playing with small string optimisation #5317
Description
@TD-er @tonhuisman puzzles here :)
Take a code from lines:
ESPEasy/src/src/Helpers/Networking.cpp
Lines 1406 to 1418 in cc2a7ae
pio run -e max_ESP32_16M1M_ETH
RAM: [== ] 19.2% (used 62768 bytes from 327680 bytes)
Flash: [====== ] 63.7% (used 2672485 bytes from 4194304 bytes)
Get rid of string and generate it in loop:
String getCNonce(const int len) {
char alphanum[26+26+10+1];
for (int i = 0; i < 26; ++i) {
if (i<10) alphanum[i] = '0'+i;
alphanum[i+10]= 'A'+i;
alphanum[i+10+26]= 'a'+i;
}
String s;
for (int i = 0; i < len; ++i) {
s += alphanum[rand() % (sizeof(alphanum) - 1)];
}
RAM: [== ] 19.2% (used 62768 bytes from 327680 bytes)
Flash: [====== ] 63.7% (used 2672469 bytes from 4194304 bytes)
So got -16 bytes of flash.
Now get rid with alphanum:
String getCNonce(const int len) {
String s;
const int numbers = '9'-'0'+ 1;
const int alphas = 'Z'-'A'+ 1;
const int allchars = 2*alphas+numbers;
char a;
for (int i = 0; i < len; ++i) {
int j = rand() % (allchars - 1);
if (j<numbers) a = ('0'+j);
else if (j<numbers+alphas) a = ('A'+j);
else a = ('a'+j);
s += a;
}
return s;
}
RAM: [== ] 19.2% (used 62768 bytes from 327680 bytes)
Flash: [====== ] 63.7% (used 2672453 bytes from 4194304 bytes)
So -32 bytes
add alphanum one more time (just for sizeof):
String getCNonce3(const int len) {
static const char alphanum[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
String s;
const int numbers = '9'-'0'+ 1;
const int alphas = 'Z'-'A'+ 1;
const int allchars = 2*alphas+numbers;
char a;
for (int i = 0; i < len; ++i) {
int j = rand() % (sizeof(alphanum) - 1);
if (j<numbers) a = ('0'+j);
else if (j<numbers+alphas) a = ('A'+j);
else a = ('a'+j);
s += a;
}
return s;
}
RAM: [== ] 19.2% (used 62768 bytes from 327680 bytes)
Flash: [====== ] 63.7% (used 2672429 bytes from 4194304 bytes)
So -56 bytes it is like a magic
how it is possible?