[llvm] Create a CharSetConverter class with both iconv and icu support (PR #74516)

[Hubert Tong via llvm-commits]

================
@@ -0,0 +1,321 @@
+//===-- CharSet.cpp - Characters sets 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
+/// set encodings.
+///
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/CharSet.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>
+
+#ifdef HAVE_ICU
+#include <unicode/ucnv.h>
+#elif defined(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 charset name to enum constant if possible.
+static std::optional<text_encoding::id> getKnownCharSet(StringRef CSName) {
+  SmallString<16> Normalized;
+  normalizeCharSetName(CSName, Normalized);
+  if (Normalized.equals("utf8"))
+    return text_encoding::id::UTF8;
+  if (Normalized.equals("ibm1047"))
+    return text_encoding::id::IBM1047;
+  return std::nullopt;
+}
+
+#if defined(HAVE_ICONV) || defined(HAVE_ICU)
+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 < std::numeric_limits<size_t>::max() / 2)
+                 ? 2 * Capacity
+                 : std::numeric_limits<size_t>::max();
+  Result.resize(0);
+  Result.resize_for_overwrite(Capacity);
+  Output = static_cast<char *>(Result.data());
+  OutputLength = Capacity;
+}
+#endif
+
+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 character sets. The use of tables for conversion is only
+// possible because EBCDIC 1047 is a single-byte, stateless encoding; other
+// character sets are not supported.
+class CharSetConverterTable : public details::CharSetConverterImplBase {
+  ConversionType ConvType;
+
+public:
+  CharSetConverterTable(ConversionType ConvType) : ConvType(ConvType) {}
+
+  std::error_code convert(StringRef Source,
+                          SmallVectorImpl<char> &Result) const override;
+};
+
+std::error_code
+CharSetConverterTable::convert(StringRef Source,
+                               SmallVectorImpl<char> &Result) const {
+  if (ConvType == IBM1047ToUTF8) {
+    ConverterEBCDIC::convertToUTF8(Source, Result);
+    return std::error_code();
+  } else if (ConvType == UTF8ToIBM1047) {
+    return ConverterEBCDIC::convertToEBCDIC(Source, Result);
+  }
+  llvm_unreachable("Invalid ConvType!");
+  return std::error_code();
+}
+
+#ifdef HAVE_ICU
+struct UConverterDeleter {
+  void operator()(UConverter *Converter) const {
+    if (Converter)
+      ucnv_close(Converter);
+  }
+};
+using UConverterUniquePtr = std::unique_ptr<UConverter, UConverterDeleter>;
+
+class CharSetConverterICU : public details::CharSetConverterImplBase {
+  UConverterUniquePtr FromConvDesc;
+  UConverterUniquePtr ToConvDesc;
+
+public:
+  CharSetConverterICU(UConverterUniquePtr FromConverter,
+                      UConverterUniquePtr ToConverter)
+      : FromConvDesc(std::move(FromConverter)),
+        ToConvDesc(std::move(ToConverter)) {}
+
+  std::error_code convert(StringRef Source,
+                          SmallVectorImpl<char> &Result) const override;
+};
+
+std::error_code
+CharSetConverterICU::convert(StringRef Source,
+                             SmallVectorImpl<char> &Result) const {
+  // Setup the input in case it has no backing data.
+  size_t InputLength = Source.size();
+  const char *In = InputLength ? const_cast<char *>(Source.data()) : "";
+
+  // Setup the output. We directly write into the SmallVector.
+  size_t Capacity = Result.capacity();
+  size_t OutputLength = Capacity;
+  Result.resize_for_overwrite(Capacity);
+  char *Output = static_cast<char *>(Result.data());
+  UErrorCode EC = U_ZERO_ERROR;
+  do {
+    EC = U_ZERO_ERROR;
+    const char *Input = In;
+
+    Output = InputLength ? static_cast<char *>(Result.data()) : nullptr;
+    ucnv_convertEx(&*ToConvDesc, &*FromConvDesc, &Output, Result.end(), &Input,
+                   In + InputLength, /*pivotStart=*/NULL,
+                   /*pivotSource=*/NULL, /*pivotTarget=*/NULL,
+                   /*pivotLimit=*/NULL, /*reset=*/true,
+                   /*flush=*/true, &EC);
+    if (U_FAILURE(EC)) {
----------------
hubert-reinterpretcast wrote:

For some reason, this is not finding any error with input that ends with an incomplete character, e.g.:
```cpp
static const char EarthUTFExtraPartial[] =
    "\x45\x61\x72\x74\x68\xe5\x9c\xb0\xe7\x90\x83\xe5";
```


https://github.com/llvm/llvm-project/pull/74516