[llvm] Create a EncodingConverter class with both iconv and icu support. (PR #138893)

[Hubert Tong via llvm-commits]

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+//===-- TextEncoding.cpp - Encoding conversion class --------------*- C++ -*-=//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file provides utility classes to convert between different character
+/// encodings.
+///
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/TextEncoding.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringExtras.h"
+#include "llvm/Support/ConvertEBCDIC.h"
+#include "llvm/Support/raw_ostream.h"
+#include <algorithm>
+#include <limits>
+#include <system_error>
+
+#if HAVE_ICU
+#include <unicode/ucnv.h>
+#elif HAVE_ICONV
+#include <iconv.h>
+#endif
+
+using namespace llvm;
+
+// Normalize the charset name with the charset alias matching algorithm proposed
+// in https://www.unicode.org/reports/tr22/tr22-8.html#Charset_Alias_Matching.
+static void normalizeCharSetName(StringRef CSName,
+                                 SmallVectorImpl<char> &Normalized) {
+  bool PrevDigit = false;
+  for (auto Ch : CSName) {
+    if (isAlnum(Ch)) {
+      Ch = toLower(Ch);
+      if (Ch != '0' || PrevDigit) {
+        PrevDigit = isDigit(Ch);
+        Normalized.push_back(Ch);
+      }
+    }
+  }
+}
+
+// Maps the encoding name to enum constant if possible.
+static std::optional<TextEncoding> getKnownEncoding(StringRef Name) {
+  SmallString<16> Normalized;
+  normalizeCharSetName(Name, Normalized);
+  if (Normalized.equals("utf8"))
+    return TextEncoding::UTF8;
+  if (Normalized.equals("ibm1047"))
+    return TextEncoding::IBM1047;
+  return std::nullopt;
+}
+
+LLVM_ATTRIBUTE_UNUSED static void
+HandleOverflow(size_t &Capacity, char *&Output, size_t &OutputLength,
+               SmallVectorImpl<char> &Result) {
+  // No space left in output buffer. Double the size of the underlying
+  // memory in the SmallVectorImpl, adjust pointer and length and continue
+  // the conversion.
+  Capacity =
+      (Capacity < Result.max_size() / 2) ? 2 * Capacity : Result.max_size();
+  Result.resize(0);
+  Result.resize_for_overwrite(Capacity);
+  Output = static_cast<char *>(Result.data());
+  OutputLength = Capacity;
+}
+
+namespace {
+enum ConversionType {
+  UTF8ToIBM1047,
+  IBM1047ToUTF8,
+};
+
+// Support conversion between EBCDIC 1047 and UTF-8. This class uses
+// built-in translation tables that allow for translation between the
+// aforementioned encodings. The use of tables for conversion is only
+// possible because EBCDIC 1047 is a single-byte, stateless encoding; other
+// encodings are not supported.
+class TextEncodingConverterTable
+    : public details::TextEncodingConverterImplBase {
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hubert-reinterpretcast wrote:

I think it makes sense for this to be marked `final`.

https://github.com/llvm/llvm-project/pull/138893