// Copyright 2024 RustFS Team
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! Backpressure management

use rustfs_io_core::{
    BackpressureConfig as CoreBackpressureConfig, BackpressureMonitor as CoreBackpressureMonitor, BackpressureState,
};
use rustfs_io_metrics::backpressure_metrics;
use std::sync::Arc;
use std::time::Instant;
use tokio::io::{DuplexStream, duplex};

/// Facade policy for duplex-pipe watermark backpressure.
#[derive(Debug, Clone, Copy)]
pub struct PipeBackpressurePolicy {
    /// Buffer size in bytes
    pub buffer_size: usize,
    /// High watermark percentage
    pub high_watermark: u32,
    /// Low watermark percentage
    pub low_watermark: u32,
}

impl Default for PipeBackpressurePolicy {
    fn default() -> Self {
        Self {
            buffer_size: 4 * 1024 * 1024, // 4MB
            high_watermark: 80,
            low_watermark: 50,
        }
    }
}

impl PipeBackpressurePolicy {
    /// Calculate high watermark threshold in bytes
    pub fn high_watermark_bytes(&self) -> usize {
        (self.buffer_size as u64 * self.high_watermark as u64 / 100) as usize
    }

    /// Calculate low watermark threshold in bytes
    pub fn low_watermark_bytes(&self) -> usize {
        (self.buffer_size as u64 * self.low_watermark as u64 / 100) as usize
    }

    /// Convert the facade policy into the reusable io-core admission-pressure config.
    ///
    /// The concurrency layer still owns duplex buffer sizing, but the shared
    /// overload/admission primitive lives in `io-core`.
    pub fn to_core_config(&self) -> CoreBackpressureConfig {
        CoreBackpressureConfig {
            max_concurrent: 32,
            high_water_mark: self.high_watermark as f64 / 100.0,
            low_water_mark: self.low_watermark as f64 / 100.0,
            cooldown: std::time::Duration::from_millis(100),
            enabled: true,
        }
    }
}

/// Backpressure manager
pub struct BackpressureManager {
    config: PipeBackpressurePolicy,
    core_config: CoreBackpressureConfig,
    monitor: Arc<CoreBackpressureMonitor>,
}

impl BackpressureManager {
    /// Create a new backpressure manager
    pub fn new(buffer_size: usize, high_watermark: u32, low_watermark: u32) -> Self {
        Self::from_policy(PipeBackpressurePolicy {
            buffer_size,
            high_watermark,
            low_watermark,
        })
    }

    /// Create a new backpressure manager from the facade policy type.
    pub fn from_policy(config: PipeBackpressurePolicy) -> Self {
        let core_config = config.to_core_config();
        Self {
            config,
            core_config: core_config.clone(),
            monitor: Arc::new(CoreBackpressureMonitor::new(core_config)),
        }
    }

    /// Get the configuration
    pub fn config(&self) -> &PipeBackpressurePolicy {
        &self.config
    }

    /// Get the derived io-core admission-pressure configuration.
    pub fn core_config(&self) -> &CoreBackpressureConfig {
        &self.core_config
    }

    /// Get the monitor
    pub fn monitor(&self) -> Arc<CoreBackpressureMonitor> {
        self.monitor.clone()
    }

    /// Create a backpressure pipe
    pub fn create_pipe(&self) -> BackpressurePipe {
        BackpressurePipe::new(self.config, self.monitor.clone())
    }

    /// Get current state
    pub fn state(&self) -> BackpressureState {
        self.monitor.state()
    }

    /// Check if backpressure is active
    pub fn is_active(&self) -> bool {
        self.monitor.is_active()
    }
}

/// Backpressure pipe wrapping tokio's duplex
pub struct BackpressurePipe {
    reader: DuplexStream,
    writer: DuplexStream,
    config: PipeBackpressurePolicy,
    monitor: Arc<CoreBackpressureMonitor>,
    created_at: Instant,
}

/// Shared pipe metadata snapshot for facade-level backpressure pipes.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct BackpressurePipeMeta {
    /// Configured duplex buffer capacity in bytes.
    pub buffer_capacity: usize,
    /// Current backpressure state reported by the shared core monitor.
    pub state: BackpressureState,
    /// Age of the pipe since creation.
    pub age: std::time::Duration,
}

impl BackpressurePipe {
    fn new(config: PipeBackpressurePolicy, monitor: Arc<CoreBackpressureMonitor>) -> Self {
        let (reader, writer) = duplex(config.buffer_size);

        Self {
            reader,
            writer,
            config,
            monitor,
            created_at: Instant::now(),
        }
    }

    /// Get the reader end
    pub fn reader(&mut self) -> &mut DuplexStream {
        &mut self.reader
    }

    /// Get the writer end
    pub fn writer(&mut self) -> &mut DuplexStream {
        &mut self.writer
    }

    /// Split into reader and writer
    pub fn into_split(self) -> (DuplexStream, DuplexStream) {
        (self.reader, self.writer)
    }

    /// Get the configuration
    pub fn config(&self) -> &PipeBackpressurePolicy {
        &self.config
    }

    /// Get current state
    pub fn state(&self) -> BackpressureState {
        self.monitor.state()
    }

    /// Get the age of this pipe
    pub fn age(&self) -> std::time::Duration {
        self.created_at.elapsed()
    }

    /// Get a compact metadata snapshot for the pipe.
    pub fn meta(&self) -> BackpressurePipeMeta {
        BackpressurePipeMeta {
            buffer_capacity: self.config.buffer_size,
            state: self.state(),
            age: self.age(),
        }
    }

    /// Check if should apply backpressure
    pub fn should_apply_backpressure(&self) -> bool {
        let should = self.monitor.should_apply_backpressure();
        if should {
            backpressure_metrics::record_backpressure_activation();
        }
        should
    }
}

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

    #[test]
    fn test_backpressure_config() {
        let config = PipeBackpressurePolicy::default();
        assert_eq!(config.buffer_size, 4 * 1024 * 1024);
        assert!(config.high_watermark > config.low_watermark);
    }

    #[test]
    fn test_backpressure_policy_to_core_config() {
        let policy = PipeBackpressurePolicy::default();
        let core = policy.to_core_config();
        assert_eq!(core.high_water_mark, policy.high_watermark as f64 / 100.0);
        assert_eq!(core.low_water_mark, policy.low_watermark as f64 / 100.0);
        assert!(core.enabled);
    }

    #[test]
    fn test_backpressure_manager() {
        let manager = BackpressureManager::new(1024, 80, 50);
        assert_eq!(manager.state(), BackpressureState::Normal);
    }

    #[test]
    fn test_backpressure_pipe() {
        let manager = BackpressureManager::new(1024, 80, 50);
        let pipe = manager.create_pipe();
        assert_eq!(pipe.state(), BackpressureState::Normal);
        assert_eq!(pipe.meta().buffer_capacity, 1024);
    }

    #[test]
    fn test_backpressure_pipe_meta_is_read_only() {
        let manager = BackpressureManager::new(2048, 80, 50);
        let pipe = manager.create_pipe();
        let first = pipe.meta();
        let second = pipe.meta();

        assert_eq!(first.buffer_capacity, 2048);
        assert_eq!(first.state, BackpressureState::Normal);
        assert_eq!(second.buffer_capacity, first.buffer_capacity);
        assert_eq!(second.state, first.state);
        assert!(second.age >= first.age);
        assert_eq!(manager.state(), BackpressureState::Normal);
    }
}