fix: mod of two negative bigints

AUTHORS

@@ -287,15 +287,15 @@ Dheemanth D <165369664+Dheemanth07@users.noreply.github.com>
 Adriano Fantini <adrfantini@users.noreply.github.com>
 Chibuike Anselm <chibuikeanselm@Chibuikes-MacBook-Pro.local>
 Jos de Jong <email@josdejong.com>
+Yarchik <spoko.dev@gmail.com>
 rrbharath <ryanrbharath@gmail.com>
 Lê Vạn Bảo Trọng <32262183+chuanghiduoc@users.noreply.github.com>
 Hugohong258 <ugfifkgfufyu@gmail.com>
-Spoko Dev <spoko.dev@gmail.com>
 Yash Goyal <24110399@iitgn.ac.in>
 Wei-Lun Li <b84330808@gmail.com>
-waw666waw666 <184799977+waw666waw666@users.noreply.github.com>
 wulimammoth <ctelestial@gmail.com>
 kevin <5299031+kevin9327@users.noreply.github.com>
 WenJing95 <126436871+WenJing95@users.noreply.github.com>
+W <184799977+waw666waw666@users.noreply.github.com>
 
 # Generated by tools/update-authors.js

HISTORY.md

@@ -2,6 +2,8 @@
 
 # unpublished changes since 15.2.0
 
+- Fix: Adjust #3710 to ensure that mod of bigints ends up in intended
+  range for all sign combinations (#3684). Thanks @spokodev.
 - Fix: #3758 correct gcd and atan2 when computed on arguments including
   sparse matrices (#3751). Thanks @waw666waw666.
 - Fix: #3160, #3202 correct TypeScript types for various cases of matrix

src/function/arithmetic/mod.js

@@ -127,8 +127,8 @@ export const createMod = /* #__PURE__ */ factory(name, dependencies, ({ typed, c
     }
 
     const m = x % y
-
-    if (x < 0 || y < 0) return m === 0n ? m : m + y
-    return m
+    // Ensure result m is always in desired range: 0 ≤ m < y for y positive,
+    // y < m ≤ 0 for y negative:
+    return m !== 0n && (m < 0n) !== (y < 0n) ? m + y : m
   }
 })

test/unit-tests/function/arithmetic/invmod.test.js

@@ -42,6 +42,8 @@ describe('invmod', function () {
   it('should calculate invmod for bigints', function () {
     assert.strictEqual(invmod(5n, 13n), 8n)
     assert.strictEqual(invmod(6n, -17n), -14n)
+    assert.strictEqual(invmod(2n, -7n), -3n)
+    assert.strictEqual(invmod(5n, -13n), -5n)
   })
 
   it('should calculate invmod for mixed BigNumbers and Numbers', function () {

test/unit-tests/function/arithmetic/mod.test.js

@@ -48,6 +48,15 @@ describe('mod', function () {
     assert.strictEqual(mod(-8n, 4n), 0n)
   })
 
+  it('should calculate the modulus of bigints for a negative divisor', function () {
+    assert.strictEqual(mod(10n, -4n), -2n)
+    assert.strictEqual(mod(-10n, -4n), -2n)
+    assert.strictEqual(mod(8n, -3n), -1n)
+    assert.strictEqual(mod(-8n, -3n), -2n)
+    assert.strictEqual(mod(8n, -4n), 0n)
+    assert.strictEqual(mod(-8n, -4n), 0n)
+  })
+
   it('should handle precise approximation of float approximation', function () {
     approxEqual(mod(0.1, 0.01), 0)
     approxEqual(mod(0.15, 0.05), 0)
@@ -111,6 +120,8 @@ describe('mod', function () {
     assert.strictEqual(mod(14, 8n), 6n)
     assert.strictEqual(mod(12.5, 37n), 12.5)
     assert.strictEqual(mod(14n, 8.5), 5.5)
+    assert.strictEqual(mod(-10, -4n), -2n)
+    assert.strictEqual(mod(-10n, -4), -2n)
   })
 
   it('should calculate the modulus of mixed booleans and bignumbers', function () {