From a8f9ae096c7458a1304a80cf7aea119729cd6293 Mon Sep 17 00:00:00 2001
From: dajiaohuang <mikewushuwen@outlook.com>
Date: Fri, 18 Sep 2026 02:05:01 +0800
Subject: [PATCH] Match Erlang semantics for div/2 and round/1

Two functions in the erlang compatibility layer diverge from Erlang on
negative operands:

- div/2 used `/`, so div(-7, 2) returned -3.5 instead of -3. Erlang's
  div truncates toward zero.
- round/1 used Math.round, which rounds half values toward +Infinity, so
  round(-2.5) returned -2 instead of -3, and round(-0.5) returned -0
  instead of -1. Erlang rounds half values away from zero.

Both are reachable from compiled Elixir: div(-7, 2) and round(-2.5)
compile to Core.erlang.div and Core.erlang.round calls.

Also drop a leftover console.log in nodes/1 that printed the accumulator
on every call.

Add tests for div, rem, round and trunc covering negative operands.
---
 .../lib/core/erlang_compat/erlang.js          |  5 ++--
 .../tests/core/erlang_compat/erlang_spec.js   | 30 +++++++++++++++++++
 2 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/src/javascript/lib/core/erlang_compat/erlang.js b/src/javascript/lib/core/erlang_compat/erlang.js
index ddece09d..a72d3805 100644
--- a/src/javascript/lib/core/erlang_compat/erlang.js
+++ b/src/javascript/lib/core/erlang_compat/erlang.js
@@ -164,7 +164,7 @@ function equals(left, right) {
 }
 
 function div(left, right) {
-  return left / right;
+  return Math.trunc(left / right);
 }
 
 function not(x) {
@@ -357,7 +357,7 @@ function min(first, second) {
 }
 
 function round(number) {
-  return Math.round(number);
+  return Math.sign(number) * Math.round(Math.abs(number));
 }
 
 function tl(list) {
@@ -463,7 +463,6 @@ function nodes(arg = []) {
   for (const nodeType of nodeTypes) {
     if (nodeType === Symbol.for('this')) {
       nodesFound.push(Symbol.for('nonode@nohost'));
-      console.log(nodesFound);
     }
   }
   return nodesFound;
diff --git a/src/javascript/tests/core/erlang_compat/erlang_spec.js b/src/javascript/tests/core/erlang_compat/erlang_spec.js
index 6e41e6ea..bc2c243d 100644
--- a/src/javascript/tests/core/erlang_compat/erlang_spec.js
+++ b/src/javascript/tests/core/erlang_compat/erlang_spec.js
@@ -83,6 +83,36 @@ test('list_subtraction', (t) => {
   t.deepEqual(Core.erlang.list_subtraction([1, 2, 3], [1, 2]), [3]);
 });
 
+test('div', (t) => {
+  t.is(Core.erlang.div(6, 3), 2);
+  t.is(Core.erlang.div(7, 2), 3);
+  t.is(Core.erlang.div(-7, 2), -3);
+  t.is(Core.erlang.div(7, -2), -3);
+  t.is(Core.erlang.div(-7, -2), 3);
+});
+
+test('rem', (t) => {
+  t.is(Core.erlang.rem(7, 2), 1);
+  t.is(Core.erlang.rem(-7, 2), -1);
+  t.is(Core.erlang.rem(7, -2), 1);
+  t.is(Core.erlang.rem(-7, -2), -1);
+});
+
+test('round', (t) => {
+  t.is(Core.erlang.round(1.4), 1);
+  t.is(Core.erlang.round(2.5), 3);
+  t.is(Core.erlang.round(-2.5), -3);
+  t.is(Core.erlang.round(0.5), 1);
+  t.is(Core.erlang.round(-0.5), -1);
+  t.is(Core.erlang.round(4), 4);
+  t.is(Core.erlang.round(-4), -4);
+});
+
+test('trunc', (t) => {
+  t.is(Core.erlang.trunc(2.7), 2);
+  t.is(Core.erlang.trunc(-2.7), -2);
+});
+
 test('node', (t) => {
   t.deepEqual(Core.erlang.node(), Symbol.for('nonode@nohost'));
 });