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diff --git a/intl/EncodingDetector.h b/intl/EncodingDetector.h
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+// Copyright 2019 Mozilla Foundation. See the COPYRIGHT
+// file at the top-level directory of this distribution.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Mostly copied and pasted from
+// third_party/rust/chardetng/src/lib.rs , so
+// "top-level directory of this distribution" above refers to
+// third_party/rust/chardetng/
+
+#ifndef mozilla_EncodingDetector_h
+#define mozilla_EncodingDetector_h
+
+#include "mozilla/Encoding.h"
+
+namespace mozilla {
+class EncodingDetector;
+};  // namespace mozilla
+
+#define CHARDETNG_ENCODING_DETECTOR mozilla::EncodingDetector
+
+#include "chardetng.h"
+
+namespace mozilla {
+
+/**
+ * A Web browser-oriented detector for guessing what character
+ * encoding a stream of bytes is encoded in.
+ *
+ * The bytes are fed to the detector incrementally using the `feed`
+ * method. The current guess of the detector can be queried using
+ * the `guess` method. The guessing parameters are arguments to the
+ * `guess` method rather than arguments to the constructor in order
+ * to enable the application to check if the arguments affect the
+ * guessing outcome. (The specific use case is to disable UI for
+ * re-running the detector with UTF-8 allowed and the top-level
+ * domain name ignored if those arguments don't change the guess.)
+ */
+class EncodingDetector final {
+ public:
+  ~EncodingDetector() = default;
+
+  static void operator delete(void* aDetector) {
+    chardetng_encoding_detector_free(
+        reinterpret_cast<EncodingDetector*>(aDetector));
+  }
+
+  /**
+   * Creates a new instance of the detector.
+   */
+  static inline UniquePtr<EncodingDetector> Create() {
+    UniquePtr<EncodingDetector> detector(chardetng_encoding_detector_new());
+    return detector;
+  }
+
+  /**
+   * Queries whether the TLD is considered non-generic and could affect the
+   * guess.
+   */
+  static inline bool TldMayAffectGuess(Span<const char> aTLD) {
+    return chardetng_encoding_detector_tld_may_affect_guess(aTLD.Elements(),
+                                                            aTLD.Length());
+  }
+
+  /**
+   * Inform the detector of a chunk of input.
+   *
+   * The byte stream is represented as a sequence of calls to this
+   * method such that the concatenation of the arguments to this
+   * method form the byte stream. It does not matter how the application
+   * chooses to chunk the stream. It is OK to call this method with
+   * a zero-length byte slice.
+   *
+   * The end of the stream is indicated by calling this method with
+   * `aLast` set to `true`. In that case, the end of the stream is
+   * considered to occur after the last byte of the `aBuffer` (which
+   * may be zero-length) passed in the same call. Once this method
+   * has been called with `last` set to `true` this method must not
+   * be called again.
+   *
+   * If you want to perform detection on just the prefix of a longer
+   * stream, do not pass `aLast=true` after the prefix if the stream
+   * actually still continues.
+   *
+   * Returns `true` if after processing `aBuffer` the stream has
+   * contained at least one non-ASCII byte and `false` if only
+   * ASCII has been seen so far.
+   *
+   * # Panics
+   *
+   * If this method has previously been called with `aLast` set to `true`.
+   */
+  inline bool Feed(Span<const uint8_t> aBuffer, bool aLast) {
+    return chardetng_encoding_detector_feed(this, aBuffer.Elements(),
+                                            aBuffer.Length(), aLast);
+  }
+
+  /**
+   * Guess the encoding given the bytes pushed to the detector so far
+   * (via `Feed()`), the top-level domain name from which the bytes were
+   * loaded, and an indication of whether to consider UTF-8 as a permissible
+   * guess.
+   *
+   * The `aTld` argument takes the rightmost DNS label of the hostname of the
+   * host the stream was loaded from in lower-case ASCII form. That is, if
+   * the label is an internationalized top-level domain name, it must be
+   * provided in its Punycode form. If the TLD that the stream was loaded
+   * from is unavalable, an empty `Spane` may be passed instead, which is
+   * equivalent to passing a `Span` for "com".
+   *
+   * If the `aAllowUTF8` argument is set to `false`, the return value of
+   * this method won't be `UTF_8_ENCODING`. When performing detection
+   * on `text/html` on non-`file:` URLs, Web browsers must pass `false`,
+   * unless the user has taken a specific contextual action to request an
+   * override. This way, Web developers cannot start depending on UTF-8
+   * detection. Such reliance would make the Web Platform more brittle.
+   *
+   * Returns the guessed encoding.
+   *
+   * # Panics
+   *
+   * If `aTld` contains non-ASCII, period, or upper-case letters. (The panic
+   * condition is intentionally limited to signs of failing to extract the
+   * label correctly, failing to provide it in its Punycode form, and failure
+   * to lower-case it. Full DNS label validation is intentionally not performed
+   * to avoid panics when the reality doesn't match the specs.)
+   */
+  inline mozilla::NotNull<const mozilla::Encoding*> Guess(
+      Span<const char> aTLD, bool aAllowUTF8) const {
+    return WrapNotNull(chardetng_encoding_detector_guess(
+        this, aTLD.Elements(), aTLD.Length(), aAllowUTF8));
+  }
+
+ private:
+  EncodingDetector() = delete;
+  EncodingDetector(const EncodingDetector&) = delete;
+  EncodingDetector& operator=(const EncodingDetector&) = delete;
+};
+
+};  // namespace mozilla
+
+#endif  // mozilla_EncodingDetector_h