Rewrite compareString() and compareStringn() for flash strings to be …

…slightly more compact, and defering (int) subtraction until the end

README.md

@@ -541,19 +541,19 @@ microcontrollers:
 ```
 Platform (resource)        |     Max | ArduinoUnit |       AUnit |
 ---------------------------+---------+-------------+-------------|
-Arduino Nano (flash)       |   30720 |       54038 |       18412 |
+Arduino Nano (flash)       |   30720 |       54038 |       18422 |
 Arduino Nano (static)      |    2048 |        1061 |         908 |
 ---------------------------+---------+-------------+-------------|
-Teensy LC (flash)          |   63488 |       36196 |       25104 |
+Teensy LC (flash)          |   63488 |       36196 |       25096 |
 Teensy LC (static)         |    8192 |        2980 |        2768 |
 ---------------------------+---------+-------------+-------------|
 Teensy 3.2 (flash)         |  262144 |       51236 |       36136 |
 Teensy 3.2 (static)        |   65536 |        5328 |        5224 |
 ---------------------------+---------+-------------+-------------|
-ESP8266 - ESP-12E (flash)  | 1044464 |    does not |      267375 |
+ESP8266 - ESP-12E (flash)  | 1044464 |    does not |      267359 |
 ESP8266 - ESP-12E (static) |   81920 |     compile |       34564 |
 ---------------------------+---------+-------------+-------------|
-ESP8266 - ESP-01 (flash)   |  499696 |    does not |      267375 |
+ESP8266 - ESP-01 (flash)   |  499696 |    does not |      267359 |
 ESP8266 - ESP-01 (static)  |   47356 |     compile |       34564 |
 ---------------------------+---------+-------------+-------------|
 ```

src/aunit/Compare.cpp

@@ -171,20 +171,17 @@ int compareString(const __FlashStringHelper* a, const char* b) {
   return -strcmp_P(b, (const char*) a);
 }
 
+// On ESP8266, pgm_read_byte() already takes care of 4-byte alignment, and
+// memcpy_P(s, p, 4) makes 4 calls to pgm_read_byte() anyway, so don't bother
+// optimizing for 4-byte alignment here.
 int compareString(const __FlashStringHelper* a, const __FlashStringHelper* b) {
   const char* aa = reinterpret_cast<const char*>(a);
   const char* bb = reinterpret_cast<const char*>(b);
 
-  // On ESP8266, pgm_read_byte() already takes care of 4-byte alignment, and
-  // memcpy_P(s, p, 4) makes 4 calls to pgm_read_byte() anyway, so don't bother
-  // optimizing for 4-byte alignment here.
   while (true) {
-    char ca = pgm_read_byte(aa);
-    char cb = pgm_read_byte(bb);
-    if (ca < cb) return -1;
-    if (ca > cb) return 1;
-    // we hit this condition only if both strings are the same length,
-    // so no need to check both strings for '\0'
+    uint8_t ca = pgm_read_byte(aa);
+    uint8_t cb = pgm_read_byte(bb);
+    if (ca != cb) return (int) ca - (int) cb;
     if (ca == '\0') return 0;
     aa++;
     bb++;
@@ -225,21 +222,18 @@ int compareStringN(const __FlashStringHelper* a, const char* b, size_t n) {
   return -strncmp_P(b, (const char*)a, n);
 }
 
+// On ESP8266, pgm_read_byte() already takes care of 4-byte alignment, and
+// memcpy_P(s, p, 4) makes 4 calls to pgm_read_byte() anyway, so don't bother
+// optimizing for 4-byte alignment here.
 int compareStringN(const __FlashStringHelper* a, const __FlashStringHelper* b,
     size_t n) {
   const char* aa = reinterpret_cast<const char*>(a);
   const char* bb = reinterpret_cast<const char*>(b);
 
-  // On ESP8266, pgm_read_byte() already takes care of 4-byte alignment, and
-  // memcpy_P(s, p, 4) makes 4 calls to pgm_read_byte() anyway, so don't bother
-  // optimizing for 4-byte alignment here.
   while (n > 0) {
-    char ca = pgm_read_byte(aa);
-    char cb = pgm_read_byte(bb);
-    if (ca < cb) return -1;
-    if (ca > cb) return 1;
-    // we hit this condition only if both strings are the same length,
-    // so no need to check both strings for '\0'
+    uint8_t ca = pgm_read_byte(aa);
+    uint8_t cb = pgm_read_byte(bb);
+    if (ca != cb) return (int) ca - (int) cb;
     if (ca == '\0') return 0;
     aa++;
     bb++;