From 4fa57a0eb9e9998644ff7b28ef31cbccd74318e6 Mon Sep 17 00:00:00 2001
From: Serge Bishyr <seriybg@gmail.com>
Date: Fri, 13 Oct 2017 17:57:11 +0300
Subject: [PATCH] synchronization/mutex: implement synchronization mutex/lock
 pattern

---
 README.md                |  2 +-
 synchronization/mutex.go | 53 +++++++++++++++++++++++++++++++++
 synchronization/mutex.md | 64 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 118 insertions(+), 1 deletion(-)
 create mode 100644 synchronization/mutex.go
 create mode 100644 synchronization/mutex.md

diff --git a/README.md b/README.md
index 7d9f5f7..ecb902d 100644
--- a/README.md
+++ b/README.md
@@ -52,7 +52,7 @@ A curated collection of idiomatic design & application patterns for Go language.
 | Pattern | Description | Status |
 |:-------:|:----------- |:------:|
 | [Condition Variable](/synchronization/condition_variable.md) | Provides a mechanism for threads to temporarily give up access in order to wait for some condition | ✘ |
-| [Lock/Mutex](/synchronization/mutex.md) | Enforces mutual exclusion limit on a resource to gain exclusive access | ✘ |
+| [Lock/Mutex](/synchronization/mutex.md) | Enforces mutual exclusion limit on a resource to gain exclusive access | ✔ |
 | [Monitor](/synchronization/monitor.md) | Combination of mutex and condition variable patterns | ✘ |
 | [Read-Write Lock](/synchronization/read_write_lock.md) | Allows parallel read access, but only exclusive access on write operations to a resource | ✘ |
 | [Semaphore](/synchronization/semaphore.md) | Allows controlling access to a common resource | ✔ |
diff --git a/synchronization/mutex.go b/synchronization/mutex.go
new file mode 100644
index 0000000..6b15a1f
--- /dev/null
+++ b/synchronization/mutex.go
@@ -0,0 +1,53 @@
+package main
+
+import (
+	"fmt"
+	"sync"
+	"time"
+)
+
+//dictionary inherits the mutex lock
+type dictionary struct {
+	sync.Mutex
+	rel map[string]string
+}
+
+//add a new key value pare if the given key is not present in the map
+//returns an error if the given key is already present
+func (d *dictionary) putIfAbsent(key string, value string) error {
+	//Locks the object, add the key-value pare to the map, and then unlock the object
+	d.Lock()
+	defer d.Unlock()
+	if _, isPresent := d.rel[key]; isPresent {
+		return fmt.Errorf("key [%s] is already present in the dictionary", key)
+	}
+	d.rel[key] = value
+	return nil
+}
+
+func newDictionary() dictionary {
+	return dictionary{
+		rel: make(map[string]string),
+	}
+}
+
+func main() {
+	d := newDictionary()
+	keys := []string{"foo", "bar", "baz", "bar"}
+	values := []string{"value1", "value2", "value3", "value4"}
+	for i := 0; i < len(keys); i++ {
+		// Starts a new goroutine
+		// that that add to the dictionary as a key string representation of i % 7
+		// and string representation of i as a value
+		go func(index int) {
+			err := d.putIfAbsent(keys[index], values[index])
+			if err != nil {
+				//print the error
+				fmt.Println(err.Error())
+			}
+		}(i)
+	}
+	time.Sleep(time.Second * 2)
+	//print the result
+	fmt.Println(d.rel)
+}
diff --git a/synchronization/mutex.md b/synchronization/mutex.md
new file mode 100644
index 0000000..3c8eeb6
--- /dev/null
+++ b/synchronization/mutex.md
@@ -0,0 +1,64 @@
+# Mutex/Lock pattern
+A mutex is a synchronization pattern that allows exclusive access to shared data by locking and unlocking.
+
+## Implementation
+
+```go
+package mutex
+
+import (
+	"errors"
+	"sync"
+	"fmt"
+)
+
+//dictionary inherits the mutex lock
+type Dictionary struct {
+	sync.Mutex
+	rel map[string]string
+}
+
+func New() Dictionary {
+	return Dictionary{
+		rel: make(map[string]string),
+	}
+}
+
+//add a new key value pare if the given key is not present in the map
+//returns an error if the given key is already present
+func (d *Dictionary) PutIfAbsent(key string, value string) error {
+	d.Lock() //lock the object
+	defer d.Unlock() //unlock the object when the work is done
+	//return the error if key is already present in the map
+	if _, isPresent := d.rel[key]; isPresent {
+		return fmt.Errorf("key [%s] is already present in the dictionary", key)
+	}
+	d.rel[key] = value
+	return nil
+}
+``` 
+
+## Usage
+
+```go
+func main() {
+    d := mutext.New()
+    keys := []string{"foo", "bar", "baz", "bar"}
+    values := []string{"value1", "value2", "value3", "value4"}
+    for i := 0; i < len(keys); i++ {
+    	// Starts a new goroutine
+    	// that that add to the dictionary as a key string representation of i % 7
+    	// and string representation of i as a value
+    	go func(index int) {
+    		err := d.putIfAbsent(keys[index], values[index])
+    		if err != nil {
+    			//print the error
+    			fmt.Println(err.Error())
+    		}
+    	}(i)
+    }
+    time.Sleep(time.Second * 2)
+    //print the result
+    fmt.Println(d.rel)
+}
+```
\ No newline at end of file