diff --git a/internal/hash/hash_test.go b/internal/hash/hash_test.go
new file mode 100644
index 0000000..1389cfd
--- /dev/null
+++ b/internal/hash/hash_test.go
@@ -0,0 +1,73 @@
+package hash
+
+import (
+	"hash/fnv"
+	"testing"
+)
+
+func TestFNV64a(t *testing.T) {
+	cases := []struct {
+		input string
+		want  uint64
+	}{
+		{"", 0xcbf29ce484222325},
+		{"hello", 0xa430d84680aabd0b},
+		{"a", 0xaf63dc4c8601ec8c},
+		{"The quick brown fox jumps over the lazy dog", 0xf3f9b7f5e7e47110},
+	}
+
+	for _, tc := range cases {
+		got := FNV64a(tc.input)
+		if got != tc.want {
+			t.Errorf("FNV64a(%q) = %#x, want %#x", tc.input, got, tc.want)
+		}
+
+		// Cross-check with stdlib FNV-1a implementation.
+		h := fnv.New64a()
+		_, _ = h.Write([]byte(tc.input))
+		if got != h.Sum64() {
+			t.Errorf("FNV64a(%q) = %#x, stdlib FNV-1a = %#x", tc.input, got, h.Sum64())
+		}
+	}
+}
+
+func TestFNV64aBytes(t *testing.T) {
+	cases := []struct {
+		input []byte
+		want  uint64
+	}{
+		{[]byte{}, 0xcbf29ce484222325},
+		{[]byte("hello"), 0xa430d84680aabd0b},
+		{[]byte{0x00, 0x01, 0xff}, 0xd94a37186c0d38bf},
+	}
+
+	for _, tc := range cases {
+		got := FNV64aBytes(tc.input)
+		if got != tc.want {
+			t.Errorf("FNV64aBytes(%q) = %#x, want %#x", tc.input, got, tc.want)
+		}
+
+		h := fnv.New64a()
+		_, _ = h.Write(tc.input)
+		if got != h.Sum64() {
+			t.Errorf("FNV64aBytes(%q) = %#x, stdlib FNV-1a = %#x", tc.input, got, h.Sum64())
+		}
+	}
+}
+
+func TestFNV64aAndBytesConsistency(t *testing.T) {
+	inputs := []string{
+		"",
+		"hello",
+		"lolly",
+		"The quick brown fox jumps over the lazy dog",
+	}
+
+	for _, s := range inputs {
+		fromString := FNV64a(s)
+		fromBytes := FNV64aBytes([]byte(s))
+		if fromString != fromBytes {
+			t.Errorf("FNV64a(%q) = %#x, FNV64aBytes(%q) = %#x", s, fromString, s, fromBytes)
+		}
+	}
+}