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Base64: constexpr initialization of alphabet and lookup table

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Problem:
- The Base64 alphabet and lookup table are initialized at
  run-time. This results in an initial start-up cost as well as a
  boolean evaluation and branch every time the function is called.

Solution:
- Provide `constexpr` functions which initialize the alphabet and
  lookup table at compile-time. These can be called and assigned to a
  `constexpr` variable so that there is no run-time cost associated
  with the initialization or lookup.
This commit is contained in:
Lenny Maiorani 2020-10-13 10:33:12 -04:00 • committed by Andreas Kling
parent b82da6b78b
commit 626bb1be9c
Notes: sideshowbarker 2024-07-19 01:54:53 +09:00
1 changed files with 32 additions and 17 deletions
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49
AK/Base64.cpp
View file

@ -24,6 +24,7 @@
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#include <AK/Array.h>
#include <AK/Base64.h>
#include <AK/ByteBuffer.h>
#include <AK/String.h>
@ -34,24 +35,37 @@
namespace AK {
static constexpr u8 s_alphabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
static u8 s_table[256] = {};
static bool s_initialized = false;
static void build_lookup_table_if_needed()
static constexpr auto make_alphabet()
{
if (s_initialized)
return;
for (size_t i = 0; i < sizeof(s_alphabet) - 1; ++i)
s_table[s_alphabet[i]] = i;
s_initialized = true;
// clang-format off
Array alphabet = {
'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
'w', 'x', 'y', 'z', '0', '1', '2', '3',
'4', '5', '6', '7', '8', '9', '+', '/'
};
// clang-format on
return alphabet;
}
static constexpr auto make_lookup_table()
{
constexpr auto alphabet = make_alphabet();
Array<u8, 256> table {};
for (size_t i = 0; i < alphabet.size() - 1; ++i) {
table[alphabet[i]] = i;
}
return table;
}
ByteBuffer decode_base64(const StringView& input)
{
build_lookup_table_if_needed();
auto get = [&](size_t offset, bool* is_padding = nullptr) -> u8 {
constexpr auto table = make_lookup_table();
if (offset >= input.length())
return 0;
if (input[offset] == '=') {
@ -59,7 +73,7 @@ ByteBuffer decode_base64(const StringView& input)
*is_padding = true;
return 0;
}
return s_table[(u8)input[offset]];
return table[input[offset]];
};
Vector<u8> output;
@ -89,6 +103,7 @@ ByteBuffer decode_base64(const StringView& input)
String encode_base64(ReadonlyBytes input)
{
constexpr auto alphabet = make_alphabet();
StringBuilder output;
auto get = [&](size_t offset, bool* need_padding = nullptr) -> u8 {
@ -113,10 +128,10 @@ String encode_base64(ReadonlyBytes input)
u8 index2 = ((in1 << 2) | (in2 >> 6)) & 0x3f;
u8 index3 = in2 & 0x3f;
u8 out0 = s_alphabet[index0];
u8 out1 = s_alphabet[index1];
u8 out2 = is_16bit ? '=' : s_alphabet[index2];
u8 out3 = is_8bit ? '=' : s_alphabet[index3];
u8 out0 = alphabet[index0];
u8 out1 = alphabet[index1];
u8 out2 = is_16bit ? '=' : alphabet[index2];
u8 out3 = is_8bit ? '=' : alphabet[index3];
output.append(out0);
output.append(out1);