From fb3fdf2262a7ced18009516f00473aa975aaa08a Mon Sep 17 00:00:00 2001
From: Christian Krinitsin <mail@krinitsin.com>
Date: Fri, 21 Mar 2025 15:11:06 +0100
Subject: implement a queue with a mutex to prevent concurrent accesses to the
 shared memory

---
 src/client/main.cpp        | 88 ++++++++++++++++++++++++++++++++++++++++------
 src/common/shared_memory.h | 13 +++++--
 2 files changed, 87 insertions(+), 14 deletions(-)

diff --git a/src/client/main.cpp b/src/client/main.cpp
index 2e4a606..6dfd48f 100644
--- a/src/client/main.cpp
+++ b/src/client/main.cpp
@@ -2,26 +2,65 @@
 #include <cstring>
 #include <fcntl.h>
 #include <iostream>
+#include <pthread.h>
 #include <sys/mman.h>
 #include <unistd.h>
 #include <utility>
 
-void sendRequest(
+bool request_processed(SharedMemory* shared_memory, int index)
+{
+    if (shared_memory->full) {
+        return false;
+    }
+    if (shared_memory->tail == shared_memory->head && !shared_memory->full) {
+        return true;
+    }
+
+    for (int i = shared_memory->head - 1; i != shared_memory->tail; i = (i - 1) % QUEUE_SIZE) {
+        if (i == index) {
+            return false;
+        }
+    }
+
+    return shared_memory->tail != index;
+}
+
+void send_request(
     SharedMemory* shared_memory,
     Operations type,
     std::pair<const std::string&, const std::string&> arguments)
 {
-    shared_memory->request.type = type;
-    strncpy(shared_memory->request.key, serialize(arguments.first).c_str(), MAX_KEY_SIZE);
-    strncpy(shared_memory->request.value, serialize(arguments.second).c_str(), MAX_VALUE_SIZE);
-    shared_memory->processed = false;
+    int index;
 
-    std::cout << "Command sent" << '\n';
+    pthread_mutex_lock(&shared_memory->mutex);
 
-    sleep(2);
+    if (shared_memory->full) {
+        std::cout << "Queue is full" << '\n';
+        pthread_cond_wait(&shared_memory->cond_var, &shared_memory->mutex);
+        return;
+    }
+
+    index = shared_memory->head;
+    Request* request = &shared_memory->request[index];
+    request->type = type;
+    strncpy(request->key, serialize(arguments.first).c_str(), MAX_KEY_SIZE);
+    strncpy(request->value, serialize(arguments.second).c_str(), MAX_VALUE_SIZE);
+    shared_memory->head = (1 + shared_memory->head) % QUEUE_SIZE;
+    shared_memory->full = shared_memory->head == shared_memory->tail;
+    pthread_cond_signal(&shared_memory->cond_var);
+
+    pthread_mutex_unlock(&shared_memory->mutex);
+
+    std::cout << "Command sent" << '\n';
 
     if (type == Operations::GET) {
-        std::string result(shared_memory->response);
+        pthread_mutex_lock(&shared_memory->mutex);
+
+        while (!request_processed(shared_memory, index)) {
+            pthread_cond_wait(&shared_memory->cond_var, &shared_memory->mutex);
+        }
+        std::string result(shared_memory->request[index].response);
+        pthread_mutex_unlock(&shared_memory->mutex);
         std::cout << "Result: " << result << std::endl;
     }
 }
@@ -38,10 +77,37 @@ int main()
         (SharedMemory*)mmap(0, sizeof(SharedMemory), PROT_READ | PROT_WRITE, MAP_SHARED, shm_fd, 0);
 
     std::cout << "Start inserting.." << '\n';
-    sendRequest(shared_memory, INSERT, std::pair(serialize(10), serialize(5)));
+    send_request(shared_memory, INSERT, std::pair(serialize(3), serialize(3)));
+
+    std::cout << "Start inserting.." << '\n';
+    send_request(shared_memory, INSERT, std::pair(serialize(4), serialize(5)));
+
+    std::cout << "Start inserting.." << '\n';
+    send_request(shared_memory, INSERT, std::pair(serialize(6), serialize(6)));
+
+    std::cout << "Start inserting.." << '\n';
+    send_request(shared_memory, INSERT, std::pair(serialize(7), serialize(3)));
+
+    std::cout << "Start inserting.." << '\n';
+    send_request(shared_memory, INSERT, std::pair(serialize(8), serialize(3)));
+
+    std::cout << "Start inserting.." << '\n';
+    send_request(shared_memory, INSERT, std::pair(serialize(9), serialize(3)));
+
+    std::cout << "Start inserting.." << '\n';
+    send_request(shared_memory, INSERT, std::pair(serialize(10), serialize(3)));
+
+    std::cout << "Start inserting.." << '\n';
+    send_request(shared_memory, INSERT, std::pair(serialize(11), serialize(3)));
+
+    std::cout << "Start inserting.." << '\n';
+    send_request(shared_memory, INSERT, std::pair(serialize(12), serialize(3)));
+
+    std::cout << "Start inserting.." << '\n';
+    send_request(shared_memory, INSERT, std::pair(serialize(13), serialize(4)));
 
-    std::cout << "Start printing.." << '\n';
-    sendRequest(shared_memory, PRINT, std::pair(serialize(10), serialize(5)));
+    std::cout << "Start PRINTING.." << '\n';
+    send_request(shared_memory, PRINT, std::pair(serialize(0), serialize(0)));
 
     munmap(shared_memory, sizeof(SharedMemory));
     close(shm_fd);
diff --git a/src/common/shared_memory.h b/src/common/shared_memory.h
index 43fe2f5..70554e4 100644
--- a/src/common/shared_memory.h
+++ b/src/common/shared_memory.h
@@ -4,6 +4,8 @@
 #include <sstream>
 
 #define SHM_NAME "/hashtable_queue"
+#define QUEUE_SIZE 10
+
 #define MAX_KEY_SIZE 64
 #define MAX_VALUE_SIZE 128
 
@@ -13,12 +15,17 @@ struct Request {
     Operations type;
     char key[MAX_KEY_SIZE];
     char value[MAX_VALUE_SIZE];
+    char response[MAX_VALUE_SIZE];
 };
 
 struct SharedMemory {
-    Request request;
-    bool processed;
-    char response[MAX_VALUE_SIZE];
+    Request request[QUEUE_SIZE];
+    pthread_mutex_t mutex;
+    pthread_cond_t cond_var;
+
+    int tail;
+    int head;
+    bool full;
 };
 
 template <typename T>
-- 
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