From 806997932ec63f2a961ee2e96ee1b7561ecae362 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Thu, 12 Feb 2026 05:40:10 +0000
Subject: [PATCH 1/2] Initial plan


From f8ce8840b504ecd27ae2433a43641796fdb30af9 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Thu, 12 Feb 2026 05:43:24 +0000
Subject: [PATCH 2/2] Fix default machine ID to respect BitsMachineID setting

Co-authored-by: YoshiyukiMineo <7577673+YoshiyukiMineo@users.noreply.github.com>
---
 v2/machine_bits_test.go | 86 +++++++++++++++++++++++++++++++++++++++++
 v2/mock/mock.go         | 10 +++++
 v2/sonyflake.go         |  7 +++-
 3 files changed, 102 insertions(+), 1 deletion(-)
 create mode 100644 v2/machine_bits_test.go

diff --git a/v2/machine_bits_test.go b/v2/machine_bits_test.go
new file mode 100644
index 0000000..1569930
--- /dev/null
+++ b/v2/machine_bits_test.go
@@ -0,0 +1,86 @@
+package sonyflake
+
+import (
+	"net"
+	"testing"
+
+	"github.com/sony/sonyflake/v2/mock"
+)
+
+// TestDefaultMachineIDWithCustomBits tests that when using default machine ID
+// with custom BitsMachineID, the machine ID is properly masked to fit within
+// the specified bit length.
+func TestDefaultMachineIDWithCustomBits(t *testing.T) {
+	testCases := []struct {
+		name          string
+		bitsMachineID int
+		mockIP        net.IP
+		expectError   bool
+	}{
+		{
+			name:          "10 bits machine ID with IP that fits",
+			bitsMachineID: 10,
+			mockIP:        net.IP{192, 168, 0, 1}, // lower 16 bits = 1, fits in 10 bits
+			expectError:   false,
+		},
+		{
+			name:          "10 bits machine ID with IP that exceeds without masking",
+			bitsMachineID: 10,
+			mockIP:        net.IP{192, 168, 255, 255}, // lower 16 bits = 65535, needs masking to fit in 10 bits
+			expectError:   false,
+		},
+		{
+			name:          "8 bits machine ID",
+			bitsMachineID: 8,
+			mockIP:        net.IP{192, 168, 100, 200}, // lower 16 bits = 25800
+			expectError:   false,
+		},
+		{
+			name:          "default 16 bits",
+			bitsMachineID: 0, // will use default 16
+			mockIP:        net.IP{192, 168, 255, 255},
+			expectError:   false,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			// Create a mock that returns our test IP
+			mockInterfaceAddrs := mock.NewInterfaceAddrsWithIP(tc.mockIP)
+
+			settings := Settings{
+				BitsMachineID: tc.bitsMachineID,
+			}
+
+			// Temporarily replace the default interface addrs function
+			oldDefaultInterfaceAddrs := defaultInterfaceAddrs
+			defaultInterfaceAddrs = mockInterfaceAddrs
+			defer func() { defaultInterfaceAddrs = oldDefaultInterfaceAddrs }()
+
+			sf, err := New(settings)
+
+			if tc.expectError {
+				if err == nil {
+					t.Errorf("expected error but got none")
+				}
+			} else {
+				if err != nil {
+					t.Errorf("unexpected error: %v", err)
+				}
+				if sf == nil {
+					t.Error("sonyflake instance should not be nil")
+				}
+
+				// Verify the machine ID fits within the specified bits
+				expectedBits := tc.bitsMachineID
+				if expectedBits == 0 {
+					expectedBits = defaultBitsMachine
+				}
+				maxMachineID := 1 << expectedBits
+				if sf.machine >= maxMachineID {
+					t.Errorf("machine ID %d exceeds max for %d bits (%d)", sf.machine, expectedBits, maxMachineID)
+				}
+			}
+		})
+	}
+}
diff --git a/v2/mock/mock.go b/v2/mock/mock.go
index 1bff6ab..639404b 100644
--- a/v2/mock/mock.go
+++ b/v2/mock/mock.go
@@ -34,3 +34,13 @@ func NewNilInterfaceAddrs() types.InterfaceAddrs {
 		return []net.Addr{}, nil
 	}
 }
+
+// NewInterfaceAddrsWithIP returns a private IP address with the given IP.
+func NewInterfaceAddrsWithIP(ip net.IP) types.InterfaceAddrs {
+	ifat := make([]net.Addr, 0, 1)
+	ifat = append(ifat, &net.IPNet{IP: ip, Mask: []byte{255, 0, 0, 0}})
+
+	return func() ([]net.Addr, error) {
+		return ifat, nil
+	}
+}
diff --git a/v2/sonyflake.go b/v2/sonyflake.go
index 0ca2be4..9ec2f6a 100644
--- a/v2/sonyflake.go
+++ b/v2/sonyflake.go
@@ -36,7 +36,8 @@ import (
 //
 // MachineID returns the unique ID of a Sonyflake instance.
 // If MachineID returns an error, the instance will not be created.
-// If MachineID is nil, the default MachineID is used, which returns the lower 16 bits of the private IP address.
+// If MachineID is nil, the default MachineID is used, which returns the lower bits
+// of the private IP address, masked to fit within BitsMachineID bits.
 //
 // CheckMachineID validates the uniqueness of a machine ID.
 // If CheckMachineID returns false, the instance will not be created.
@@ -154,6 +155,10 @@ func New(st Settings) (*Sonyflake, error) {
 	var err error
 	if st.MachineID == nil {
 		sf.machine, err = lower16BitPrivateIP(defaultInterfaceAddrs)
+		if err == nil {
+			// Mask to use only the required number of bits
+			sf.machine = sf.machine & (1<<sf.bitsMachine - 1)
+		}
 	} else {
 		sf.machine, err = st.MachineID()
 	}