build the blog with nix

8 files changed, 1207 insertions, 0 deletions

diff --git a/src/assets/component-oriented-design/v1/main.go b/src/assets/component-oriented-design/v1/main.go
new file mode 100644
index 0000000..490a516
--- /dev/null
+++ b/src/assets/component-oriented-design/v1/main.go
@@ -0,0 +1,314 @@
+package main
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"log"
+	"math/rand"
+	"net"
+	"net/http"
+	"os"
+	"sort"
+	"strconv"
+	"sync"
+	"time"
+)
+
+// Logger describes a simple component used for printing log lines.
+type Logger interface {
+	Printf(string, ...interface{})
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// The scoreboard component
+
+// File wraps the standard os.File type.
+type File interface {
+	io.ReadWriter
+	Truncate(int64) error
+	Seek(int64, int) (int64, error)
+}
+
+// scoreboard loads player scores from a save file, tracks score updates, and
+// periodically saves those scores back to the save file.
+type scoreboard struct {
+	file       File
+	scoresM    map[string]int
+	scoresLock sync.Mutex
+
+	// this field will only be set in tests, and is used to synchronize with the
+	// the for-select loop in saveLoop.
+	saveLoopWaitCh chan struct{}
+}
+
+// newScoreboard initializes a scoreboard using scores saved in the given File
+// (which may be empty). The scoreboard will rewrite the save file with the
+// latest scores everytime saveTicker is written to.
+func newScoreboard(file File, saveTicker <-chan time.Time, logger Logger) (*scoreboard, error) {
+	fileBody, err := ioutil.ReadAll(file)
+	if err != nil {
+		return nil, fmt.Errorf("reading saved scored: %w", err)
+	}
+
+	scoresM := map[string]int{}
+	if len(fileBody) > 0 {
+		if err := json.Unmarshal(fileBody, &scoresM); err != nil {
+			return nil, fmt.Errorf("decoding saved scores: %w", err)
+		}
+	}
+
+	scoreboard := &scoreboard{
+		file:           file,
+		scoresM:        scoresM,
+		saveLoopWaitCh: make(chan struct{}),
+	}
+
+	go scoreboard.saveLoop(saveTicker, logger)
+
+	return scoreboard, nil
+}
+
+func (s *scoreboard) guessedCorrect(name string) int {
+	s.scoresLock.Lock()
+	defer s.scoresLock.Unlock()
+
+	s.scoresM[name] += 1000
+	return s.scoresM[name]
+}
+
+func (s *scoreboard) guessedIncorrect(name string) int {
+	s.scoresLock.Lock()
+	defer s.scoresLock.Unlock()
+
+	s.scoresM[name] -= 1
+	return s.scoresM[name]
+}
+
+func (s *scoreboard) scores() map[string]int {
+	s.scoresLock.Lock()
+	defer s.scoresLock.Unlock()
+
+	scoresCp := map[string]int{}
+	for name, score := range s.scoresM {
+		scoresCp[name] = score
+	}
+
+	return scoresCp
+}
+
+func (s *scoreboard) save() error {
+	scores := s.scores()
+	if _, err := s.file.Seek(0, 0); err != nil {
+		return fmt.Errorf("seeking to start of save file: %w", err)
+	} else if err := s.file.Truncate(0); err != nil {
+		return fmt.Errorf("truncating save file: %w", err)
+	} else if err := json.NewEncoder(s.file).Encode(scores); err != nil {
+		return fmt.Errorf("encoding scores to save file: %w", err)
+	}
+	return nil
+}
+
+func (s *scoreboard) saveLoop(ticker <-chan time.Time, logger Logger) {
+	for {
+		select {
+		case <-ticker:
+			if err := s.save(); err != nil {
+				logger.Printf("error saving scoreboard to file: %v", err)
+			}
+		case <-s.saveLoopWaitCh:
+			// test will unblock, nothing to do here.
+		}
+	}
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// The httpHandlers component
+
+// Scoreboard describes the scoreboard component from the point of view of the
+// httpHandler component (which only needs a subset of scoreboard's methods).
+type Scoreboard interface {
+	guessedCorrect(name string) int
+	guessedIncorrect(name string) int
+	scores() map[string]int
+}
+
+// RandSrc describes a randomness component which can produce random integers.
+type RandSrc interface {
+	Int() int
+}
+
+// httpHandlers implements the http.HandlerFuncs used by the httpServer.
+type httpHandlers struct {
+	scoreboard Scoreboard
+	randSrc    RandSrc
+	logger     Logger
+
+	mux   *http.ServeMux
+	n     int
+	nLock sync.Mutex
+}
+
+func newHTTPHandlers(scoreboard Scoreboard, randSrc RandSrc, logger Logger) *httpHandlers {
+	n := randSrc.Int()
+	logger.Printf("first n is %v", n)
+
+	httpHandlers := &httpHandlers{
+		scoreboard: scoreboard,
+		randSrc:    randSrc,
+		logger:     logger,
+		mux:        http.NewServeMux(),
+		n:          n,
+	}
+
+	httpHandlers.mux.HandleFunc("/guess", httpHandlers.handleGuess)
+	httpHandlers.mux.HandleFunc("/scores", httpHandlers.handleScores)
+
+	return httpHandlers
+}
+
+func (h *httpHandlers) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
+	h.mux.ServeHTTP(rw, r)
+}
+
+func (h *httpHandlers) handleGuess(rw http.ResponseWriter, r *http.Request) {
+	r.Header.Set("Content-Type", "text/plain")
+
+	name := r.FormValue("name")
+	nStr := r.FormValue("n")
+	if name == "" || nStr == "" {
+		http.Error(rw, `"name" and "n" GET args are required`, http.StatusBadRequest)
+		return
+	}
+
+	n, err := strconv.Atoi(nStr)
+	if err != nil {
+		http.Error(rw, err.Error(), http.StatusBadRequest)
+		return
+	}
+
+	h.nLock.Lock()
+	defer h.nLock.Unlock()
+
+	if h.n == n {
+		newScore := h.scoreboard.guessedCorrect(name)
+		h.n = h.randSrc.Int()
+		h.logger.Printf("new n is %v", h.n)
+		rw.WriteHeader(http.StatusOK)
+		fmt.Fprintf(rw, "Correct! Your score is now %d\n", newScore)
+		return
+	}
+
+	hint := "higher"
+	if h.n < n {
+		hint = "lower"
+	}
+
+	newScore := h.scoreboard.guessedIncorrect(name)
+	rw.WriteHeader(http.StatusBadRequest)
+	fmt.Fprintf(rw, "Try %s. Your score is now %d\n", hint, newScore)
+}
+
+func (h *httpHandlers) handleScores(rw http.ResponseWriter, r *http.Request) {
+	r.Header.Set("Content-Type", "text/plain")
+
+	h.nLock.Lock()
+	defer h.nLock.Unlock()
+
+	type scoreTup struct {
+		name  string
+		score int
+	}
+
+	scores := h.scoreboard.scores()
+	scoresTups := make([]scoreTup, 0, len(scores))
+	for name, score := range scores {
+		scoresTups = append(scoresTups, scoreTup{name, score})
+	}
+
+	sort.Slice(scoresTups, func(i, j int) bool {
+		return scoresTups[i].score > scoresTups[j].score
+	})
+
+	for _, scoresTup := range scoresTups {
+		fmt.Fprintf(rw, "%s: %d\n", scoresTup.name, scoresTup.score)
+	}
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// The httpServer component.
+
+type httpServer struct {
+	httpServer *http.Server
+	errCh      chan error
+}
+
+func newHTTPServer(listener net.Listener, httpHandlers *httpHandlers, logger Logger) *httpServer {
+	loggingHandler := http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+		ip, _, _ := net.SplitHostPort(r.RemoteAddr)
+		logger.Printf("HTTP request -> %s %s %s", ip, r.Method, r.URL.String())
+		httpHandlers.ServeHTTP(rw, r)
+	})
+
+	server := &httpServer{
+		httpServer: &http.Server{
+			Handler: loggingHandler,
+		},
+		errCh: make(chan error, 1),
+	}
+
+	go func() {
+		err := server.httpServer.Serve(listener)
+		if errors.Is(err, http.ErrServerClosed) {
+			err = nil
+		}
+		server.errCh <- err
+	}()
+
+	return server
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// main
+
+const (
+	saveFilePath = "./save.json"
+	listenAddr   = ":8888"
+	saveInterval = 5 * time.Second
+)
+
+func main() {
+	logger := log.New(os.Stdout, "", log.LstdFlags)
+
+	logger.Printf("opening scoreboard save file %q", saveFilePath)
+	file, err := os.OpenFile(saveFilePath, os.O_RDWR|os.O_CREATE, 0644)
+	if err != nil {
+		logger.Fatalf("failed to open file %q: %v", saveFilePath, err)
+	}
+
+	saveTicker := time.NewTicker(saveInterval)
+	randSrc := rand.New(rand.NewSource(time.Now().UnixNano()))
+
+	logger.Printf("initializing scoreboard")
+	scoreboard, err := newScoreboard(file, saveTicker.C, logger)
+	if err != nil {
+		logger.Fatalf("failed to initialize scoreboard: %v", err)
+	}
+
+	logger.Printf("listening on %q", listenAddr)
+	listener, err := net.Listen("tcp", listenAddr)
+	if err != nil {
+		logger.Fatalf("failed to listen on %q: %v", listenAddr, err)
+	}
+
+	logger.Printf("setting up HTTP handlers")
+	httpHandlers := newHTTPHandlers(scoreboard, randSrc, logger)
+
+	logger.Printf("serving HTTP requests")
+	newHTTPServer(listener, httpHandlers, logger)
+
+	logger.Printf("initialization done")
+	select {} // block forever
+}
diff --git a/src/assets/component-oriented-design/v1/main.md b/src/assets/component-oriented-design/v1/main.md
new file mode 100644
index 0000000..37346c6
--- /dev/null
+++ b/src/assets/component-oriented-design/v1/main.md
@@ -0,0 +1,4 @@
+---
+layout: code
+include: main.go
+---
diff --git a/src/assets/component-oriented-design/v1/main_test.go b/src/assets/component-oriented-design/v1/main_test.go
new file mode 100644
index 0000000..6cfd9fb
--- /dev/null
+++ b/src/assets/component-oriented-design/v1/main_test.go
@@ -0,0 +1,167 @@
+package main
+
+import (
+	"bytes"
+	"net/http"
+	"net/http/httptest"
+	"reflect"
+	"testing"
+	"time"
+)
+
+type nullLogger struct{}
+
+func (nullLogger) Printf(string, ...interface{}) {}
+
+////////////////////////////////////////////////////////////////////////////////
+// Test scoreboard component
+
+type fileStub struct {
+	*bytes.Buffer
+}
+
+func newFileStub(init string) *fileStub {
+	return &fileStub{Buffer: bytes.NewBufferString(init)}
+}
+
+func (fs *fileStub) Truncate(i int64) error {
+	fs.Buffer.Truncate(int(i))
+	return nil
+}
+
+func (fs *fileStub) Seek(i int64, whence int) (int64, error) {
+	return i, nil
+}
+
+func TestScoreboard(t *testing.T) {
+	newScoreboard := func(t *testing.T, fileStub *fileStub, saveTicker <-chan time.Time) *scoreboard {
+		t.Helper()
+		scoreboard, err := newScoreboard(fileStub, saveTicker, nullLogger{})
+		if err != nil {
+			t.Errorf("unexpected error checking saved scored: %v", err)
+		}
+		return scoreboard
+	}
+
+	assertScores := func(t *testing.T, expScores, gotScores map[string]int) {
+		t.Helper()
+		if !reflect.DeepEqual(expScores, gotScores) {
+			t.Errorf("expected scores of %+v, but instead got %+v", expScores, gotScores)
+		}
+	}
+
+	assertSavedScores := func(t *testing.T, expScores map[string]int, fileStub *fileStub) {
+		t.Helper()
+		fileStubCp := newFileStub(fileStub.String())
+		tmpScoreboard := newScoreboard(t, fileStubCp, nil)
+		assertScores(t, expScores, tmpScoreboard.scores())
+	}
+
+	t.Run("loading", func(t *testing.T) {
+		// make sure loading scoreboards with various file contents works
+		assertSavedScores(t, map[string]int{}, newFileStub(""))
+		assertSavedScores(t, map[string]int{"foo": 1}, newFileStub(`{"foo":1}`))
+		assertSavedScores(t, map[string]int{"foo": 1, "bar": -2}, newFileStub(`{"foo":1,"bar":-2}`))
+	})
+
+	t.Run("tracking", func(t *testing.T) {
+		scoreboard := newScoreboard(t, newFileStub(""), nil)
+		assertScores(t, map[string]int{}, scoreboard.scores()) // sanity check
+
+		scoreboard.guessedCorrect("foo")
+		assertScores(t, map[string]int{"foo": 1000}, scoreboard.scores())
+
+		scoreboard.guessedIncorrect("bar")
+		assertScores(t, map[string]int{"foo": 1000, "bar": -1}, scoreboard.scores())
+
+		scoreboard.guessedIncorrect("foo")
+		assertScores(t, map[string]int{"foo": 999, "bar": -1}, scoreboard.scores())
+	})
+
+	t.Run("saving", func(t *testing.T) {
+		// this test tests scoreboard's periodic save feature using a ticker
+		// channel which will be written to manually. The saveLoopWaitCh is used
+		// here to ensure that each ticker has been fully processed.
+		ticker := make(chan time.Time)
+		fileStub := newFileStub("")
+		scoreboard := newScoreboard(t, fileStub, ticker)
+
+		tick := func() {
+			ticker <- time.Time{}
+			scoreboard.saveLoopWaitCh <- struct{}{}
+		}
+
+		// this should not effect the save file at first
+		scoreboard.guessedCorrect("foo")
+		assertSavedScores(t, map[string]int{}, fileStub)
+
+		// after the ticker the new score should get saved
+		tick()
+		assertSavedScores(t, map[string]int{"foo": 1000}, fileStub)
+
+		// ticker again after no changes should save the same thing as before
+		tick()
+		assertSavedScores(t, map[string]int{"foo": 1000}, fileStub)
+
+		// buffer a bunch of changes, shouldn't get saved till after tick
+		scoreboard.guessedCorrect("foo")
+		scoreboard.guessedCorrect("bar")
+		scoreboard.guessedCorrect("bar")
+		assertSavedScores(t, map[string]int{"foo": 1000}, fileStub)
+		tick()
+		assertSavedScores(t, map[string]int{"foo": 2000, "bar": 2000}, fileStub)
+	})
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Test httpHandlers component
+
+type mockScoreboard map[string]int
+
+func (mockScoreboard) guessedCorrect(name string) int { return 1 }
+
+func (mockScoreboard) guessedIncorrect(name string) int { return -1 }
+
+func (m mockScoreboard) scores() map[string]int { return m }
+
+type mockRandSrc struct{}
+
+func (m mockRandSrc) Int() int { return 666 }
+
+func TestHTTPHandlers(t *testing.T) {
+	mockScoreboard := mockScoreboard{"foo": 1, "bar": 2}
+	httpHandlers := newHTTPHandlers(mockScoreboard, mockRandSrc{}, nullLogger{})
+
+	assertRequest := func(t *testing.T, expCode int, expBody string, r *http.Request) {
+		t.Helper()
+		rw := httptest.NewRecorder()
+		httpHandlers.ServeHTTP(rw, r)
+		if rw.Code != expCode {
+			t.Errorf("expected HTTP response code %d, got %d", expCode, rw.Code)
+		} else if rw.Body.String() != expBody {
+			t.Errorf("expected HTTP response body %q, got %q", expBody, rw.Body.String())
+		}
+	}
+
+	r := httptest.NewRequest("GET", "/guess?name=foo&n=665", nil)
+	assertRequest(t, 400, "Try higher. Your score is now -1\n", r)
+
+	r = httptest.NewRequest("GET", "/guess?name=foo&n=667", nil)
+	assertRequest(t, 400, "Try lower. Your score is now -1\n", r)
+
+	r = httptest.NewRequest("GET", "/guess?name=foo&n=666", nil)
+	assertRequest(t, 200, "Correct! Your score is now 1\n", r)
+
+	r = httptest.NewRequest("GET", "/scores", nil)
+	assertRequest(t, 200, "bar: 2\nfoo: 1\n", r)
+}
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// httpServer is NOT tested, for the following reasons:
+// * It depends on a `net.Listener`, which is not trivial to mock.
+// * It does very little besides passing an httpHandlers along to an http.Server
+//   and managing cleanup.
+// * It isn't likely to be changed often.
+// * If it were to break it would be very apparent in subsequent testing stages.
+//
diff --git a/src/assets/component-oriented-design/v1/main_test.md b/src/assets/component-oriented-design/v1/main_test.md
new file mode 100644
index 0000000..b0a0751
--- /dev/null
+++ b/src/assets/component-oriented-design/v1/main_test.md
@@ -0,0 +1,4 @@
+---
+layout: code
+include: main_test.go
+---
diff --git a/src/assets/component-oriented-design/v2/main.go b/src/assets/component-oriented-design/v2/main.go
new file mode 100644
index 0000000..fb5773c
--- /dev/null
+++ b/src/assets/component-oriented-design/v2/main.go
@@ -0,0 +1,320 @@
+package main
+
+import (
+	"encoding/json"
+	"errors"
+	"flag"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"log"
+	"math/rand"
+	"net"
+	"net/http"
+	"os"
+	"sort"
+	"strconv"
+	"sync"
+	"time"
+)
+
+// Logger describes a simple component used for printing log lines.
+type Logger interface {
+	Printf(string, ...interface{})
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// The scoreboard component
+
+// File wraps the standard os.File type.
+type File interface {
+	io.ReadWriter
+	Truncate(int64) error
+	Seek(int64, int) (int64, error)
+}
+
+// scoreboard loads player scores from a save file, tracks score updates, and
+// periodically saves those scores back to the save file.
+type scoreboard struct {
+	file       File
+	scoresM    map[string]int
+	scoresLock sync.Mutex
+
+	pointsOnCorrect, pointsOnIncorrect int
+
+	// this field will only be set in tests, and is used to synchronize with the
+	// the for-select loop in saveLoop.
+	saveLoopWaitCh chan struct{}
+}
+
+// newScoreboard initializes a scoreboard using scores saved in the given File
+// (which may be empty). The scoreboard will rewrite the save file with the
+// latest scores everytime saveTicker is written to.
+func newScoreboard(file File, saveTicker <-chan time.Time, logger Logger, pointsOnCorrect, pointsOnIncorrect int) (*scoreboard, error) {
+	fileBody, err := ioutil.ReadAll(file)
+	if err != nil {
+		return nil, fmt.Errorf("reading saved scored: %w", err)
+	}
+
+	scoresM := map[string]int{}
+	if len(fileBody) > 0 {
+		if err := json.Unmarshal(fileBody, &scoresM); err != nil {
+			return nil, fmt.Errorf("decoding saved scores: %w", err)
+		}
+	}
+
+	scoreboard := &scoreboard{
+		file:              file,
+		scoresM:           scoresM,
+		pointsOnCorrect:   pointsOnCorrect,
+		pointsOnIncorrect: pointsOnIncorrect,
+		saveLoopWaitCh:    make(chan struct{}),
+	}
+
+	go scoreboard.saveLoop(saveTicker, logger)
+
+	return scoreboard, nil
+}
+
+func (s *scoreboard) guessedCorrect(name string) int {
+	s.scoresLock.Lock()
+	defer s.scoresLock.Unlock()
+
+	s.scoresM[name] += s.pointsOnCorrect
+	return s.scoresM[name]
+}
+
+func (s *scoreboard) guessedIncorrect(name string) int {
+	s.scoresLock.Lock()
+	defer s.scoresLock.Unlock()
+
+	s.scoresM[name] += s.pointsOnIncorrect
+	return s.scoresM[name]
+}
+
+func (s *scoreboard) scores() map[string]int {
+	s.scoresLock.Lock()
+	defer s.scoresLock.Unlock()
+
+	scoresCp := map[string]int{}
+	for name, score := range s.scoresM {
+		scoresCp[name] = score
+	}
+
+	return scoresCp
+}
+
+func (s *scoreboard) save() error {
+	scores := s.scores()
+	if _, err := s.file.Seek(0, 0); err != nil {
+		return fmt.Errorf("seeking to start of save file: %w", err)
+	} else if err := s.file.Truncate(0); err != nil {
+		return fmt.Errorf("truncating save file: %w", err)
+	} else if err := json.NewEncoder(s.file).Encode(scores); err != nil {
+		return fmt.Errorf("encoding scores to save file: %w", err)
+	}
+	return nil
+}
+
+func (s *scoreboard) saveLoop(ticker <-chan time.Time, logger Logger) {
+	for {
+		select {
+		case <-ticker:
+			if err := s.save(); err != nil {
+				logger.Printf("error saving scoreboard to file: %v", err)
+			}
+		case <-s.saveLoopWaitCh:
+			// test will unblock, nothing to do here.
+		}
+	}
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// The httpHandlers component
+
+// Scoreboard describes the scoreboard component from the point of view of the
+// httpHandler component (which only needs a subset of scoreboard's methods).
+type Scoreboard interface {
+	guessedCorrect(name string) int
+	guessedIncorrect(name string) int
+	scores() map[string]int
+}
+
+// RandSrc describes a randomness component which can produce random integers.
+type RandSrc interface {
+	Int() int
+}
+
+// httpHandlers implements the http.HandlerFuncs used by the httpServer.
+type httpHandlers struct {
+	scoreboard Scoreboard
+	randSrc    RandSrc
+	logger     Logger
+
+	mux   *http.ServeMux
+	n     int
+	nLock sync.Mutex
+}
+
+func newHTTPHandlers(scoreboard Scoreboard, randSrc RandSrc, logger Logger) *httpHandlers {
+	n := randSrc.Int()
+	logger.Printf("first n is %v", n)
+
+	httpHandlers := &httpHandlers{
+		scoreboard: scoreboard,
+		randSrc:    randSrc,
+		logger:     logger,
+		mux:        http.NewServeMux(),
+		n:          n,
+	}
+
+	httpHandlers.mux.HandleFunc("/guess", httpHandlers.handleGuess)
+	httpHandlers.mux.HandleFunc("/scores", httpHandlers.handleScores)
+
+	return httpHandlers
+}
+
+func (h *httpHandlers) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
+	h.mux.ServeHTTP(rw, r)
+}
+
+func (h *httpHandlers) handleGuess(rw http.ResponseWriter, r *http.Request) {
+	r.Header.Set("Content-Type", "text/plain")
+
+	name := r.FormValue("name")
+	nStr := r.FormValue("n")
+	if name == "" || nStr == "" {
+		http.Error(rw, `"name" and "n" GET args are required`, http.StatusBadRequest)
+		return
+	}
+
+	n, err := strconv.Atoi(nStr)
+	if err != nil {
+		http.Error(rw, err.Error(), http.StatusBadRequest)
+		return
+	}
+
+	h.nLock.Lock()
+	defer h.nLock.Unlock()
+
+	if h.n == n {
+		newScore := h.scoreboard.guessedCorrect(name)
+		h.n = h.randSrc.Int()
+		h.logger.Printf("new n is %v", h.n)
+		rw.WriteHeader(http.StatusOK)
+		fmt.Fprintf(rw, "Correct! Your score is now %d\n", newScore)
+		return
+	}
+
+	hint := "higher"
+	if h.n < n {
+		hint = "lower"
+	}
+
+	newScore := h.scoreboard.guessedIncorrect(name)
+	rw.WriteHeader(http.StatusBadRequest)
+	fmt.Fprintf(rw, "Try %s. Your score is now %d\n", hint, newScore)
+}
+
+func (h *httpHandlers) handleScores(rw http.ResponseWriter, r *http.Request) {
+	r.Header.Set("Content-Type", "text/plain")
+
+	h.nLock.Lock()
+	defer h.nLock.Unlock()
+
+	type scoreTup struct {
+		name  string
+		score int
+	}
+
+	scores := h.scoreboard.scores()
+	scoresTups := make([]scoreTup, 0, len(scores))
+	for name, score := range scores {
+		scoresTups = append(scoresTups, scoreTup{name, score})
+	}
+
+	sort.Slice(scoresTups, func(i, j int) bool {
+		return scoresTups[i].score > scoresTups[j].score
+	})
+
+	for _, scoresTup := range scoresTups {
+		fmt.Fprintf(rw, "%s: %d\n", scoresTup.name, scoresTup.score)
+	}
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// The httpServer component.
+
+type httpServer struct {
+	httpServer *http.Server
+	errCh      chan error
+}
+
+func newHTTPServer(listener net.Listener, httpHandlers *httpHandlers, logger Logger) *httpServer {
+	loggingHandler := http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+		ip, _, _ := net.SplitHostPort(r.RemoteAddr)
+		logger.Printf("HTTP request -> %s %s %s", ip, r.Method, r.URL.String())
+		httpHandlers.ServeHTTP(rw, r)
+	})
+
+	server := &httpServer{
+		httpServer: &http.Server{
+			Handler: loggingHandler,
+		},
+		errCh: make(chan error, 1),
+	}
+
+	go func() {
+		err := server.httpServer.Serve(listener)
+		if errors.Is(err, http.ErrServerClosed) {
+			err = nil
+		}
+		server.errCh <- err
+	}()
+
+	return server
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// main
+
+func main() {
+	saveFilePath := flag.String("save-file", "./save.json", "File used to save scores")
+	listenAddr := flag.String("listen-addr", ":8888", "Address to listen for HTTP requests on")
+	saveInterval := flag.Duration("save-interval", 5*time.Second, "How often to resave scores")
+	pointsOnCorrect := flag.Int("points-on-correct", 1000, "Amount to change a user's score by upon a correct score")
+	pointsOnIncorrect := flag.Int("points-on-incorrect", -1, "Amount to change a user's score by upon an incorrect score")
+	flag.Parse()
+
+	logger := log.New(os.Stdout, "", log.LstdFlags)
+
+	logger.Printf("opening scoreboard save file %q", *saveFilePath)
+	file, err := os.OpenFile(*saveFilePath, os.O_RDWR|os.O_CREATE, 0644)
+	if err != nil {
+		logger.Fatalf("failed to open file %q: %v", *saveFilePath, err)
+	}
+
+	saveTicker := time.NewTicker(*saveInterval)
+	randSrc := rand.New(rand.NewSource(time.Now().UnixNano()))
+
+	logger.Printf("initializing scoreboard")
+	scoreboard, err := newScoreboard(file, saveTicker.C, logger, *pointsOnCorrect, *pointsOnIncorrect)
+	if err != nil {
+		logger.Fatalf("failed to initialize scoreboard: %v", err)
+	}
+
+	logger.Printf("listening on %q", *listenAddr)
+	listener, err := net.Listen("tcp", *listenAddr)
+	if err != nil {
+		logger.Fatalf("failed to listen on %q: %v", *listenAddr, err)
+	}
+
+	logger.Printf("setting up HTTP handlers")
+	httpHandlers := newHTTPHandlers(scoreboard, randSrc, logger)
+
+	logger.Printf("serving HTTP requests")
+	newHTTPServer(listener, httpHandlers, logger)
+
+	logger.Printf("initialization done")
+	select {} // block forever
+}
diff --git a/src/assets/component-oriented-design/v2/main.md b/src/assets/component-oriented-design/v2/main.md
new file mode 100644
index 0000000..37346c6
--- /dev/null
+++ b/src/assets/component-oriented-design/v2/main.md
@@ -0,0 +1,4 @@
+---
+layout: code
+include: main.go
+---
diff --git a/src/assets/component-oriented-design/v3/main.go b/src/assets/component-oriented-design/v3/main.go
new file mode 100644
index 0000000..afe8bab
--- /dev/null
+++ b/src/assets/component-oriented-design/v3/main.go
@@ -0,0 +1,390 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"flag"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"log"
+	"math/rand"
+	"net"
+	"net/http"
+	"os"
+	"os/signal"
+	"sort"
+	"strconv"
+	"sync"
+	"time"
+)
+
+// Logger describes a simple component used for printing log lines.
+type Logger interface {
+	Printf(string, ...interface{})
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// The scoreboard component
+
+// File wraps the standard os.File type.
+type File interface {
+	io.ReadWriter
+	Truncate(int64) error
+	Seek(int64, int) (int64, error)
+}
+
+// scoreboard loads player scores from a save file, tracks score updates, and
+// periodically saves those scores back to the save file.
+type scoreboard struct {
+	file       File
+	scoresM    map[string]int
+	scoresLock sync.Mutex
+
+	pointsOnCorrect, pointsOnIncorrect int
+
+	// The cleanup method closes cleanupCh to signal to all scoreboard's running
+	// go-routines to clean themselves up, and cleanupWG is then used to wait
+	// for those goroutines to do so.
+	cleanupCh chan struct{}
+	cleanupWG sync.WaitGroup
+
+	// this field will only be set in tests, and is used to synchronize with the
+	// the for-select loop in saveLoop.
+	saveLoopWaitCh chan struct{}
+}
+
+// newScoreboard initializes a scoreboard using scores saved in the given File
+// (which may be empty). The scoreboard will rewrite the save file with the
+// latest scores everytime saveTicker is written to.
+func newScoreboard(file File, saveTicker <-chan time.Time, logger Logger, pointsOnCorrect, pointsOnIncorrect int) (*scoreboard, error) {
+	fileBody, err := ioutil.ReadAll(file)
+	if err != nil {
+		return nil, fmt.Errorf("reading saved scored: %w", err)
+	}
+
+	scoresM := map[string]int{}
+	if len(fileBody) > 0 {
+		if err := json.Unmarshal(fileBody, &scoresM); err != nil {
+			return nil, fmt.Errorf("decoding saved scores: %w", err)
+		}
+	}
+
+	scoreboard := &scoreboard{
+		file:              file,
+		scoresM:           scoresM,
+		pointsOnCorrect:   pointsOnCorrect,
+		pointsOnIncorrect: pointsOnIncorrect,
+		cleanupCh:         make(chan struct{}),
+		saveLoopWaitCh:    make(chan struct{}),
+	}
+
+	scoreboard.cleanupWG.Add(1)
+	go func() {
+		scoreboard.saveLoop(saveTicker, logger)
+		scoreboard.cleanupWG.Done()
+	}()
+
+	return scoreboard, nil
+}
+
+func (s *scoreboard) cleanup() error {
+	close(s.cleanupCh)
+	s.cleanupWG.Wait()
+
+	if err := s.save(); err != nil {
+		return fmt.Errorf("saving scores during cleanup: %w", err)
+	}
+	return nil
+}
+
+func (s *scoreboard) guessedCorrect(name string) int {
+	s.scoresLock.Lock()
+	defer s.scoresLock.Unlock()
+
+	s.scoresM[name] += s.pointsOnCorrect
+	return s.scoresM[name]
+}
+
+func (s *scoreboard) guessedIncorrect(name string) int {
+	s.scoresLock.Lock()
+	defer s.scoresLock.Unlock()
+
+	s.scoresM[name] += s.pointsOnIncorrect
+	return s.scoresM[name]
+}
+
+func (s *scoreboard) scores() map[string]int {
+	s.scoresLock.Lock()
+	defer s.scoresLock.Unlock()
+
+	scoresCp := map[string]int{}
+	for name, score := range s.scoresM {
+		scoresCp[name] = score
+	}
+
+	return scoresCp
+}
+
+func (s *scoreboard) save() error {
+	scores := s.scores()
+	if _, err := s.file.Seek(0, 0); err != nil {
+		return fmt.Errorf("seeking to start of save file: %w", err)
+	} else if err := s.file.Truncate(0); err != nil {
+		return fmt.Errorf("truncating save file: %w", err)
+	} else if err := json.NewEncoder(s.file).Encode(scores); err != nil {
+		return fmt.Errorf("encoding scores to save file: %w", err)
+	}
+	return nil
+}
+
+func (s *scoreboard) saveLoop(ticker <-chan time.Time, logger Logger) {
+	for {
+		select {
+		case <-ticker:
+			if err := s.save(); err != nil {
+				logger.Printf("error saving scoreboard to file: %v", err)
+			}
+		case <-s.cleanupCh:
+			return
+		case <-s.saveLoopWaitCh:
+			// test will unblock, nothing to do here.
+		}
+	}
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// The httpHandlers component
+
+// Scoreboard describes the scoreboard component from the point of view of the
+// httpHandler component (which only needs a subset of scoreboard's methods).
+type Scoreboard interface {
+	guessedCorrect(name string) int
+	guessedIncorrect(name string) int
+	scores() map[string]int
+}
+
+// RandSrc describes a randomness component which can produce random integers.
+type RandSrc interface {
+	Int() int
+}
+
+// httpHandlers implements the http.HandlerFuncs used by the httpServer.
+type httpHandlers struct {
+	scoreboard Scoreboard
+	randSrc    RandSrc
+	logger     Logger
+
+	mux   *http.ServeMux
+	n     int
+	nLock sync.Mutex
+}
+
+func newHTTPHandlers(scoreboard Scoreboard, randSrc RandSrc, logger Logger) *httpHandlers {
+	n := randSrc.Int()
+	logger.Printf("first n is %v", n)
+
+	httpHandlers := &httpHandlers{
+		scoreboard: scoreboard,
+		randSrc:    randSrc,
+		logger:     logger,
+		mux:        http.NewServeMux(),
+		n:          n,
+	}
+
+	httpHandlers.mux.HandleFunc("/guess", httpHandlers.handleGuess)
+	httpHandlers.mux.HandleFunc("/scores", httpHandlers.handleScores)
+
+	return httpHandlers
+}
+
+func (h *httpHandlers) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
+	h.mux.ServeHTTP(rw, r)
+}
+
+func (h *httpHandlers) handleGuess(rw http.ResponseWriter, r *http.Request) {
+	r.Header.Set("Content-Type", "text/plain")
+
+	name := r.FormValue("name")
+	nStr := r.FormValue("n")
+	if name == "" || nStr == "" {
+		http.Error(rw, `"name" and "n" GET args are required`, http.StatusBadRequest)
+		return
+	}
+
+	n, err := strconv.Atoi(nStr)
+	if err != nil {
+		http.Error(rw, err.Error(), http.StatusBadRequest)
+		return
+	}
+
+	h.nLock.Lock()
+	defer h.nLock.Unlock()
+
+	if h.n == n {
+		newScore := h.scoreboard.guessedCorrect(name)
+		h.n = h.randSrc.Int()
+		h.logger.Printf("new n is %v", h.n)
+		rw.WriteHeader(http.StatusOK)
+		fmt.Fprintf(rw, "Correct! Your score is now %d\n", newScore)
+		return
+	}
+
+	hint := "higher"
+	if h.n < n {
+		hint = "lower"
+	}
+
+	newScore := h.scoreboard.guessedIncorrect(name)
+	rw.WriteHeader(http.StatusBadRequest)
+	fmt.Fprintf(rw, "Try %s. Your score is now %d\n", hint, newScore)
+}
+
+func (h *httpHandlers) handleScores(rw http.ResponseWriter, r *http.Request) {
+	r.Header.Set("Content-Type", "text/plain")
+
+	h.nLock.Lock()
+	defer h.nLock.Unlock()
+
+	type scoreTup struct {
+		name  string
+		score int
+	}
+
+	scores := h.scoreboard.scores()
+	scoresTups := make([]scoreTup, 0, len(scores))
+	for name, score := range scores {
+		scoresTups = append(scoresTups, scoreTup{name, score})
+	}
+
+	sort.Slice(scoresTups, func(i, j int) bool {
+		return scoresTups[i].score > scoresTups[j].score
+	})
+
+	for _, scoresTup := range scoresTups {
+		fmt.Fprintf(rw, "%s: %d\n", scoresTup.name, scoresTup.score)
+	}
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// The httpServer component.
+
+type httpServer struct {
+	httpServer *http.Server
+	errCh      chan error
+}
+
+func newHTTPServer(listener net.Listener, httpHandlers *httpHandlers, logger Logger) *httpServer {
+	loggingHandler := http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+		ip, _, _ := net.SplitHostPort(r.RemoteAddr)
+		logger.Printf("HTTP request -> %s %s %s", ip, r.Method, r.URL.String())
+		httpHandlers.ServeHTTP(rw, r)
+	})
+
+	server := &httpServer{
+		httpServer: &http.Server{
+			Handler: loggingHandler,
+		},
+		errCh: make(chan error, 1),
+	}
+
+	go func() {
+		err := server.httpServer.Serve(listener)
+		if errors.Is(err, http.ErrServerClosed) {
+			err = nil
+		}
+		server.errCh <- err
+	}()
+
+	return server
+}
+
+func (s *httpServer) cleanup() error {
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+	if err := s.httpServer.Shutdown(ctx); err != nil {
+		return fmt.Errorf("shutting down http server: %w", err)
+	}
+	return <-s.errCh
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// main
+
+func main() {
+	saveFilePath := flag.String("save-file", "./save.json", "File used to save scores")
+	listenAddr := flag.String("listen-addr", ":8888", "Address to listen for HTTP requests on")
+	saveInterval := flag.Duration("save-interval", 5*time.Second, "How often to resave scores")
+	pointsOnCorrect := flag.Int("points-on-correct", 1000, "Amount to change a user's score by upon a correct score")
+	pointsOnIncorrect := flag.Int("points-on-incorrect", -1, "Amount to change a user's score by upon an incorrect score")
+	flag.Parse()
+
+	logger := log.New(os.Stdout, "", log.LstdFlags)
+
+	logger.Printf("opening scoreboard save file %q", *saveFilePath)
+	file, err := os.OpenFile(*saveFilePath, os.O_RDWR|os.O_CREATE, 0644)
+	if err != nil {
+		logger.Fatalf("failed to open file %q: %v", *saveFilePath, err)
+	}
+
+	saveTicker := time.NewTicker(*saveInterval)
+	randSrc := rand.New(rand.NewSource(time.Now().UnixNano()))
+
+	logger.Printf("initializing scoreboard")
+	scoreboard, err := newScoreboard(file, saveTicker.C, logger, *pointsOnCorrect, *pointsOnIncorrect)
+	if err != nil {
+		logger.Fatalf("failed to initialize scoreboard: %v", err)
+	}
+
+	logger.Printf("listening on %q", *listenAddr)
+	listener, err := net.Listen("tcp", *listenAddr)
+	if err != nil {
+		logger.Fatalf("failed to listen on %q: %v", *listenAddr, err)
+	}
+
+	logger.Printf("setting up HTTP handlers")
+	httpHandlers := newHTTPHandlers(scoreboard, randSrc, logger)
+
+	logger.Printf("serving HTTP requests")
+	httpServer := newHTTPServer(listener, httpHandlers, logger)
+
+	logger.Printf("initialization done, waiting for interrupt signal")
+	sigCh := make(chan os.Signal)
+	signal.Notify(sigCh, os.Interrupt)
+	<-sigCh
+	logger.Printf("interrupt signal received, cleaning up")
+	go func() {
+		<-sigCh
+		log.Fatalf("interrupt signal received again, forcing shutdown")
+	}()
+
+	if err := httpServer.cleanup(); err != nil {
+		logger.Fatalf("cleaning up http server: %v", err)
+	}
+
+	// NOTE go's builtin http server does not follow component property 5a, and
+	// instead closes the net.Listener given to it as a parameter when Shutdown
+	// is called. Because of that inconsistency this Close would error if it
+	// were called.
+	//
+	// While there are ways to work around this, it's instead highlighted in
+	// this example as an instance of a language making the component-oriented
+	// pattern more difficult.
+	//
+	//if err := listener.Close(); err != nil {
+	//	logger.Fatalf("closing listener %q: %v", listenAddr, err)
+	//}
+
+	if err := scoreboard.cleanup(); err != nil {
+		logger.Fatalf("cleaning up scoreboard: %v", err)
+	}
+
+	saveTicker.Stop()
+
+	if err := file.Close(); err != nil {
+		logger.Fatalf("closing file %q: %v", *saveFilePath, err)
+	}
+
+	os.Stdout.Sync()
+}
diff --git a/src/assets/component-oriented-design/v3/main.md b/src/assets/component-oriented-design/v3/main.md
new file mode 100644
index 0000000..37346c6
--- /dev/null
+++ b/src/assets/component-oriented-design/v3/main.md
@@ -0,0 +1,4 @@
+---
+layout: code
+include: main.go
+---