Allow configuring iOS build with CMake (#3234)

platform/darwin/core/collator.mm

@@ -0,0 +1,67 @@
+#include <mbgl/i18n/collator.hpp>
+
+#include <sstream>
+
+#import <Foundation/Foundation.h>
+
+namespace mbgl {
+namespace platform {
+
+class Collator::Impl {
+public:
+    Impl(bool caseSensitive, bool diacriticSensitive, const std::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 static_cast<int>([nsLhs compare:nsRhs options:options range:compareRange locale:locale]);
+    }
+
+    std::string resolvedLocale() const {
+        // Following documentation at:
+        // https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPInternational/LanguageandLocaleIDs/LanguageandLocaleIDs.html#//apple_ref/doc/uid/10000171i-CH15-SW9
+        // We expect NSLocale to accept BCP 47 tags as localeIdentifier inputs, but the output format
+        // may append the region tag with an "_". Changing that to a "-" makes the identifier BCP 47 compliant.
+        // Experimentally, "zh-Hans-HK" and "zh-Hans_HK" both round trip -- if the second is used by
+        //  `currentLocale`, we don't want to return the underscore.
+        return [[locale localeIdentifier] stringByReplacingOccurrencesOfString:@"_" withString:@"-"].UTF8String;
+    }
+private:
+    NSStringCompareOptions options;
+    NSLocale* locale;
+};
+
+Collator::Collator(bool caseSensitive, bool diacriticSensitive, const std::optional<std::string>& locale_)
+    : impl(std::make_shared<Impl>(caseSensitive, diacriticSensitive, 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 platform
+} // namespace mbgl