builtins: refactor bit shifting logic in mapToUnorderedUniqueInt

This patch refactors the bit shifting logic in `mapToUnorderedUniqueInt`
to use constants instead of magic numbers.

Release note: None

Author: andyyang890

pkg/sql/sem/builtins/builtins.go

@@ -9796,11 +9796,13 @@ func GenerateUniqueUnorderedID(instanceID base.SQLInstanceID) tree.DInt {
 // serial unique uint64 to an unordered unique int64. It accomplishes this by
 // reversing the timestamp portion of the unique ID. This bit manipulation
 // should preserve the number of 1-bits.
-func mapToUnorderedUniqueInt(val uint64) uint64 {
+func mapToUnorderedUniqueInt(uniqueInt uint64) uint64 {
 	// val is [0][48 bits of ts][15 bits of node id]
-	ts := (val & ((uint64(math.MaxUint64) >> 16) << 15)) >> 15
-	v := (bits.Reverse64(ts) >> 1) | (val & (1<<15 - 1))
-	return v
+	ts := uniqueInt & builtinconstants.UniqueIntTimestampMask
+	nodeID := uniqueInt & builtinconstants.UniqueIntNodeIDMask
+	reversedTS := bits.Reverse64(ts<<builtinconstants.UniqueIntLeadingZeroBits) << builtinconstants.UniqueIntNodeIDBits
+	unorderedUniqueInt := reversedTS | nodeID
+	return unorderedUniqueInt
 }
 
 // ProcessUniqueID is an ID which is unique to this process in the cluster.

pkg/sql/sem/builtins/builtins_test.go

@@ -80,20 +80,54 @@ func TestGenerateUniqueIDOrder(t *testing.T) {
 	}
 }
 
-// TestMapToUniqueUnorderedID verifies that the mapping preserves the ones count.
-func TestMapToUniqueUnorderedID(t *testing.T) {
+func TestMapToUnorderedUniqueInt(t *testing.T) {
 	defer leaktest.AfterTest(t)()
-	for i := 0; i < 30; i++ {
-		// RandInput is [0][63 random bits].
-		randInput := uint64(rand.Int63())
-		output := mapToUnorderedUniqueInt(randInput)
-
-		inputOnesCount := bits.OnesCount64(randInput)
-		outputOnesCount := bits.OnesCount64(output)
-		require.Equalf(t, inputOnesCount, outputOnesCount, "input: %b, output: "+
-			"%b\nExpected: %d, got: %d", randInput, output, inputOnesCount,
-			outputOnesCount)
-	}
+	defer log.Scope(t).Close(t)
+
+	t.Run("preserves number of one bits", func(t *testing.T) {
+		for i := 0; i < 30; i++ {
+			// RandInput is [0][63 random bits].
+			randInput := uint64(rand.Int63())
+			output := mapToUnorderedUniqueInt(randInput)
+
+			inputOnesCount := bits.OnesCount64(randInput)
+			outputOnesCount := bits.OnesCount64(output)
+			require.Equalf(t, inputOnesCount, outputOnesCount, "input: %b, output: "+
+				"%b\nExpected: %d, got: %d", randInput, output, inputOnesCount,
+				outputOnesCount)
+		}
+	})
+
+	t.Run("correctly reverses timestamp", func(t *testing.T) {
+		for name, tc := range map[string]struct {
+			input    uint64
+			expected uint64
+		}{
+			"asymmetrical timestamp": {
+				input:    0b0101100000000000000000000000000000000000000000000000000000000001,
+				expected: 0b0000000000000000000000000000000000000000000001101000000000000001,
+			},
+			"symmetrical timestamp": {
+				input:    0b0100000000000000000000000000000000000000000000001000000000000101,
+				expected: 0b0100000000000000000000000000000000000000000000001000000000000101,
+			},
+			"max timestamp": {
+				input:    0b0111111111111111111111111111111111111111111111111000000000000001,
+				expected: 0b0111111111111111111111111111111111111111111111111000000000000001,
+			},
+		} {
+			t.Run(name, func(t *testing.T) {
+				actual := mapToUnorderedUniqueInt(tc.input)
+				require.Equalf(t,
+					tc.expected, actual,
+					"actual unordered unique int does not match expected:\n"+
+						"%064b (expected)\n"+
+						"%064b (actual)",
+					tc.expected, actual,
+				)
+			})
+		}
+	})
 }
 
 // TestSerialNormalizationWithUniqueUnorderedID makes sure that serial