package srt

import (
	"math"
	"sort"
	"io"
)

type Datum struct {
	seq_num uint32
	timestamp uint32
	data []byte
	next *Datum
}

// linked chain of datums, specifically to store continuous segments
// when a packet is missed, start and end can be used to generate lost
// packet reports, and easily link when missing is received
// 1..2..3..4  6..7..8..9
// chain_1	chain_2
// nack: get 5
type DatumLink struct {
	queued int // remove eventually, was to be used for ACK recv rate calcs, not needed
	root *Datum
	end *Datum
}

// data type and function to allow sorting so order can be ignored during
// linking since each is sequential on outset
type chains []*DatumLink

func (c chains) Len() (int) {
	return len(c)
}

func (c chains) Swap(i, j int) {
	c[i], c[j] = c[j], c[i]
}

// chain_1 is less then chain_2 when chain_1 ends before chain_2 starts
func (c chains) Less(i, j int) (bool) {
	x_1 := c[i].end.seq_num
	x_2 := c[j].root.seq_num
	if serial_less(x_1, x_2, 31) {
		return true
	}
	return false
}

type DatumStorage struct {
	main *DatumLink
	offshoots chains
}

// append new packet to end of buffer
func (buffer *DatumLink) NewDatum(pkt *Packet) {
	datum := new(Datum)
	datum.seq_num = pkt.header_info.(*DataHeader).seq_num
	datum.timestamp = pkt.timestamp
	datum.data = pkt.cif.([]byte)
	buffer.queued += 1
	buffer.end.next = datum
	buffer.end = datum
}

// create a new datumlink with root and end at the given packet
func NewDatumLink(pkt *Packet) (*DatumLink) {
	buffer := new(DatumLink)
	root_datum := new(Datum)
	root_datum.seq_num = pkt.header_info.(*DataHeader).seq_num
	root_datum.timestamp = pkt.timestamp
	root_datum.data = pkt.cif.([]byte)

	buffer.root = root_datum
	buffer.end = root_datum
	buffer.queued = 1
	return buffer
}

// initialize storage with the given data packet, in the main chain
func NewDatumStorage(packet *Packet) (*DatumStorage) {
	storage := new(DatumStorage)
	storage.main = NewDatumLink(packet)
	return storage
}

// purge all packets in the main chain except the last for future linkage
func (storage *DatumStorage) Expunge(output io.WriteCloser) (error) {
	curr_datum := storage.main.root
	seq_num_end := storage.main.end.seq_num
	for curr_datum.seq_num != seq_num_end {
		_, err := output.Write(curr_datum.data)
		if err != nil {
			return err
		}
		curr_datum = curr_datum.next
	}
	storage.main.root = curr_datum
	return nil
}

// check if the given sequence number should already be inside the given buffer
func (buffer *DatumLink) Holds(new_seq_num uint32) (bool) {
	start := buffer.root.seq_num
	end := buffer.end.seq_num
	if new_seq_num == start || new_seq_num == end {
		return true
	}
	if buffer.Before(new_seq_num) || buffer.After(new_seq_num) {
		return false
	}
	return true
}

// check if the given seq num lies before the given buffer starts
// buffer is After seq num?
func (buffer *DatumLink) After(new_seq_num uint32) (bool) {
	start := buffer.root.seq_num
	if serial_less(new_seq_num, start, 31) {
		return true
	}
	return false
}

// check if the given seq num lies after the given buffer starts
// buffer is Before seq num?
func (buffer *DatumLink) Before(new_seq_num uint32) (bool) {
	end := buffer.end.seq_num
	if serial_less(end, new_seq_num, 31) {
		return true
	}
	return false
}

// add the given packet to the appropriate buffer or add a new offshoot buffer
func (storage *DatumStorage) NewDatum(pkt *Packet) {
	new_pkt_num := pkt.header_info.(*DataHeader).seq_num
	prev_num := (new_pkt_num - 1) % uint32(1 << 31)
	if storage.main.end.seq_num == prev_num {
		storage.main.NewDatum(pkt)
	} else if storage.main.Holds(new_pkt_num) {
		return
	} else {
		oldest := storage.main.root.seq_num
		if serial_less(new_pkt_num, oldest, 31) {
			return
		}
		for _, v := range storage.offshoots {
			if v.end.seq_num == prev_num {
				v.NewDatum(pkt)
				return
			} else if v.Holds(new_pkt_num) {
				return
			}
		}
		new_link := NewDatumLink(pkt)
		storage.offshoots = append(storage.offshoots, new_link)
	}
}

// Link 2 buffers to each other
func (buffer *DatumLink) Link(buffer_next *DatumLink) {
	buffer.end.next = buffer_next.root
	buffer.end = buffer_next.end
	buffer.queued += buffer_next.queued
}

// check if the start of the buffer_next is sequentially next to the end of buffer
// or it is too late to get the true next packets to link and the packets must be linked
// given a 31-bit wrap around
func check_append_serial_next(buffer *DatumLink, buffer_next *DatumLink, curr_time uint32) (bool) {
	seq_1 := buffer.end.seq_num
	seq_2 := (seq_1 + 1) % uint32(math.Pow(2, 31))
	if buffer_next.root.seq_num == seq_2 || buffer.end.timestamp + 500000 < curr_time {
		buffer.Link(buffer_next)
		return true
	}
	return false
}

// Given the current storage state, check what packets need to added to fully link main
func (storage *DatumStorage) GenNACKCIF() (*NACKCIF, bool) {
	if len(storage.offshoots) == 0 {
		return nil, false
	}
	cif := new(NACKCIF)
	init_range := new(pckts_range)
	init_range.start = (storage.main.end.seq_num + 1) % (1 << 31)
	init_range.end = (storage.offshoots[0].root.seq_num - 1) % (1 << 31)
	cif.lost_pkts = append(cif.lost_pkts, init_range)
	for i := 0; i < len(storage.offshoots) - 1; i++ {
		new_range := new(pckts_range)
		new_range.start = (storage.offshoots[i].end.seq_num + 1) % (1 << 31)
		new_range.end = (storage.offshoots[i + 1].root.seq_num - 1) % (1 << 31)
		cif.lost_pkts = append(cif.lost_pkts, new_range)
	}
	return cif, true
}

// Try to relink all chains wherever possible or necessary due to TLPKTDROP
func (storage *DatumStorage) Relink(curr_time uint32) {
	sort.Sort(storage.offshoots)
	buffer := storage.main
	i := 0
	for i < len(storage.offshoots) {
		if check_append_serial_next(buffer, storage.offshoots[i], curr_time) {
			storage.offshoots = append(storage.offshoots[:i], storage.offshoots[i + 1:]...) // nuke the chain since it is not needed anymore
		} else {
			buffer = storage.offshoots[i]
			i++
		}
	}
}

// check if a is less than b under serial arithmetic (modulo operations)
func serial_less(a uint32, b uint32, bits int) (bool) {
	if (a < b && b-a < (1 << (bits - 1))) || (a > b && a-b > (1 << (bits - 1))) {
		return true
	}
	return false
}