# ==================================================
# NGINX REVERSE PROXY CONFIGURATION
# Math Platform - Load Balancer + Rate Limiting + Security
# ==================================================

user nginx;
worker_processes auto;
error_log /var/log/nginx/error.log warn;
pid /var/run/nginx.pid;

events {
    worker_connections 1024;
    use epoll;
    multi_accept on;
}

http {
    include /etc/nginx/mime.types;
    default_type application/octet-stream;

    # Logging format
    log_format main '$remote_addr - $remote_user [$time_local] "$request" '
                    '$status $body_bytes_sent "$http_referer" '
                    '"$http_user_agent" "$http_x_forwarded_for" '
                    'rt=$request_time uct="$upstream_connect_time" '
                    'uht="$upstream_header_time" urt="$upstream_response_time"';

    access_log /var/log/nginx/access.log main;

    # Performance optimizations
    sendfile on;
    tcp_nopush on;
    tcp_nodelay on;
    keepalive_timeout 65;
    types_hash_max_size 2048;
    client_max_body_size 20M;

    # Gzip compression
    gzip on;
    gzip_vary on;
    gzip_proxied any;
    gzip_comp_level 6;
    gzip_types text/plain text/css text/xml text/javascript
               application/json application/javascript application/xml+rss
               application/rss+xml font/truetype font/opentype
               application/vnd.ms-fontobject image/svg+xml;
    gzip_disable "msie6";

    # Rate limiting zones
    limit_req_zone $binary_remote_addr zone=auth_limit:10m rate=5r/s;
    limit_req_zone $binary_remote_addr zone=api_limit:10m rate=10r/s;
    limit_req_zone $binary_remote_addr zone=general_limit:10m rate=20r/s;
    limit_conn_zone $binary_remote_addr zone=conn_limit:10m;

    # Upstream servers
    upstream frontend {
        server frontend:3000;
        keepalive 64;
    }

    upstream backend {
        server backend:3001;
        keepalive 64;
    }

    # Security headers
    add_header X-Frame-Options "SAMEORIGIN" always;
    add_header X-Content-Type-Options "nosniff" always;
    add_header X-XSS-Protection "1; mode=block" always;
    add_header Referrer-Policy "no-referrer-when-downgrade" always;

    # Main server block
    server {
        listen 80;
        listen [::]:80;
        server_name localhost;

        # Client body size limit for file uploads
        client_max_body_size 20M;

        # Rate limiting
        limit_conn conn_limit 10;

        # Frontend routes
        location / {
            proxy_pass http://frontend;
            proxy_http_version 1.1;

            # Headers
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection 'upgrade';
            proxy_set_header Host $host;
            proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header X-Forwarded-Proto $scheme;

            # Timeouts
            proxy_connect_timeout 60s;
            proxy_send_timeout 60s;
            proxy_read_timeout 60s;

            # Caching
            proxy_cache_bypass $http_upgrade;

            # Hide sensitive upstream headers
            proxy_hide_header X-Powered-By;
            proxy_hide_header Server;
            proxy_hide_header X-AspNet-Version;
            proxy_hide_header X-AspNetMvc-Version;

            # Rate limiting
            limit_req zone=general_limit burst=30 nodelay;
        }

        # API routes - Auth endpoints (stricter rate limit)
        location /api/auth {
            proxy_pass http://backend;
            proxy_http_version 1.1;

            # Headers
            proxy_set_header Host $host;
            proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header X-Forwarded-Proto $scheme;

            # Timeouts
            proxy_connect_timeout 30s;
            proxy_send_timeout 30s;
            proxy_read_timeout 30s;

            # Stricter rate limiting for auth
            limit_req zone=auth_limit burst=10 nodelay;
            limit_req_status 429;
        }

        # API routes - General
        location /api/ {
            proxy_pass http://backend;
            proxy_http_version 1.1;

            # Headers
            proxy_set_header Host $host;
            proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header X-Forwarded-Proto $scheme;

            # Timeouts
            proxy_connect_timeout 60s;
            proxy_send_timeout 60s;
            proxy_read_timeout 60s;

            # Rate limiting
            limit_req zone=api_limit burst=20 nodelay;
            limit_req_status 429;
        }

        # Health check endpoint
        location /health {
            access_log off;
            return 200 "healthy\n";
            add_header Content-Type text/plain;
        }

        # Nginx status (for monitoring)
        location /nginx_status {
            stub_status on;
            access_log off;
            allow 127.0.0.1;
            allow 172.16.0.0/12;
            allow 10.0.0.0/8;
            deny all;
        }

        # Error pages
        error_page 429 /429.html;
        error_page 500 502 503 504 /50x.html;

        location = /429.html {
            internal;
            return 429 '{"error": "Too many requests. Please slow down."}';
            add_header Content-Type application/json;
        }

        location = /50x.html {
            internal;
            return 500 '{"error": "Internal server error. Please try again later."}';
            add_header Content-Type application/json;
        }

        # Static files caching
        location ~* \.(jpg|jpeg|png|gif|ico|css|js|svg|woff|woff2|ttf|eot)$ {
            proxy_pass http://frontend;
            expires 1y;
            add_header Cache-Control "public, immutable";
        }

        # WebSocket support for Next.js HMR (development only)
        location /_next/webpack-hmr {
            proxy_pass http://frontend;
            proxy_http_version 1.1;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection "upgrade";
        }
    }

    # HTTPS server block (uncomment for production with SSL certificates)
    # server {
    #     listen 443 ssl http2;
    #     listen [::]:443 ssl http2;
    #     server_name localhost;
    #
    #     ssl_certificate /etc/nginx/ssl/cert.pem;
    #     ssl_certificate_key /etc/nginx/ssl/key.pem;
    #     ssl_protocols TLSv1.2 TLSv1.3;
    #     ssl_ciphers HIGH:!aNULL:!MD5;
    #     ssl_prefer_server_ciphers on;
    #
    #     # Same location blocks as above
    #     # ...
    # }
}

# ==================================================
# CONFIGURATION NOTES
# ==================================================
#
# Rate Limiting:
# - /api/auth: 5 requests/second (stricter for security)
# - /api/*: 10 requests/second (general API)
# - /*: 20 requests/second (frontend pages)
#
# Connection Limits:
# - Max 10 concurrent connections per IP
#
# Upstream Health:
# - Frontend: frontend:3000
# - Backend: backend:3001
#
# Monitoring:
# - Health check: http://localhost/health
# - Nginx status: http://localhost/nginx_status (local only)