Add UTF-8 BOM support for package.json files

src/lib.rs

@@ -20,7 +20,12 @@ pub fn sort_package_json_with_options(
     input: &str,
     options: &SortOptions,
 ) -> Result<String, serde_json::Error> {
-    let value: Value = serde_json::from_str(input)?;
+    // Check for UTF-8 BOM and strip it for parsing
+    const BOM: char = '\u{FEFF}';
+    let input_without_bom = input.strip_prefix(BOM).unwrap_or(input);
+    let has_bom = input_without_bom.len() != input.len();
+
+    let value: Value = serde_json::from_str(input_without_bom)?;
 
     let sorted_value = if let Value::Object(obj) = value {
         Value::Object(sort_object_keys(obj, options))
@@ -36,7 +41,15 @@ pub fn sort_package_json_with_options(
         serde_json::to_string(&sorted_value)?
     };
 
-    Ok(result)
+    // Preserve BOM if it was present in the input
+    if has_bom {
+        let mut output = String::with_capacity(BOM.len_utf8() + result.len());
+        output.push(BOM);
+        output.push_str(&result);
+        Ok(output)
+    } else {
+        Ok(result)
+    }
 }
 
 /// Sorts a package.json string with default options (pretty-printed)

tests/fixtures/package-bom.json

@@ -0,0 +1,8 @@
+{
+  "version": "1.0.0",
+  "exports": {
+    ".": "./index.mjs"
+  },
+  "name": "@vitejs/test-utf8-bom-package",
+  "private": true
+}

tests/integration_test.rs

@@ -68,3 +68,31 @@ fn test_size_limit_preservation() {
     let ignore_array = ignore.as_array().unwrap();
     assert_eq!(ignore_array.len(), 3, "ignore should have 3 elements");
 }
+
+#[test]
+fn test_utf8_bom_preservation() {
+    // Test case based on https://github.com/vitejs/vite/blob/main/playground/resolve/utf8-bom-package/package.json
+    const BOM: char = '\u{FEFF}';
+
+    // Test 1: Files with BOM preserve it
+    let input =
+        fs::read_to_string("tests/fixtures/package-bom.json").expect("Failed to read BOM fixture");
+    assert!(input.starts_with(BOM), "Fixture should have BOM");
+
+    let result = sort(&input);
+    assert!(result.starts_with(BOM), "BOM should be preserved in output");
+
+    let json_without_bom = &result[BOM.len_utf8()..];
+    let parsed: Value =
+        serde_json::from_str(json_without_bom).expect("Output should be valid JSON after BOM");
+    assert_eq!(parsed.get("name").and_then(|v| v.as_str()), Some("@vitejs/test-utf8-bom-package"));
+
+    // Test 2: Files without BOM don't get BOM added
+    let input_no_bom = r#"{"version": "1.0.0", "name": "test"}"#;
+    let result_no_bom = sort(input_no_bom);
+    assert!(!result_no_bom.starts_with(BOM), "BOM should not be added if not present");
+
+    // Test 3: Idempotency - sorting twice produces same result
+    let second_sort = sort(&result);
+    assert_eq!(result, second_sort, "Sorting BOM files should be idempotent");
+}