      symNumer === 'd' ||
+      symNumer === 'd_upright' ||
+      symNumer === 'differentialD';
+
+    if (!isDifferentialNumer) {
+      // Also detect sequences like ['Sequence', 'd'] or ['Sequence', ...]
+      if (operator(numer) === 'Sequence') {
+        const ops = operands(numer);
+        for (const op of ops) {
+          const s = symbol(op);
+          if (s === 'd' || s === 'd_upright' || s === 'differentialD') {
+            // consider it a differential numerator
+            // mark isDifferentialNumer true by setting symNumer
+            // (we don't reassign const, so just proceed)
+            // fallback to treat as differential
+            // break out and handle below
+            // We'll treat existence of any 'd' token as a match
+            // by setting isDifferentialNumer via side-effect variable isDifferentialNumer
+          }
+        }
+      }
+    }
+
+    // If the numerator is a plain 'd' differential operator:
+    if (isDifferentialNumer || operator(numer) === 'Sequence') {
+      // Extract variable(s) from the denominator. Typical forms:
+      // - 'dx' -> Sequence ['d','x']
+      // - ['Sequence','d','x']
+      // - ['Multiply', 'd', 'x']
+      const vars: Expression[] = [];
+      const collectVar = (expr: Expression | null) => {
+        if (!expr) return;
+        const s = symbol(expr);
+        if (s && s !== 'd' && s !== 'd_upright' && s !== 'differentialD') {
+          vars.push(expr);
+          return;
+        }
+        // If it's a sequence/multiply, inspect operands
+        const h = operator(expr);
+        if (h === 'Sequence' || h === 'Multiply' || h === 'InvisibleOperator') {
+          for (const op of operands(expr)) collectVar(op);
+        }
+      };
+
+      collectVar(denom);
+
+      // If we didn't find vars in denom, try denom as a single symbol token like 'x' or 'dx' where 'dx' may be parsed as a single symbol whose last char is the variable
+      if (vars.length === 0) {
+        const s = symbol(denom);
+        if (s && s.length > 1 && s[0] === 'd') {
+          // take the remainder as variable name, e.g. 'dx' -> 'x'
+          vars.push(s.slice(1));
+        }
+      }
+
+      if (vars.length > 0) {
+        // If numerator contained an explicit function (rare), reuse it, otherwise parse the following expression as the function to differentiate
+        let fnExpr = operand(numer, 1);
+        if (fnExpr === null || fnExpr === undefined)
+          fnExpr = missingIfEmpty(parser.parseExpression());
+
+        // If multiple vars, return a list, otherwise single var
+        const varExpr: Expression | null =
+          vars.length > 1 ? ['List', ...vars] : vars[0];
+        return ['D', fnExpr, varExpr];
+      }
+    }
+  } catch (e) {
+    // Defensive: any parsing error should fall through to default Divide
+  }
+
+  // Handle Leibniz notation for ordinary derivatives using 'd', e.g.
+  // \frac{d}{dx} f(x)  -> D(f(x), x)
+  // \frac{d^2}{dx^2} f(x) -> D(f(x), x, x)  (higher orders handled by caller)
+  const numerSym = symbol(numer);
+  if (numerSym === 'd' || numerSym === 'd_upright') {
+    // Attempt to extract the variable(s) from the denominator.
+    const vars: Expression[] = [];
+
+    if (operator(denom) === 'Multiply' || operator(denom) === 'Sequence') {
+      const args = operands(denom);
+      for (let i = 0; i < args.length; i++) {
+        const a = args[i];
+        const s = symbol(a);
+        if (s === 'd' || s === 'd_upright') {
+          const v = args[i + 1];
+          if (v) vars.push(v);
+        }
+      }
+    } else {
+      // denom could be a single symbol like 'dx'
+      const denomSym = symbol(denom);
+      if (denomSym && denomSym.length > 1 && denomSym[0] === 'd') {
+        vars.push(denomSym.slice(1));
+      }
+    }
+
+    if (vars.length > 0) {
+      let fn = operand(numer, 1);
+      if (fn === null || fn === undefined)
+        fn = missingIfEmpty(parser.parseExpression());
+      if (vars.length > 1) vars.unshift('List');
+      return ['D', fn, ...vars];
+    }
+  }
+
   return ['Divide', numer, denom];
 }
 
diff --git a/test/compute-engine/calculus.test.ts b/test/compute-engine/calculus.test.ts
index f28dafcf..2db731ef 100644
--- a/test/compute-engine/calculus.test.ts
+++ b/test/compute-engine/calculus.test.ts
@@ -9,6 +9,28 @@ function N(expr: string): number {
   return result.re;
 }
 
+describe('DERIVATION', () => {
+  test('basic derivative', () =>
+    expect(evaluate('\\frac{d}{dx} x^2')).toMatchInlineSnapshot(
+      `2x`
+    ));
+
+  test('partial derivative', () =>
+    expect(evaluate('\\frac{d}{dx} tx^2')).toMatchInlineSnapshot(
+      `2t * x`
+    ));
+
+  test('to constant', () =>
+    expect(evaluate('\\frac{d}{dx} 3x')).toMatchInlineSnapshot(
+      `3`
+    ));
+
+  test('no variable', () =>
+    expect(evaluate('\\frac{d}{dx} 3t')).toMatchInlineSnapshot(
+      `0`
+    ));
+});
+
 describe('INDEFINITE INTEGRATION', () => {
   test('basic integration', () =>
     expect(evaluate('\\int x^2 dx')).toMatchInlineSnapshot(`1/3 * x^3`));