[libc-commits] [libc] [libc] implement l64a (PR #129099)

[via libc-commits]

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@@ -0,0 +1,64 @@
+//===-- Implementation of l64a --------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/stdlib/l64a.h"
+#include "hdr/types/size_t.h"
+#include "src/__support/common.h"
+#include "src/__support/ctype_utils.h"
+#include "src/__support/libc_assert.h"
+#include "src/__support/macros/config.h"
+
+#include <stdint.h>
+
+namespace LIBC_NAMESPACE_DECL {
+
+// the standard says to only use up to 6 characters. Null terminator is
+// unnecessary, but we'll add it for ease-of-use. Also going from 48 -> 56 bits
+// probably won't matter since it's likely 32-bit aligned anyways.
+constexpr size_t MAX_BASE64_LENGTH = 6;
+LIBC_THREAD_LOCAL char BASE64_BUFFER[MAX_BASE64_LENGTH + 1];
+
+constexpr static char b64_int_to_char(uint32_t num) {
+  // from the standard: "The characters used to represent digits are '.' (dot)
+  // for 0, '/' for 1, '0' through '9' for [2,11], 'A' through 'Z' for [12,37],
+  // and 'a' through 'z' for [38,63]."
+  LIBC_ASSERT(num < 64);
+  if (num == 0)
+    return '.';
+  if (num == 1)
+    return '/';
+  if (num < 38)
+    return static_cast<char>(
+        internal::toupper(internal::int_to_b36_char(num - 2)));
+
+  // this tolower is technically unnecessary, but it provides safety if we
+  // change the default behavior of int_to_b36_char. Also the compiler
+  // completely elides it so there's no performance penalty, see:
+  // https://godbolt.org/z/sfxdhYvYb TODO: UPDATE THIS LINK
+  return static_cast<char>(
+      internal::tolower(internal::int_to_b36_char(num - 2 - 26)));
+}
+
+// This function takes a long and converts the low 32 bits of it into at most 6
----------------
lntue wrote:

Can you leave a TODO with an issue to use `LIBC_ADD_NULL_CHECKS` for the UB with null pointer input?

https://github.com/llvm/llvm-project/pull/129099