Add encode method to convert Sections into GPP String

.gitignore

@@ -13,3 +13,8 @@
 
 # Dependency directories (remove the comment below to include it)
 # vendor/
+
+release/
+.idea/
+.vscode/
+.DS_Store

README.md

@@ -1,2 +1,8 @@
 # go-gpp
-Golang support for the IAB's GPP framework
+Golang support for the IAB's GPP framework.
+
+Provided the basic *Parse* and *Encode* methods for conversions between IAB-supported 
+sections and GPP strings, people other than CMPs are not encouraged to generate GPP strings 
+in production according to the IAB documentation.
+
+For more information, please refer to https://github.com/InteractiveAdvertisingBureau/Global-Privacy-Platform

encode.go

@@ -0,0 +1,73 @@
+package gpp
+
+import (
+	"errors"
+	"fmt"
+	"github.com/prebid/go-gpp/constants"
+	"github.com/prebid/go-gpp/util"
+	"sort"
+	"strings"
+)
+
+const (
+	// the first 6 bits of the header must always evaluate to the integer '3'.
+	gppType    byte = 0x3
+	gppVersion byte = 0x1
+	// the range of SectionID must start with 1 and end with the maximum value represented by uint16.
+	minSectionId constants.SectionID = 1
+	maxSectionId constants.SectionID = 0xffff
+)
+
+var (
+	sectionIdOutOfRangeErr = errors.New("section ID out of range")
+	duplicatedSectionErr   = errors.New("duplicated sections")
+)
+
+func Encode(sections []Section) (string, error) {
+	bs := util.NewBitStreamForWrite()
+	builder := strings.Builder{}
+
+	bs.WriteByte6(gppType)
+	bs.WriteByte6(gppVersion)
+
+	sort.Slice(sections, func(i, j int) bool {
+		return sections[i].GetID() < sections[j].GetID()
+	})
+
+	if len(sections) > 0 && (sections[0].GetID() < minSectionId ||
+		sections[len(sections)-1].GetID() > maxSectionId) {
+		return "", sectionIdOutOfRangeErr
+	}
+	// Generate int range object.
+	intRange := new(util.IntRange)
+	// Since the minimum sectionID is 1, the previous one should start with -1, which makes it not continuous.
+	var prevID constants.SectionID = -1
+	for _, sec := range sections {
+		id := sec.GetID()
+		if id == prevID {
+			return "", duplicatedSectionErr
+		}
+		if prevID+1 == id {
+			intRange.Range[len(intRange.Range)-1].EndID = uint16(id)
+		} else {
+			intRange.Range = append(intRange.Range, util.IRange{StartID: uint16(id), EndID: uint16(id)})
+		}
+		prevID = id
+	}
+	intRange.Size = uint16(len(intRange.Range))
+
+	err := bs.WriteIntRange(intRange)
+	if err != nil {
+		return "", fmt.Errorf("write int range error: %v", err)
+	}
+
+	builder.Write(bs.Base64Encode())
+
+	for _, sec := range sections {
+		builder.WriteByte('~')
+		// By default, GPP is included.
+		builder.Write(sec.Encode(true))
+	}
+
+	return builder.String(), nil
+}