go project - add a migration and rename the dockerfiles so that Go is the primary

.dockerignore

@@ -0,0 +1,8 @@
+.git
+.idea
+.DS_Store
+**/bin/
+**/obj/
+output/
+device-simulator-mac-arm64
+*.md

Dockerfile

@@ -0,0 +1,27 @@
+# --- Build the Go API ---
+FROM golang:1.26-alpine AS build
+WORKDIR /src
+COPY go/go.mod ./
+COPY go/ ./
+RUN CGO_ENABLED=0 go build -o /out/server ./cmd/server
+
+# --- API runtime ---
+FROM alpine:3.20 AS api
+RUN apk add --no-cache curl
+WORKDIR /app
+COPY --from=build /out/server /app/server
+COPY --from=build /src/devices.csv /app/devices.csv
+EXPOSE 6733
+HEALTHCHECK --interval=2s --timeout=2s --start-period=5s --retries=15 \
+    CMD curl -sf http://127.0.0.1:6733/healthz || exit 1
+ENTRYPOINT ["/app/server"]
+
+# --- Device simulator (writes results.txt into /output) ---
+FROM alpine:3.20 AS simulator
+COPY device-simulator-linux-arm64 /usr/local/bin/device-simulator
+RUN chmod +x /usr/local/bin/device-simulator \
+    && mkdir -p /output \
+    && chmod 0777 /output
+WORKDIR /output
+VOLUME ["/output"]
+ENTRYPOINT ["device-simulator"]

Dockerfile.dotnet

@@ -0,0 +1,30 @@
+# --- Build the .NET API ---
+FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
+WORKDIR /src
+COPY dotnet/SafelyYou/SafelyYou.csproj dotnet/SafelyYou/
+RUN dotnet restore dotnet/SafelyYou/SafelyYou.csproj
+COPY dotnet/SafelyYou/ dotnet/SafelyYou/
+RUN dotnet publish dotnet/SafelyYou/SafelyYou.csproj -c Release -o /app/publish --no-restore
+
+# --- API runtime ---
+FROM mcr.microsoft.com/dotnet/aspnet:10.0 AS api
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends curl \
+    && rm -rf /var/lib/apt/lists/*
+WORKDIR /app
+COPY --from=build /app/publish .
+ENV ASPNETCORE_URLS=http://+:6733
+EXPOSE 6733
+HEALTHCHECK --interval=2s --timeout=2s --start-period=5s --retries=15 \
+    CMD curl -s -o /dev/null http://127.0.0.1:6733/ || exit 1
+ENTRYPOINT ["dotnet", "SafelyYou.dll"]
+
+# --- Device simulator (writes results.txt into /output) ---
+FROM alpine:3.20 AS simulator
+COPY device-simulator-linux-arm64 /usr/local/bin/device-simulator
+RUN chmod +x /usr/local/bin/device-simulator \
+    && mkdir -p /output \
+    && chmod 0777 /output
+WORKDIR /output
+VOLUME ["/output"]
+ENTRYPOINT ["device-simulator"]

docker-compose.dotnet.yml

@@ -0,0 +1,27 @@
+services:
+  api:
+    build:
+      context: .
+      target: api
+    ports:
+      - "6733:6733"
+    healthcheck:
+      test: ["CMD", "curl", "-s", "-o", "/dev/null", "http://127.0.0.1:6733/"]
+      interval: 2s
+      timeout: 2s
+      retries: 15
+      start_period: 5s
+
+  simulator:
+    build:
+      context: .
+      target: simulator
+    depends_on:
+      api:
+        condition: service_healthy
+    command: ["-host", "api", "-port", "6733"]
+    working_dir: /output
+    volumes:
+      - ./output:/output
+    # Ensure the bind-mounted host dir is writable for results.txt
+    user: "0:0"

docker-compose.yml

@@ -0,0 +1,27 @@
+services:
+  api:
+    build:
+      context: .
+      target: api
+    ports:
+      - "6733:6733"
+    healthcheck:
+      test: ["CMD", "curl", "-sf", "http://127.0.0.1:6733/healthz"]
+      interval: 2s
+      timeout: 2s
+      retries: 15
+      start_period: 5s
+
+  simulator:
+    build:
+      context: .
+      target: simulator
+    depends_on:
+      api:
+        condition: service_healthy
+    command: ["-host", "api", "-port", "6733"]
+    working_dir: /output
+    volumes:
+      - ./output:/output
+    # Ensure the bind-mounted host dir is writable for results.txt
+    user: "0:0"

go/cmd/server/main.go

@@ -0,0 +1,65 @@
+package main
+
+import (
+	"encoding/csv"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"os"
+
+	"github.com/michaeldileo/SafelyYouCodingChallenge/internal/api"
+	"github.com/michaeldileo/SafelyYouCodingChallenge/internal/store"
+)
+
+func main() {
+	deviceIDs, err := loadDeviceIDs("devices.csv")
+	if err != nil {
+		log.Fatalf("load devices: %v", err)
+	}
+
+	s := store.New(deviceIDs)
+	log.Printf("loaded %d known devices", len(deviceIDs))
+
+	srv := api.NewServer(s)
+
+	addr := ":6733"
+	fmt.Println("listening on", addr)
+	if err := http.ListenAndServe(addr, srv.Handler()); err != nil {
+		log.Fatal(err)
+	}
+}
+
+// loadDeviceIDs reads devices.csv (header + one device_id per row).
+// Lives in main on purpose: startup I/O stays at the process edge;
+// the store only receives already-parsed IDs.
+func loadDeviceIDs(path string) ([]store.DeviceID, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	r := csv.NewReader(f)
+
+	// Skip header: device_id
+	if _, err := r.Read(); err != nil {
+		return nil, fmt.Errorf("read header: %w", err)
+	}
+
+	var ids []store.DeviceID
+	for {
+		record, err := r.Read()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			return nil, err
+		}
+		if len(record) == 0 || record[0] == "" {
+			continue
+		}
+		ids = append(ids, store.DeviceID(record[0]))
+	}
+	return ids, nil
+}

go/devices.csv

@@ -0,0 +1,6 @@
+device_id
+60-6b-44-84-dc-64
+b4-45-52-a2-f1-3c
+26-9a-66-01-33-83
+18-b8-87-e7-1f-06
+38-4e-73-e0-33-59

go/go.mod

@@ -0,0 +1,3 @@
+module github.com/michaeldileo/SafelyYouCodingChallenge
+
+go 1.26.3

go/internal/api/handlers.go

@@ -0,0 +1,130 @@
+package api
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/http"
+	"time"
+
+	"github.com/michaeldileo/SafelyYouCodingChallenge/internal/store"
+)
+
+// Server wires HTTP handlers to the in-memory store.
+// Constructed in main and passed dependencies explicitly (no DI container).
+type Server struct {
+	store *store.Store
+}
+
+func NewServer(s *store.Store) *Server {
+	return &Server{store: s}
+}
+
+// Handler returns the HTTP routes for this API.
+func (s *Server) Handler() http.Handler {
+	mux := http.NewServeMux()
+	mux.HandleFunc("GET /healthz", s.healthz)
+	mux.HandleFunc("POST /api/v1/devices/{device_id}/heartbeat", s.postHeartbeat)
+	mux.HandleFunc("POST /api/v1/devices/{device_id}/stats", s.postStats)
+	mux.HandleFunc("GET /api/v1/devices/{device_id}/stats", s.getStats)
+	return mux
+}
+
+type heartbeatRequest struct {
+	SentAt time.Time `json:"sent_at"`
+}
+
+type uploadStatsRequest struct {
+	SentAt     time.Time `json:"sent_at"`     // accepted for contract; ignored in average
+	UploadTime int64     `json:"upload_time"` // nanoseconds
+}
+
+type getDeviceStatsResponse struct {
+	AvgUploadTime string  `json:"avg_upload_time"`
+	Uptime        float64 `json:"uptime"`
+}
+
+// OpenAPI NotFoundResponse / ErrorResponse both use {"msg": "..."}.
+type msgResponse struct {
+	Msg string `json:"msg"`
+}
+
+func (s *Server) healthz(w http.ResponseWriter, _ *http.Request) {
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write([]byte("ok"))
+}
+
+func (s *Server) postHeartbeat(w http.ResponseWriter, r *http.Request) {
+	id := store.DeviceID(r.PathValue("device_id"))
+	if !s.store.Known(id) {
+		writeJSON(w, http.StatusNotFound, msgResponse{
+			Msg: fmt.Sprintf("Device ID %s not found", id),
+		})
+		return
+	}
+
+	var req heartbeatRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeJSON(w, http.StatusInternalServerError, msgResponse{
+			Msg: "invalid request body",
+		})
+		return
+	}
+
+	if err := s.store.AddHeartbeat(id, req.SentAt); err != nil {
+		if errors.Is(err, store.ErrOutOfSequence) {
+			writeJSON(w, http.StatusInternalServerError, msgResponse{
+				Msg: "Attempted to add a timestamp that was out of sequence or invalid.",
+			})
+			return
+		}
+		writeJSON(w, http.StatusInternalServerError, msgResponse{Msg: err.Error()})
+		return
+	}
+
+	w.WriteHeader(http.StatusNoContent)
+}
+
+func (s *Server) postStats(w http.ResponseWriter, r *http.Request) {
+	id := store.DeviceID(r.PathValue("device_id"))
+	if !s.store.Known(id) {
+		writeJSON(w, http.StatusNotFound, msgResponse{
+			Msg: fmt.Sprintf("Device ID %s not found", id),
+		})
+		return
+	}
+
+	var req uploadStatsRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeJSON(w, http.StatusInternalServerError, msgResponse{
+			Msg: "invalid request body",
+		})
+		return
+	}
+
+	// sent_at is decoded above but intentionally unused for the average.
+	s.store.AddUpload(id, time.Duration(req.UploadTime))
+	w.WriteHeader(http.StatusNoContent)
+}
+
+func (s *Server) getStats(w http.ResponseWriter, r *http.Request) {
+	id := store.DeviceID(r.PathValue("device_id"))
+	if !s.store.Known(id) {
+		writeJSON(w, http.StatusNotFound, msgResponse{
+			Msg: fmt.Sprintf("Device ID %s not found", id),
+		})
+		return
+	}
+
+	avg := s.store.AverageUploadTime(id)
+	writeJSON(w, http.StatusOK, getDeviceStatsResponse{
+		AvgUploadTime: avg.String(), // HTTP-edge formatting (Q13)
+		Uptime:        s.store.Uptime(id),
+	})
+}
+
+func writeJSON(w http.ResponseWriter, status int, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(v)
+}

go/internal/api/handlers_test.go

@@ -0,0 +1,80 @@
+package api
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/michaeldileo/SafelyYouCodingChallenge/internal/store"
+)
+
+func TestPostHeartbeat_UnknownDevice_404(t *testing.T) {
+	s := store.New([]store.DeviceID{"known"})
+	srv := NewServer(s)
+
+	req := httptest.NewRequest(http.MethodPost, "/api/v1/devices/missing/heartbeat", strings.NewReader(`{"sent_at":"2024-01-01T12:00:00Z"}`))
+	rec := httptest.NewRecorder()
+	srv.Handler().ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusNotFound {
+		t.Fatalf("status = %d, want 404", rec.Code)
+	}
+	var body msgResponse
+	if err := json.NewDecoder(rec.Body).Decode(&body); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if body.Msg == "" {
+		t.Fatal("expected non-empty msg")
+	}
+}
+
+func TestPostAndGetStats_RoundTrip(t *testing.T) {
+	id := store.DeviceID("device-1")
+	s := store.New([]store.DeviceID{id})
+	srv := NewServer(s)
+	h := srv.Handler()
+
+	t0 := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+	// two heartbeats one minute apart → uptime 200
+	postJSON(t, h, http.MethodPost, "/api/v1/devices/device-1/heartbeat",
+		`{"sent_at":"`+t0.Format(time.RFC3339)+`"}`, http.StatusNoContent)
+	postJSON(t, h, http.MethodPost, "/api/v1/devices/device-1/heartbeat",
+		`{"sent_at":"`+t0.Add(time.Minute).Format(time.RFC3339)+`"}`, http.StatusNoContent)
+
+	// upload 197331667813 ns → "3m17.331667813s"
+	postJSON(t, h, http.MethodPost, "/api/v1/devices/device-1/stats",
+		`{"sent_at":"2024-01-01T12:00:00Z","upload_time":197331667813}`, http.StatusNoContent)
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/devices/device-1/stats", nil)
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200", rec.Code)
+	}
+	var got getDeviceStatsResponse
+	if err := json.NewDecoder(rec.Body).Decode(&got); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if got.Uptime != 200 {
+		t.Fatalf("uptime = %v, want 200", got.Uptime)
+	}
+	if got.AvgUploadTime != "3m17.331667813s" {
+		t.Fatalf("avg_upload_time = %q, want 3m17.331667813s", got.AvgUploadTime)
+	}
+}
+
+func postJSON(t *testing.T, h http.Handler, method, path, body string, wantStatus int) {
+	t.Helper()
+	req := httptest.NewRequest(method, path, strings.NewReader(body))
+	req.Header.Set("Content-Type", "application/json")
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	if rec.Code != wantStatus {
+		t.Fatalf("%s %s status = %d, want %d; body=%s", method, path, rec.Code, wantStatus, rec.Body.String())
+	}
+}

go/internal/store/heartbeat.go

@@ -0,0 +1,94 @@
+package store
+
+import (
+	"errors"
+	"sync"
+	"time"
+)
+
+// ErrOutOfSequence is returned when a heartbeat timestamp is earlier than
+// the latest accepted heartbeat for that device.
+var ErrOutOfSequence = errors.New("heartbeat out of sequence")
+
+// heartbeatHistory tracks one device's heartbeat timeline.
+// Unexported: only Store methods expose behavior.
+type heartbeatHistory struct {
+	mu    sync.Mutex
+	count int64
+	first time.Time
+	last  time.Time
+}
+
+func (h *heartbeatHistory) add(sentAt time.Time) error {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	if h.first.IsZero() {
+		h.first = sentAt
+		h.last = sentAt
+		h.count = 1
+		return nil
+	}
+
+	// Equal timestamps are allowed (matches C#: sentAt < LastHeartBeat only).
+	if sentAt.Before(h.last) {
+		return ErrOutOfSequence
+	}
+
+	h.last = sentAt
+	h.count++
+	return nil
+}
+
+func (h *heartbeatHistory) uptime() float64 {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	if h.first.IsZero() {
+		return 0
+	}
+
+	minutes := h.last.Sub(h.first).Minutes()
+	// Single heartbeat (or identical timestamps): treat span as one minute.
+	if minutes == 0 {
+		minutes = 1
+	}
+
+	return float64(h.count) / minutes * 100
+}
+
+// historyFor returns the per-device history, creating it if needed.
+// The store-level lock only covers map insert/lookup; callers then use
+// the history's own lock for mutations/reads.
+func (s *Store) historyFor(id DeviceID) *heartbeatHistory {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	h, ok := s.heartbeats[id]
+	if !ok {
+		h = &heartbeatHistory{}
+		s.heartbeats[id] = h
+	}
+	return h
+}
+
+// AddHeartbeat records a heartbeat for deviceID.
+// Returns ErrOutOfSequence if sentAt is earlier than the last accepted beat.
+func (s *Store) AddHeartbeat(deviceID DeviceID, sentAt time.Time) error {
+	return s.historyFor(deviceID).add(sentAt)
+}
+
+// Uptime returns uptime percent for deviceID:
+//
+//	(heartbeatCount / minutesBetweenFirstAndLast) * 100
+//
+// Unknown / never-seen devices return 0. A zero-length span is treated as 1 minute.
+func (s *Store) Uptime(deviceID DeviceID) float64 {
+	s.mu.Lock()
+	h, ok := s.heartbeats[deviceID]
+	s.mu.Unlock()
+	if !ok {
+		return 0
+	}
+	return h.uptime()
+}

go/internal/store/heartbeat_test.go

@@ -0,0 +1,106 @@
+package store
+
+import (
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestAddHeartbeat_FirstSetsFirstLastAndCount(t *testing.T) {
+	s := New(nil)
+	id := DeviceID("device-1")
+	t0 := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+	if err := s.AddHeartbeat(id, t0); err != nil {
+		t.Fatalf("AddHeartbeat: %v", err)
+	}
+	if got := s.Uptime(id); got != 100 {
+		t.Fatalf("uptime = %v, want 100 (1 beat / 1 minute)", got)
+	}
+}
+
+func TestAddHeartbeat_LaterUpdatesLast(t *testing.T) {
+	s := New(nil)
+	id := DeviceID("device-1")
+	t0 := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+	t1 := t0.Add(time.Minute)
+
+	_ = s.AddHeartbeat(id, t0)
+	if err := s.AddHeartbeat(id, t1); err != nil {
+		t.Fatalf("AddHeartbeat: %v", err)
+	}
+	// (2 beats / 1 minute) * 100
+	if got := s.Uptime(id); got != 200 {
+		t.Fatalf("uptime = %v, want 200", got)
+	}
+}
+
+func TestAddHeartbeat_SameTimestampAccepted(t *testing.T) {
+	s := New(nil)
+	id := DeviceID("device-1")
+	t0 := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+	_ = s.AddHeartbeat(id, t0)
+	if err := s.AddHeartbeat(id, t0); err != nil {
+		t.Fatalf("same timestamp should be accepted: %v", err)
+	}
+	// span still zero → 1 minute floor; 2 beats → 200
+	if got := s.Uptime(id); got != 200 {
+		t.Fatalf("uptime = %v, want 200", got)
+	}
+}
+
+func TestAddHeartbeat_OutOfSequence(t *testing.T) {
+	s := New(nil)
+	id := DeviceID("device-1")
+	t0 := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+	t1 := t0.Add(time.Minute)
+
+	_ = s.AddHeartbeat(id, t1)
+	err := s.AddHeartbeat(id, t0)
+	if !errors.Is(err, ErrOutOfSequence) {
+		t.Fatalf("err = %v, want ErrOutOfSequence", err)
+	}
+	// rejected beat must not affect uptime: still 1 beat → 100
+	if got := s.Uptime(id); got != 100 {
+		t.Fatalf("uptime = %v, want 100", got)
+	}
+}
+
+func TestUptime_TwoHeartbeatsOverTwoMinutes(t *testing.T) {
+	s := New(nil)
+	id := DeviceID("device-1")
+	t0 := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+	_ = s.AddHeartbeat(id, t0)
+	_ = s.AddHeartbeat(id, t0.Add(2*time.Minute))
+
+	if got := s.Uptime(id); got != 100 {
+		t.Fatalf("uptime = %v, want 100", got)
+	}
+}
+
+func TestUptime_UnknownDevice(t *testing.T) {
+	s := New(nil)
+	if got := s.Uptime(DeviceID("missing")); got != 0 {
+		t.Fatalf("uptime = %v, want 0", got)
+	}
+}
+
+func TestUptime_IsPerDevice(t *testing.T) {
+	s := New(nil)
+	a := DeviceID("device-1")
+	b := DeviceID("device-2")
+	t0 := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
+
+	_ = s.AddHeartbeat(a, t0)
+	_ = s.AddHeartbeat(a, t0.Add(time.Minute))
+	_ = s.AddHeartbeat(b, t0)
+
+	if got := s.Uptime(a); got != 200 {
+		t.Fatalf("device-1 uptime = %v, want 200", got)
+	}
+	if got := s.Uptime(b); got != 100 {
+		t.Fatalf("device-2 uptime = %v, want 100 (single beat = 1 minute)", got)
+	}
+}

go/internal/store/store.go

@@ -0,0 +1,38 @@
+package store
+
+import "sync"
+
+// DeviceID is a typed device identifier.
+// Using a named type (instead of plain string) makes call sites clearer
+// and prevents accidentally mixing unrelated string values.
+type DeviceID string
+
+// Store holds in-memory fleet metrics for known devices.
+type Store struct {
+	known map[DeviceID]struct{}
+
+	// mu protects the heartbeats/uploads maps (insert/lookup of entries only).
+	// Each per-device history has its own mutex for mutations.
+	mu         sync.Mutex
+	heartbeats map[DeviceID]*heartbeatHistory
+	uploads    map[DeviceID]*uploadHistory
+}
+
+// New creates a Store seeded with the given known device IDs.
+func New(known []DeviceID) *Store {
+	m := make(map[DeviceID]struct{}, len(known))
+	for _, id := range known {
+		m[id] = struct{}{}
+	}
+	return &Store{
+		known:      m,
+		heartbeats: make(map[DeviceID]*heartbeatHistory),
+		uploads:    make(map[DeviceID]*uploadHistory),
+	}
+}
+
+// Known reports whether deviceID was present in the startup device list.
+func (s *Store) Known(deviceID DeviceID) bool {
+	_, ok := s.known[deviceID]
+	return ok
+}

go/internal/store/upload.go

@@ -0,0 +1,61 @@
+package store
+
+import (
+	"sync"
+	"time"
+)
+
+// uploadHistory tracks running total/count of upload durations for one device.
+type uploadHistory struct {
+	mu    sync.Mutex
+	total time.Duration
+	count int64
+}
+
+func (u *uploadHistory) add(d time.Duration) {
+	u.mu.Lock()
+	defer u.mu.Unlock()
+	u.total += d
+	u.count++
+}
+
+func (u *uploadHistory) average() time.Duration {
+	u.mu.Lock()
+	defer u.mu.Unlock()
+	if u.count == 0 {
+		return 0
+	}
+	// Truncating integer division — same idea as C# BigInteger total / count.
+	return u.total / time.Duration(u.count)
+}
+
+func (s *Store) uploadFor(id DeviceID) *uploadHistory {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	u, ok := s.uploads[id]
+	if !ok {
+		u = &uploadHistory{}
+		s.uploads[id] = u
+	}
+	return u
+}
+
+// AddUpload records an upload duration (nanoseconds as time.Duration) for deviceID.
+// sent_at from the API is intentionally ignored here; the HTTP layer still accepts it.
+func (s *Store) AddUpload(deviceID DeviceID, uploadTime time.Duration) {
+	s.uploadFor(deviceID).add(uploadTime)
+}
+
+// AverageUploadTime returns the mean upload duration for deviceID.
+// Formatting for the API (duration.String()) belongs at the HTTP edge.
+// Unknown / never-seen devices return 0.
+func (s *Store) AverageUploadTime(deviceID DeviceID) time.Duration {
+	s.mu.Lock()
+	u, ok := s.uploads[deviceID]
+	s.mu.Unlock()
+	if !ok {
+		return 0
+	}
+	return u.average()
+}

go/internal/store/upload_test.go

@@ -0,0 +1,69 @@
+package store
+
+import (
+	"testing"
+	"time"
+)
+
+func TestAddUpload_AverageOfOne(t *testing.T) {
+	s := New(nil)
+	id := DeviceID("device-1")
+	d := 197331667813 * time.Nanosecond // ~3m17.331667813s
+
+	s.AddUpload(id, d)
+
+	if got := s.AverageUploadTime(id); got != d {
+		t.Fatalf("avg = %v, want %v", got, d)
+	}
+}
+
+func TestAddUpload_AverageOfMany(t *testing.T) {
+	s := New(nil)
+	id := DeviceID("device-1")
+
+	s.AddUpload(id, 100*time.Nanosecond)
+	s.AddUpload(id, 200*time.Nanosecond)
+	s.AddUpload(id, 300*time.Nanosecond)
+
+	// (100+200+300)/3 = 200
+	if got := s.AverageUploadTime(id); got != 200*time.Nanosecond {
+		t.Fatalf("avg = %v, want 200ns", got)
+	}
+}
+
+func TestAverageUploadTime_UnknownDevice(t *testing.T) {
+	s := New(nil)
+	if got := s.AverageUploadTime(DeviceID("missing")); got != 0 {
+		t.Fatalf("avg = %v, want 0", got)
+	}
+}
+
+func TestAverageUploadTime_IsPerDevice(t *testing.T) {
+	s := New(nil)
+	a := DeviceID("device-1")
+	b := DeviceID("device-2")
+
+	s.AddUpload(a, 100*time.Nanosecond)
+	s.AddUpload(a, 300*time.Nanosecond)
+	s.AddUpload(b, 50*time.Nanosecond)
+
+	if got := s.AverageUploadTime(a); got != 200*time.Nanosecond {
+		t.Fatalf("device-1 avg = %v, want 200ns", got)
+	}
+	if got := s.AverageUploadTime(b); got != 50*time.Nanosecond {
+		t.Fatalf("device-2 avg = %v, want 50ns", got)
+	}
+}
+
+func TestAverageUploadTime_TruncatingDivision(t *testing.T) {
+	s := New(nil)
+	id := DeviceID("device-1")
+
+	// 10+11 = 21; 21/2 = 10 in integer division
+	s.AddUpload(id, 10*time.Nanosecond)
+	s.AddUpload(id, 11*time.Nanosecond)
+
+	if got := s.AverageUploadTime(id); got != 10*time.Nanosecond {
+		t.Fatalf("avg = %v, want 10ns (truncating)", got)
+	}
+}