first commit.

way too late.

doorman.go

@@ -0,0 +1,153 @@
+package doorman
+
+import (
+	"encoding/json"
+	"errors"
+	"log"
+	"math"
+	mathrand "math/rand"
+	"net/http"
+	"sync"
+	"time"
+
+	"github.com/dchest/siphash"
+	"github.com/didiercrunch/doorman/shared"
+	"gopkg.in/mgo.v2/bson"
+)
+
+// the smallest value that makes two floats the same.  Consequently x == y
+// if |x - y| < epsilon.  It is a known issue that one can find three numbers
+// a,b,c such that a == b, b == c and a != c. Hence, it is the programmer role
+// to be very careful about equalities and inequalities.
+const epsilon float64 = 0.000000001
+
+var rand *mathrand.Rand
+
+func initRandomSeed() {
+	kindOfRandomSeed := mathrand.NewSource(time.Now().Unix())
+	rand = mathrand.New(kindOfRandomSeed)
+}
+
+func init() {
+	initRandomSeed()
+}
+
+func IsZero(f float64) bool {
+	if math.Abs(f) > epsilon {
+		return false
+	}
+	return true
+}
+
+func IsEqual(f1, f2 float64) bool {
+	return IsZero(f1 - f2)
+}
+
+type Doorman struct {
+	Id                  bson.ObjectId  // the id of the doorman
+	LastChangeTimestamp int64          // an always increasing int that represent the last time the doorman has beed updated
+	Probabilities       []float64      //  The probability of each cases.  The sum of probabilities needs to be one
+	wg                  sync.WaitGroup // waitgroup for goroutine safety
+}
+
+func NewDoorman(id bson.ObjectId, Probabilities []float64) *Doorman {
+	wab := &Doorman{}
+	wab.Id = id
+	wab.Probabilities = Probabilities
+	return wab
+}
+
+func (w *Doorman) Length() int {
+	return len(w.Probabilities)
+}
+
+func (w *Doorman) UpdateHard(baseURL string) error {
+	r, err := http.Get(baseURL + "/" + w.Id.Hex())
+	if err != nil {
+		return err
+	}
+	message := new(shared.DoormanUpdater)
+	d := json.NewDecoder(r.Body)
+	if err := d.Decode(message); err != nil {
+		return err
+	} else {
+		return w.Update(message)
+	}
+}
+
+func (w *Doorman) Update(wu *shared.DoormanUpdater) error {
+	if wu.Timestamp <= w.LastChangeTimestamp {
+		return nil
+	}
+	if wu.Id != w.Id.Hex() {
+		return errors.New("bad doorman id")
+	}
+	w.wg.Add(1)
+	defer w.wg.Done()
+	w.LastChangeTimestamp = wu.Timestamp
+
+	if !IsEqual(w.sum(wu.Probabilities), 1) {
+		return errors.New("the sum of probabilities cannot be different than 1")
+	}
+	w.Probabilities = wu.Probabilities
+	log.Printf("Updated doorman %v with new probabilities %v with timestamp %v", wu.Id, wu.Probabilities, wu.Timestamp)
+	return nil
+}
+
+func (w *Doorman) sum(prob []float64) float64 {
+	var ret float64 = 0
+	for _, p := range prob {
+		ret += p
+	}
+	return ret
+}
+
+func (w *Doorman) Validate() error {
+	if len(w.Probabilities) == 0 {
+		return errors.New("not initiated")
+	}
+	if !IsZero(w.sum(w.Probabilities) - 1.0) {
+		return errors.New("The sum of probabilities is not one")
+	}
+	return nil
+}
+
+func (w *Doorman) GetCase(choosenRandomPosition float64) uint {
+	w.wg.Wait()
+	var prob float64 = 0
+	for i, p := range w.Probabilities {
+		prob += p
+		if choosenRandomPosition-epsilon < prob {
+			return uint(i)
+		}
+	}
+	panic("cannot have a probability above 1")
+}
+
+func (w *Doorman) GenerateRandomProbabilityFromInteger(data uint64) float64 {
+	var ret float64 = 0
+	for i := 0; data > 0; i++ {
+		if data&1 == 1 {
+			ret += 1.0 / math.Pow(2, float64(i+1))
+		}
+		data >>= 1
+	}
+	return ret
+}
+
+func (w *Doorman) Hash(data ...[]byte) uint64 {
+	h := siphash.New(make([]byte, 16))
+	for _, datum := range data {
+		h.Write(datum)
+	}
+	return h.Sum64()
+}
+
+func (w *Doorman) GetCaseFromData(data ...[]byte) uint {
+	random := w.GenerateRandomProbabilityFromInteger(w.Hash(data...))
+	return w.GetCase(random)
+}
+
+func (w *Doorman) GetRandomCase() uint {
+	return w.GetCase(rand.Float64())
+}