Fix SingleMapping.spanFor to use previous line

pkgs/source_maps/CHANGELOG.md

@@ -1,5 +1,9 @@
 ## 0.10.14-wip
 
+* Fix `SingleMapping.spanFor` to use the entry from a previous line as specified
+  by the sourcemap specification
+  (https://tc39.es/ecma426/#sec-GetOriginalPositions),
+
 ## 0.10.13
 
 * Require Dart 3.3

pkgs/source_maps/lib/parser.dart

@@ -500,31 +500,37 @@ class SingleMapping extends Mapping {
       StateError('Invalid entry in sourcemap, expected 1, 4, or 5'
           ' values, but got $seen.\ntargeturl: $targetUrl, line: $line');
 
-  /// Returns [TargetLineEntry] which includes the location in the target [line]
-  /// number. In particular, the resulting entry is the last entry whose line
-  /// number is lower or equal to [line].
-  TargetLineEntry? _findLine(int line) {
-    var index = binarySearch(lines, (e) => e.line > line);
-    return (index <= 0) ? null : lines[index - 1];
-  }
-
-  /// Returns [TargetEntry] which includes the location denoted by
-  /// [line], [column]. If [lineEntry] corresponds to [line], then this will be
-  /// the last entry whose column is lower or equal than [column]. If
-  /// [lineEntry] corresponds to a line prior to [line], then the result will be
-  /// the very last entry on that line.
-  TargetEntry? _findColumn(int line, int column, TargetLineEntry? lineEntry) {
-    if (lineEntry == null || lineEntry.entries.isEmpty) return null;
-    if (lineEntry.line != line) return lineEntry.entries.last;
-    var entries = lineEntry.entries;
-    var index = binarySearch(entries, (e) => e.column > column);
-    return (index <= 0) ? null : entries[index - 1];
+  /// Returns the last [TargetEntry] which includes the location denoted by
+  /// [line], [column].
+  ///
+  /// This corresponds to the computation of _last_ in [GetOriginalPositions][1]
+  /// in the sourcemap specification.
+  ///
+  ///  [1]: https://tc39.es/ecma426/#sec-GetOriginalPositions
+  TargetEntry? _findEntry(int line, int column) {
+    // To find the *last* TargetEntry, we scan backwards, starting from the
+    // first line after our target line, or the end of [lines].
+    var lineIndex = binarySearch(lines, (e) => e.line > line);
+    while (--lineIndex >= 0) {
+      final lineEntry = lines[lineIndex];
+      final entries = lineEntry.entries;
+      if (entries.isEmpty) continue;
+      // If we scan to a line before the target line, the last entry extends to
+      // cover our search location.
+      if (lineEntry.line != line) return entries.last;
+      final index = binarySearch(entries, (e) => e.column > column);
+      if (index > 0) return entries[index - 1];
+      // We get here when the line has entries, but they are all after the
+      // column. When this happens, the line and column correspond to the
+      // previous entry, usually the last entry at the previous `lineIndex`.
+    }
+    return null;
   }
 
   @override
   SourceMapSpan? spanFor(int line, int column,
       {Map<String, SourceFile>? files, String? uri}) {
-    var entry = _findColumn(line, column, _findLine(line));
+    final entry = _findEntry(line, column);
     if (entry == null) return null;
 
     var sourceUrlId = entry.sourceUrlId;

pkgs/source_maps/test/continued_region_test.dart

@@ -0,0 +1,110 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:source_maps/source_maps.dart';
+import 'package:source_span/source_span.dart';
+import 'package:test/test.dart';
+
+void main() {
+  /// This is a test for spans of the generated file that continue over several
+  /// lines.
+  ///
+  /// In a sourcemap, a span continues from the start encoded position until the
+  /// next position, regardless of whether the second position in on the same
+  /// line in the generated file or a subsequent line.
+  void testSpans(int lineA, int columnA, int lineB, int columnB) {
+    // Create a sourcemap describing a 'rectangular' generated file with three
+    // spans, each potentially over several lines: (1) an initial span that is
+    // unmapped, (2) a span that maps to file 'A', the span continuing until (3)
+    // a span that maps to file 'B'.
+    //
+    // We can describe the mapping by an 'image' of the generated file, where
+    // the positions marked as 'A' in the 'image' correspond to locations in the
+    // generated file that map to locations in source file 'A'. Lines and
+    // columns are zero-based.
+    //
+    //       0123456789
+    //    0: ----------
+    //    1: ----AAAAAA         lineA: 1, columnA: 4, i.e. locationA
+    //    2: AABBBBBBBB         lineB: 2, columnB: 2, i.e. locationB
+    //    3: BBBBBBBBBB
+    //
+    // Once we have the mapping, we probe every position in a 8x10 rectangle to
+    // validate that it maps to the intended original source file.
+
+    expect(isBefore(lineB, columnB, lineA, columnA), isFalse,
+        reason: 'Test valid only for ordered positions');
+
+    SourceLocation location(Uri? uri, int line, int column) {
+      final offset = line * 10 + column;
+      return SourceLocation(offset, sourceUrl: uri, line: line, column: column);
+    }
+
+    // Locations in the generated file.
+    final uriMap = Uri.parse('output.js.map');
+    final locationA = location(uriMap, lineA, columnA);
+    final locationB = location(uriMap, lineB, columnB);
+
+    // Original source locations.
+    final sourceA = location(Uri.parse('A'), 0, 0);
+    final sourceB = location(Uri.parse('B'), 0, 0);
+
+    final json = (SourceMapBuilder()
+          ..addLocation(sourceA, locationA, null)
+          ..addLocation(sourceB, locationB, null))
+        .build(uriMap.toString());
+
+    final mapping = parseJson(json);
+
+    // Validate by comparing 'images' of the generate file.
+    final expectedImage = StringBuffer();
+    final actualImage = StringBuffer();
+
+    for (var line = 0; line < 8; line++) {
+      for (var column = 0; column < 10; column++) {
+        final span = mapping.spanFor(line, column);
+        final expected = isBefore(line, column, lineA, columnA)
+            ? '-'
+            : isBefore(line, column, lineB, columnB)
+                ? 'A'
+                : 'B';
+        final actual = span?.start.sourceUrl?.path ?? '-'; // Unmapped -> '-'.
+
+        expectedImage.write(expected);
+        actualImage.write(actual);
+      }
+      expectedImage.writeln();
+      actualImage.writeln();
+    }
+    expect(actualImage.toString(), expectedImage.toString());
+  }
+
+  test('continued span, same position', () {
+    testSpans(2, 4, 2, 4);
+  });
+
+  test('continued span, same line', () {
+    testSpans(2, 4, 2, 7);
+  });
+
+  test('continued span, next line, earlier column', () {
+    testSpans(2, 4, 3, 2);
+  });
+
+  test('continued span, next line, later column', () {
+    testSpans(2, 4, 3, 6);
+  });
+
+  test('continued span, later line, earlier column', () {
+    testSpans(2, 4, 5, 2);
+  });
+
+  test('continued span, later line, later column', () {
+    testSpans(2, 4, 5, 6);
+  });
+}
+
+bool isBefore(int line1, int column1, int line2, int column2) {
+  return line1 < line2 || line1 == line2 && column1 < column2;
+}

pkgs/source_maps/test/refactor_test.dart

@@ -126,9 +126,16 @@ void main() {
         '  |      ^\n'
         "  '");
 
-    // Lines added have no mapping (they should inherit the last mapping),
-    // but the end of the edit region continues were we left off:
-    expect(_span(4, 1, map, file), isNull);
+    // Newly added lines had no additional mapping, so they inherit the last
+    // position on the previously mapped line. The end of the region continues
+    // where the previous mapping left off.
+    expect(
+        _span(4, 1, map, file),
+        'line 3, column 6: \n'
+        '  ,\n'
+        '3 | 01*3456789\n'
+        '  |      ^\n'
+        '  \'');
     expect(
         _span(4, 5, map, file),
         'line 3, column 8: \n'