package shadowsocks

import (
	"crypto/sha1"
	"fmt"
	"io"
	"log"
	"net/http"
	"sync"

	"github.com/daeuniverse/softwind/common"
	"github.com/daeuniverse/softwind/pkg/fastrand"
	"github.com/daeuniverse/softwind/pool"
	"golang.org/x/crypto/hkdf"
)

type (
	SaltGeneratorType int
)

const (
	IodizedSaltGeneratorType SaltGeneratorType = iota
	RandomSaltGeneratorType
)

const DefaultBucketSize = 300

var (
	DefaultSaltGeneratorType = RandomSaltGeneratorType
	DefaultIodizedSource     = "https://github.com/explore"
	saltGenerators           = make(map[int]SaltGenerator)
	muGenerators             sync.Mutex
)

func GetSaltGenerator(masterKey []byte, saltLen int) (sg SaltGenerator, err error) {
	muGenerators.Lock()
	sg, ok := saltGenerators[saltLen]
	if !ok {
		dummy := NewDummySaltGenerator()
		saltGenerators[saltLen] = dummy
		muGenerators.Unlock()
		defer func() {
			dummy.Success = err == nil
			close(dummy.Closed)
		}()
		switch DefaultSaltGeneratorType {
		case IodizedSaltGeneratorType:
			sg, err = NewIodizedSaltGenerator(masterKey, saltLen, DefaultBucketSize, true)
			if err != nil {
				return nil, err
			}
		case RandomSaltGeneratorType:
			sg, err = NewRandomSaltGenerator(DefaultBucketSize, true)
			if err != nil {
				return nil, err
			}
		}
		muGenerators.Lock()
		saltGenerators[saltLen] = sg
		muGenerators.Unlock()
	} else {
		muGenerators.Unlock()
		if g, isBuilding := sg.(*DummySaltGenerator); isBuilding {
			<-g.Closed
			if g.Success {
				muGenerators.Lock()
				sg = saltGenerators[saltLen]
				muGenerators.Unlock()
			} else {
				return GetSaltGenerator(masterKey, saltLen)
			}
		}
	}
	return sg, nil
}

type SaltGenerator interface {
	Get() []byte
	Close() error
}

type IodizedSaltGenerator struct {
	tokenBucket chan []byte
	saltSize    int
	fromPool    bool
	muSource    sync.Mutex
	source      []byte
	begin       int
	tokenLen    int
	kdfInfo     []byte
	salt        []byte
	cnt         [32]byte
	closed      chan struct{}
}

func NewIodizedSaltGenerator(salt []byte, saltSize, bucketSize int, fromPool bool) (*IodizedSaltGenerator, error) {
	resp, err := http.Get(DefaultIodizedSource)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	if resp.StatusCode != 200 {
		return nil, fmt.Errorf("error when fetching entropy source: %v %v", resp.StatusCode, resp.Status)
	}
	b, err := io.ReadAll(resp.Body)
	if err != nil {
		return nil, err
	}
	var rnd [2]byte
	fastrand.Read(rnd[:])
	h := sha1.New()
	h.Write(rnd[:])
	h.Write(salt)
	kdfInfo := h.Sum(b)
	g := IodizedSaltGenerator{
		tokenBucket: make(chan []byte, bucketSize),
		saltSize:    saltSize,
		fromPool:    fromPool,
		source:      b,
		begin:       0,
		tokenLen:    5,
		kdfInfo:     kdfInfo[:],
		salt:        salt,
		closed:      make(chan struct{}),
	}
	go g.start()
	return &g, nil
}

func (g *IodizedSaltGenerator) start() {
	var salt []byte
	for {
		if g.fromPool {
			salt = pool.Get(g.saltSize)
		} else {
			salt = make([]byte, g.saltSize)
		}
		// lock has low cost for single thread
		g.muSource.Lock()
		tokenEnd := g.begin + g.tokenLen
		if tokenEnd > len(g.source) {
			g.begin = 0
			g.tokenLen++
			tokenEnd = g.begin + g.tokenLen
		}
		kdf := hkdf.New(sha1.New, g.source[g.begin:tokenEnd], g.cnt[:], g.kdfInfo)
		g.begin += g.tokenLen / 3
		common.BytesIncBigEndian(g.cnt[:])
		g.muSource.Unlock()
		if g.tokenLen >= 100 {
			go func() {
				// fetch the new source
				if ns, e := NewIodizedSaltGenerator(g.salt, g.saltSize, 0, false); e == nil {
					ns.Close()
					g.muSource.Lock()
					g.source = ns.source
					g.kdfInfo = ns.kdfInfo
					g.begin = ns.begin
					g.tokenLen = ns.tokenLen
					g.muSource.Unlock()
				}
			}()
		}
		_, err := io.ReadFull(kdf, salt)
		if err != nil {
			log.Fatal("IodizedSaltGenerator.start:", err)
		}
		select {
		case <-g.closed:
			break
		case g.tokenBucket <- salt:
		}
	}
}

func (g *IodizedSaltGenerator) Get() []byte {
	return <-g.tokenBucket
}

func (g *IodizedSaltGenerator) Close() error {
	close(g.closed)
	return nil
}

type RandomSaltGenerator struct {
	saltSize int
	fromPool bool
}

func NewRandomSaltGenerator(saltSize int, fromPool bool) (*RandomSaltGenerator, error) {
	return &RandomSaltGenerator{
		saltSize: saltSize,
		fromPool: fromPool,
	}, nil
}

func (g *RandomSaltGenerator) Get() []byte {
	var salt []byte
	if g.fromPool {
		salt = pool.Get(g.saltSize)
	} else {
		salt = make([]byte, g.saltSize)
	}
	_, _ = fastrand.Read(salt)
	return salt
}

func (g *RandomSaltGenerator) Close() error {
	return nil
}

type DummySaltGenerator struct {
	Closed  chan struct{}
	Success bool
}

func NewDummySaltGenerator() *DummySaltGenerator {
	return &DummySaltGenerator{Closed: make(chan struct{})}
}

func (g *DummySaltGenerator) Get() []byte {
	return nil
}

func (g *DummySaltGenerator) Close() error {
	close(g.Closed)
	return nil
}
