Make pretty print O(max size)

pkgs/checks/CHANGELOG.md

@@ -1,6 +1,7 @@
 ## 0.3.2-wip
 
 - Require Dart 3.7
+- Improve speed of pretty printing for large collections.
 
 ## 0.3.1
 
@@ -13,7 +14,7 @@
     combined functionality in `containsInOrder`.
 -   Replace `pairwiseComparesTo` with `pairwiseMatches`.
 -   Fix a bug where printing the result of a failed deep quality check would
-    fail with a `TypeError` when comparing large `Map` instances 
+    fail with a `TypeError` when comparing large `Map` instances.
 -   Increase SDK constraint to ^3.5.0.
 -   Clarify this package is experimental.
 

pkgs/checks/lib/src/describe.dart

@@ -42,24 +42,23 @@ Iterable<String> _prettyPrint(
       open = '(';
       close = ')';
     }
-    final elements = object.map(prettyPrintNested).toList();
+    final elements = object.map(prettyPrintNested);
     return _prettyPrintCollection(
       open,
       close,
       elements,
       _maxLineLength - indentSize,
     );
   } else if (object is Map) {
-    final entries =
-        object.entries.map((entry) {
-          final key = prettyPrintNested(entry.key);
-          final value = prettyPrintNested(entry.value);
-          return [
-            ...key.take(key.length - 1),
-            '${key.last}: ${value.first}',
-            ...value.skip(1),
-          ];
-        }).toList();
+    final entries = object.entries.map((entry) {
+      final key = prettyPrintNested(entry.key);
+      final value = prettyPrintNested(entry.value);
+      return [
+        ...key.take(key.length - 1),
+        '${key.last}: ${value.first}',
+        ...value.skip(1),
+      ];
+    });
     return _prettyPrintCollection(
       '{',
       '}',
@@ -86,15 +85,19 @@ Iterable<String> _prettyPrint(
 Iterable<String> _prettyPrintCollection(
   String open,
   String close,
-  List<Iterable<String>> elements,
+  Iterable<Iterable<String>> elements,
   int maxLength,
 ) {
-  if (elements.length > _maxItems) {
-    const ellipsisElement = [
-      ['...'],
-    ];
-    elements.replaceRange(_maxItems - 1, elements.length, ellipsisElement);
+  final trimmedElements = <Iterable<String>>[];
+  for (var element in elements) {
+    if (trimmedElements.length >= _maxItems) {
+      trimmedElements[_maxItems - 1] = ['...'];
+      break;
+    }
+    trimmedElements.add(element);
   }
+  elements = trimmedElements;
+
   if (elements.every((e) => e.length == 1)) {
     final singleLine = '$open${elements.map((e) => e.single).join(', ')}$close';
     if (singleLine.length <= maxLength) {

pkgs/checks/test/pretty_print_test.dart

@@ -23,10 +23,22 @@ void main() {
       test('in iterables', () {
         check(literal(largeList.followedBy([]))).last.equals('...)');
       });
+      test('without processing truncated elements', () {
+        final poisonedList = [...largeList, _PoisonToString()];
+        check(() {
+          literal(poisonedList);
+        }).returnsNormally();
+      });
       test('in maps', () {
         final map = Map<int, int>.fromIterables(largeList, largeList);
         check(literal(map)).last.equals('...}');
       });
     });
   });
 }
+
+class _PoisonToString {
+  @override
+  String toString() =>
+      throw StateError('Truncated entry should not be processed');
+}

pkgs/matcher/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.12.19-wip
+
+* Improve speed of pretty printing for large collections.
+
 ## 0.12.18
 
 * Add `isSorted` and related matchers for iterables.

pkgs/matcher/lib/src/pretty_print.dart

@@ -39,9 +39,13 @@ String prettyPrint(Object? object, {int? maxLineLength, int? maxItems}) {
       var type = object is List ? '' : '${_typeName(object)}:';
 
       // Truncate the list of strings if it's longer than [maxItems].
-      var strings = object.map(pp).toList();
-      if (maxItems != null && strings.length > maxItems) {
-        strings.replaceRange(maxItems - 1, strings.length, ['...']);
+      var strings = <String>[];
+      for (var item in object) {
+        if (maxItems != null && strings.length >= maxItems) {
+          strings[maxItems - 1] = '...';
+          break;
+        }
+        strings.add(pp(item));
       }
 
       // If the printed string is short and doesn't contain a newline, print it
@@ -58,15 +62,14 @@ String prettyPrint(Object? object, {int? maxLineLength, int? maxItems}) {
         return _indent(indent + 2) + string;
       }).join(',\n')}\n${_indent(indent)}]';
     } else if (object is Map) {
-      // Convert the contents of the map to string representations.
-      var strings =
-          object.keys.map((key) {
-            return '${pp(key)}: ${pp(object[key])}';
-          }).toList();
-
       // Truncate the list of strings if it's longer than [maxItems].
-      if (maxItems != null && strings.length > maxItems) {
-        strings.replaceRange(maxItems - 1, strings.length, ['...']);
+      var strings = <String>[];
+      for (var key in object.keys) {
+        if (maxItems != null && strings.length >= maxItems) {
+          strings[maxItems - 1] = '...';
+          break;
+        }
+        strings.add('${pp(key)}: ${pp(object[key])}');
       }
 
       // If the printed string is short and doesn't contain a newline, print it

pkgs/matcher/pubspec.yaml

@@ -1,5 +1,5 @@
 name: matcher
-version: 0.12.18
+version: 0.12.19-wip
 description: >-
   Support for specifying test expectations via an extensible Matcher class.
   Also includes a number of built-in Matcher implementations for common cases.