static const char kBase64Chars[] =

"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

static const char kBase64PadChar = '=';

int in_i = 0;

int out_i = 0;

size_t out_size = ((in_size + 2) / 3) * 4;

string out(out_size, 0);

while (in_i + 2 < in_size) {

uint32 in_bytes = in[in_i] << 16 | in[in_i + 1] << 8 | in[in_i + 2];

out[out_i++] = kBase64Chars[in_bytes >> 18];

out[out_i++] = kBase64Chars[(in_bytes >> 12) & 0x3F];

out[out_i++] = kBase64Chars[(in_bytes >> 6) & 0x3F];

out[out_i++] = kBase64Chars[in_bytes & 0x3F];

in_i += 3;

}

if (in_i + 1 == in_size) {

uint32 in_bytes = in[in_i] << 16;

out[out_i++] = kBase64Chars[in_bytes >> 18];

out[out_i++] = kBase64Chars[(in_bytes >> 12) & 0x3F];

out[out_i++] = kBase64PadChar;

out[out_i++] = kBase64PadChar;

} else if (in_i + 2 == in_size) {

uint32 in_bytes = in[in_i] << 16 | in[in_i + 1] << 8;

out[out_i++] = kBase64Chars[in_bytes >> 18];

out[out_i++] = kBase64Chars[(in_bytes >> 12) & 0x3F];

out[out_i++] = kBase64Chars[(in_bytes >> 6) & 0x3F];

out[out_i++] = kBase64PadChar;

}

return out;

int valid_bytes_read = 0;

uint32 code_point = 0;

int code_point_length_remaining = 0;

for (int i = 0; i < in_size; i++) {

char cur = in[i];

if (code_point_length_remaining) {

if ((cur & 0xC0) != 0x80) {

// Continuation byte not of the form 0b10XXXXXX.

return valid_bytes_read;

}

code_point = (code_point << 6) | (cur & 0x3F);

code_point_length_remaining--;

if (code_point_length_remaining == 0) {

int code_point_length = i + 1 - valid_bytes_read;

if ((code_point_length == 2 && code_point < 0x80) ||

(code_point_length == 3 && code_point < 0x800) ||

(code_point_length == 4 && code_point < 0x10000)) {

// https://en.wikipedia.org/wiki/UTF-8#Overlong_encodings

return valid_bytes_read;

}

if ((code_point >= 0xD800 && code_point <= 0xDFFF) ||

code_point > 0x10FFFF) {

// https://en.wikipedia.org/wiki/UTF-8#Invalid_code_points

return valid_bytes_read;

}

valid_bytes_read = i + 1;

code_point = 0;

}

} else if (!(cur & 0x80)) {

valid_bytes_read = i + 1;

} else if ((cur & 0xE0) == 0xC0) {

code_point = cur & 0x1F;

code_point_length_remaining = 1;

} else if ((cur & 0xF0) == 0xE0) {

code_point = cur & 0x0F;

code_point_length_remaining = 2;

} else if ((cur & 0xF8) == 0xf0) {

code_point = cur & 0x07;

code_point_length_remaining = 3;

} else {

// Unexpected continuation byte or invalid leadoff byte.

return valid_bytes_read;

}

}

return valid_bytes_read;