dbnexus 0.5.1

// Copyright (c) 2026 Kirky.X
// SPDX-License-Identifier: MIT
//! 跨分片查询引擎 — Scatter-Gather 执行器
//!
//! 向所有已注册分片并行发送查询，收集并聚合结果。

use std::sync::Arc;
use std::time::Duration;

use futures::stream::{FuturesUnordered, StreamExt};

use crate::database::sharding::ShardRouter;

// ============================================================================
// 类型定义
// ============================================================================

/// 部分失败策略
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum PartialFailurePolicy {
    /// 任何分片失败则整体失败
    Fail,
    /// 返回已成功分片的结果 + 失败分片信息
    BestEffort,
}

/// 聚合函数类型
#[derive(Debug, Clone)]
pub enum AggregateFunction {
    /// COUNT 聚合
    Count,
    /// SUM 聚合
    Sum(String),
    /// AVG 聚合
    Avg(String),
    /// MIN 聚合
    Min(String),
    /// MAX 聚合
    Max(String),
}

/// 聚合值
#[derive(Debug, Clone)]
pub enum AggregateValue {
    /// COUNT 结果
    Count(i64),
    /// SUM 结果
    Sum(f64),
    /// AVG 结果
    Avg(f64),
    /// MIN 结果
    Min(f64),
    /// MAX 结果
    Max(f64),
}

/// 单分片错误
#[derive(Debug, Clone)]
pub struct ShardError {
    /// 分片 ID
    pub shard_id: u32,
    /// 错误信息
    pub error: String,
}

/// 跨分片查询结果
#[derive(Debug)]
pub struct ScatterResult {
    /// 各分片返回的行数 (shard_id, row_count)
    pub shard_row_counts: Vec<(u32, u64)>,
    /// 失败分片列表
    pub failed_shards: Vec<ShardError>,
    /// 聚合结果（可选）
    pub aggregated: Option<AggregateValue>,
}

// ============================================================================
// ScatterGatherExecutor
// ============================================================================

/// Scatter-Gather 查询执行器
pub struct ScatterGatherExecutor {
    router: Arc<ShardRouter>,
    timeout: Duration,
    partial_failure: PartialFailurePolicy,
}

impl ScatterGatherExecutor {
    /// 创建执行器
    pub fn new(router: Arc<ShardRouter>, timeout: Duration, partial_failure: PartialFailurePolicy) -> Self {
        Self {
            router,
            timeout,
            partial_failure,
        }
    }

    /// 执行 scatter-gather 查询
    ///
    /// 向所有已注册分片并行发送 SQL，收集结果。
    pub async fn scatter_query(&self, sql: &str, role: &str) -> Result<ScatterResult, String> {
        let shards = self.router.all_shards();
        let mut futures = FuturesUnordered::new();

        for shard_info in shards {
            let shard_id = shard_info.shard_id;
            if let Some(pool) = self.router.get_pool(shard_id) {
                let sql = sql.to_string();
                let role = role.to_string();
                futures.push(async move {
                    match pool.get_session(&role).await {
                        Ok(session) => match session.execute_raw(&sql).await {
                            Ok(exec_result) => Ok((shard_id, exec_result.rows_affected())),
                            Err(e) => Err(ShardError {
                                shard_id,
                                error: e.to_string(),
                            }),
                        },
                        Err(e) => Err(ShardError {
                            shard_id,
                            error: e.to_string(),
                        }),
                    }
                });
            }
        }

        let mut shard_row_counts = Vec::new();
        let mut failed_shards = Vec::new();

        let collect_future = async {
            while let Some(result) = futures.next().await {
                match result {
                    Ok((shard_id, rows)) => shard_row_counts.push((shard_id, rows)),
                    Err(err) => failed_shards.push(err),
                }
            }
        };

        // 超时控制
        match tokio::time::timeout(self.timeout, collect_future).await {
            Ok(()) => {}
            Err(_) => return Err("Scatter-gather query timed out".to_string()),
        }

        // 部分失败策略
        if !failed_shards.is_empty() && self.partial_failure == PartialFailurePolicy::Fail {
            return Err(format!(
                "Scatter-gather failed: {} shard(s) failed",
                failed_shards.len()
            ));
        }

        Ok(ScatterResult {
            shard_row_counts,
            failed_shards,
            aggregated: None,
        })
    }

    /// 对 scatter 结果执行 COUNT 聚合
    pub fn aggregate_count(result: &ScatterResult) -> AggregateValue {
        let total: u64 = result.shard_row_counts.iter().map(|(_, count)| count).sum();
        AggregateValue::Count(total as i64)
    }

    /// 对 scatter 结果执行 SUM 聚合
    pub fn aggregate_sum(values: &[f64]) -> AggregateValue {
        AggregateValue::Sum(values.iter().sum())
    }

    /// 对 scatter 结果执行 AVG 聚合
    pub fn aggregate_avg(values: &[f64]) -> AggregateValue {
        if values.is_empty() {
            AggregateValue::Avg(0.0)
        } else {
            AggregateValue::Avg(values.iter().sum::<f64>() / values.len() as f64)
        }
    }

    /// 对 scatter 结果执行 MIN 聚合
    pub fn aggregate_min(values: &[f64]) -> AggregateValue {
        AggregateValue::Min(values.iter().copied().fold(f64::INFINITY, f64::min))
    }

    /// 对 scatter 结果执行 MAX 聚合
    pub fn aggregate_max(values: &[f64]) -> AggregateValue {
        AggregateValue::Max(values.iter().copied().fold(f64::NEG_INFINITY, f64::max))
    }
}