Add Expander<P,I>.SpanBasedConcatenator

src/Build/Evaluation/Expander.cs

@@ -123,6 +123,123 @@ internal class Expander<P, I>
         where P : class, IProperty
         where I : class, IItem
     {
+        /// <summary>
+        /// A helper struct wrapping a <see cref="SpanBasedStringBuilder"/> and providing file path conversion
+        /// as used in e.g. property expansion.
+        /// </summary>
+        /// <remarks>
+        /// If exactly one value is added and no concatenation takes places, this value is returned without
+        /// conversion. In other cases values are stringified and attempted to be interpreted as file paths
+        /// before concatenation.
+        /// </remarks>
+        private struct SpanBasedConcatenator : IDisposable
+        {
+            /// <summary>
+            /// The backing <see cref="SpanBasedStringBuilder"/>, null until the second value is added.
+            /// </summary>
+            private SpanBasedStringBuilder _builder;
+
+            /// <summary>
+            /// The first value added to the concatenator. Tracked in its own field so it can be returned
+            /// without conversion if no concatenation takes place.
+            /// </summary>
+            private object _firstObject;
+
+            /// <summary>
+            /// The first value added to the concatenator if it is a span. Tracked in its own field so the
+            /// <see cref="SpanBasedStringBuilder"/> functionality doesn't have to be invoked if no concatenation
+            /// takes place.
+            /// </summary>
+            private ReadOnlyMemory<char> _firstSpan;
+
+            /// <summary>
+            /// Adds an object to be concatenated.
+            /// </summary>
+            public void Add(object obj)
+            {
+                FlushFirstValueIfNeeded();
+                if (_builder != null)
+                {
+                    _builder.Append(FileUtilities.MaybeAdjustFilePath(obj.ToString()));
+                }
+                else
+                {
+                    _firstObject = obj;
+                }
+            }
+
+            /// <summary>
+            /// Adds a span to be concatenated.
+            /// </summary>
+            public void Add(ReadOnlyMemory<char> span)
+            {
+                FlushFirstValueIfNeeded();
+                if (_builder != null)
+                {
+                    _builder.Append(FileUtilities.MaybeAdjustFilePath(span));
+                }
+                else
+                {
+                    _firstSpan = span;
+                }
+            }
+
+            /// <summary>
+            /// Lazily initializes <see cref="_builder"/> and populates it with the first value
+            /// when the second value is being added.
+            /// </summary>
+            private void FlushFirstValueIfNeeded()
+            {
+                if (_firstObject != null)
+                {
+                    _builder = Strings.GetSpanBasedStringBuilder();
+                    _builder.Append(FileUtilities.MaybeAdjustFilePath(_firstObject.ToString()));
+                    _firstObject = null;
+                }
+                else if (!_firstSpan.IsEmpty)
+                {
+                    _builder = Strings.GetSpanBasedStringBuilder();
+#if FEATURE_SPAN
+                    _builder.Append(FileUtilities.MaybeAdjustFilePath(_firstSpan));
+#else
+                    _builder.Append(FileUtilities.MaybeAdjustFilePath(_firstSpan.ToString()));
+#endif
+                    _firstSpan = new ReadOnlyMemory<char>();
+                }
+            }
+
+            /// <summary>
+            /// Returns the result of the concatenation.
+            /// </summary>
+            /// <returns>
+            /// If only one value has been added and it is not a string, it is returned unchanged.
+            /// In all other cases (no value, one string value, multiple values) the result is a
+            /// concatenation of the string representation of the values, each additionally subjected
+            /// to file path adjustment.
+            /// </returns>
+            public object GetResult()
+            {
+                if (_firstObject != null)
+                {
+                    if (_firstObject is string stringValue)
+                    {
+                        return FileUtilities.MaybeAdjustFilePath(stringValue);
+                    }
+                    return _firstObject;
+                }
+                if (!_firstSpan.IsEmpty)
+                {
+                    return FileUtilities.MaybeAdjustFilePath(_firstSpan).ToString();
+                }
+                return _builder?.ToString() ?? string.Empty;
+            }
+
+            /// <summary>
+            /// Disposes of the struct by delegating the call to the underlying <see cref="SpanBasedStringBuilder"/>.
+            /// </summary>
+            public void Dispose() => _builder?.Dispose();
+        }
+
         /// <summary>
         /// A limit for truncating string expansions within an evaluated Condition. Properties, item metadata, or item groups will be truncated to N characters such as 'N...'.
         /// Enabled by ExpanderOptions.Truncate.