package activity

import (
    "context"
    "crypto/hmac"
    "crypto/rand"
    "crypto/sha256"
    "encoding/binary"
    "time"
)

func (s *service) ExecuteDraw(ctx context.Context, issueID int64) (*Receipt, error) {
    cm, err := s.GetIssueRandomCommit(ctx, issueID)
    if err != nil {
        return nil, err
    }
    if cm == nil {
        return nil, nil
    }
    master := unmaskSeed(cm.ServerSeedMaster, cm.IssueID, cm.StateVersion)
    items, err := s.readDB.ActivityRewardSettings.WithContext(ctx).
        Where(s.readDB.ActivityRewardSettings.IssueID.Eq(issueID)).
        Order(s.readDB.ActivityRewardSettings.Sort).
        Find()
    if err != nil {
        return nil, err
    }
    var snapshot []ReceiptItem
    var total int64
    for _, it := range items {
        snapshot = append(snapshot, ReceiptItem{ID: it.ID, Name: it.Name, Weight: it.Weight, QuantityBefore: it.Quantity})
        if it.Weight > 0 && (it.Quantity == -1 || it.Quantity > 0) {
            total += int64(it.Weight)
        }
    }
    if total <= 0 {
        return nil, nil
    }
    drawId := time.Now().UnixNano()
    clientSeed := make([]byte, 32)
    _, _ = rand.Read(clientSeed)
    nonce := uint64(1)
    subInput := make([]byte, 16)
    binary.BigEndian.PutUint64(subInput[:8], uint64(issueID))
    binary.BigEndian.PutUint64(subInput[8:16], uint64(drawId))
    mac := hmac.New(sha256.New, master)
    mac.Write(subInput)
    serverSubSeed := mac.Sum(nil)
    enc := encodeMessage(cm.AlgoVersion, issueID, drawId, 0, clientSeed, nonce, cm.ItemsRoot[:], uint64(total))
    entropy := hmacSha256(serverSubSeed, enc)
    pos, proof := rejectSample(entropy, serverSubSeed, enc, uint64(total))
    var acc uint64
    var selIndex int
    var selID int64
    for i, it := range items {
        if it.Weight <= 0 || !(it.Quantity == -1 || it.Quantity > 0) {
            continue
        }
        w := uint64(it.Weight)
        if pos < acc+w {
            selIndex = i
            selID = it.ID
            break
        }
        acc += w
    }
    rec := &Receipt{
        AlgoVersion:    cm.AlgoVersion,
        RoundId:        issueID,
        DrawId:         drawId,
        ClientId:       0,
        Timestamp:      time.Now().UnixMilli(),
        ServerSeedHash: cm.ServerSeedHash[:],
        ServerSubSeed:  serverSubSeed,
        ClientSeed:     clientSeed,
        Nonce:          nonce,
        Items:          snapshot,
        ItemsRoot:      cm.ItemsRoot[:],
        WeightsTotal:   uint64(total),
        SelectedIndex:  selIndex,
        SelectedItemId: selID,
        RandProof:      proof,
        Signature:      nil,
    }
    return rec, nil
}

func encodeMessage(algo string, roundId int64, drawId int64, clientId int64, clientSeed []byte, nonce uint64, itemsRoot []byte, weightsTotal uint64) []byte {
    var buf []byte
    buf = appendUint32String(buf, algo)
    buf = appendUint64(buf, uint64(roundId))
    buf = appendUint64(buf, uint64(drawId))
    buf = appendUint64(buf, uint64(clientId))
    buf = appendUint32Bytes(buf, clientSeed)
    buf = appendUint64(buf, nonce)
    buf = append(buf, itemsRoot...)
    buf = appendUint64(buf, weightsTotal)
    return buf
}

func appendUint32String(b []byte, s string) []byte {
    bs := []byte(s)
    nb := make([]byte, 4)
    binary.BigEndian.PutUint32(nb, uint32(len(bs)))
    b = append(b, nb...)
    b = append(b, bs...)
    return b
}

func appendUint32Bytes(b []byte, bs []byte) []byte {
    nb := make([]byte, 4)
    binary.BigEndian.PutUint32(nb, uint32(len(bs)))
    b = append(b, nb...)
    b = append(b, bs...)
    return b
}

func appendUint64(b []byte, v uint64) []byte {
    nb := make([]byte, 8)
    binary.BigEndian.PutUint64(nb, v)
    b = append(b, nb...)
    return b
}

func hmacSha256(key []byte, msg []byte) []byte {
    m := hmac.New(sha256.New, key)
    m.Write(msg)
    return m.Sum(nil)
}

func rejectSample(entropy []byte, subSeed []byte, enc []byte, W uint64) (uint64, []byte) {
    var counter uint64
    for {
        R := binary.BigEndian.Uint64(entropy[:8])
        M := (uint64(^uint64(0)) / W) * W
        if R < M {
            return R % W, entropy
        }
        counter++
        cenc := make([]byte, len(enc)+8)
        copy(cenc, enc)
        binary.BigEndian.PutUint64(cenc[len(enc):], counter)
        entropy = hmacSha256(subSeed, cenc)
    }
}