Fix control-byte computation at boundary sizes

`writeCtrlByte` writes the left-over value using:

   byte((leftOver >> (8 * i)) & 0xFF)

This means that, after the `leftOver` value has been shifted, it should have a value in the [0,255] range so that the bitmask operation can work properly.

Currently, because the switch case determining `leftOver` is inclusive (<=) rather than inclusive (<), the shifted `leftOver` value can reach the value of 256, making an MMDB file invalid.

mmdbtype/types.go

@@ -723,23 +723,23 @@ func writeCtrlByte(w writer, t DataType) (int64, error) {
 	switch {
 	case size < firstSize:
 		firstByte |= byte(size)
-	case size <= secondSize:
+	case size < secondSize:
 		firstByte |= 29
 		leftOver = size - firstSize
 		leftOverSize = 1
-	case size <= thirdSize:
+	case size < thirdSize:
 		firstByte |= 30
 		leftOver = size - secondSize
 		leftOverSize = 2
-	case size <= maxSize:
+	case size < maxSize:
 		firstByte |= 31
 		leftOver = size - thirdSize
 		leftOverSize = 3
 	default:
 		return 0, fmt.Errorf(
 			"cannot store %d bytes; max size is %d",
 			size,
-			maxSize,
+			maxSize-1,
 		)
 	}
 

tree_test.go

@@ -6,6 +6,7 @@ import (
 	"math/big"
 	"net"
 	"os"
+	"strings"
 	"testing"
 	"time"
 
@@ -110,6 +111,36 @@ func TestTreeInsertAndGet(t *testing.T) {
 		"utf8_string": "unicode! ☯ - ♫",
 	}
 
+	stringsGetRecord := mmdbtype.Map{
+		// firstSize
+		"size28": mmdbtype.String(strings.Repeat("*", 28)),
+		"size29": mmdbtype.String(strings.Repeat("*", 29)),
+		"size30": mmdbtype.String(strings.Repeat("*", 30)),
+		// secondSize
+		"size284": mmdbtype.String(strings.Repeat("*", 284)),
+		"size285": mmdbtype.String(strings.Repeat("*", 285)),
+		"size286": mmdbtype.String(strings.Repeat("*", 286)),
+		// thirdSize
+		"size65820": mmdbtype.String(strings.Repeat("*", 65820)),
+		"size65821": mmdbtype.String(strings.Repeat("*", 65821)),
+		"size65822": mmdbtype.String(strings.Repeat("*", 65822)),
+		// maxSize
+		"maxSizeMinus1": mmdbtype.String(strings.Repeat("*", 16843036)),
+	}
+
+	var stringsLookupRecord any = map[string]any{
+		"size28":        strings.Repeat("*", 28),
+		"size29":        strings.Repeat("*", 29),
+		"size30":        strings.Repeat("*", 30),
+		"size284":       strings.Repeat("*", 284),
+		"size285":       strings.Repeat("*", 285),
+		"size286":       strings.Repeat("*", 286),
+		"size65820":     strings.Repeat("*", 65820),
+		"size65821":     strings.Repeat("*", 65821),
+		"size65822":     strings.Repeat("*", 65822),
+		"maxSizeMinus1": strings.Repeat("*", 16843036),
+	}
+
 	tests := []struct {
 		name                    string
 		disableIPv4Aliasing     bool
@@ -536,6 +567,26 @@ func TestTreeInsertAndGet(t *testing.T) {
 			},
 			expectedNodeCount: 375,
 		},
+		{
+			name: "insertion of strings at boundary control byte size",
+			inserts: []testInsert{
+				{
+					network: "1.1.1.1/32",
+					start:   "1.1.1.1",
+					end:     "1.1.1.1",
+					value:   stringsGetRecord,
+				},
+			},
+			gets: []testGet{
+				{
+					ip:                  "1.1.1.1",
+					expectedNetwork:     "1.1.1.1/32",
+					expectedGetValue:    stringsGetRecord,
+					expectedLookupValue: &stringsLookupRecord,
+				},
+			},
+			expectedNodeCount: 375,
+		},
 	}
 
 	for _, recordSize := range []int{24, 28, 32} {