[ios, macos] Darwin "collator" implementation

- Uses NSString for comparison
 - Uses NSLocale for loading locales

platform/darwin/src/collator.mm

@@ -0,0 +1,64 @@
+#include <mbgl/style/expression/collator.hpp>
+
+#include <sstream>
+
+#import <Foundation/Foundation.h>
+
+namespace mbgl {
+namespace style {
+namespace expression {
+
+class Collator::Impl {
+public:
+    Impl(bool caseSensitive, bool diacriticSensitive, optional<std::string> locale_)
+    : options((caseSensitive ? 0 : NSCaseInsensitiveSearch) |
+              (diacriticSensitive ? 0 : NSDiacriticInsensitiveSearch))
+    , locale(locale_ ?
+                [[NSLocale alloc] initWithLocaleIdentifier:@((*locale_).c_str())] :
+                [NSLocale currentLocale])
+    {}
+
+    bool operator==(const Impl& other) const {
+        return options == other.options &&
+            [[locale localeIdentifier] isEqualToString:[other.locale localeIdentifier]];
+    }
+
+    int compare(const std::string& lhs, const std::string& rhs) const {
+        NSString* nsLhs = @(lhs.c_str());
+        NSString* nsRhs = @(rhs.c_str());
+        // Limiting the compare range to the length of the LHS seems weird, but
+        // experimentally we've checked that if LHS is a prefix of RHS compare returns -1
+        // https://developer.apple.com/documentation/foundation/nsstring/1414561-compare
+        NSRange compareRange = NSMakeRange(0, nsLhs.length);
+
+        return [nsLhs compare:nsRhs options:options range:compareRange locale:locale];
+    }
+
+    std::string resolvedLocale() const {
+        return [locale localeIdentifier].UTF8String;
+    }
+private:
+    NSStringCompareOptions options;
+    NSLocale* locale;
+};
+
+
+Collator::Collator(bool caseSensitive, bool diacriticSensitive, optional<std::string> locale_)
+    : impl(std::make_shared<Impl>(caseSensitive, diacriticSensitive, std::move(locale_)))
+{}
+
+bool Collator::operator==(const Collator& other) const {
+    return *impl == *(other.impl);
+}
+
+int Collator::compare(const std::string& lhs, const std::string& rhs) const {
+    return impl->compare(lhs, rhs);
+}
+
+std::string Collator::resolvedLocale() const {
+    return impl->resolvedLocale();
+}
+
+} // namespace expression
+} // namespace style
+} // namespace mbgl

platform/ios/config.cmake

@@ -29,6 +29,7 @@ macro(mbgl_platform_core)
         PRIVATE platform/darwin/mbgl/storage/reachability.h
         PRIVATE platform/darwin/mbgl/storage/reachability.m
         PRIVATE platform/darwin/src/CFHandle.hpp
+        PRIVATE platform/darwin/src/collator.mm
         PRIVATE platform/darwin/src/local_glyph_rasterizer.mm
         PRIVATE platform/darwin/src/logging_nslog.mm
         PRIVATE platform/darwin/src/nsthread.mm

platform/macos/config.cmake

@@ -14,6 +14,7 @@ macro(mbgl_platform_core)
         PRIVATE platform/darwin/mbgl/storage/reachability.h
         PRIVATE platform/darwin/mbgl/storage/reachability.m
         PRIVATE platform/darwin/src/CFHandle.hpp
+        PRIVATE platform/darwin/src/collator.mm
         PRIVATE platform/darwin/src/local_glyph_rasterizer.mm
         PRIVATE platform/darwin/src/logging_nslog.mm
         PRIVATE platform/darwin/src/nsthread.mm