good good study, day day up!

vendor/gopl.io/ch9/bank2/bank.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 262.
+
+// Package bank provides a concurrency-safe bank with one account.
+package bank
+
+//!+
+var (
+	sema    = make(chan struct{}, 1) // a binary semaphore guarding balance
+	balance int
+)
+
+func Deposit(amount int) {
+	sema <- struct{}{} // acquire token
+	balance = balance + amount
+	<-sema // release token
+}
+
+func Balance() int {
+	sema <- struct{}{} // acquire token
+	b := balance
+	<-sema // release token
+	return b
+}
+
+//!-

vendor/gopl.io/ch9/bank2/bank_test.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package bank_test
+
+import (
+	"sync"
+	"testing"
+
+	"gopl.io/ch9/bank2"
+)
+
+func TestBank(t *testing.T) {
+	// Deposit [1..1000] concurrently.
+	var n sync.WaitGroup
+	for i := 1; i <= 1000; i++ {
+		n.Add(1)
+		go func(amount int) {
+			bank.Deposit(amount)
+			n.Done()
+		}(i)
+	}
+	n.Wait()
+
+	if got, want := bank.Balance(), (1000+1)*1000/2; got != want {
+		t.Errorf("Balance = %d, want %d", got, want)
+	}
+}