// SPDX-FileCopyrightText: Copyright (c) 2024-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
// SPDX-License-Identifier: Apache-2.0

//! NIXL agent wrapper and configuration.
//!
//! This module provides:
//! - `NixlAgent`: Wrapper around nixl_sys::Agent that tracks initialized backends
//! - `NixlBackendConfig`: Configuration for NIXL backends from environment variables

use anyhow::Result;
use nixl_sys::{Agent, is_stub};
use std::collections::{HashMap, HashSet};

use crate::nixl::NixlBackendConfig;

/// A NIXL agent wrapper that tracks which backends were successfully initialized.
///
/// This wrapper provides:
/// - Runtime validation of backend availability
/// - Clear error messages when operations need unavailable backends
/// - Single source of truth for backend state in tests and production
///
/// # Backend Tracking
///
/// Since `nixl_sys::Agent` doesn't provide a method to query active backends,
/// we track them during initialization. The `available_backends` set is populated
/// based on successful `create_backend()` calls.
#[derive(Clone, Debug)]
pub struct NixlAgent {
    agent: Agent,
    available_backends: HashSet<String>,
}

impl NixlAgent {
    /// Create a NIXL agent without any backends.
    pub fn new(name: &str) -> Result<Self> {
        if is_stub() {
            return Err(anyhow::anyhow!("NIXL is not supported in stub mode"));
        }
        let agent = Agent::new(name)?;

        Ok(Self {
            agent,
            available_backends: HashSet::new(),
        })
    }

    /// Creates a new agent configured with backends from the given config.
    ///
    /// This method iterates over all backends in the config and initializes them
    /// with their associated parameters. If a backend has custom parameters defined
    /// in the config, those are used; otherwise, default plugin parameters are used.
    pub fn from_nixl_backend_config(name: &str, config: NixlBackendConfig) -> Result<Self> {
        let mut agent = Self::new(name)?;
        for (backend, params) in config.iter() {
            agent.add_backend_with_params(backend, params)?;
        }
        Ok(agent)
    }

    /// Add a backend to the agent with default parameters.
    pub fn add_backend(&mut self, backend: &str) -> Result<()> {
        self.add_backend_with_params(backend, &HashMap::new())
    }

    /// Add a backend to the agent with optional custom parameters.
    ///
    /// If `custom_params` is non-empty, those parameters are used instead of
    /// the plugin defaults. If empty, default parameters from the plugin are used.
    ///
    /// # Errors
    /// Returns an error if custom parameters are provided (not yet supported until nixl_sys 0.9).
    pub fn add_backend_with_params(
        &mut self,
        backend: &str,
        custom_params: &HashMap<String, String>,
    ) -> Result<()> {
        let backend_upper = backend.to_uppercase();
        if self.available_backends.contains(&backend_upper) {
            return Ok(());
        }

        // TODO(DIS-1310): Custom params require nixl_sys 0.9+ which adds nixl_capi_params_add
        if !custom_params.is_empty() {
            anyhow::bail!(
                "Custom NIXL backend parameters for {} are not yet supported. \
                 This feature requires nixl_sys 0.9+. Params provided: {:?}",
                backend_upper,
                custom_params.keys().collect::<Vec<_>>()
            );
        }

        // Get default params from plugin
        let (_, params) = match self.agent.get_plugin_params(&backend_upper) {
            Ok(result) => result,
            Err(_) => anyhow::bail!("No {} plugin found", backend_upper),
        };

        match self.agent.create_backend(&backend_upper, &params) {
            Ok(_) => {
                self.available_backends.insert(backend_upper);
                Ok(())
            }
            Err(e) => anyhow::bail!("Failed to create nixl backend: {}", e),
        }
    }

    /// Create a NIXL agent requiring ALL specified backends to be available.
    ///
    /// Unlike `new_with_backends()` which continues if some backends fail, this method
    /// will return an error if ANY backend fails to initialize. Use this in production
    /// when specific backends are mandatory.
    ///
    /// # Arguments
    /// * `name` - Agent name
    /// * `backends` - List of backend names that MUST be available
    ///
    /// # Returns
    /// A `NixlAgent` with all requested backends initialized.
    ///
    /// # Errors
    /// Returns an error if:
    /// - Agent creation fails
    /// - Any backend fails to initialize
    pub fn with_backends(name: &str, backends: &[&str]) -> Result<Self> {
        let mut agent = Self::new(name)?;
        let mut failed_backends = Vec::new();

        for backend in backends {
            let backend_upper = backend.to_uppercase();
            match agent.add_backend(&backend_upper) {
                Ok(_) => {
                    tracing::debug!("Initialized NIXL backend: {}", backend_upper);
                }
                Err(e) => {
                    tracing::error!("Failed to initialize {} backend: {}", backend_upper, e);
                    failed_backends.push((backend_upper, e.to_string()));
                }
            }
        }

        if !failed_backends.is_empty() {
            let error_details: Vec<String> = failed_backends
                .iter()
                .map(|(name, reason)| format!("{}: {}", name, reason))
                .collect();

            anyhow::bail!(
                "Failed to initialize required backends: [{}]",
                error_details.join(", ")
            );
        }

        Ok(agent)
    }

    /// Get a reference to the underlying raw NIXL agent.
    pub fn raw_agent(&self) -> &Agent {
        &self.agent
    }

    /// Consume and return the underlying raw NIXL agent.
    ///
    /// **Warning**: Once consumed, backend tracking is lost. Use this only when
    /// interfacing with code that requires `nixl_sys::Agent` directly.
    pub fn into_raw_agent(self) -> Agent {
        self.agent
    }

    /// Check if a specific backend is available.
    pub fn has_backend(&self, backend: &str) -> bool {
        self.available_backends.contains(&backend.to_uppercase())
    }

    /// Get all available backends.
    pub fn backends(&self) -> &HashSet<String> {
        &self.available_backends
    }

    /// Require a specific backend, returning an error if unavailable.
    ///
    /// Use this at the start of operations that need specific backends.
    ///
    /// Note: In general, you want to instantiate all your backends before you start registering memory.
    /// We may change this to a builder pattern in the future to enforce all backends are instantiated
    /// before you start registering memory.
    pub fn require_backend(&self, backend: &str) -> Result<()> {
        let backend_upper = backend.to_uppercase();
        if self.has_backend(&backend_upper) {
            Ok(())
        } else {
            anyhow::bail!(
                "Operation requires {} backend, but it was not initialized. Available backends: {:?}",
                backend_upper,
                self.available_backends
            )
        }
    }
}

// Delegate common methods to the underlying agent
impl std::ops::Deref for NixlAgent {
    type Target = Agent;

    fn deref(&self) -> &Self::Target {
        &self.agent
    }
}

#[cfg(all(test, feature = "testing-nixl"))]
mod tests {
    use super::*;

    #[test]
    fn test_agent_backend_tracking() {
        // Try to create agent with UCX
        let agent = NixlAgent::with_backends("test", &["UCX"]).expect("Need UCX for test");

        // Should succeed if UCX is available
        assert!(agent.has_backend("UCX"));
        assert!(agent.has_backend("ucx")); // Case insensitive
    }

    #[test]
    fn test_require_backend() {
        let agent = NixlAgent::with_backends("test", &["UCX"]).expect("Need UCX for test");

        // Should succeed for available backend
        assert!(agent.require_backend("UCX").is_ok());

        // Should fail for unavailable backend
        assert!(agent.require_backend("GDS_MT").is_err());
    }

    #[test]
    fn test_require_backends_strict() {
        // Should succeed if UCX is available
        let agent =
            NixlAgent::with_backends("test_strict", &["UCX"]).expect("Failed to require backends");
        assert!(agent.has_backend("UCX"));

        // Should fail if any backend is missing (GDS likely not available)
        let result = NixlAgent::with_backends("test_strict_fail", &["UCX", "DUDE"]);
        assert!(result.is_err());
    }

    #[test]
    fn test_add_backend_with_empty_params() {
        let mut agent = NixlAgent::new("test_empty_params").expect("Failed to create agent");

        // Empty params should work (uses plugin defaults)
        let result = agent.add_backend_with_params("UCX", &HashMap::new());
        assert!(result.is_ok());
        assert!(agent.has_backend("UCX"));
    }

    #[test]
    fn test_add_backend_with_custom_params_fails() {
        let mut agent = NixlAgent::new("test_custom_params").expect("Failed to create agent");

        // Custom params should fail until nixl_sys 0.9
        let mut params = HashMap::new();
        params.insert("some_key".to_string(), "some_value".to_string());

        let result = agent.add_backend_with_params("UCX", &params);
        assert!(result.is_err());

        let err_msg = result.unwrap_err().to_string();
        assert!(err_msg.contains("not yet supported"));
        assert!(err_msg.contains("nixl_sys 0.9"));
        assert!(err_msg.contains("some_key"));
    }

    #[test]
    fn test_from_nixl_backend_config_with_custom_params_fails() {
        // Config with custom params should fail
        let mut params = HashMap::new();
        params.insert("threads".to_string(), "4".to_string());

        let config = NixlBackendConfig::default().with_backend_params("UCX", params);

        let result = NixlAgent::from_nixl_backend_config("test_config_params", config);
        assert!(result.is_err());

        let err_msg = result.unwrap_err().to_string();
        assert!(err_msg.contains("not yet supported"));
        assert!(err_msg.contains("threads"));
    }

    #[test]
    fn test_from_nixl_backend_config_with_empty_params() {
        // Config with empty params should work
        let config = NixlBackendConfig::default().with_backend("UCX");

        let result = NixlAgent::from_nixl_backend_config("test_config_empty", config);
        assert!(result.is_ok());

        let agent = result.unwrap();
        assert!(agent.has_backend("UCX"));
    }
}
