constexpr size_t bufSize = 128;
uint16_t indexes[64];
char buf[bufSize];

int16_t i, line_len, line_idx;

void setup() {
  delay(250);
  Serial.begin(115200); Serial.println();

  memset(buf, '\0', bufSize);
  strcpy(buf, "hello me mate\nthis is a text!\nwith mult lines and then some more.\nline1\nline2\nline3\nabcdefghijklmno 1234567890\n");
  Serial.println("buf: "); Serial.println(buf); Serial.println("\n---------------------\n");

  line_len = 0;
  line_idx = 0;

  indexes[0] = 0;

  for (i=0; i<strlen(buf); i++) {
    Serial.print(line_idx);
    Serial.print(":");
    Serial.print(line_len);
    Serial.print(" '");
    Serial.print(buf[i]);
    Serial.println("'");

    line_len++;

    if ((buf[i] == '\n' || buf[i] == '\n') || (line_len > 21)) {
      line_len = 0,
      indexes[++line_idx] = i+1;
    }
  }
  Serial.println("\n---------------------\n");

  for (int n=0; n<line_idx; n++) {
    Serial.print(n); Serial.print(" "); Serial.print(indexes[n]);
    Serial.print(" -> "); Serial.println(buf[(indexes[n])]);
  }
}

void loop() {
  delay(10);
}