segment.go

package segcalculator

// Segment represents a single SMS segment. A segment may contain a
// UserDataHeader and one or more EncodedChar values.
type Segment struct {
	Items                 []interface{} // Items can be UserDataHeader or EncodedChar
	HasTwilioReservedBits bool
	HasUserDataHeader     bool
}

// NewSegment creates a new Segment with or without a user data header.
func NewSegment(withUserDataHeader bool) *Segment {
	seg := &Segment{
		Items:                 make([]interface{}, 0),
		HasTwilioReservedBits: withUserDataHeader,
		HasUserDataHeader:     withUserDataHeader,
	}
	if withUserDataHeader {
		for i := 0; i < 6; i++ {
			seg.Items = append(seg.Items, NewUserDataHeader()) // Assume NewUserDataHeader exists
		}
	}
	return seg
}

// SizeInBits is implemented by types that can report their size in bits.
type SizeInBits interface {
	SizeInBits() int
}

func (s *Segment) SizeInBits() int {
	total := 0
	for _, item := range s.Items {
		if si, ok := item.(SizeInBits); ok {
			total += si.SizeInBits()
		}
	}
	return total
}

func (s *Segment) MessageSizeInBits() int {
	total := 0
	for _, item := range s.Items {
		// Skip UserDataHeader items
		if _, ok := item.(*UserDataHeader); !ok {
			if si, ok := item.(SizeInBits); ok {
				total += si.SizeInBits()
			}
		}
	}
	return total
}

func (s *Segment) FreeSizeInBits() int {
	const maxBitsInSegment = 1120 // max size of a SMS is 140 octets -> 140 * 8bits = 1120 bits
	return maxBitsInSegment - s.SizeInBits()
}

func (s *Segment) AddHeader() []*EncodedChar {
	var leftOverChars []*EncodedChar
	if s.HasUserDataHeader {
		return leftOverChars
	}
	s.HasTwilioReservedBits = true
	s.HasUserDataHeader = false
	for i := 0; i < 6; i++ {
		s.Items = append([]interface{}{NewUserDataHeader()}, s.Items...)
	}
	// Ensure there's enough free space, remove items as needed
	for s.FreeSizeInBits() < 0 && len(s.Items) > 0 {
		item := s.Items[len(s.Items)-1]
		s.Items = s.Items[:len(s.Items)-1]
		if ec, ok := item.(*EncodedChar); ok {
			leftOverChars = append([]*EncodedChar{ec}, leftOverChars...)
		}
	}
	return leftOverChars
}

// Push adds an item to the segment. The item should be either UserDataHeader or EncodedChar.
func (s *Segment) Push(item interface{}) {
	s.Items = append(s.Items, item)
}