From b7358c84932514736dab937eb45c2d24051dcd0d Mon Sep 17 00:00:00 2001
From: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Date: Fri, 10 Jul 2026 11:11:36 -0700
Subject: [PATCH] fix: restore BigNumber precision state when decimal.js trig
 throws precision-limit errors

Fixes #3676
---
 src/type/bignumber/BigNumber.js               | 31 +++++++++++++++++++
 .../function/trigonometry/tan.test.js         | 11 +++++++
 .../type/bignumber/BigNumber.test.js          | 28 +++++++++++++++++
 3 files changed, 70 insertions(+)

diff --git a/src/type/bignumber/BigNumber.js b/src/type/bignumber/BigNumber.js
index 9b424f97ac..38ceaabf22 100644
--- a/src/type/bignumber/BigNumber.js
+++ b/src/type/bignumber/BigNumber.js
@@ -3,11 +3,42 @@ import { factory } from '../../utils/factory.js'
 
 const name = 'BigNumber'
 const dependencies = ['?on', 'config']
+const trigonometricMethods = [
+  'acos', 'acosh', 'asin', 'asinh', 'atan', 'atanh',
+  'cos', 'cosh', 'sin', 'sinh', 'tan', 'tanh',
+  'cosine', 'hyperbolicCosine', 'hyperbolicSine', 'hyperbolicTangent',
+  'inverseCosine', 'inverseHyperbolicCosine', 'inverseHyperbolicSine',
+  'inverseHyperbolicTangent', 'inverseSine', 'inverseTangent', 'sine', 'tangent'
+]
 
 export const createBigNumberClass = /* #__PURE__ */ factory(name, dependencies, ({ on, config }) => {
   const BigNumber = Decimal.clone({ precision: config.precision, modulo: Decimal.EUCLID })
   BigNumber.prototype = Object.create(BigNumber.prototype)
 
+  trigonometricMethods.forEach(method => {
+    const originalMethod = BigNumber.prototype[method]
+
+    BigNumber.prototype[method] = function (...args) {
+      const precision = BigNumber.precision
+      const rounding = BigNumber.rounding
+
+      try {
+        return originalMethod.apply(this, args)
+      } catch (error) {
+        if (error instanceof Error &&
+          error.message.includes('Precision limit exceeded') &&
+          !error.message.includes('decimal.js trigonometric functions are limited')) {
+          error.message += '; decimal.js trigonometric functions are limited to approximately ' +
+            `1000 digits of working precision (configured precision is ${config.precision})`
+        }
+        throw error
+      } finally {
+        BigNumber.precision = precision
+        BigNumber.rounding = rounding
+      }
+    }
+  })
+
   /**
    * Attach type information
    */
diff --git a/test/unit-tests/function/trigonometry/tan.test.js b/test/unit-tests/function/trigonometry/tan.test.js
index 90b6e6c13a..10d97601d7 100644
--- a/test/unit-tests/function/trigonometry/tan.test.js
+++ b/test/unit-tests/function/trigonometry/tan.test.js
@@ -41,6 +41,17 @@ describe('tan', function () {
     assert.deepStrictEqual(bigTan(bigPi.div(4)).toString(), '0.999999999999999999999')
   })
 
+  it('should restore BigNumber precision after a decimal.js precision error', function () {
+    const highPrecisionMath = math.create({ number: 'BigNumber', precision: 509 })
+
+    assert.throws(
+      () => highPrecisionMath.evaluate('tan(pi/2)'),
+      /decimal\.js trigonometric functions are limited/
+    )
+    assert.strictEqual(highPrecisionMath.BigNumber.precision, 509)
+    assert.strictEqual(highPrecisionMath.evaluate('1.1 + 2.2').toString(), '3.3')
+  })
+
   it('should return the tangent of a complex number', function () {
     const re = 0.00376402564150425
     const im = 1.00323862735360980
diff --git a/test/unit-tests/type/bignumber/BigNumber.test.js b/test/unit-tests/type/bignumber/BigNumber.test.js
index a6dcd03eec..23efb8c88c 100644
--- a/test/unit-tests/type/bignumber/BigNumber.test.js
+++ b/test/unit-tests/type/bignumber/BigNumber.test.js
@@ -32,4 +32,32 @@ describe('BigNumber', function () {
 
     assert.strictEqual(a.toJSON(), '2')
   })
+
+  it('should restore precision and rounding when a trigonometric method throws', function () {
+    const bigmath = math.create({ number: 'BigNumber', precision: 509 })
+    const BigNumber = bigmath.BigNumber
+
+    assert.throws(
+      () => bigmath.pi.div(2).tangent(),
+      error => {
+        assert.ok(error instanceof Error)
+        assert.match(error.message, /decimal\.js trigonometric functions are limited/)
+        assert.match(error.message, /configured precision is 509/)
+        return true
+      }
+    )
+    assert.strictEqual(BigNumber.precision, 509)
+    assert.strictEqual(BigNumber.rounding, Decimal.ROUND_HALF_UP)
+
+    bigmath.config({ precision: 100 })
+
+    const UnwrappedBigNumber = Decimal.clone({ precision: 100, modulo: Decimal.EUCLID })
+    const value = bigmath.pi.div(2)
+    const expected = new UnwrappedBigNumber(value.toString()).tan()
+
+    assert.strictEqual(BigNumber.precision, 100)
+    assert.strictEqual(bigmath.tan(value).toString(), expected.toString())
+
+    bigmath.config({ precision: 509 })
+  })
 })