1 files changed, 146 insertions, 0 deletions
diff --git a/intl/components/src/RelativeTimeFormat.h b/intl/components/src/RelativeTimeFormat.h
new file mode 100644
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+++ b/intl/components/src/RelativeTimeFormat.h
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+/* 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 intl_components_RelativeTimeFormat_h_
+#define intl_components_RelativeTimeFormat_h_
+
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/intl/NumberPart.h"
+
+#include "unicode/ureldatefmt.h"
+#include "unicode/utypes.h"
+
+namespace mozilla::intl {
+
+struct RelativeTimeFormatOptions {
+  enum class Style { Short, Narrow, Long };
+  Style style = Style::Long;
+
+  enum class Numeric {
+    /**
+     * Only strings with numeric components like `1 day ago`.
+     */
+    Always,
+    /**
+     * Natural-language strings like `yesterday` when possible,
+     * otherwise strings with numeric components as in `7 months ago`.
+     */
+    Auto,
+  };
+  Numeric numeric = Numeric::Always;
+};
+
+/**
+ * A RelativeTimeFormat implementation that roughly mirrors the API provided by
+ * the ECMA-402 Intl.RelativeTimeFormat object.
+ *
+ * https://tc39.es/ecma402/#relativetimeformat-objects
+ */
+class RelativeTimeFormat final {
+ public:
+  /**
+   *
+   * Initialize a new RelativeTimeFormat for the provided locale and using the
+   * provided options.
+   *
+   * https://tc39.es/ecma402/#sec-InitializeRelativeTimeFormat
+   */
+  static Result<UniquePtr<RelativeTimeFormat>, ICUError> TryCreate(
+      const char* aLocale, const RelativeTimeFormatOptions& aOptions);
+
+  RelativeTimeFormat() = default;
+
+  RelativeTimeFormat(RelativeTimeFormatOptions::Numeric aNumeric,
+                     URelativeDateTimeFormatter* aFormatter,
+                     UFormattedRelativeDateTime* aFormattedRelativeDateTime);
+
+  RelativeTimeFormat(const RelativeTimeFormat&) = delete;
+  RelativeTimeFormat& operator=(const RelativeTimeFormat&) = delete;
+  ~RelativeTimeFormat();
+
+  enum class FormatUnit {
+    Second,
+    Minute,
+    Hour,
+    Day,
+    Week,
+    Month,
+    Quarter,
+    Year
+  };
+
+  /**
+   * Formats a double to the provider buffer (either utf-8 or utf-16)
+   *
+   * https://tc39.es/ecma402/#sec-FormatRelativeTime
+   */
+  template <typename B>
+  Result<Ok, ICUError> format(double aNumber, FormatUnit aUnit,
+                              B& aBuffer) const {
+    static_assert(
+        std::is_same_v<typename B::CharType, char> ||
+            std::is_same_v<typename B::CharType, char16_t>,
+        "The only buffer CharTypes supported by RelativeTimeFormat are char "
+        "(for UTF-8 support) and char16_t (for UTF-16 support).");
+
+    auto fmt = mNumeric == RelativeTimeFormatOptions::Numeric::Auto
+                   ? ureldatefmt_format
+                   : ureldatefmt_formatNumeric;
+
+    if constexpr (std::is_same_v<typename B::CharType, char>) {
+      mozilla::Vector<char16_t, StackU16VectorSize> u16Vec;
+
+      MOZ_TRY(FillBufferWithICUCall(
+          u16Vec, [this, aNumber, aUnit, fmt](UChar* target, int32_t length,
+                                              UErrorCode* status) {
+            return fmt(mFormatter, aNumber, ToURelativeDateTimeUnit(aUnit),
+                       target, length, status);
+          }));
+
+      if (!FillBuffer(u16Vec, aBuffer)) {
+        return Err(ICUError::OutOfMemory);
+      }
+      return Ok{};
+    } else {
+      static_assert(std::is_same_v<typename B::CharType, char16_t>);
+
+      return FillBufferWithICUCall(
+          aBuffer, [this, aNumber, aUnit, fmt](UChar* target, int32_t length,
+                                               UErrorCode* status) {
+            return fmt(mFormatter, aNumber, ToURelativeDateTimeUnit(aUnit),
+                       target, length, status);
+          });
+    }
+  }
+
+  /**
+   * Formats the relative time to a utf-16 string, and fills the provided parts
+   * vector. The string view is valid until another time is formatted.
+   * Accessing the string view after this event is undefined behavior.
+   *
+   * This is utf-16 only because the only current use case is in
+   * SpiderMonkey. Supporting utf-8 would require recalculating the offsets
+   * in NumberPartVector from fixed width to variable width, which might be
+   * tricky to get right and is work that won't be necessary if we switch to
+   * ICU4X (see Bug 1723120).
+   *
+   * https://tc39.es/ecma402/#sec-FormatRelativeTimeToParts
+   */
+  Result<Span<const char16_t>, ICUError> formatToParts(
+      double aNumber, FormatUnit aUnit, NumberPartVector& aParts) const;
+
+ private:
+  RelativeTimeFormatOptions::Numeric mNumeric =
+      RelativeTimeFormatOptions::Numeric::Always;
+  URelativeDateTimeFormatter* mFormatter = nullptr;
+  UFormattedRelativeDateTime* mFormattedRelativeDateTime = nullptr;
+
+  static constexpr size_t StackU16VectorSize = 128;
+
+  URelativeDateTimeUnit ToURelativeDateTimeUnit(FormatUnit unit) const;
+};
+
+}  // namespace mozilla::intl
+
+#endif