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test/lines.test.js

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+'use strict';
+
+/**
+ * Tests for splitLines in src/lines.js.
+ *
+ * Run with:  node --test
+ * (or)       node --test test/lines.test.js
+ *
+ * The helper must count lines the way editors and `wc -l` do: a trailing
+ * newline terminates the final line rather than starting an extra empty one.
+ */
+
+import test from 'node:test';
+import assert from 'node:assert/strict';
+
+import { splitLines } from '../src/lines.js';
+
+test('splitLines: empty file has no lines', () => {
+    assert.deepEqual(splitLines(''), []);
+});
+
+test('splitLines: no trailing newline is unchanged', () => {
+    assert.deepEqual(splitLines('a\nb\nc'), ['a', 'b', 'c']);
+});
+
+test('splitLines: trailing newline does not add a phantom line', () => {
+    // The reported bug: this used to yield 4 elements (['a','b','c','']).
+    assert.deepEqual(splitLines('a\nb\nc\n'), ['a', 'b', 'c']);
+    assert.equal(splitLines('a\nb\nc\n').length, 3);
+});
+
+test('splitLines: terminated and non-terminated agree on line count', () => {
+    assert.equal(splitLines('a\nb\nc\n').length, splitLines('a\nb\nc').length);
+});
+
+test('splitLines: a real trailing blank line is preserved', () => {
+    assert.deepEqual(splitLines('a\n\n'), ['a', '']);
+    assert.equal(splitLines('a\n\n').length, 2);
+});
+
+test('splitLines: a lone newline is one empty line', () => {
+    assert.deepEqual(splitLines('\n'), ['']);
+    assert.equal(splitLines('\n').length, 1);
+});
+
+test('splitLines: multiple trailing newlines keep the real blank lines', () => {
+    assert.deepEqual(splitLines('a\n\n\n'), ['a', '', '']);
+    assert.equal(splitLines('a\n\n\n').length, 3);
+});