use std::collections::HashMap;

use secrecy::{ExposeSecret, SecretString};
use serde::{Deserialize, Serialize};

use crate::infra::llm::ProviderKind;
use crate::module::DEFAULT_URL_PREFIX;

pub mod background;
pub use background::{CleanupWorkerConfig, OrphanWatchdogConfig, ThreadSummaryWorkerConfig};

#[derive(Debug, Clone, Serialize, Deserialize, modkit_macros::ExpandVars)]
#[serde(deny_unknown_fields)]
pub struct MiniChatConfig {
    #[serde(default = "default_url_prefix")]
    pub url_prefix: String,
    #[serde(default)]
    pub streaming: StreamingConfig,
    #[serde(default = "default_vendor")]
    pub vendor: String,
    #[serde(default)]
    pub estimation_budgets: EstimationBudgets,
    #[serde(default)]
    pub quota: QuotaConfig,
    #[serde(default)]
    pub outbox: OutboxConfig,
    #[serde(default)]
    pub context: ContextConfig,
    #[serde(default)]
    pub rag: RagConfig,
    /// `OAuth2` client credentials for OAGW upstream provisioning.
    /// Mini-chat exchanges these via the `AuthN` resolver to obtain
    /// a `SecurityContext` for OAGW API calls.
    #[expand_vars]
    #[serde(skip_serializing)]
    pub client_credentials: ClientCredentialsConfig,
    #[serde(default)]
    pub metrics: MetricsConfig,
    /// Provider registry. Key = `provider_id` (matches [`ModelCatalogEntry::provider_id`]).
    #[expand_vars]
    #[serde(default = "default_providers")]
    pub providers: HashMap<String, ProviderEntry>,
    /// Orphan watchdog background worker.
    #[serde(default)]
    pub orphan_watchdog: OrphanWatchdogConfig,
    /// Thread summary background worker.
    #[serde(default)]
    pub thread_summary_worker: ThreadSummaryWorkerConfig,
    /// Cleanup background worker for soft-deleted chat resources.
    #[serde(default)]
    pub cleanup_worker: CleanupWorkerConfig,
    /// Image thumbnail generation settings.
    #[serde(default)]
    pub thumbnail: ThumbnailConfig,
}

/// Which file/vector-store implementation to use for RAG operations.
///
/// Controls URI patterns (`/v1/…` vs `/openai/…?api-version=…`) and
/// dispatch to provider-specific `FileStorageProvider` / `VectorStoreProvider`.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
pub enum StorageKind {
    /// OpenAI-native: `/{alias}/v1/{path}`, no query params.
    #[serde(rename = "openai")]
    OpenAi,
    /// Azure `OpenAI`: `/{alias}/openai/{path}?api-version={ver}`.
    /// Requires `api_version` to be set on the `ProviderEntry`.
    #[serde(rename = "azure")]
    Azure,
}

/// Metrics configuration.
#[derive(Debug, Clone, Default, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct MetricsConfig {
    /// Metric name prefix. When empty (the default), derived from the module
    /// name by converting it to `snake_case` (e.g., `"mini-chat"` → `"mini_chat"`).
    #[serde(default)]
    pub prefix: String,
}

impl MetricsConfig {
    /// Resolve the effective prefix: explicit config value, or
    /// `snake_case(module_name)`.
    #[must_use]
    pub fn effective_prefix(&self, module_name: &str) -> String {
        let trimmed = self.prefix.trim();
        if trimmed.is_empty() {
            heck::ToSnakeCase::to_snake_case(module_name)
        } else {
            trimmed.to_owned()
        }
    }
}

/// Configuration for a single LLM provider.
#[derive(Debug, Clone, Serialize, Deserialize, modkit_macros::ExpandVars)]
#[serde(deny_unknown_fields)]
pub struct ProviderEntry {
    /// Which adapter to use (e.g., `openai_responses`, `openai_chat_completions`).
    pub kind: ProviderKind,
    /// OAGW upstream alias (used in proxy URI: `/{alias}/...`).
    ///
    /// In config: only required for IP-based hosts. For hostname-based
    /// hosts OAGW auto-derives the alias — leave this unset.
    ///
    /// At runtime: overwritten with the OAGW-assigned alias after
    /// `create_upstream` succeeds.
    #[serde(default)]
    pub upstream_alias: Option<String>,
    /// Upstream hostname (e.g., `api.openai.com`). Used for OAGW upstream
    /// registration during module init.
    #[expand_vars]
    pub host: String,
    /// Upstream port. Defaults to `443` (HTTPS). Set to a non-standard port
    /// for local/mock providers.
    #[serde(default)]
    pub port: Option<u16>,
    /// Use plain HTTP instead of HTTPS for this upstream. Defaults to `false`.
    /// Only effective when the oagw `allow_http_upstream` option is also enabled.
    #[serde(default)]
    pub use_http: bool,
    /// API path template for the responses endpoint.
    /// Use `{model}` as placeholder for the deployment/model name.
    /// Defaults to `/v1/responses` (`OpenAI` native).
    /// Azure example: `/openai/deployments/{model}/responses?api-version=2025-03-01-preview`
    #[serde(default = "default_api_path")]
    pub api_path: String,
    /// OAGW auth plugin type for this upstream (optional).
    /// Example: `gts.x.core.oagw.auth_plugin.v1~x.core.oagw.apikey.v1`
    #[serde(default)]
    pub auth_plugin_type: Option<String>,
    /// Auth plugin config (e.g., `header`, `prefix`, `secret_ref`).
    /// Values support `${VAR}` env expansion via [`config_expanded()`].
    #[expand_vars]
    #[serde(default)]
    pub auth_config: Option<HashMap<String, String>>,
    /// Storage backend label persisted in attachment/vector-store DB rows.
    /// Used by cleanup workers to determine which provider API to target.
    /// When `None`, falls back to the `provider_id` key as-is.
    /// Example: `"azure"` for `azure_openai` providers.
    #[serde(default)]
    pub storage_backend: Option<String>,
    /// Whether this provider supports `file_search` metadata filters.
    /// Azure `OpenAI` does not support filters — `FilteredByAttachmentIds`
    /// is degraded to `UnrestrictedChatSearch`. Defaults to `true`.
    #[serde(default = "default_true")]
    pub supports_file_search_filters: bool,
    /// Which file/vector-store implementation to use for RAG operations.
    /// Controls URI patterns and dispatch to provider-specific impls.
    /// Required — no default; forces explicit configuration per provider.
    pub storage_kind: StorageKind,
    /// Optional API version query parameter appended to RAG requests.
    /// Required for Azure (`?api-version=…`). Ignored for `OpenAI`.
    #[serde(default)]
    pub api_version: Option<String>,
    /// Per-tenant overrides. Key = tenant ID (UUID string).
    /// Overrides host and/or auth for specific tenants while sharing
    /// the same adapter kind and API path.
    #[expand_vars]
    #[serde(default)]
    pub tenant_overrides: HashMap<String, ProviderTenantOverride>,
}

/// Per-tenant override for a [`ProviderEntry`].
///
/// All fields are optional — omitted fields inherit from the parent
/// [`ProviderEntry`]. Keyed by tenant ID (UUID string) in the config.
#[derive(Debug, Clone, Serialize, Deserialize, modkit_macros::ExpandVars)]
#[serde(deny_unknown_fields)]
pub struct ProviderTenantOverride {
    /// Override upstream hostname for this tenant.
    #[serde(default)]
    pub host: Option<String>,
    /// OAGW upstream alias for this tenant.
    ///
    /// In config: only required for IP-based hosts. For hostname-based
    /// hosts OAGW auto-derives the alias — leave this unset.
    ///
    /// At runtime: overwritten with the OAGW-assigned alias after
    /// `create_upstream` succeeds.
    #[serde(default)]
    pub upstream_alias: Option<String>,
    /// Override auth plugin type for this tenant.
    #[serde(default)]
    pub auth_plugin_type: Option<String>,
    /// Override auth plugin config for this tenant.
    #[expand_vars]
    #[serde(default)]
    pub auth_config: Option<HashMap<String, String>>,
}

impl ProviderEntry {
    /// Effective host for a given tenant. Returns the tenant override host
    /// if configured, otherwise the root host.
    #[must_use]
    pub fn effective_host_for_tenant(&self, tenant_id: &str) -> &str {
        self.tenant_overrides
            .get(tenant_id)
            .and_then(|o| o.host.as_deref())
            .unwrap_or(&self.host)
    }

    /// Effective auth plugin type for a given tenant.
    #[must_use]
    pub fn effective_auth_plugin_type_for_tenant(&self, tenant_id: &str) -> Option<&str> {
        self.tenant_overrides
            .get(tenant_id)
            .and_then(|o| o.auth_plugin_type.as_deref())
            .or(self.auth_plugin_type.as_deref())
    }

    /// Effective auth config for a given tenant.
    #[must_use]
    pub fn effective_auth_config_for_tenant(
        &self,
        tenant_id: &str,
    ) -> Option<&HashMap<String, String>> {
        self.tenant_overrides
            .get(tenant_id)
            .and_then(|o| o.auth_config.as_ref())
            .or(self.auth_config.as_ref())
    }

    /// Validate provider entry at startup.
    pub fn validate(&self, provider_id: &str) -> Result<(), String> {
        if self.host.trim().is_empty() {
            return Err(format!("provider '{provider_id}': host must not be empty"));
        }
        if self.port == Some(0) {
            return Err(format!("provider '{provider_id}': port must not be 0"));
        }
        for (tid, tenant_override) in &self.tenant_overrides {
            if let Some(h) = &tenant_override.host
                && h.trim().is_empty()
            {
                return Err(format!(
                    "provider '{provider_id}': tenant override '{tid}' host must not be empty"
                ));
            }

            let overrides_auth =
                tenant_override.auth_plugin_type.is_some() || tenant_override.auth_config.is_some();
            let has_distinct_upstream =
                tenant_override.host.is_some() || tenant_override.upstream_alias.is_some();

            if overrides_auth && !has_distinct_upstream {
                return Err(format!(
                    "provider '{provider_id}': tenant override '{tid}' overrides auth \
                     without host or upstream_alias - \
                     set one to create a distinct upstream"
                ));
            }
        }
        Ok(())
    }
}

const fn default_true() -> bool {
    true
}

fn default_api_path() -> String {
    "/v1/responses".to_owned()
}

fn default_providers() -> HashMap<String, ProviderEntry> {
    let mut m = HashMap::new();
    m.insert(
        "openai".to_owned(),
        ProviderEntry {
            kind: ProviderKind::OpenAiResponses,
            upstream_alias: None,
            host: "api.openai.com".to_owned(),
            port: None,
            use_http: false,
            api_path: default_api_path(),
            auth_plugin_type: Some(
                "gts.x.core.oagw.auth_plugin.v1~x.core.oagw.apikey.v1".to_owned(),
            ),
            auth_config: Some({
                let mut c = HashMap::new();
                c.insert("header".to_owned(), "Authorization".to_owned());
                c.insert("prefix".to_owned(), "Bearer ".to_owned());
                c.insert("secret_ref".to_owned(), "cred://openai-key".to_owned());
                c
            }),
            storage_backend: None,
            supports_file_search_filters: true,
            storage_kind: StorageKind::OpenAi,
            api_version: None,
            tenant_overrides: HashMap::new(),
        },
    );
    m
}

/// `OAuth2` client credentials for authenticating OAGW provisioning calls.
#[derive(Clone, Deserialize, modkit_macros::ExpandVars)]
#[serde(deny_unknown_fields)]
pub struct ClientCredentialsConfig {
    /// `OAuth2` client identifier. Supports `${VAR}` env expansion.
    #[expand_vars]
    pub client_id: String,
    /// `OAuth2` client secret. Supports `${VAR}` env expansion.
    /// Redacted in `Debug` output.
    #[expand_vars]
    pub client_secret: SecretString,
}

impl std::fmt::Debug for ClientCredentialsConfig {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("ClientCredentialsConfig")
            .field("client_id", &self.client_id)
            .field("client_secret", &"[REDACTED]")
            .finish()
    }
}

/// SSE streaming tuning parameters.
#[derive(Debug, Clone, Copy, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct StreamingConfig {
    /// Bounded mpsc channel capacity between provider task and SSE writer.
    /// Valid range: 16–64 (default 32).
    #[serde(default = "default_channel_capacity")]
    pub sse_channel_capacity: u16,

    /// Ping keepalive interval in seconds.
    /// Valid range: 5–60 (default 15).
    #[serde(default = "default_ping_interval")]
    pub sse_ping_interval_seconds: u16,

    /// Maximum output tokens sent to the preflight reserve.
    /// Default 32768 (matching common model limits).
    #[serde(default = "default_max_output_tokens")]
    pub max_output_tokens: u32,

    /// Search context size passed to the `web_search` tool.
    /// Valid values: "low", "medium", "high" (default "low").
    #[serde(default)]
    pub web_search_context_size: crate::domain::llm::WebSearchContextSize,
}

impl Default for StreamingConfig {
    fn default() -> Self {
        Self {
            sse_channel_capacity: default_channel_capacity(),
            sse_ping_interval_seconds: default_ping_interval(),
            max_output_tokens: default_max_output_tokens(),
            web_search_context_size: crate::domain::llm::WebSearchContextSize::default(),
        }
    }
}

fn default_max_output_tokens() -> u32 {
    32_768
}

impl StreamingConfig {
    /// Validate configuration values at startup. Returns an error message
    /// describing the first invalid value found.
    pub fn validate(&self) -> Result<(), String> {
        if !(16..=64).contains(&self.sse_channel_capacity) {
            return Err(format!(
                "sse_channel_capacity must be 16-64, got {}",
                self.sse_channel_capacity
            ));
        }
        if !(5..=60).contains(&self.sse_ping_interval_seconds) {
            return Err(format!(
                "sse_ping_interval_seconds must be 5-60, got {}",
                self.sse_ping_interval_seconds
            ));
        }
        // web_search_context_size validated by serde at parse time (enum).
        Ok(())
    }
}

fn default_channel_capacity() -> u16 {
    32
}

fn default_ping_interval() -> u16 {
    15
}

impl Default for MiniChatConfig {
    fn default() -> Self {
        Self {
            url_prefix: default_url_prefix(),
            streaming: StreamingConfig::default(),
            vendor: default_vendor(),
            estimation_budgets: EstimationBudgets::default(),
            quota: QuotaConfig::default(),
            outbox: OutboxConfig::default(),
            context: ContextConfig::default(),
            rag: RagConfig::default(),
            client_credentials: ClientCredentialsConfig::default(),
            metrics: MetricsConfig::default(),
            providers: default_providers(),
            orphan_watchdog: OrphanWatchdogConfig::default(),
            thread_summary_worker: ThreadSummaryWorkerConfig::default(),
            cleanup_worker: CleanupWorkerConfig::default(),
            thumbnail: ThumbnailConfig::default(),
        }
    }
}

impl Default for ClientCredentialsConfig {
    fn default() -> Self {
        Self {
            client_id: String::new(),
            client_secret: SecretString::from(String::new()),
        }
    }
}

impl ClientCredentialsConfig {
    /// Validate that S2S credentials are configured.
    pub fn validate(&self) -> Result<(), String> {
        if self.client_id.trim().is_empty() {
            return Err("client_credentials client_id must not be empty".to_owned());
        }
        if self.client_secret.expose_secret().trim().is_empty() {
            return Err("client_credentials client_secret must not be empty".to_owned());
        }
        Ok(())
    }
}

/// Token estimation parameters sourced from `ConfigMap` (P1).
#[derive(Debug, Clone, Copy, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct EstimationBudgets {
    #[serde(default = "default_bytes_per_token")]
    pub bytes_per_token_conservative: u32,
    #[serde(default = "default_fixed_overhead")]
    pub fixed_overhead_tokens: u32,
    #[serde(default = "default_safety_margin")]
    pub safety_margin_pct: u32,
    #[serde(default = "default_image_budget")]
    pub image_token_budget: u32,
    #[serde(default = "default_tool_surcharge")]
    pub tool_surcharge_tokens: u32,
    #[serde(default = "default_web_surcharge")]
    pub web_search_surcharge_tokens: u32,
    #[serde(default = "default_code_interpreter_surcharge")]
    pub code_interpreter_surcharge_tokens: u32,
    #[serde(default = "default_min_gen_floor")]
    pub minimal_generation_floor: u32,
}

impl Default for EstimationBudgets {
    fn default() -> Self {
        Self {
            bytes_per_token_conservative: default_bytes_per_token(),
            fixed_overhead_tokens: default_fixed_overhead(),
            safety_margin_pct: default_safety_margin(),
            image_token_budget: default_image_budget(),
            tool_surcharge_tokens: default_tool_surcharge(),
            web_search_surcharge_tokens: default_web_surcharge(),
            code_interpreter_surcharge_tokens: default_code_interpreter_surcharge(),
            minimal_generation_floor: default_min_gen_floor(),
        }
    }
}

impl EstimationBudgets {
    pub fn validate(self) -> Result<(), String> {
        if self.bytes_per_token_conservative == 0 {
            return Err("bytes_per_token_conservative must be > 0".to_owned());
        }
        if self.minimal_generation_floor == 0 {
            return Err("minimal_generation_floor must be > 0".to_owned());
        }
        Ok(())
    }
}

fn default_bytes_per_token() -> u32 {
    4
}
fn default_fixed_overhead() -> u32 {
    100
}
fn default_safety_margin() -> u32 {
    10
}
fn default_image_budget() -> u32 {
    1000
}
fn default_tool_surcharge() -> u32 {
    500
}
fn default_web_surcharge() -> u32 {
    500
}
fn default_code_interpreter_surcharge() -> u32 {
    1000
}
fn default_min_gen_floor() -> u32 {
    50
}
fn default_web_search_max_calls() -> u32 {
    2
}
fn default_web_search_daily_quota() -> u32 {
    75
}
fn default_ci_max_calls() -> u32 {
    10
}
fn default_ci_daily_quota() -> u32 {
    50
}
fn default_warning_threshold_pct() -> u8 {
    80
}

/// Quota enforcement configuration.
#[derive(Debug, Clone, Copy, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct QuotaConfig {
    #[serde(default = "default_overshoot_tolerance")]
    pub overshoot_tolerance_factor: f64,
    #[serde(default = "default_web_search_max_calls")]
    pub web_search_max_calls_per_message: u32,
    #[serde(default = "default_web_search_daily_quota")]
    pub web_search_daily_quota: u32,
    #[serde(default = "default_ci_max_calls")]
    pub code_interpreter_max_calls_per_message: u32,
    #[serde(default = "default_ci_daily_quota")]
    pub code_interpreter_daily_quota: u32,
    #[serde(default = "default_warning_threshold_pct")]
    pub warning_threshold_pct: u8,
}

impl Default for QuotaConfig {
    fn default() -> Self {
        Self {
            overshoot_tolerance_factor: default_overshoot_tolerance(),
            web_search_max_calls_per_message: default_web_search_max_calls(),
            web_search_daily_quota: default_web_search_daily_quota(),
            code_interpreter_max_calls_per_message: default_ci_max_calls(),
            code_interpreter_daily_quota: default_ci_daily_quota(),
            warning_threshold_pct: default_warning_threshold_pct(),
        }
    }
}

impl QuotaConfig {
    pub fn validate(self) -> Result<(), String> {
        if !(1.0..=1.5).contains(&self.overshoot_tolerance_factor) {
            return Err(format!(
                "overshoot_tolerance_factor must be 1.0-1.5, got {}",
                self.overshoot_tolerance_factor
            ));
        }
        if self.web_search_max_calls_per_message == 0 {
            return Err("web_search_max_calls_per_message must be > 0".to_owned());
        }
        if self.web_search_daily_quota == 0 {
            return Err("web_search_daily_quota must be > 0".to_owned());
        }
        if self.code_interpreter_max_calls_per_message == 0 {
            return Err("code_interpreter_max_calls_per_message must be > 0".to_owned());
        }
        if self.code_interpreter_daily_quota == 0 {
            return Err("code_interpreter_daily_quota must be > 0".to_owned());
        }
        if self.warning_threshold_pct == 0 || self.warning_threshold_pct >= 100 {
            return Err(format!(
                "warning_threshold_pct must be 1-99, got {}",
                self.warning_threshold_pct
            ));
        }
        Ok(())
    }
}

/// Outbox configuration for usage and audit event publishing.
#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct OutboxConfig {
    /// Queue name for usage events.
    #[serde(default = "default_outbox_queue_name")]
    pub queue_name: String,

    /// Queue name for attachment cleanup events.
    #[serde(default = "default_outbox_cleanup_queue_name")]
    pub cleanup_queue_name: String,
    /// Queue name for thread summary task events.
    #[serde(default = "default_thread_summary_queue_name")]
    pub thread_summary_queue_name: String,
    /// Queue name for chat-deletion cleanup events.
    #[serde(default = "default_chat_cleanup_queue_name")]
    pub chat_cleanup_queue_name: String,
    /// Queue name for audit events.
    #[serde(default = "default_audit_queue_name")]
    pub audit_queue_name: String,

    /// Number of outbox partitions. Must be 1–64.
    #[serde(default = "default_outbox_num_partitions")]
    pub num_partitions: u32,
}

impl Default for OutboxConfig {
    fn default() -> Self {
        Self {
            queue_name: default_outbox_queue_name(),
            cleanup_queue_name: default_outbox_cleanup_queue_name(),
            thread_summary_queue_name: default_thread_summary_queue_name(),
            chat_cleanup_queue_name: default_chat_cleanup_queue_name(),
            audit_queue_name: default_audit_queue_name(),
            num_partitions: default_outbox_num_partitions(),
        }
    }
}

impl OutboxConfig {
    pub fn validate(&self) -> Result<(), String> {
        if self.queue_name.trim().is_empty() {
            return Err("outbox queue_name must not be empty".to_owned());
        }
        if self.cleanup_queue_name.trim().is_empty() {
            return Err("outbox cleanup_queue_name must not be empty".to_owned());
        }
        if self.chat_cleanup_queue_name.trim().is_empty() {
            return Err("outbox chat_cleanup_queue_name must not be empty".to_owned());
        }
        if self.thread_summary_queue_name.trim().is_empty() {
            return Err("outbox thread_summary_queue_name must not be empty".to_owned());
        }
        if self.audit_queue_name.trim().is_empty() {
            return Err("outbox audit_queue_name must not be empty".to_owned());
        }
        if !(1..=64).contains(&self.num_partitions) || !self.num_partitions.is_power_of_two() {
            return Err(format!(
                "outbox num_partitions must be a power of 2 in 1-64, got {}",
                self.num_partitions
            ));
        }
        Ok(())
    }
}

fn default_outbox_queue_name() -> String {
    "mini-chat.usage_snapshot".to_owned()
}

fn default_outbox_cleanup_queue_name() -> String {
    "mini-chat.attachment_cleanup".to_owned()
}

fn default_chat_cleanup_queue_name() -> String {
    "mini-chat.chat_cleanup".to_owned()
}

fn default_thread_summary_queue_name() -> String {
    "mini-chat.thread_summary".to_owned()
}

fn default_audit_queue_name() -> String {
    "mini-chat.audit".to_owned()
}

fn default_outbox_num_partitions() -> u32 {
    4
}

fn default_overshoot_tolerance() -> f64 {
    1.10
}

/// Context assembly configuration.
#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct ContextConfig {
    /// Soft-guideline instruction appended to system prompt when `web_search` is enabled.
    #[serde(default = "default_web_search_guard")]
    pub web_search_guard: String,

    /// Soft-guideline instruction appended to system prompt when `file_search` is enabled.
    #[serde(default = "default_file_search_guard")]
    pub file_search_guard: String,

    /// Maximum number of recent messages to include in context. Range: 0–100.
    #[serde(default = "default_recent_messages_limit")]
    pub recent_messages_limit: u32,
}

impl Default for ContextConfig {
    fn default() -> Self {
        Self {
            web_search_guard: default_web_search_guard(),
            file_search_guard: default_file_search_guard(),
            recent_messages_limit: default_recent_messages_limit(),
        }
    }
}

impl ContextConfig {
    pub fn validate(&self) -> Result<(), String> {
        if self.recent_messages_limit > 100 {
            return Err(format!(
                "context recent_messages_limit must be 0-100, got {}",
                self.recent_messages_limit
            ));
        }
        Ok(())
    }
}

fn default_web_search_guard() -> String {
    "Use web_search only if the answer cannot be obtained from the provided context or your training data. Never use it for general knowledge questions. At most one web_search call per request.".to_owned()
}

fn default_file_search_guard() -> String {
    "Use file_search to find relevant information in the user's uploaded documents. Prefer file_search over general knowledge when documents are available.".to_owned()
}

fn default_recent_messages_limit() -> u32 {
    10
}

// ── RAG config ───────────────────────────────────────────────────────────

fn default_max_documents_per_chat() -> u32 {
    50
}

fn default_max_total_upload_mb_per_chat() -> u32 {
    100
}

fn default_uploaded_file_max_size_kb() -> u32 {
    // 25 MB in KB
    25 * 1024
}

fn default_uploaded_image_max_size_kb() -> u32 {
    // 5 MB in KB
    5 * 1024
}

#[derive(Debug, Clone, Deserialize, Serialize)]
#[serde(deny_unknown_fields)]
#[allow(clippy::struct_field_names)]
pub struct RagConfig {
    /// Maximum number of document attachments per chat.
    #[serde(default = "default_max_documents_per_chat")]
    pub max_documents_per_chat: u32,

    /// Maximum total upload size per chat in MB.
    #[serde(default = "default_max_total_upload_mb_per_chat")]
    pub max_total_upload_mb_per_chat: u32,

    /// Maximum single uploaded file (document) size in KB.
    #[serde(default = "default_uploaded_file_max_size_kb")]
    pub uploaded_file_max_size_kb: u32,

    /// Maximum single uploaded image size in KB.
    #[serde(default = "default_uploaded_image_max_size_kb")]
    pub uploaded_image_max_size_kb: u32,

    /// Accept `text/csv` uploads remapped to `text/plain` for `file_search`.
    #[serde(default = "default_true")]
    pub allow_csv_upload: bool,

    /// Maximum number of image attachments per message (DESIGN.md B.8).
    #[serde(default = "default_max_images_per_message")]
    pub max_images_per_message: u32,

    /// Maximum concurrent in-flight uploads **per process** (across all tenants
    /// within this pod). Each upload can buffer up to `uploaded_file_max_size_kb`
    /// in memory; this semaphore prevents OOM under burst load.
    ///
    /// Note: cluster-wide concurrency scales with the number of replicas
    /// (`max_concurrent_uploads × num_pods`).
    /// Valid range: 1–256 (default 10).
    #[serde(default = "default_max_concurrent_uploads")]
    pub max_concurrent_uploads: u16,
}

fn default_max_images_per_message() -> u32 {
    4
}

fn default_max_concurrent_uploads() -> u16 {
    10
}

impl Default for RagConfig {
    fn default() -> Self {
        Self {
            max_documents_per_chat: default_max_documents_per_chat(),
            max_total_upload_mb_per_chat: default_max_total_upload_mb_per_chat(),
            uploaded_file_max_size_kb: default_uploaded_file_max_size_kb(),
            uploaded_image_max_size_kb: default_uploaded_image_max_size_kb(),
            allow_csv_upload: true,
            max_images_per_message: default_max_images_per_message(),
            max_concurrent_uploads: default_max_concurrent_uploads(),
        }
    }
}

impl RagConfig {
    pub fn validate(&self) -> Result<(), String> {
        if self.max_documents_per_chat == 0 {
            return Err("rag max_documents_per_chat must be > 0".into());
        }
        if self.max_total_upload_mb_per_chat == 0 {
            return Err("rag max_total_upload_mb_per_chat must be > 0".into());
        }
        if self.uploaded_file_max_size_kb == 0 {
            return Err("rag uploaded_file_max_size_kb must be > 0".into());
        }
        if self.uploaded_image_max_size_kb == 0 {
            return Err("rag uploaded_image_max_size_kb must be > 0".into());
        }
        if self.max_images_per_message == 0 {
            return Err("rag max_images_per_message must be > 0".into());
        }
        if self.max_concurrent_uploads == 0 || self.max_concurrent_uploads > 256 {
            return Err(format!(
                "rag max_concurrent_uploads must be 1-256, got {}",
                self.max_concurrent_uploads
            ));
        }
        Ok(())
    }
}

// ── Thumbnail config ────────────────────────────────────────────────────

fn default_thumbnail_width() -> u32 {
    128
}
fn default_thumbnail_height() -> u32 {
    128
}
fn default_thumbnail_max_bytes() -> usize {
    131_072 // 128 KiB
}
fn default_thumbnail_max_pixels() -> u64 {
    100_000_000
}
fn default_thumbnail_max_decode_bytes() -> usize {
    33_554_432 // 32 MiB
}

/// Configuration for server-generated image thumbnails (DESIGN.md §B.6).
#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct ThumbnailConfig {
    /// Target thumbnail width in pixels.
    #[serde(default = "default_thumbnail_width")]
    pub width: u32,
    /// Target thumbnail height in pixels.
    #[serde(default = "default_thumbnail_height")]
    pub height: u32,
    /// Maximum decoded thumbnail size in bytes (128 KiB).
    #[serde(default = "default_thumbnail_max_bytes")]
    pub max_bytes: usize,
    /// Maximum source image pixel count before skipping thumbnail generation.
    #[serde(default = "default_thumbnail_max_pixels")]
    pub max_pixels: u64,
    /// Maximum bytes the image decoder may allocate before aborting.
    #[serde(default = "default_thumbnail_max_decode_bytes")]
    pub max_decode_bytes: usize,
}

impl Default for ThumbnailConfig {
    fn default() -> Self {
        Self {
            width: default_thumbnail_width(),
            height: default_thumbnail_height(),
            max_bytes: default_thumbnail_max_bytes(),
            max_pixels: default_thumbnail_max_pixels(),
            max_decode_bytes: default_thumbnail_max_decode_bytes(),
        }
    }
}

impl ThumbnailConfig {
    pub fn validate(&self) -> Result<(), String> {
        if self.width == 0 {
            return Err("thumbnail width must be > 0".into());
        }
        if self.height == 0 {
            return Err("thumbnail height must be > 0".into());
        }
        if self.max_bytes == 0 {
            return Err("thumbnail max_bytes must be > 0".into());
        }
        if self.max_pixels == 0 {
            return Err("thumbnail max_pixels must be > 0".into());
        }
        if self.max_decode_bytes == 0 {
            return Err("thumbnail max_decode_bytes must be > 0".into());
        }
        Ok(())
    }
}

fn default_url_prefix() -> String {
    DEFAULT_URL_PREFIX.to_owned()
}

fn default_vendor() -> String {
    "hyperspot".to_owned()
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn default_config_is_valid() {
        StreamingConfig::default().validate().unwrap();
        EstimationBudgets::default().validate().unwrap();
        QuotaConfig::default().validate().unwrap();
        OutboxConfig::default().validate().unwrap();
        ContextConfig::default().validate().unwrap();
        RagConfig::default().validate().unwrap();
        ThumbnailConfig::default().validate().unwrap();
    }

    #[test]
    fn estimation_budgets_validation() {
        let valid = EstimationBudgets::default();

        assert!(
            (EstimationBudgets {
                bytes_per_token_conservative: 0,
                ..valid
            })
            .validate()
            .is_err()
        );
        assert!(
            (EstimationBudgets {
                minimal_generation_floor: 0,
                ..valid
            })
            .validate()
            .is_err()
        );
    }

    #[test]
    fn quota_config_validation() {
        assert!(
            (QuotaConfig {
                overshoot_tolerance_factor: 0.99,
                ..QuotaConfig::default()
            })
            .validate()
            .is_err()
        );
        assert!(
            (QuotaConfig {
                overshoot_tolerance_factor: 1.0,
                ..QuotaConfig::default()
            })
            .validate()
            .is_ok()
        );
        assert!(
            (QuotaConfig {
                overshoot_tolerance_factor: 1.5,
                ..QuotaConfig::default()
            })
            .validate()
            .is_ok()
        );
        assert!(
            (QuotaConfig {
                overshoot_tolerance_factor: 1.51,
                ..QuotaConfig::default()
            })
            .validate()
            .is_err()
        );
        assert!(
            (QuotaConfig {
                web_search_max_calls_per_message: 0,
                ..QuotaConfig::default()
            })
            .validate()
            .is_err()
        );
        assert!(
            (QuotaConfig {
                web_search_daily_quota: 0,
                ..QuotaConfig::default()
            })
            .validate()
            .is_err()
        );
        assert!(
            (QuotaConfig {
                code_interpreter_max_calls_per_message: 0,
                ..QuotaConfig::default()
            })
            .validate()
            .is_err()
        );
        assert!(
            (QuotaConfig {
                code_interpreter_daily_quota: 0,
                ..QuotaConfig::default()
            })
            .validate()
            .is_err()
        );
    }

    #[test]
    fn channel_capacity_boundaries() {
        let valid = StreamingConfig::default();

        assert!(
            (StreamingConfig {
                sse_channel_capacity: 15,
                ..valid
            })
            .validate()
            .is_err()
        );
        assert!(
            (StreamingConfig {
                sse_channel_capacity: 16,
                ..valid
            })
            .validate()
            .is_ok()
        );
        assert!(
            (StreamingConfig {
                sse_channel_capacity: 64,
                ..valid
            })
            .validate()
            .is_ok()
        );
        assert!(
            (StreamingConfig {
                sse_channel_capacity: 65,
                ..valid
            })
            .validate()
            .is_err()
        );
    }

    #[test]
    fn ping_interval_boundaries() {
        let valid = StreamingConfig::default();

        assert!(
            (StreamingConfig {
                sse_ping_interval_seconds: 4,
                ..valid
            })
            .validate()
            .is_err()
        );
        assert!(
            (StreamingConfig {
                sse_ping_interval_seconds: 5,
                ..valid
            })
            .validate()
            .is_ok()
        );
        assert!(
            (StreamingConfig {
                sse_ping_interval_seconds: 60,
                ..valid
            })
            .validate()
            .is_ok()
        );
        assert!(
            (StreamingConfig {
                sse_ping_interval_seconds: 61,
                ..valid
            })
            .validate()
            .is_err()
        );
    }

    #[test]
    fn streaming_config_web_search_context_size_enum() {
        use crate::domain::llm::WebSearchContextSize;

        // Default is Low
        let cfg = StreamingConfig::default();
        assert_eq!(cfg.web_search_context_size, WebSearchContextSize::Low);

        // Valid values deserialize correctly
        for (json_val, expected) in [
            ("\"low\"", WebSearchContextSize::Low),
            ("\"medium\"", WebSearchContextSize::Medium),
            ("\"high\"", WebSearchContextSize::High),
        ] {
            let json = format!(r#"{{"web_search_context_size": {json_val}}}"#);
            let cfg: StreamingConfig = serde_json::from_str(&json).unwrap();
            assert_eq!(cfg.web_search_context_size, expected);
        }

        // Invalid values rejected at parse time
        for bad in ["\"Low\"", "\"med\"", "\"HIGH\"", "\"none\"", "\"\""] {
            let json = format!(r#"{{"web_search_context_size": {bad}}}"#);
            assert!(
                serde_json::from_str::<StreamingConfig>(&json).is_err(),
                "expected parse error for {bad}"
            );
        }
    }

    #[test]
    fn provider_entry_deser_with_alias() {
        let json = r#"{
            "kind": "openai_responses",
            "storage_kind": "openai",
            "host": "10.0.0.1",
            "upstream_alias": "my-llm-service"
        }"#;
        let entry: ProviderEntry = serde_json::from_str(json).unwrap();
        assert_eq!(entry.host, "10.0.0.1");
        assert_eq!(entry.upstream_alias.as_deref(), Some("my-llm-service"));
        assert!(entry.auth_plugin_type.is_none());
    }

    #[test]
    fn provider_entry_deser_without_alias() {
        let json = r#"{
            "kind": "openai_responses",
            "storage_kind": "azure",
            "host": "my-azure.openai.azure.com",
            "api_path": "/openai/v1/responses"
        }"#;
        let entry: ProviderEntry = serde_json::from_str(json).unwrap();
        assert!(entry.upstream_alias.is_none());
        assert_eq!(entry.host, "my-azure.openai.azure.com");
        assert_eq!(entry.api_path, "/openai/v1/responses");
    }

    #[test]
    fn provider_entry_deser_with_auth() {
        let json = r#"{
            "kind": "openai_responses",
            "storage_kind": "openai",
            "host": "api.openai.com",
            "auth_plugin_type": "gts.x.core.oagw.auth_plugin.v1~x.core.oagw.apikey.v1",
            "auth_config": {
                "header": "Authorization",
                "prefix": "Bearer ",
                "secret_ref": "cred://openai-key"
            }
        }"#;
        let entry: ProviderEntry = serde_json::from_str(json).unwrap();
        assert!(entry.auth_plugin_type.is_some());
        let config = entry.auth_config.unwrap();
        assert_eq!(config.get("header").unwrap(), "Authorization");
        assert_eq!(config.get("secret_ref").unwrap(), "cred://openai-key");
    }

    #[test]
    fn default_providers_has_openai() {
        let cfg = MiniChatConfig::default();
        assert!(cfg.providers.contains_key("openai"));
        let openai = &cfg.providers["openai"];
        assert_eq!(openai.host, "api.openai.com");
        assert_eq!(openai.api_path, "/v1/responses");
    }

    #[test]
    fn provider_entry_deser_with_tenant_overrides() {
        let json = r#"{
            "kind": "openai_responses",
            "storage_kind": "azure",
            "host": "default.openai.azure.com",
            "api_path": "/openai/v1/responses",
            "tenant_overrides": {
                "tenant-a": {
                    "host": "tenant-a.openai.azure.com"
                },
                "tenant-b": {
                    "host": "tenant-b.openai.azure.com"
                }
            }
        }"#;
        let entry: ProviderEntry = serde_json::from_str(json).unwrap();
        assert_eq!(entry.tenant_overrides.len(), 2);
        assert_eq!(
            entry.tenant_overrides["tenant-a"].host.as_deref(),
            Some("tenant-a.openai.azure.com")
        );
        assert!(entry.tenant_overrides["tenant-b"].host.is_some());
    }

    #[test]
    fn effective_host_for_tenant_fallback() {
        let entry = ProviderEntry {
            kind: crate::infra::llm::ProviderKind::OpenAiResponses,
            upstream_alias: None,
            host: "default.openai.azure.com".to_owned(),
            port: None,
            use_http: false,
            api_path: "/v1/responses".to_owned(),
            auth_plugin_type: None,
            auth_config: None,
            storage_backend: None,
            supports_file_search_filters: true,
            storage_kind: StorageKind::Azure,
            api_version: Some("2024-10-21".to_owned()),
            tenant_overrides: {
                let mut m = HashMap::new();
                m.insert(
                    "tenant-a".to_owned(),
                    ProviderTenantOverride {
                        host: Some("tenant-a.openai.azure.com".to_owned()),
                        upstream_alias: None,
                        auth_plugin_type: None,
                        auth_config: None,
                    },
                );
                // Tenant with no host override — inherits root.
                m.insert(
                    "tenant-c".to_owned(),
                    ProviderTenantOverride {
                        host: None,
                        upstream_alias: None,
                        auth_plugin_type: Some("custom-plugin".to_owned()),
                        auth_config: None,
                    },
                );
                m
            },
        };
        assert_eq!(
            entry.effective_host_for_tenant("tenant-a"),
            "tenant-a.openai.azure.com"
        );
        assert_eq!(
            entry.effective_host_for_tenant("tenant-c"),
            "default.openai.azure.com"
        );
        assert_eq!(
            entry.effective_host_for_tenant("unknown"),
            "default.openai.azure.com"
        );
    }

    #[test]
    fn effective_auth_for_tenant() {
        let root_auth: HashMap<String, String> = {
            let mut c = HashMap::new();
            c.insert("header".to_owned(), "api-key".to_owned());
            c.insert("secret_ref".to_owned(), "cred://root-key".to_owned());
            c
        };
        let tenant_auth: HashMap<String, String> = {
            let mut c = HashMap::new();
            c.insert("header".to_owned(), "api-key".to_owned());
            c.insert("secret_ref".to_owned(), "cred://tenant-a-key".to_owned());
            c
        };
        let entry = ProviderEntry {
            kind: crate::infra::llm::ProviderKind::OpenAiResponses,
            upstream_alias: None,
            host: "default.openai.azure.com".to_owned(),
            port: None,
            use_http: false,
            api_path: "/v1/responses".to_owned(),
            auth_plugin_type: Some("root-plugin".to_owned()),
            auth_config: Some(root_auth),
            storage_backend: None,
            supports_file_search_filters: true,
            storage_kind: StorageKind::Azure,
            api_version: Some("2024-10-21".to_owned()),
            tenant_overrides: {
                let mut m = HashMap::new();
                m.insert(
                    "tenant-a".to_owned(),
                    ProviderTenantOverride {
                        host: None,
                        upstream_alias: None,
                        auth_plugin_type: Some("tenant-plugin".to_owned()),
                        auth_config: Some(tenant_auth),
                    },
                );
                m
            },
        };
        // Tenant with auth override.
        assert_eq!(
            entry.effective_auth_plugin_type_for_tenant("tenant-a"),
            Some("tenant-plugin")
        );
        assert_eq!(
            entry
                .effective_auth_config_for_tenant("tenant-a")
                .unwrap()
                .get("secret_ref")
                .unwrap(),
            "cred://tenant-a-key"
        );
        // Unknown tenant → falls back to root.
        assert_eq!(
            entry.effective_auth_plugin_type_for_tenant("unknown"),
            Some("root-plugin")
        );
        assert_eq!(
            entry
                .effective_auth_config_for_tenant("unknown")
                .unwrap()
                .get("secret_ref")
                .unwrap(),
            "cred://root-key"
        );
    }

    #[test]
    fn validate_rejects_empty_tenant_override_host() {
        let entry = ProviderEntry {
            kind: crate::infra::llm::ProviderKind::OpenAiResponses,
            upstream_alias: None,
            host: "default.openai.azure.com".to_owned(),
            port: None,
            use_http: false,
            api_path: "/v1/responses".to_owned(),
            auth_plugin_type: None,
            auth_config: None,
            storage_backend: None,
            supports_file_search_filters: true,
            storage_kind: StorageKind::Azure,
            api_version: Some("2024-10-21".to_owned()),
            tenant_overrides: {
                let mut m = HashMap::new();
                m.insert(
                    "bad-tenant".to_owned(),
                    ProviderTenantOverride {
                        host: Some("  ".to_owned()),
                        upstream_alias: None,
                        auth_plugin_type: None,
                        auth_config: None,
                    },
                );
                m
            },
        };
        let err = entry.validate("azure_openai").unwrap_err();
        assert!(err.contains("bad-tenant"));
        assert!(err.contains("host must not be empty"));
    }

    #[test]
    fn validate_rejects_auth_only_override_without_alias() {
        let entry = ProviderEntry {
            kind: crate::infra::llm::ProviderKind::OpenAiResponses,
            upstream_alias: None,
            host: "default.openai.azure.com".to_owned(),
            port: None,
            use_http: false,
            api_path: "/v1/responses".to_owned(),
            auth_plugin_type: None,
            auth_config: None,
            storage_backend: None,
            supports_file_search_filters: true,
            storage_kind: StorageKind::Azure,
            api_version: Some("2024-10-21".to_owned()),
            tenant_overrides: {
                let mut m = HashMap::new();
                m.insert(
                    "tenant-a".to_owned(),
                    ProviderTenantOverride {
                        host: None,
                        upstream_alias: None,
                        auth_plugin_type: Some("custom-plugin".to_owned()),
                        auth_config: Some({
                            let mut c = HashMap::new();
                            c.insert("secret_ref".to_owned(), "tenant-a-key".to_owned());
                            c
                        }),
                    },
                );
                m
            },
        };
        let err = entry.validate("azure_openai").unwrap_err();
        assert!(err.contains("tenant-a"));
        assert!(err.contains("overrides auth"));
    }

    #[test]
    fn validate_rejects_auth_plugin_type_only_override_without_alias() {
        let entry = ProviderEntry {
            kind: crate::infra::llm::ProviderKind::OpenAiResponses,
            upstream_alias: None,
            host: "default.openai.azure.com".to_owned(),
            port: None,
            use_http: false,
            api_path: "/v1/responses".to_owned(),
            auth_plugin_type: None,
            auth_config: None,
            storage_backend: None,
            supports_file_search_filters: true,
            storage_kind: StorageKind::Azure,
            api_version: Some("2024-10-21".to_owned()),
            tenant_overrides: {
                let mut m = HashMap::new();
                m.insert(
                    "tenant-b".to_owned(),
                    ProviderTenantOverride {
                        host: None,
                        upstream_alias: None,
                        auth_plugin_type: Some("different-plugin".to_owned()),
                        auth_config: None,
                    },
                );
                m
            },
        };
        let err = entry.validate("azure_openai").unwrap_err();
        assert!(err.contains("tenant-b"));
        assert!(err.contains("overrides auth"));
    }

    #[test]
    fn validate_accepts_auth_only_override_with_explicit_alias() {
        let entry = ProviderEntry {
            kind: crate::infra::llm::ProviderKind::OpenAiResponses,
            upstream_alias: None,
            host: "default.openai.azure.com".to_owned(),
            port: None,
            use_http: false,
            api_path: "/v1/responses".to_owned(),
            auth_plugin_type: None,
            auth_config: None,
            storage_backend: None,
            supports_file_search_filters: true,
            storage_kind: StorageKind::Azure,
            api_version: Some("2024-10-21".to_owned()),
            tenant_overrides: {
                let mut m = HashMap::new();
                m.insert(
                    "tenant-a".to_owned(),
                    ProviderTenantOverride {
                        host: None,
                        upstream_alias: Some("azure-tenant-a".to_owned()),
                        auth_plugin_type: Some("custom-plugin".to_owned()),
                        auth_config: None,
                    },
                );
                m
            },
        };
        assert!(entry.validate("azure_openai").is_ok());
    }

    #[test]
    fn validate_accepts_host_differing_override_with_auth() {
        let entry = ProviderEntry {
            kind: crate::infra::llm::ProviderKind::OpenAiResponses,
            upstream_alias: None,
            host: "default.openai.azure.com".to_owned(),
            port: None,
            use_http: false,
            api_path: "/v1/responses".to_owned(),
            auth_plugin_type: None,
            auth_config: None,
            storage_backend: None,
            supports_file_search_filters: true,
            storage_kind: StorageKind::Azure,
            api_version: Some("2024-10-21".to_owned()),
            tenant_overrides: {
                let mut m = HashMap::new();
                m.insert(
                    "tenant-a".to_owned(),
                    ProviderTenantOverride {
                        host: Some("tenant-a.openai.azure.com".to_owned()),
                        upstream_alias: None,
                        auth_plugin_type: Some("custom-plugin".to_owned()),
                        auth_config: None,
                    },
                );
                m
            },
        };
        assert!(entry.validate("azure_openai").is_ok());
    }

    #[test]
    fn metrics_effective_prefix_uses_module_name_when_empty() {
        let cfg = MetricsConfig {
            prefix: String::new(),
        };
        assert_eq!(cfg.effective_prefix("mini-chat"), "mini_chat");
    }

    #[test]
    fn metrics_effective_prefix_uses_module_name_when_whitespace() {
        let cfg = MetricsConfig {
            prefix: "   ".to_owned(),
        };
        assert_eq!(cfg.effective_prefix("mini-chat"), "mini_chat");
    }

    #[test]
    fn metrics_effective_prefix_uses_trimmed_explicit_prefix() {
        let cfg = MetricsConfig {
            prefix: "  custom.prefix  ".to_owned(),
        };
        assert_eq!(cfg.effective_prefix("mini-chat"), "custom.prefix");
    }

    #[test]
    fn rag_config_max_concurrent_uploads_validation() {
        // zero is rejected
        assert!(
            RagConfig {
                max_concurrent_uploads: 0,
                ..RagConfig::default()
            }
            .validate()
            .is_err()
        );

        // over 256 is rejected
        assert!(
            RagConfig {
                max_concurrent_uploads: 257,
                ..RagConfig::default()
            }
            .validate()
            .is_err()
        );

        // boundary values are accepted
        assert!(
            RagConfig {
                max_concurrent_uploads: 1,
                ..RagConfig::default()
            }
            .validate()
            .is_ok()
        );
        assert!(
            RagConfig {
                max_concurrent_uploads: 256,
                ..RagConfig::default()
            }
            .validate()
            .is_ok()
        );
    }

    #[test]
    fn upload_semaphore_exhaustion_blocks_further_acquires() {
        use tokio::sync::Semaphore;

        let sem = Semaphore::new(2);

        // Acquire all permits.
        let p1 = sem.try_acquire().expect("first permit");
        let _p2 = sem.try_acquire().expect("second permit");

        // Third acquire must fail — this is the 503 path.
        assert!(
            sem.try_acquire().is_err(),
            "semaphore should be exhausted after all permits are taken"
        );

        // Releasing a permit makes the semaphore available again.
        drop(p1);
        assert!(
            sem.try_acquire().is_ok(),
            "permit should be available after release"
        );
    }
}
