/// Streaming PostgreSQL ingest.
///
/// The original `db_ingest::import_postgres_table` loads every row into
/// memory before emitting Parquet — fine for small tables, blows up on
/// 1M+ rows. This module paginates via `ORDER BY <pk> LIMIT N OFFSET M`
/// and streams batches into an `ArrowWriter`, closing once exhausted.
///
/// Pagination is OFFSET-based (not keyset) — simpler, works on any table
/// without needing to know the PK type. OFFSET N scans N rows per fetch,
/// which becomes quadratic for very large tables. Upgrade to keyset when
/// a user actually ingests something that hurts.

use arrow::array::{ArrayRef, BooleanArray, Float64Array, Int32Array, Int64Array, RecordBatch, StringArray};
use arrow::datatypes::{DataType, Field, Schema};
use parquet::arrow::ArrowWriter;
use std::sync::Arc;
use tokio_postgres::{Client, NoTls, types::Type as PgType};

use crate::db_ingest::DbConfig;

/// Request shape for streaming ingest — takes a DSN string and optional
/// tuning knobs.
#[derive(Debug, Clone, serde::Deserialize)]
pub struct PgStreamRequest {
    /// postgresql://user:pass@host:port/db — alternative to DbConfig.
    pub dsn: String,
    pub table: String,
    #[serde(default)]
    pub dataset_name: Option<String>,
    /// Rows per fetch. Default 10_000.
    #[serde(default)]
    pub batch_size: Option<usize>,
    /// Column to ORDER BY for stable pagination. If omitted, the first
    /// column returned by the schema probe is used.
    #[serde(default)]
    pub order_by: Option<String>,
    /// Hard cap on total rows (for sampling / previews).
    #[serde(default)]
    pub limit: Option<usize>,
}

#[derive(Debug, Clone, serde::Serialize)]
pub struct PgStreamResult {
    pub table: String,
    pub rows: usize,
    pub batches: usize,
    pub columns: usize,
    pub schema: Vec<String>,
    pub parquet_bytes: u64,
    pub duration_secs: f32,
}

/// Parse a postgresql:// DSN into a DbConfig.
/// Supports: postgresql://[user[:password]@]host[:port][/db]
/// Does NOT support: query parameters (sslmode etc) — add if needed.
pub fn parse_dsn(dsn: &str) -> Result<DbConfig, String> {
    let rest = dsn
        .strip_prefix("postgresql://")
        .or_else(|| dsn.strip_prefix("postgres://"))
        .ok_or_else(|| "DSN must start with postgresql:// or postgres://".to_string())?;

    // Split off path (database) first.
    let (auth_host, database) = match rest.split_once('/') {
        Some((ah, db)) => (ah, db.split('?').next().unwrap_or(db).to_string()),
        None => (rest, "postgres".to_string()),
    };

    // Split user[:password] from host[:port].
    let (userpass, hostport) = match auth_host.rsplit_once('@') {
        Some((up, hp)) => (Some(up), hp),
        None => (None, auth_host),
    };

    let (user, password) = match userpass {
        Some(up) => match up.split_once(':') {
            Some((u, p)) => (u.to_string(), p.to_string()),
            None => (up.to_string(), String::new()),
        },
        None => ("postgres".to_string(), String::new()),
    };

    let (host, port) = match hostport.rsplit_once(':') {
        Some((h, p)) => (
            h.to_string(),
            p.parse::<u16>().map_err(|_| format!("invalid port in DSN: {p}"))?,
        ),
        None => (hostport.to_string(), 5432),
    };

    if host.is_empty() {
        return Err("DSN has no host".into());
    }

    Ok(DbConfig { host, port, database, user, password })
}

/// Stream a Postgres table as Parquet bytes.
///
/// Returns the full Parquet payload + summary stats. The payload is
/// written to an in-memory buffer via `ArrowWriter` so each batch gets
/// columnar compression as it arrives — memory footprint stays roughly
/// at one batch plus Parquet footer state.
pub async fn stream_table_to_parquet(
    req: &PgStreamRequest,
) -> Result<(bytes::Bytes, PgStreamResult), String> {
    let t0 = std::time::Instant::now();
    let config = parse_dsn(&req.dsn)?;
    let batch_size = req.batch_size.unwrap_or(10_000).max(1);

    let (client, connection) = tokio_postgres::connect(&config.connection_string(), NoTls)
        .await
        .map_err(|e| format!("postgres connect: {e}"))?;
    tokio::spawn(async move {
        if let Err(e) = connection.await {
            tracing::error!("pg connection: {e}");
        }
    });

    // Probe schema.
    let columns = probe_columns(&client, &req.table).await?;
    if columns.is_empty() {
        return Err(format!("table '{}' not found or has no columns", req.table));
    }

    let arrow_fields: Vec<Field> = columns
        .iter()
        .map(|(name, pg)| Field::new(name, pg_type_to_arrow(pg), true))
        .collect();
    let schema = Arc::new(Schema::new(arrow_fields));
    let schema_report: Vec<String> = columns
        .iter()
        .map(|(n, t)| format!("{}:{}", n, t))
        .collect();

    // Pagination key: user-specified or first column.
    let order_col = req.order_by.clone().unwrap_or_else(|| columns[0].0.clone());

    // Stream batches into ArrowWriter.
    let mut buf: Vec<u8> = Vec::with_capacity(1024 * 1024);
    let mut writer = ArrowWriter::try_new(&mut buf, schema.clone(), None)
        .map_err(|e| format!("ArrowWriter init: {e}"))?;

    let mut total_rows: usize = 0;
    let mut batch_count: usize = 0;
    let row_cap = req.limit.unwrap_or(usize::MAX);

    loop {
        let remaining = row_cap.saturating_sub(total_rows);
        if remaining == 0 { break; }
        let fetch = remaining.min(batch_size);

        let sql = format!(
            "SELECT * FROM \"{}\" ORDER BY \"{}\" LIMIT {} OFFSET {}",
            req.table, order_col, fetch, total_rows,
        );

        let rows = client
            .query(&sql, &[])
            .await
            .map_err(|e| format!("fetch batch at offset {total_rows}: {e}"))?;

        if rows.is_empty() { break; }

        let n = rows.len();
        let arrays: Vec<ArrayRef> = columns
            .iter()
            .enumerate()
            .map(|(idx, (_, pg))| rows_to_column(&rows, idx, pg))
            .collect::<Result<_, _>>()?;
        let batch = RecordBatch::try_new(schema.clone(), arrays)
            .map_err(|e| format!("RecordBatch: {e}"))?;
        writer.write(&batch).map_err(|e| format!("ArrowWriter::write: {e}"))?;

        total_rows += n;
        batch_count += 1;
        tracing::info!(
            "pg stream '{}': fetched batch {} ({} rows, total {})",
            req.table, batch_count, n, total_rows,
        );

        if n < fetch { break; } // short read = end of table
    }

    writer.close().map_err(|e| format!("ArrowWriter::close: {e}"))?;
    let parquet_bytes = buf.len() as u64;
    let duration = t0.elapsed().as_secs_f32();

    let result = PgStreamResult {
        table: req.table.clone(),
        rows: total_rows,
        batches: batch_count,
        columns: columns.len(),
        schema: schema_report,
        parquet_bytes,
        duration_secs: duration,
    };

    Ok((bytes::Bytes::from(buf), result))
}

async fn probe_columns(client: &Client, table: &str) -> Result<Vec<(String, PgType)>, String> {
    let stmt = client
        .prepare(&format!("SELECT * FROM \"{}\" LIMIT 0", table))
        .await
        .map_err(|e| format!("prepare schema probe: {e}"))?;
    Ok(stmt
        .columns()
        .iter()
        .map(|c| (c.name().to_string(), c.type_().clone()))
        .collect())
}

fn pg_type_to_arrow(pg: &PgType) -> DataType {
    match *pg {
        PgType::BOOL => DataType::Boolean,
        PgType::INT2 | PgType::INT4 => DataType::Int32,
        PgType::INT8 | PgType::OID => DataType::Int64,
        PgType::FLOAT4 | PgType::FLOAT8 | PgType::NUMERIC => DataType::Float64,
        _ => DataType::Utf8,
    }
}

fn rows_to_column(
    rows: &[tokio_postgres::Row],
    idx: usize,
    pg: &PgType,
) -> Result<ArrayRef, String> {
    match *pg {
        PgType::BOOL => {
            let v: Vec<Option<bool>> = rows.iter().map(|r| r.try_get(idx).ok()).collect();
            Ok(Arc::new(BooleanArray::from(v)))
        }
        PgType::INT2 => {
            let v: Vec<Option<i32>> = rows.iter()
                .map(|r| r.try_get::<_, i16>(idx).ok().map(|x| x as i32)).collect();
            Ok(Arc::new(Int32Array::from(v)))
        }
        PgType::INT4 => {
            let v: Vec<Option<i32>> = rows.iter().map(|r| r.try_get(idx).ok()).collect();
            Ok(Arc::new(Int32Array::from(v)))
        }
        PgType::INT8 | PgType::OID => {
            let v: Vec<Option<i64>> = rows.iter().map(|r| r.try_get(idx).ok()).collect();
            Ok(Arc::new(Int64Array::from(v)))
        }
        PgType::FLOAT4 => {
            let v: Vec<Option<f64>> = rows.iter()
                .map(|r| r.try_get::<_, f32>(idx).ok().map(|x| x as f64)).collect();
            Ok(Arc::new(Float64Array::from(v)))
        }
        PgType::FLOAT8 => {
            let v: Vec<Option<f64>> = rows.iter().map(|r| r.try_get(idx).ok()).collect();
            Ok(Arc::new(Float64Array::from(v)))
        }
        _ => {
            // Safe-default per ADR-010: everything else becomes string.
            let v: Vec<Option<String>> = rows.iter().map(|r| {
                r.try_get::<_, String>(idx).ok()
                    .or_else(|| r.try_get::<_, serde_json::Value>(idx).ok().map(|j| j.to_string()))
                    .or_else(|| {
                        // Timestamps + uuid, rendered via Display.
                        r.try_get::<_, chrono::DateTime<chrono::Utc>>(idx).ok()
                            .map(|t| t.to_rfc3339())
                    })
                    .or_else(|| r.try_get::<_, uuid::Uuid>(idx).ok().map(|u| u.to_string()))
                    .or(Some(String::new()))
            }).collect();
            Ok(Arc::new(StringArray::from(v)))
        }
    }
}

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

    #[test]
    fn parse_dsn_full() {
        let c = parse_dsn("postgresql://daisy:secret@db.example.com:6543/my_db").unwrap();
        assert_eq!(c.host, "db.example.com");
        assert_eq!(c.port, 6543);
        assert_eq!(c.user, "daisy");
        assert_eq!(c.password, "secret");
        assert_eq!(c.database, "my_db");
    }

    #[test]
    fn parse_dsn_minimal() {
        let c = parse_dsn("postgresql://localhost/knowledge_base").unwrap();
        assert_eq!(c.host, "localhost");
        assert_eq!(c.port, 5432);
        assert_eq!(c.user, "postgres");
        assert_eq!(c.password, "");
        assert_eq!(c.database, "knowledge_base");
    }

    #[test]
    fn parse_dsn_no_password() {
        let c = parse_dsn("postgres://postgres@127.0.0.1:5432/mydb").unwrap();
        assert_eq!(c.user, "postgres");
        assert_eq!(c.password, "");
    }

    #[test]
    fn parse_dsn_rejects_non_pg() {
        assert!(parse_dsn("http://host/db").is_err());
    }
}