Rewrite script in Swift instead of Python.

In response to: swiftlang#1286 (comment)

Author: YOCKOW

utils/update-iana-charset-names

@@ -53,7 +53,7 @@ declare generatedCode
 generatedCode=$(
   echo "${swiftLicenseHeader##$'\n'}"
   echo "$warningComment"
-  python3 "${utilsDir}/${commandName}-impl.py"
+  swift -D PRINT_CODE "${utilsDir}/${commandName}-impl.swift"
 )
 
 echo "Writing the code to '${targetSwiftFileRelativePath}'..." 1>&2

utils/update-iana-charset-names-impl.py

@@ -0,0 +1,195 @@
+#!/usr/bin/env swift
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+/*
+
+This is a Swift script that converts an XML file containing the list of IANA
+"Character Sets" to Swift source code.
+This script generates minimum code and is intended to be executed by other shell
+script.
+
+ */
+
+import Foundation
+#if canImport(FoundationXML)
+import FoundationXML
+#endif
+
+// MARK: - Constants
+
+let requiredCharsetNames = [
+    "UTF-8",
+    "US-ASCII",
+    "EUC-JP",
+    "ISO-8859-1",
+    "Shift_JIS",
+    "ISO-8859-2",
+    "UTF-16",
+    "windows-1251",
+    "windows-1252",
+    "windows-1253",
+    "windows-1254",
+    "windows-1250",
+    "ISO-2022-JP",
+    "macintosh",
+    "UTF-16BE",
+    "UTF-16LE",
+    "UTF-32",
+    "UTF-32BE",
+    "UTF-32LE",
+]
+let charsetsXMLURL = URL(
+    string: "https://www.iana.org/assignments/character-sets/character-sets.xml"
+)!
+let charsetsXMLNamespace = "http://www.iana.org/assignments"
+let swiftCodeIndent = "    "
+
+
+// MARK: - Implementation
+
+enum CodeGenerationError: Swift.Error {
+    case missingName
+    case missingAliasValue
+    case noRootElement
+}
+
+/// Representation of <record> element in 'character-sets.xml'
+///
+/// The structure of <record> element is as blow:
+/// ```xml
+/// <record>
+///     <name>US-ASCII</name>
+///     <xref type="rfc" data="rfc2046"/>
+///     <value>3</value>
+///     <description>ANSI X3.4-1986</description>
+///     <alias>iso-ir-6</alias>
+///     <alias>ANSI_X3.4-1968</alias>
+///     <alias>ANSI_X3.4-1986</alias>
+///     <alias>ISO_646.irv:1991</alias>
+///     <alias>ISO646-US</alias>
+///     <alias>US-ASCII</alias>
+///     <alias>us</alias>
+///     <alias>IBM367</alias>
+///     <alias>cp367</alias>
+///     <alias>csASCII</alias>
+///     <preferred_alias>US-ASCII</preferred_alias>
+/// </record>
+/// ```
+struct IANACharsetNameRecord {
+    /// Preferred MIME Name
+    let preferredMIMEName: String?
+
+    /// The name of this charset
+    let name: String
+
+    /// The aliases of this charset
+    let aliases: Array<String>
+
+    var representativeName: String {
+        return preferredMIMEName ?? name
+    }
+
+    var swiftCodeLines: [String] {
+        var lines: [String] = []
+        lines.append("/// IANA Charset `\(representativeName)`.")
+        lines.append("static let \(representativeName._camelcased()) = IANACharset(")
+        lines.append("\(swiftCodeIndent)preferredMIMEName: \(preferredMIMEName.map { #""\#($0)""# } ?? "nil"),")
+        lines.append("\(swiftCodeIndent)name: \"\(name)\",")
+        lines.append("\(swiftCodeIndent)aliases: [")
+        for alias in aliases {
+            lines.append("\(swiftCodeIndent)\(swiftCodeIndent)\"\(alias)\",")
+        }
+        lines.append("\(swiftCodeIndent)]")
+        lines.append(")")
+        return lines
+    }
+
+    init(_ node: XMLNode) throws {
+        guard let name = try node.nodes(forXPath: "./name").first?.stringValue else {
+            throw CodeGenerationError.missingName
+        }
+        self.name = name
+        self.preferredMIMEName = try node.nodes(forXPath: "./preferred_alias").first?.stringValue
+        self.aliases = try node.nodes(forXPath: "./alias").map {
+            guard let alias = $0.stringValue else {
+                throw CodeGenerationError.missingAliasValue
+            }
+            return alias
+        }
+    }
+}
+
+func generateSwiftCode() throws -> String {
+    let charsetsXMLDocument = try XMLDocument(contentsOf: charsetsXMLURL)
+    guard let charsetsXMLRoot = charsetsXMLDocument.rootElement() else {
+        throw CodeGenerationError.noRootElement
+    }
+    let charsetsXMLRecordElements = try charsetsXMLRoot.nodes(forXPath: "./registry/record")
+
+    var result = "extension IANACharset {"
+
+    for record in try charsetsXMLRecordElements.map({
+        try IANACharsetNameRecord($0)
+    }) where requiredCharsetNames.contains(record.representativeName) {
+        result += "\n"
+        result += record.swiftCodeLines.map({ swiftCodeIndent + $0 }).joined(separator: "\n")
+        result += "\n"
+    }
+
+    result += "}\n"
+    return result
+}
+
+#if PRINT_CODE
+print(try generateSwiftCode())
+#endif
+
+// MARK: - Extensions
+
+extension UTF8.CodeUnit {
+    var _isASCIINumeric: Bool { (0x30...0x39).contains(self) }
+    var _isASCIIUppercase: Bool { (0x41...0x5A).contains(self) }
+    var _isASCIILowercase: Bool { (0x61...0x7A).contains(self) }
+}
+
+extension String {
+    func _camelcased() -> String {
+        var result = ""
+        var previousWord: Substring.UTF8View? = nil
+        for wordUTF8 in self.utf8.split(whereSeparator: {
+            !$0._isASCIINumeric &&
+            !$0._isASCIIUppercase &&
+            !$0._isASCIILowercase
+        }) {
+            defer {
+                previousWord = wordUTF8
+            }
+            let word = String(Substring(wordUTF8))
+            guard let previousWord else {
+                result += word.lowercased()
+                continue
+            }
+            if previousWord.last!._isASCIINumeric && wordUTF8.first!._isASCIINumeric {
+                result += "_"
+            }
+            if let firstNonNumericIndex = wordUTF8.firstIndex(where: { !$0._isASCIINumeric }),
+               wordUTF8[firstNonNumericIndex...].allSatisfy({ $0._isASCIIUppercase }) {
+                result += word
+            } else {
+                result += word.capitalized(with: nil)
+            }
+
+        }
+        return result
+    }
+}