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diff --git a/dom/base/ResponsiveImageSelector.h b/dom/base/ResponsiveImageSelector.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_responsiveimageselector_h__
+#define mozilla_dom_responsiveimageselector_h__
+
+#include "mozilla/UniquePtr.h"
+#include "mozilla/ServoBindingTypes.h"
+#include "mozilla/FunctionRef.h"
+#include "nsISupports.h"
+#include "nsIContent.h"
+#include "nsString.h"
+#include "nsCycleCollectionParticipant.h"
+
+class nsMediaQuery;
+class nsCSSValue;
+
+namespace mozilla::dom {
+
+class ResponsiveImageCandidate;
+
+class ResponsiveImageSelector {
+  friend class ResponsiveImageCandidate;
+
+ public:
+  NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING(ResponsiveImageSelector)
+  NS_DECL_CYCLE_COLLECTION_NATIVE_CLASS(ResponsiveImageSelector)
+
+  explicit ResponsiveImageSelector(nsIContent* aContent);
+  explicit ResponsiveImageSelector(dom::Document* aDocument);
+
+  // Parses the raw candidates and calls into the callback for each one of them.
+  static void ParseSourceSet(const nsAString& aSrcSet,
+                             FunctionRef<void(ResponsiveImageCandidate&&)>);
+
+  // NOTE ABOUT CURRENT SELECTION
+  //
+  // The best candidate is selected lazily when GetSelectedImage*() is
+  // called, or when SelectImage() is called explicitly. This result
+  // is then cached until either invalidated by further Set*() calls,
+  // or explicitly by replaced by SelectImage(aReselect = true).
+  //
+  // Because the selected image depends on external variants like
+  // viewport size and device pixel ratio, the time at which image
+  // selection occurs can affect the result.
+
+  // Given a srcset string, parse and replace current candidates (does not
+  // replace default source)
+  bool SetCandidatesFromSourceSet(const nsAString& aSrcSet,
+                                  nsIPrincipal* aTriggeringPrincipal = nullptr);
+
+  // Fill the source sizes from a valid sizes descriptor. Returns false if
+  // descriptor is invalid.
+  bool SetSizesFromDescriptor(const nsAString& aSizesDescriptor);
+
+  // Set the default source, treated as the least-precedence 1.0 density source.
+  void SetDefaultSource(const nsAString& aURLString,
+                        nsIPrincipal* aPrincipal = nullptr);
+
+  uint32_t NumCandidates(bool aIncludeDefault = true);
+
+  // If this was created for a specific content. May be null if we were only
+  // created for a document.
+  nsIContent* Content();
+
+  // The document we were created for, or the owner document of the content if
+  // we were created for a specific nsIContent.
+  dom::Document* Document();
+
+  // Get the url and density for the selected best candidate. These
+  // implicitly cause an image to be selected if necessary.
+  already_AddRefed<nsIURI> GetSelectedImageURL();
+  // Returns false if there is no selected image
+  bool GetSelectedImageURLSpec(nsAString& aResult);
+  double GetSelectedImageDensity();
+  nsIPrincipal* GetSelectedImageTriggeringPrincipal();
+
+  // Runs image selection now if necessary. If an image has already
+  // been choosen, takes no action unless aReselect is true.
+  //
+  // aReselect - Always re-run selection, replacing the previously
+  //             choosen image.
+  // return - true if the selected image result changed.
+  bool SelectImage(bool aReselect = false);
+
+ protected:
+  virtual ~ResponsiveImageSelector();
+
+ private:
+  // Append a candidate unless its selector is duplicated by a higher priority
+  // candidate
+  void AppendCandidateIfUnique(ResponsiveImageCandidate&& aCandidate);
+
+  // Append a default candidate with this URL if necessary. Does not check if
+  // the array already contains one, use SetDefaultSource instead.
+  void MaybeAppendDefaultCandidate();
+
+  // Get index of selected candidate, triggering selection if necessary.
+  int GetSelectedCandidateIndex();
+
+  // Forget currently selected candidate. (See "NOTE ABOUT CURRENT SELECTION"
+  // above.)
+  void ClearSelectedCandidate();
+
+  // Compute a density from a Candidate width. Returns false if sizes were not
+  // specified for this selector.
+  //
+  // aContext is the presContext to use for current viewport sizing, null will
+  // use the associated content's context.
+  bool ComputeFinalWidthForCurrentViewport(double* aWidth);
+
+  nsCOMPtr<nsINode> mOwnerNode;
+  // The cached URL for default candidate.
+  nsString mDefaultSourceURL;
+  nsCOMPtr<nsIPrincipal> mDefaultSourceTriggeringPrincipal;
+  // If this array contains an eCandidateType_Default, it should be the last
+  // element, such that the Setters can preserve/replace it respectively.
+  nsTArray<ResponsiveImageCandidate> mCandidates;
+  int mSelectedCandidateIndex;
+  // The cached resolved URL for mSelectedCandidateIndex, such that we only
+  // resolve the absolute URL at selection time
+  nsCOMPtr<nsIURI> mSelectedCandidateURL;
+
+  // Servo bits.
+  UniquePtr<StyleSourceSizeList> mServoSourceSizeList;
+};
+
+class ResponsiveImageCandidate {
+ public:
+  ResponsiveImageCandidate();
+  ResponsiveImageCandidate(const ResponsiveImageCandidate&) = delete;
+  ResponsiveImageCandidate(ResponsiveImageCandidate&&) = default;
+
+  void SetURLSpec(const nsAString& aURLString);
+  void SetTriggeringPrincipal(nsIPrincipal* aPrincipal);
+  // Set this as a default-candidate. This behaves the same as density 1.0, but
+  // has a differing type such that it can be replaced by subsequent
+  // SetDefaultSource calls.
+  void SetParameterDefault();
+
+  // Set this candidate as a by-density candidate with specified density.
+  void SetParameterAsDensity(double aDensity);
+  void SetParameterAsComputedWidth(int32_t aWidth);
+
+  void SetParameterInvalid();
+
+  // Consume descriptors from a string defined by aIter and aIterEnd, adjusts
+  // aIter to the end of data consumed.
+  // Returns false if descriptors string is invalid, but still parses to the end
+  // of descriptors microsyntax.
+  bool ConsumeDescriptors(nsAString::const_iterator& aIter,
+                          const nsAString::const_iterator& aIterEnd);
+
+  // Check if our parameter (which does not include the url) is identical
+  bool HasSameParameter(const ResponsiveImageCandidate& aOther) const;
+
+  const nsAString& URLString() const { return mURLString; }
+  nsIPrincipal* TriggeringPrincipal() const { return mTriggeringPrincipal; }
+
+  // Compute and return the density relative to a selector.
+  double Density(ResponsiveImageSelector* aSelector) const;
+  // If the width is already known. Useful when iterating over candidates to
+  // avoid having each call re-compute the width.
+  double Density(double aMatchingWidth) const;
+
+  // Append the descriptors for this candidate serialized as a string.
+  void AppendDescriptors(nsAString&) const;
+
+  bool IsValid() const { return mType != CandidateType::Invalid; }
+
+  // If this selector is computed from the selector's matching width.
+  bool IsComputedFromWidth() const {
+    return mType == CandidateType::ComputedFromWidth;
+  }
+
+  bool IsDefault() const { return mType == CandidateType::Default; }
+
+  enum class CandidateType : uint8_t {
+    Invalid,
+    Density,
+    // Treated as 1.0 density, but a separate type so we can update the
+    // responsive candidates and default separately
+    Default,
+    ComputedFromWidth
+  };
+
+  CandidateType Type() const { return mType; }
+
+ private:
+  nsString mURLString;
+  nsCOMPtr<nsIPrincipal> mTriggeringPrincipal;
+  CandidateType mType;
+  union {
+    double mDensity;
+    int32_t mWidth;
+  } mValue;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_responsiveimageselector_h__