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ch9-7: fmt code
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chai2010
parent
687549b05d
commit
86650d2d26
234
ch9/ch9-07.md
234
ch9/ch9-07.md
@ -6,12 +6,12 @@
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```go
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func httpGetBody(url string) (interface{}, error) {
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resp, err := http.Get(url)
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if err != nil {
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return nil, err
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}
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defer resp.Body.Close()
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return ioutil.ReadAll(resp.Body)
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resp, err := http.Get(url)
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if err != nil {
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return nil, err
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}
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defer resp.Body.Close()
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return ioutil.ReadAll(resp.Body)
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}
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```
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@ -27,30 +27,30 @@ package memo
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// A Memo caches the results of calling a Func.
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type Memo struct {
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f Func
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cache map[string]result
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f Func
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cache map[string]result
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}
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// Func is the type of the function to memoize.
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type Func func(key string) (interface{}, error)
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type result struct {
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value interface{}
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err error
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value interface{}
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err error
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}
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func New(f Func) *Memo {
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return &Memo{f: f, cache: make(map[string]result)}
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return &Memo{f: f, cache: make(map[string]result)}
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}
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// NOTE: not concurrency-safe!
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func (memo *Memo) Get(key string) (interface{}, error) {
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res, ok := memo.cache[key]
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if !ok {
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res.value, res.err = memo.f(key)
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memo.cache[key] = res
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}
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return res.value, res.err
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res, ok := memo.cache[key]
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if !ok {
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res.value, res.err = memo.f(key)
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memo.cache[key] = res
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}
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return res.value, res.err
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}
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```
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@ -61,13 +61,13 @@ Memo實例會記録需要緩存的函數f(類型爲Func)，以及緩存內容(
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```go
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m := memo.New(httpGetBody)
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for url := range incomingURLs() {
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start := time.Now()
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value, err := m.Get(url)
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if err != nil {
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log.Print(err)
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}
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fmt.Printf("%s, %s, %d bytes\n",
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url, time.Since(start), len(value.([]byte)))
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start := time.Now()
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value, err := m.Get(url)
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if err != nil {
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log.Print(err)
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}
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fmt.Printf("%s, %s, %d bytes\n",
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url, time.Since(start), len(value.([]byte)))
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}
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```
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@ -97,17 +97,17 @@ ok gopl.io/ch9/memo1 1.257s
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m := memo.New(httpGetBody)
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var n sync.WaitGroup
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for url := range incomingURLs() {
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n.Add(1)
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go func(url string) {
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start := time.Now()
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value, err := m.Get(url)
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if err != nil {
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log.Print(err)
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}
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fmt.Printf("%s, %s, %d bytes\n",
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url, time.Since(start), len(value.([]byte)))
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n.Done()
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}(url)
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n.Add(1)
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go func(url string) {
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start := time.Now()
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value, err := m.Get(url)
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if err != nil {
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log.Print(err)
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}
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fmt.Printf("%s, %s, %d bytes\n",
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url, time.Since(start), len(value.([]byte)))
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n.Done()
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}(url)
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}
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n.Wait()
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```
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@ -156,24 +156,24 @@ memo.go的32行出現了兩次，説明有兩個goroutine在沒有同步榦預
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```go
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gopl.io/ch9/memo2
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type Memo struct {
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f Func
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mu sync.Mutex // guards cache
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cache map[string]result
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f Func
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mu sync.Mutex // guards cache
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cache map[string]result
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}
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// Get is concurrency-safe.
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func (memo *Memo) Get(key string) (value interface{}, err error) {
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res, ok := memo.cache[key] if!ok{
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res.value, res.err = memo.f(key)
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memo.cache[key] = res
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memo.mu.Lock()
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res, ok := memo.cache[key]
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if !ok {
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res.value, res.err = memo.f(key)
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memo.cache[key] = res
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}
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memo.mu.Unlock()
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return res.value, res.err
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res, ok := memo.cache[key] if!ok{
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res.value, res.err = memo.f(key)
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memo.cache[key] = res
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memo.mu.Lock()
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res, ok := memo.cache[key]
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if !ok {
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res.value, res.err = memo.f(key)
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memo.cache[key] = res
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}
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memo.mu.Unlock()
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return res.value, res.err
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}
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```
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@ -184,19 +184,19 @@ func (memo *Memo) Get(key string) (value interface{}, err error) {
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```go
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gopl.io/ch9/memo3
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func (memo *Memo) Get(key string) (value interface{}, err error) {
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memo.mu.Lock()
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res, ok := memo.cache[key]
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memo.mu.Unlock()
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if !ok {
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res.value, res.err = memo.f(key)
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memo.mu.Lock()
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res, ok := memo.cache[key]
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memo.mu.Unlock()
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if !ok {
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res.value, res.err = memo.f(key)
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// Between the two critical sections, several goroutines
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// may race to compute f(key) and update the map.
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memo.mu.Lock()
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memo.cache[key] = res
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memo.mu.Unlock()
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}
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return res.value, res.err
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// Between the two critical sections, several goroutines
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// may race to compute f(key) and update the map.
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memo.mu.Lock()
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memo.cache[key] = res
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memo.mu.Unlock()
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}
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return res.value, res.err
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}
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```
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@ -207,41 +207,41 @@ func (memo *Memo) Get(key string) (value interface{}, err error) {
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```go
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gopl.io/ch9/memo4
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type entry struct {
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res result
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ready chan struct{} // closed when res is ready
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res result
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ready chan struct{} // closed when res is ready
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}
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func New(f Func) *Memo {
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return &Memo{f: f, cache: make(map[string]*entry)}
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return &Memo{f: f, cache: make(map[string]*entry)}
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}
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type Memo struct {
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f Func
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mu sync.Mutex // guards cache
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cache map[string]*entry
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f Func
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mu sync.Mutex // guards cache
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cache map[string]*entry
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}
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func (memo *Memo) Get(key string) (value interface{}, err error) {
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memo.mu.Lock()
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e := memo.cache[key]
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if e == nil {
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// This is the first request for this key.
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// This goroutine becomes responsible for computing
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// the value and broadcasting the ready condition.
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e = &entry{ready: make(chan struct{})}
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memo.cache[key] = e
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memo.mu.Unlock()
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memo.mu.Lock()
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e := memo.cache[key]
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if e == nil {
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// This is the first request for this key.
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// This goroutine becomes responsible for computing
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// the value and broadcasting the ready condition.
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e = &entry{ready: make(chan struct{})}
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memo.cache[key] = e
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memo.mu.Unlock()
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e.res.value, e.res.err = memo.f(key)
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e.res.value, e.res.err = memo.f(key)
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close(e.ready) // broadcast ready condition
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} else {
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// This is a repeat request for this key.
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memo.mu.Unlock()
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close(e.ready) // broadcast ready condition
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} else {
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// This is a repeat request for this key.
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memo.mu.Unlock()
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<-e.ready // wait for ready condition
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}
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return e.res.value, e.res.err
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<-e.ready // wait for ready condition
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}
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return e.res.value, e.res.err
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}
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```
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@ -263,13 +263,13 @@ type Func func(key string) (interface{}, error)
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// A result is the result of calling a Func.
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type result struct {
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value interface{}
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err error
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value interface{}
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err error
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}
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type entry struct {
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res result
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ready chan struct{} // closed when res is ready
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res result
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ready chan struct{} // closed when res is ready
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}
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```
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@ -279,23 +279,23 @@ type entry struct {
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gopl.io/ch9/memo5
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// A request is a message requesting that the Func be applied to key.
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type request struct {
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key string
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response chan<- result // the client wants a single result
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key string
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response chan<- result // the client wants a single result
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}
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type Memo struct{ requests chan request }
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// New returns a memoization of f. Clients must subsequently call Close.
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func New(f Func) *Memo {
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memo := &Memo{requests: make(chan request)}
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go memo.server(f)
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return memo
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memo := &Memo{requests: make(chan request)}
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go memo.server(f)
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return memo
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}
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func (memo *Memo) Get(key string) (interface{}, error) {
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response := make(chan result)
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memo.requests <- request{key, response}
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res := <-response
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return res.value, res.err
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response := make(chan result)
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memo.requests <- request{key, response}
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res := <-response
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return res.value, res.err
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}
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func (memo *Memo) Close() { close(memo.requests) }
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@ -307,31 +307,31 @@ cache變量被限製在了monitor goroutine (\*Memo).server中，下面會看到
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```go
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func (memo *Memo) server(f Func) {
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cache := make(map[string]*entry)
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for req := range memo.requests {
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e := cache[req.key]
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if e == nil {
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// This is the first request for this key.
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e = &entry{ready: make(chan struct{})}
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cache[req.key] = e
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go e.call(f, req.key) // call f(key)
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}
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go e.deliver(req.response)
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}
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cache := make(map[string]*entry)
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for req := range memo.requests {
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e := cache[req.key]
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if e == nil {
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// This is the first request for this key.
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e = &entry{ready: make(chan struct{})}
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cache[req.key] = e
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go e.call(f, req.key) // call f(key)
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}
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go e.deliver(req.response)
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}
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}
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func (e *entry) call(f Func, key string) {
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// Evaluate the function.
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e.res.value, e.res.err = f(key)
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// Broadcast the ready condition.
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close(e.ready)
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// Evaluate the function.
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e.res.value, e.res.err = f(key)
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// Broadcast the ready condition.
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close(e.ready)
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}
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func (e *entry) deliver(response chan<- result) {
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// Wait for the ready condition.
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<-e.ready
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// Send the result to the client.
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response <- e.res
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// Wait for the ready condition.
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<-e.ready
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// Send the result to the client.
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response <- e.res
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}
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```
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@ -343,5 +343,5 @@ func (e *entry) deliver(response chan<- result) {
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上面的兩種方案併不好説特定情境下哪種更好，不過了解他們還是有價值的。有時候從一種方式切換到另一種可以使你的代碼更爲簡潔。(譯註：不是説好的golang推崇通信併發麽)
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練習 9.3: 擴展Func類型和(\*Memo).Get方法，支持調用方提供一個可選的done channel，使其具備通過該channel來取消整個操作的能力(§8.9)。一個被取消了的Func的調用結果不應該被緩存。
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**練習 9.3：** 擴展Func類型和(\*Memo).Get方法，支持調用方提供一個可選的done channel，使其具備通過該channel來取消整個操作的能力(§8.9)。一個被取消了的Func的調用結果不應該被緩存。
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