AP_Common: Make hexadecimal character number conversion method common

libraries/AP_Common/AP_Common.cpp

@@ -36,3 +36,37 @@ bool is_bounded_int32(int32_t value, int32_t lower_bound, int32_t upper_bound)
     return false;
 
 }
+
+/**
+ * return the numeric value of an ascii hex character
+ *
+ * @param [in]  a   Hexa character
+ * @param [out] res uint8 value
+ * @retval true  Conversion OK
+ * @retval false Input value error
+ * @Note Input character is 0-9, A-F, a-f
+ *  A 0x41, a 0x61, 0 0x30
+ */
+bool hex_to_uint8(uint8_t a, uint8_t &res)
+{
+    uint8_t nibble_low  = a & 0xf;
+
+    switch (a & 0xf0) {
+    case 0x30:  // 0-
+        if (nibble_low > 9) {
+            return false;
+        }
+        res = nibble_low;
+        break;
+    case 0x40:  // uppercase A-
+    case 0x60:  // lowercase a-
+        if (nibble_low == 0 || nibble_low > 6) {
+            return false;
+        }
+        res = nibble_low + 9;
+        break;
+    default:
+        return false;
+    }
+    return true;
+}

libraries/AP_Common/AP_Common.h

@@ -133,3 +133,5 @@ template<typename s, size_t t> struct assert_storage_size {
   False otherwise.
 */
 bool is_bounded_int32(int32_t value, int32_t lower_bound, int32_t upper_bound);
+
+bool hex_to_uint8(uint8_t a, uint8_t &res);  // return the uint8 value of an ascii hex character