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[core] Optimize format_hex_internal by eliminating per-byte length check

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Remove the per-byte `i < length - 1` check in the separator path by
writing the separator unconditionally after each byte and overwriting
the last one with the null terminator.

Single-loop approach keeps code smaller than the original while
eliminating the hot-path branch. Flash: -16 bytes vs baseline.
This commit is contained in:
J. Nick Koston
2026-04-08 20:41:09 -10:00
parent 671e434400
commit bd66f26144
1 changed files with 19 additions and 33 deletions
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esphome/core/helpers.cpp
+19 -33
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@@ -348,48 +348,34 @@ std::string format_mac_address_pretty(const uint8_t *mac) {
}
// Internal helper for hex formatting - base is 'a' for lowercase or 'A' for uppercase.
// Splits into two loops to eliminate per-byte branch on separator.
// When separator is set, it is written unconditionally after each byte and the last
// one is overwritten with '\0', eliminating the per-byte `i < length - 1` check.
static char *format_hex_internal(char *buffer, size_t buffer_size, const uint8_t *data, size_t length, char separator,
char base) {
if (length == 0) {
buffer[0] = '\0';
return buffer;
}
if (separator) {
// With separator: "XX:XX:...:XX\0" = length * 3 bytes total
size_t max_bytes = buffer_size / 3;
if (max_bytes == 0) {
buffer[0] = '\0';
return buffer;
}
if (length > max_bytes) {
length = max_bytes;
}
for (size_t i = 0; i < length; i++) {
size_t pos = i * 3;
buffer[pos] = format_hex_char(data[i] >> 4, base);
buffer[pos + 1] = format_hex_char(data[i] & 0x0F, base);
uint8_t stride = separator ? 3 : 2;
size_t max_bytes = separator ? (buffer_size / 3) : ((buffer_size - 1) / 2);
if (max_bytes == 0) {
buffer[0] = '\0';
return buffer;
}
if (length > max_bytes) {
length = max_bytes;
}
for (size_t i = 0; i < length; i++) {
size_t pos = i * stride;
buffer[pos] = format_hex_char(data[i] >> 4, base);
buffer[pos + 1] = format_hex_char(data[i] & 0x0F, base);
if (separator) {
buffer[pos + 2] = separator;
}
// Overwrite last separator with null terminator
buffer[length * 3 - 1] = '\0';
} else {
// Without separator: "XXXX...XX\0" = length * 2 + 1 bytes total
size_t max_bytes = (buffer_size - 1) / 2;
if (max_bytes == 0) {
buffer[0] = '\0';
return buffer;
}
if (length > max_bytes) {
length = max_bytes;
}
for (size_t i = 0; i < length; i++) {
size_t pos = i * 2;
buffer[pos] = format_hex_char(data[i] >> 4, base);
buffer[pos + 1] = format_hex_char(data[i] & 0x0F, base);
}
buffer[length * 2] = '\0';
}
// With separator: overwrite last separator with '\0'
// Without: write '\0' after last hex char
buffer[length * stride - (separator ? 1 : 0)] = '\0';
return buffer;
}