Fixed code smells

protocol/adsb/fields/util.go

@@ -0,0 +1,78 @@
+package fields
+
+import "errors"
+
+// GillhamToAltitude convert an altitude given in Gillham bits to an altitude in feet.
+func GillhamToAltitude(d1, d2, d4, a1, a2, a4, b1, b2, b4, c1, c2, c4 bool) (int32, error) {
+	fiveHundredBits := uint16(0)
+	if d1 {
+		fiveHundredBits |= 0x0100
+	}
+	if d2 {
+		fiveHundredBits |= 0x0080
+	}
+	if d4 {
+		fiveHundredBits |= 0x0040
+	}
+	if a1 {
+		fiveHundredBits |= 0x0020
+	}
+	if a2 {
+		fiveHundredBits |= 0x0010
+	}
+	if a4 {
+		fiveHundredBits |= 0x0008
+	}
+	if b1 {
+		fiveHundredBits |= 0x0004
+	}
+	if b2 {
+		fiveHundredBits |= 0x0002
+	}
+	if b4 {
+		fiveHundredBits |= 0x0001
+	}
+
+	oneHundredBits := uint16(0)
+	if c1 {
+		oneHundredBits |= 0x0004
+	}
+	if c2 {
+		oneHundredBits |= 0x0002
+	}
+	if c4 {
+		oneHundredBits |= 0x0001
+	}
+
+	oneHundred := int32(grayToBinary(oneHundredBits))
+	fiveHundred := int32(grayToBinary(fiveHundredBits))
+
+	// Check for invalid codes.
+	if oneHundred == 5 || oneHundred == 6 || oneHundred == 0 {
+		return 0, errors.New("the bits C1 to to C4 are incorrect")
+	}
+
+	// Remove 7s from OneHundreds.
+	if oneHundred == 7 {
+		oneHundred = 5
+	}
+
+	// Correct order of OneHundreds.
+	if fiveHundred%2 != 0 {
+		oneHundred = 6 - oneHundred
+	}
+
+	// Convert to feet and apply altitude datum offset.
+	return (int32(fiveHundred)*500 + int32(oneHundred)*100) - 1300, nil
+}
+
+func grayToBinary(num uint16) uint16 {
+	temp := uint16(0)
+
+	temp = num ^ (num >> 8)
+	temp ^= temp >> 4
+	temp ^= temp >> 2
+	temp ^= temp >> 1
+
+	return temp
+}