Convert every printf specifier in .stringsdict plural forms to ICU arguments

Sources/SkipFoundation/PropertyListSerialization.swift

@@ -106,14 +106,8 @@ public class PropertyListSerialization {
             let categories = ["zero", "one", "two", "few", "many", "other"]
             let rules = categories.compactMap { (category: String) -> String? in
                 guard let pluralString = varDict[category] as? String else { return nil }
-                var cleaned = pluralString!
-                for index in 1...10 {
-                    cleaned = cleaned
-                        .replacingOccurrences(of: "%\(index)$@", with: "{\(index-1)}")
-                        .replacingOccurrences(of: "%\(index)$d", with: "{\(index-1)}")
-                }
                 let icuCategory = category == "zero" ? "=0" : category
-                return "\(icuCategory){\(cleaned)}"
+                return "\(icuCategory){\(icuMessage(fromFormat: pluralString!))}"
             }.joined(separator: " ")
 
             if !rules.isEmpty {
@@ -140,6 +134,43 @@ public class PropertyListSerialization {
         return result
     }
 
+    /// Converts a printf-style plural form into ICU MessageFormat text: each specifier becomes an argument
+    /// (`%lld` and `%1$d` become `{0,number,#}`, without grouping like Darwin, and `%@` becomes `{0}`),
+    /// and ICU syntax characters in the literal text are quoted.
+    static func icuMessage(fromFormat format: String) -> String {
+        let specifier = java.util.regex.Pattern.compile("%(?:(\\d+)\\$)?[-+ 0#']*\\d*(?:\\.\\d+)?(?:hh|h|ll|l|q|z|t|j)?([@dDiuUxXoOfFeEgGaAcCsSp%])")
+        let matcher = specifier.matcher(format)
+        var result = ""
+        var literalStart = 0
+        var nextArgument = 0
+        while matcher.find() {
+            result += icuLiteral(format.substring(literalStart, matcher.start()))
+            literalStart = matcher.end()
+            let conversion = matcher.group(2)!
+            if conversion == "%" {
+                result += "%"
+                continue
+            }
+            var argument = nextArgument
+            if let position = matcher.group(1) {
+                argument = position.toInt() - 1
+            } else {
+                nextArgument += 1
+            }
+            result += "dDiuU".contains(conversion) ? "{\(argument),number,#}" : "{\(argument)}"
+        }
+        result += icuLiteral(format.substring(literalStart))
+        return result
+    }
+
+    private static func icuLiteral(_ text: String) -> String {
+        text.replace("'", "''")
+            .replace("{", "'{'")
+            .replace("}", "'}'")
+            .replace("#", "'#'")
+            .replace("|", "'|'")
+    }
+
     private static func openStepPropertyList(from data: Data, options: PropertyListSerialization.ReadOptions = []) throws -> [String: String]? {
         var dict: Dictionary<String, String> = [:]
 

Tests/SkipFoundationTests/Foundation/LocaleTests.swift

@@ -178,6 +178,47 @@ final class LocaleTests: XCTestCase {
         XCTAssertNotNil(dict["repeats_every_day"])
     }
 
+    func testLocalizableStringsDictPrintfSpecifiers() throws {
+        #if !SKIP
+        throw XCTSkip("Darwin formats .stringsdict plurals natively")
+        #else
+        let locstr = #"""
+        <?xml version="1.0" encoding="UTF-8"?>
+        <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+        <plist version="1.0">
+        <dict>
+            <key>%lld files</key>
+            <dict>
+                <key>NSStringLocalizedFormatKey</key>
+                <string>%#@value@</string>
+                <key>value</key>
+                <dict>
+                    <key>NSStringFormatSpecTypeKey</key>
+                    <string>NSStringPluralRuleType</string>
+                    <key>NSStringFormatValueTypeKey</key>
+                    <string>lld</string>
+                    <key>zero</key>
+                    <string>No files</string>
+                    <key>one</key>
+                    <string>%lld file, it's #1</string>
+                    <key>other</key>
+                    <string>%lld files {100%%}</string>
+                </dict>
+            </dict>
+        </dict>
+        </plist>
+        """#
+
+        let data = try XCTUnwrap(locstr.data(using: .utf8))
+        let plist = try XCTUnwrap(PropertyListSerialization.propertyList(from: data, options: [], format: nil))
+        let format = try XCTUnwrap(plist["%lld files"])
+
+        XCTAssertEqual(String.localizedStringWithFormat(format, 0), "No files")
+        XCTAssertEqual(String.localizedStringWithFormat(format, 1), "1 file, it's #1")
+        XCTAssertEqual(String.localizedStringWithFormat(format, 1234), "1234 files {100%}")
+        #endif
+    }
+
     func testLocaleFormats() throws {
         #if !SKIP
         // TODO