server {
    listen 80;
    server_name localhost;

    root /usr/share/nginx/html;
    index index.html;

    # Prevent HTTP protocol downgrade behind reverse proxy on redirects
    absolute_redirect off;

    # Security Headers (Defense in depth)
    add_header X-Content-Type-Options "nosniff" always;
    add_header X-Frame-Options "DENY" always;
    add_header Referrer-Policy "strict-origin-when-cross-origin" always;
    add_header Cross-Origin-Opener-Policy "same-origin" always;
    add_header Strict-Transport-Security "max-age=63072000; includeSubDomains; preload" always;

    # Gzip compression
    gzip on;
    gzip_vary on;
    gzip_proxied any;
    gzip_comp_level 6;
    gzip_types text/plain text/css text/xml application/json application/javascript application/rss+xml application/atom+xml image/svg+xml;

    # Root redirect: detect browser language, fallback to /en/
    location = / {
        set $locale "en";
        if ($http_accept_language ~* "(^|,\s*)ru") {
            set $locale "ru";
        }
        return 302 /$locale/;
    }

    # Cache hashed static assets (JS, CSS, fonts) — matched before locale prefix blocks
    # root is inherited from server block, so /en/static/js/file.js resolves correctly
    location ~* \.(js|css|woff|woff2|ttf|eot)$ {
        expires 1y;
        add_header Cache-Control "public, immutable" always;
        add_header Access-Control-Allow-Origin "*" always;
    }

    # Cache images
    location ~* \.(png|jpg|jpeg|gif|ico|svg)$ {
        expires 1y;
        add_header Cache-Control "public, no-transform" always;
        add_header Access-Control-Allow-Origin "*" always;
    }

    # English locale — SPA fallback for non-asset routes
    location /en/ {
        try_files $uri $uri/ $uri.html /en/index.html;
    }

    # Russian locale — SPA fallback for non-asset routes
    location /ru/ {
        try_files $uri $uri/ $uri.html /ru/index.html;
    }

    # Version probe for release rollout verification
    location = /version.json {
        add_header Cache-Control "no-store, no-cache, must-revalidate, proxy-revalidate, max-age=0" always;
        add_header Content-Type "application/json" always;
        add_header Access-Control-Allow-Origin "*" always;
        try_files /version.json =404;
    }

    # Error pages
    error_page 404 /en/index.html;
}