[ip6] update Ip6::Prefix comparison (#7813)

This commit updates `Ip6::Prefix` comparison (overload of `<`
operator). If the two prefixes have same length N, then the bytes are
compared directly (as two big-endian N-bit numbers). If the two
prefix have different lengths, the shorter prefix is padded by `0`
bit up to the longer prefix length N before the bytes are compared
(as big-endian N-bit numbers). If all bytes are equal, the prefix
with shorter length is considered smaller.

This commit also updates `test_ip_address` unit test to validate
the new comparison behavior.

src/core/net/ip6_address.cpp

@@ -77,13 +77,17 @@ bool Prefix::IsEqual(const uint8_t *aPrefixBytes, uint8_t aPrefixLength) const
 bool Prefix::operator<(const Prefix &aOther) const
 {
     bool    isSmaller;
+    uint8_t minLength;
     uint8_t matchedLength;
 
-    VerifyOrExit(GetLength() == aOther.GetLength(), isSmaller = GetLength() < aOther.GetLength());
+    minLength     = OT_MIN(GetLength(), aOther.GetLength());
+    matchedLength = MatchLength(GetBytes(), aOther.GetBytes(), SizeForLength(minLength));
 
-    matchedLength = MatchLength(GetBytes(), aOther.GetBytes(), GetBytesSize());
-
-    VerifyOrExit(matchedLength < GetLength(), isSmaller = false);
+    if (matchedLength >= minLength)
+    {
+        isSmaller = (GetLength() < aOther.GetLength());
+        ExitNow();
+    }
 
     isSmaller = GetBytes()[matchedLength / CHAR_BIT] < aOther.GetBytes()[matchedLength / CHAR_BIT];
 

src/core/net/ip6_address.hpp

@@ -266,8 +266,10 @@ public:
     /**
      * This method overloads operator `<` to compare two prefixes.
      *
-     * A prefix with shorter length is considered smaller than the one with longer length. If the prefix lengths are
-     * equal, then the prefix bytes are compared directly.
+     * If the two prefixes have the same length N, then the bytes are compared directly (as two big-endian N-bit
+     * numbers). If the two prefix have different lengths, the shorter prefix is padded by `0` bit up to the longer
+     * prefix length N before the bytes are compared (as big-endian N-bit numbers). If all bytes are equal, the prefix
+     * with shorter length is considered smaller.
      *
      * @param[in] aOther  The other prefix to compare against.
      *

tests/unit/test_ip_address.cpp

@@ -274,6 +274,17 @@ void TestIp6AddressSetPrefix(void)
     }
 }
 
+ot::Ip6::Prefix PrefixFrom(const char *aAddressString, uint8_t aPrefixLength)
+{
+    ot::Ip6::Prefix  prefix;
+    ot::Ip6::Address address;
+
+    SuccessOrQuit(address.FromString(aAddressString));
+    prefix.Set(address.GetBytes(), aPrefixLength);
+
+    return prefix;
+}
+
 void TestIp6Prefix(void)
 {
     const uint8_t kPrefixes[][OT_IP6_ADDRESS_SIZE] = {
@@ -351,6 +362,38 @@ void TestIp6Prefix(void)
             }
         }
     }
+
+    {
+        struct TestCase
+        {
+            ot::Ip6::Prefix mPrefixA;
+            ot::Ip6::Prefix mPrefixB;
+        };
+
+        TestCase kTestCases[] = {
+            {PrefixFrom("fd00::", 16), PrefixFrom("fd01::", 16)},
+            {PrefixFrom("fc00::", 16), PrefixFrom("fd00::", 16)},
+            {PrefixFrom("fd00::", 15), PrefixFrom("fd00::", 16)},
+            {PrefixFrom("fd00::", 16), PrefixFrom("fd00:0::", 32)},
+            {PrefixFrom("2001:0:0:0::", 64), PrefixFrom("fd00::", 8)},
+            {PrefixFrom("2001:dba::", 32), PrefixFrom("fd12:3456:1234:abcd::", 64)},
+            {PrefixFrom("910b:1000:0::", 48), PrefixFrom("910b:2000::", 32)},
+            {PrefixFrom("::", 0), PrefixFrom("fd00::", 8)},
+            {PrefixFrom("::", 0), PrefixFrom("::", 16)},
+            {PrefixFrom("fd00:2:2::", 33), PrefixFrom("fd00:2:2::", 35)},
+            {PrefixFrom("1:2:3:ffff::", 62), PrefixFrom("1:2:3:ffff::", 63)},
+        };
+
+        printf("\nCompare Prefixes:\n");
+
+        for (const TestCase &testCase : kTestCases)
+        {
+            printf(" %26s  <  %s\n", testCase.mPrefixA.ToString().AsCString(),
+                   testCase.mPrefixB.ToString().AsCString());
+            VerifyOrQuit(testCase.mPrefixA < testCase.mPrefixB);
+            VerifyOrQuit(!(testCase.mPrefixB < testCase.mPrefixA));
+        }
+    }
 }
 
 void TestIp4Ip6Translation(void)