Add support for creating a subspan view of a cel::Source.

PiperOrigin-RevId: 939969141

Author: jnthntatum

common/source.cc

@@ -585,6 +585,51 @@ absl::optional<std::pair<int32_t, SourcePosition>> Source::FindLine(
   return std::make_pair(line, line_offsets[static_cast<size_t>(line) - 2]);
 }
 
+SourceSubrange::SourceSubrange(const Source& source, SourceRange range)
+    : source_(source), range_(range) {
+  SourcePosition size = source_.content().size();
+  ABSL_DCHECK(range_.begin >= 0);
+  ABSL_DCHECK(range_.begin <= size);
+  ABSL_DCHECK(range_.end >= range_.begin);
+  ABSL_DCHECK(range_.end <= size);
+  if (range_.begin < 0) {
+    range_.begin = 0;
+  }
+  if (range_.begin > size) {
+    range_.begin = size;
+  }
+  if (range_.end < range_.begin) {
+    range_.end = range_.begin;
+  }
+  if (range_.end > size) {
+    range_.end = size;
+  }
+  for (const auto& line_offset : source_.line_offsets()) {
+    if (line_offset > range_.begin && line_offset <= range_.end) {
+      line_offsets_.push_back(line_offset - range_.begin);
+    }
+  }
+  line_offsets_.push_back(range_.end - range_.begin + 1);
+}
+
+SourceContentView SourceSubrange::content() const {
+  auto parent_content = source_.content();
+  if (parent_content.empty() || range_.begin >= range_.end) {
+    return EmptyContentView();
+  }
+  return absl::visit(
+      [this](auto view) {
+        return SourceContentView(
+            view.subspan(static_cast<size_t>(range_.begin),
+                         static_cast<size_t>(range_.end - range_.begin)));
+      },
+      parent_content.view_);
+}
+
+absl::Span<const SourcePosition> SourceSubrange::line_offsets() const {
+  return absl::MakeConstSpan(line_offsets_);
+}
+
 absl::StatusOr<absl_nonnull SourcePtr> NewSource(absl::string_view content,
                                                  std::string description) {
   return common_internal::NewSourceImpl(std::move(description), content,

common/source.h

@@ -22,6 +22,7 @@
 
 #include "absl/base/attributes.h"
 #include "absl/base/nullability.h"
+#include "absl/container/inlined_vector.h"
 #include "absl/status/statusor.h"
 #include "absl/strings/cord.h"
 #include "absl/strings/string_view.h"
@@ -36,6 +37,7 @@ class SourceImpl;
 }  // namespace common_internal
 
 class Source;
+class SourceSubrange;
 
 // SourcePosition represents an offset in source text.
 using SourcePosition = int32_t;
@@ -94,6 +96,7 @@ class SourceContentView final {
 
  private:
   friend class Source;
+  friend class SourceSubrange;
 
   constexpr SourceContentView() = default;
 
@@ -178,6 +181,7 @@ class Source {
 
  private:
   friend class common_internal::SourceImpl;
+  friend class SourceSubrange;
 
   Source() = default;
 
@@ -187,6 +191,34 @@ class Source {
       SourcePosition position) const;
 };
 
+// `SourceSubrange` is a view of a subrange fo an underlying `Source` object.
+// Intended to be used when the CEL expression is embedded in a larger text
+// representation (such as a.celpolicy).
+//
+// The parent `Source` must outlive this object.
+class SourceSubrange final : public Source {
+ public:
+  SourceSubrange(const Source& source ABSL_ATTRIBUTE_LIFETIME_BOUND,
+                 SourceRange range);
+
+  absl::string_view description() const ABSL_ATTRIBUTE_LIFETIME_BOUND override {
+    return source_.description();
+  }
+
+  // Returns a view of the underlying expression text.
+  ContentView content() const ABSL_ATTRIBUTE_LIFETIME_BOUND override;
+
+  // Returns a `absl::Span` of `SourcePosition` which represent the positions
+  // where new lines occur.
+  absl::Span<const SourcePosition> line_offsets() const
+      ABSL_ATTRIBUTE_LIFETIME_BOUND override;
+
+ private:
+  const Source& source_;
+  SourceRange range_;
+  absl::InlinedVector<SourcePosition, 1> line_offsets_;
+};
+
 using SourcePtr = std::unique_ptr<Source>;
 
 absl::StatusOr<absl_nonnull SourcePtr> NewSource(

common/source_test.cc

@@ -223,5 +223,44 @@ TEST(Source, DisplayErrorLocationFullWidth) {
             "\n | .．＾");
 }
 
+TEST(SourceSubrange, Description) {
+  ASSERT_OK_AND_ASSIGN(auto source, NewSource("hello world", "subrange-test"));
+  SourceSubrange subrange(*source, SourceRange{0, 5});
+  EXPECT_THAT(subrange.description(), Eq("subrange-test"));
+}
+
+TEST(SourceSubrange, Content) {
+  ASSERT_OK_AND_ASSIGN(auto source, NewSource("hello world", "subrange-test"));
+  SourceSubrange subrange(*source, SourceRange{6, 11});
+  EXPECT_THAT(subrange.content().ToString(), Eq("world"));
+}
+
+TEST(SourceSubrange, ContentEmpty) {
+  ASSERT_OK_AND_ASSIGN(auto source, NewSource("hello world", "subrange-test"));
+  SourceSubrange subrange(*source, SourceRange{5, 5});
+  EXPECT_THAT(subrange.content().ToString(), Eq(""));
+}
+
+TEST(SourceSubrange, LineOffsetsNoNewlines) {
+  ASSERT_OK_AND_ASSIGN(auto source,
+                       NewSource("hello\nworld\n", "subrange-test"));
+  SourceSubrange subrange(*source, SourceRange{0, 5});
+  EXPECT_THAT(subrange.line_offsets(), ElementsAre(6));
+}
+
+TEST(SourceSubrange, LineOffsetsWithNewlines) {
+  ASSERT_OK_AND_ASSIGN(auto source,
+                       NewSource("hello\nworld\ncel", "subrange-test"));
+  SourceSubrange subrange(*source, SourceRange{0, 11});
+  EXPECT_THAT(subrange.line_offsets(), ElementsAre(6, 12));
+}
+
+TEST(SourceSubrange, LineOffsetsMiddleSubrange) {
+  ASSERT_OK_AND_ASSIGN(auto source,
+                       NewSource("hello\nworld\ncel\ncpp", "subrange-test"));
+  SourceSubrange subrange(*source, SourceRange{6, 15});
+  EXPECT_THAT(subrange.line_offsets(), ElementsAre(6, 10));
+}
+
 }  // namespace
 }  // namespace cel