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GH-49720: [C++] Optimize base64_decode validation using lookup table #49748

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GH-49720: [C++] Optimize base64_decode validation using lookup table #49748

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22 changes: 14 additions & 8 deletions cpp/src/arrow/vendored/base64.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -30,6 +30,7 @@
*/

#include "arrow/util/base64.h"
#include <array>
#include <iostream>

namespace arrow {
Expand All @@ -40,6 +41,17 @@ static const std::string base64_chars =
"abcdefghijklmnopqrstuvwxyz"
"0123456789+/";

static const std::array<int8_t, 256> kBase64Lookup = [] {
std::array<int8_t, 256> table{};
table.fill(-1);

for (size_t i = 0; i < base64_chars.size(); ++i) {
table[static_cast<uint8_t>(base64_chars[i])] = i;
}

return table;
}();

static std::string base64_encode(unsigned char const* bytes_to_encode, unsigned int in_len) {
std::string ret;
int i = 0;
Expand Down Expand Up @@ -119,22 +131,16 @@ Result<std::string> base64_decode(std::string_view encoded_string) {
return Status::Invalid("Invalid base64 input: padding in wrong position");
}

if (base64_chars.find(c) == std::string::npos) {
if (kBase64Lookup[static_cast<uint8_t>(c)] == -1) {
return Status::Invalid("Invalid base64 input: character is not valid base64 character");
}

char_array_4[i++] = c;
char_array_4[i++] = kBase64Lookup[static_cast<uint8_t>(c)];
}

in_++;

if (i == 4) {
for (i = 0; i < 4; i++) {
if (char_array_4[i] != 0) {
char_array_4[i] = base64_chars.find(char_array_4[i]) & 0xff;
}
}

char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];
Expand Down
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