Fix the position of the end of a file

.changeset/offense-end-at-eof.md

@@ -0,0 +1,5 @@
+---
+'@shopify/theme-check-common': patch
+---
+
+Fix the line and character of an offense that ends at the end of a file. They were one character short, or on the previous line when the file ends with a newline.

packages/theme-check-common/src/utils/position.spec.ts

@@ -0,0 +1,25 @@
+import { describe, expect, it } from 'vitest';
+import { getPosition } from './position';
+
+describe('Unit: getPosition', () => {
+  it('returns the line and character of an index', () => {
+    expect(getPosition('a\nbc', 3)).toEqual({ index: 3, line: 1, character: 1 });
+  });
+
+  it('returns the position of the end of the file', () => {
+    expect(getPosition('a\nbc', 4)).toEqual({ index: 4, line: 1, character: 2 });
+    expect(getPosition('a\n', 2)).toEqual({ index: 2, line: 1, character: 0 });
+    expect(getPosition('', 0)).toEqual({ index: 0, line: 0, character: 0 });
+    expect(getPosition('a\nbc', 9)).toEqual({ index: 9, line: 1, character: 2 });
+  });
+
+  it('ends a line at the \\n of a Windows line break', () => {
+    expect(getPosition('a\r\nbc', 2)).toEqual({ index: 2, line: 0, character: 2 });
+    expect(getPosition('a\r\nbc', 4)).toEqual({ index: 4, line: 1, character: 1 });
+    expect(getPosition('a\r\n', 3)).toEqual({ index: 3, line: 1, character: 0 });
+  });
+
+  it('returns -1 for a negative index', () => {
+    expect(getPosition('a\nbc', -1)).toEqual({ index: -1, line: -1, character: -1 });
+  });
+});

packages/theme-check-common/src/utils/position.ts

@@ -1,14 +1,23 @@
 import lineColumn from 'line-column';
 import { Position } from '../types';
 
+/**
+ * The line and character of an index, both 0-based. An index past the end of
+ * the source is at its end, and a negative one, like the end of an unclosed
+ * block, has line and character -1.
+ */
 export function getPosition(source: string, index: number): Position {
-  const lineCol = lineColumn(source, { origin: 0 }).fromIndex(Math.min(index, source.length - 1));
-
-  return {
-    index,
-    line: lineCol ? lineCol.line : -1,
-    character: lineCol ? lineCol.col : -1,
-  };
+  if (index < 0) return { index, line: -1, character: -1 };
+  // Not lineColumn's fromIndex, which has no position for the end of the file,
+  // and splits the whole source on every call.
+  const end = Math.min(index, source.length);
+  let line = 0;
+  let lineStart = 0;
+  for (let i = source.indexOf('\n'); i !== -1 && i < end; i = source.indexOf('\n', i + 1)) {
+    line++;
+    lineStart = i + 1;
+  }
+  return { index, line, character: end - lineStart };
 }
 
 export function getOffset(source: string, line: number, column: number): number {