Use typed errors for aliased and reserved inserts

These are two errors that it may make sense to handle on the
application side.

errors.go

@@ -0,0 +1,124 @@
+package mmdbwriter
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"net/netip"
+)
+
+// AliasedNetworkError is returned when inserting a aliased network into
+// a Tree where DisableIPv4Aliasing in Options is false.
+type AliasedNetworkError struct {
+	// AliasedNetwork is the aliased network being inserted into.
+	AliasedNetwork netip.Prefix
+	// InsertedNetwork is the network being inserted into the Tree.
+	InsertedNetwork netip.Prefix
+}
+
+func newAliasedNetworkError(netIP net.IP, curPrefixLen, recPrefixLen int) error {
+	anErr := &AliasedNetworkError{}
+	ip, ok := netip.AddrFromSlice(netIP)
+	if !ok {
+		return errors.Join(
+			fmt.Errorf("creating netip.Addr from %s", netIP),
+			anErr,
+		)
+	}
+	var err error
+	// We are using netip here despite using net.IP/net.IPNet internally as
+	// it seems quite likely that we will switch to netip throughout.
+	anErr.InsertedNetwork, err = ip.Prefix(recPrefixLen)
+	if err != nil {
+		return errors.Join(
+			fmt.Errorf(
+				"creating prefix from addr %s and prefix length %d: %w",
+				ip,
+				recPrefixLen,
+				err,
+			),
+			anErr,
+		)
+	}
+
+	anErr.AliasedNetwork, err = ip.Prefix(curPrefixLen)
+	if err != nil {
+		return errors.Join(
+			fmt.Errorf(
+				"creating prefix from addr %s and prefix length %d: %w",
+				ip,
+				curPrefixLen,
+				err,
+			),
+			anErr,
+		)
+	}
+	return anErr
+}
+
+func (r *AliasedNetworkError) Error() string {
+	return fmt.Sprintf(
+		"attempt to insert %s into %s, which is an aliased network",
+		r.InsertedNetwork,
+		r.AliasedNetwork,
+	)
+}
+
+// ReservedNetworkError is returned when inserting a reserved network into
+// a Tree where IncludeReservedNetworks in Options is false.
+type ReservedNetworkError struct {
+	// InsertedNetwork is the network being inserted into the Tree.
+	InsertedNetwork netip.Prefix
+	// ReservedNetwork is the reserved network being inserted into.
+	ReservedNetwork netip.Prefix
+}
+
+var _ error = &ReservedNetworkError{}
+
+func newReservedNetworkError(netIP net.IP, curPrefixLen, recPrefixLen int) error {
+	rnErr := &ReservedNetworkError{}
+	ip, ok := netip.AddrFromSlice(netIP)
+	if !ok {
+		return errors.Join(
+			fmt.Errorf("creating netip.Addr from %s", netIP),
+			rnErr,
+		)
+	}
+	var err error
+	// We are using netip here despite using net.IP/net.IPNet internally as
+	// it seems quite likely that we will switch to netip throughout.
+	rnErr.InsertedNetwork, err = ip.Prefix(recPrefixLen)
+	if err != nil {
+		return errors.Join(
+			fmt.Errorf(
+				"creating prefix from addr %s and prefix length %d: %w",
+				ip,
+				recPrefixLen,
+				err,
+			),
+			rnErr,
+		)
+	}
+
+	rnErr.ReservedNetwork, err = ip.Prefix(curPrefixLen)
+	if err != nil {
+		return errors.Join(
+			fmt.Errorf(
+				"creating prefix from addr %s and prefix length %d: %w",
+				ip,
+				curPrefixLen,
+				err,
+			),
+			rnErr,
+		)
+	}
+	return rnErr
+}
+
+func (r *ReservedNetworkError) Error() string {
+	return fmt.Sprintf(
+		"attempt to insert %s into %s, which is a reserved network",
+		r.InsertedNetwork,
+		r.ReservedNetwork,
+	)
+}

node.go

@@ -117,11 +117,7 @@ func (r *record) insert(
 		return r.maybeMergeChildren(iRec)
 	case recordTypeReserved:
 		if iRec.prefixLen >= newDepth {
-			return fmt.Errorf(
-				"attempt to insert %s/%d, which is in a reserved network",
-				iRec.ip,
-				iRec.prefixLen,
-			)
+			return newReservedNetworkError(iRec.ip, newDepth, iRec.prefixLen)
 		}
 		// If we are inserting a network that contains a reserved network,
 		// we silently remove the reserved network.
@@ -133,11 +129,7 @@ func (r *record) insert(
 			return nil
 		}
 		// attempting to insert _into_ an aliased network
-		return fmt.Errorf(
-			"attempt to insert %s/%d, which is in an aliased network",
-			iRec.ip,
-			iRec.prefixLen,
-		)
+		return newAliasedNetworkError(iRec.ip, newDepth, iRec.prefixLen)
 	default:
 		return fmt.Errorf("inserting into record type %d is not implemented", r.recordType)
 	}

tree_test.go

@@ -341,19 +341,19 @@ func TestTreeInsertAndGet(t *testing.T) {
 					network:          "10.0.0.0/8",
 					start:            "10.0.0.0",
 					end:              "10.255.255.255",
-					expectedErrorMsg: "attempt to insert ::a00:0/104, which is in a reserved network",
+					expectedErrorMsg: "attempt to insert ::a00:0/104 into ::a00:0/104, which is a reserved network",
 				},
 				{
 					network:          "10.0.0.1/32",
 					start:            "10.0.0.1",
 					end:              "10.0.0.1",
-					expectedErrorMsg: "attempt to insert ::a00:1/128, which is in a reserved network",
+					expectedErrorMsg: "attempt to insert ::a00:1/128 into ::a00:0/104, which is a reserved network",
 				},
 				{
 					network:          "2002:100::/24",
 					start:            "2002:100::",
 					end:              "2002:1ff:ffff:ffff:ffff:ffff:ffff:ffff",
-					expectedErrorMsg: "attempt to insert 2002:100::/24, which is in an aliased network",
+					expectedErrorMsg: "attempt to insert 2002:100::/24 into 2002::/16, which is an aliased network",
 				},
 			},
 			gets: []testGet{