/*
    This file is part of gnix (https://codeberg.org/metamuffin/gnix)
    which is licensed under the GNU Affero General Public License (version 3); see /COPYING.
    Copyright (C) 2025 metamuffin <metamuffin.org>
*/
//! Response caching module
//!
//! Considerations:
//! - check cache header
//! - ignore responses that get too large
//! - ignore requests with body (or too large body)
//! - LRU cache pruning
//! - different backends:
//!     - in memory (HashMap)
//!     - on disk (redb? filesystem?)
//!     - external db?
use super::{Node, NodeContext, NodeKind, NodeRequest, NodeResponse};
use crate::{config::DynNode, error::ServiceError};
use anyhow::Result;
use bytes::Bytes;
use headers::{CacheControl, HeaderMapExt};
use http::Response;
use http_body_util::{BodyExt, Full};
use log::debug;
use serde::Deserialize;
use serde_yml::Value;
use sha2::{Digest, Sha256};
use std::{collections::HashMap, future::Future, pin::Pin, sync::Arc, time::Duration};
use tokio::sync::RwLock;

pub struct CacheKind;

#[derive(Deserialize)]
struct CacheConfig {
    next: DynNode,
}

struct Cache {
    entries: RwLock<HashMap<[u8; 32], Response<Bytes>>>,
    config: CacheConfig,
}

impl NodeKind for CacheKind {
    fn name(&self) -> &'static str {
        "cache"
    }
    fn instanciate(&self, config: Value) -> Result<Arc<dyn Node>> {
        let config = serde_yml::from_value::<CacheConfig>(config)?;
        Ok(Arc::new(Cache {
            config,
            entries: HashMap::new().into(),
        }))
    }
}
impl Node for Cache {
    fn handle<'a>(
        &'a self,
        context: &'a mut NodeContext,
        request: NodeRequest,
    ) -> Pin<Box<dyn Future<Output = Result<NodeResponse, ServiceError>> + Send + Sync + 'a>> {
        Box::pin(async move {
            let allow_cache = request.method().is_safe();

            if !allow_cache {
                return self.config.next.handle(context, request).await;
            }

            // not very fast
            let mut hasher = Sha256::new();
            hasher.update(request.method().as_str().len().to_be_bytes());
            hasher.update(request.method().as_str());
            hasher.update(request.uri().path().len().to_be_bytes());
            hasher.update(request.uri().path());
            hasher.update([request.uri().query().is_some() as u8]);
            if let Some(q) = request.uri().query() {
                hasher.update(q.len().to_be_bytes());
                hasher.update(q);
            }
            // TODO which headers are important to caching?
            hasher.update(request.headers().len().to_be_bytes());
            for (k, v) in request.headers() {
                hasher.update(k.as_str().len().to_be_bytes());
                hasher.update(v.as_bytes().len().to_be_bytes());
                hasher.update(k);
                hasher.update(v);
            }
            let key: [u8; 32] = hasher.finalize().into();

            if let Some(resp) = self.entries.read().await.get(&key) {
                debug!("hit");
                return Ok(resp
                    .to_owned()
                    .map(|b| Full::new(b).map_err(|e| match e {}).boxed()));
            }
            debug!("miss");

            let response = self.config.next.handle(context, request).await?;

            let cache_control = response
                .headers()
                .typed_get::<CacheControl>()
                .unwrap_or(CacheControl::new().with_max_age(Duration::from_secs(120))); // TODO what is the correct default?

            let allow_store = !cache_control.no_store() && !cache_control.no_cache();

            let response = if allow_store {
                debug!("store");
                let h = response.headers().to_owned();
                let s = response.status().to_owned();
                let body = response.collect().await?.to_bytes();

                let mut r1 = Response::new(Full::new(body.clone()).map_err(|e| match e {}).boxed());
                *r1.headers_mut() = h.clone();
                *r1.status_mut() = s;

                let mut r2 = Response::new(body);
                *r2.headers_mut() = h;
                *r2.status_mut() = s;
                self.entries.write().await.insert(key, r2);
                r1
            } else {
                debug!("no store");
                response
            };

            Ok(response)
        })
    }
}