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[core] Optimize base64 decoding fast path

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CodSpeed Bot
2026-10-08 00:20:39 +00:00
parent 883af036a9
commit 0523725530
1 changed files with 39 additions and 6 deletions
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esphome/core/helpers.cpp
+39 -6
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@@ -646,11 +646,40 @@ size_t base64_decode(const std::string &encoded_string, uint8_t *buf, size_t buf
}
size_t base64_decode(const uint8_t *encoded_data, size_t encoded_len, uint8_t *buf, size_t buf_len) {
size_t in = 0;
size_t out = 0;
bool truncated = false;
// Fast path: a full group of 4 characters becomes 3 bytes with no bit bookkeeping
// and a single bounds check. Bails out to the loop below on the last partial group,
// on padding ('=') and on any other character outside the alphabet.
while (in + 4 <= encoded_len && out + 3 <= buf_len) {
uint8_t v0 = base64_char_value(encoded_data[in]);
uint8_t v1 = base64_char_value(encoded_data[in + 1]);
uint8_t v2 = base64_char_value(encoded_data[in + 2]);
uint8_t v3 = base64_char_value(encoded_data[in + 3]);
// Alphabet values are <= 63, so any bit above bit 5 means one of the four
// characters was INVALID_BASE64_CHAR.
if (((v0 | v1 | v2 | v3) & 0xC0) != 0)
break;
uint32_t group =
(static_cast<uint32_t>(v0) << 18) | (static_cast<uint32_t>(v1) << 12) | (static_cast<uint32_t>(v2) << 6) | v3;
buf[out] = static_cast<uint8_t>(group >> 16);
buf[out + 1] = static_cast<uint8_t>(group >> 8);
buf[out + 2] = static_cast<uint8_t>(group);
in += 4;
out += 3;
}
// Bit accumulator: characters contribute 6 bits each and a byte is emitted as soon
// as 8 bits are available. Padding ('=') and any other character outside the
// alphabet stop the decode, leaving the leftover bits unused - this matches
// zero-padding a trailing partial group, so a group of 2 chars yields 1 byte and a
// group of 3 chars yields 2 bytes.
uint32_t accum = 0;
uint32_t bits = 0;
// Stops at '=' or any non-alphabet char; leftover bits of a partial group are dropped.
for (size_t in = 0; in < encoded_len; in++) {
for (; in < encoded_len; in++) {
uint8_t value = base64_char_value(encoded_data[in]);
if (value == INVALID_BASE64_CHAR)
break;
@@ -658,13 +687,17 @@ size_t base64_decode(const uint8_t *encoded_data, size_t encoded_len, uint8_t *b
bits += 6;
if (bits >= 8) {
bits -= 8;
if (out == buf_len) {
ESP_LOGW(TAG, "Base64 decode: buffer too small, truncating");
return out;
if (out < buf_len) {
buf[out++] = static_cast<uint8_t>(accum >> bits);
} else {
truncated = true;
}
buf[out++] = static_cast<uint8_t>(accum >> bits);
}
}
if (truncated) {
ESP_LOGW(TAG, "Base64 decode: buffer too small, truncating");
}
return out;
}