//! resolver implementations implementing different strategies for answering
//! incoming queries

use std::sync::Arc;
use std::time::Instant;
use std::vec::Vec;

use derive_more::{Display, Error, From};

use crate::dns::context::ServerContext;
use crate::dns::protocol::{DnsPacket, QueryType, ResultCode};

#[derive(Debug, Display, From, Error)]
pub enum ResolveError {
    Client(crate::dns::client::ClientError),
    Cache(crate::dns::cache::CacheError),
    Io(std::io::Error),
    NoServerFound
}

type Result<T> = std::result::Result<T, ResolveError>;

pub trait DnsResolver {
    fn get_context(&self) -> Arc<ServerContext>;

    fn resolve(&mut self, qname: &str, qtype: QueryType, recursive: bool) -> Result<DnsPacket> {
        if let QueryType::UNKNOWN(_) = qtype {
            let mut packet = DnsPacket::new();
            packet.header.rescode = ResultCode::NOTIMP;
            return Ok(packet);
        }

        let context = self.get_context();

        if let Some(qr) = context.authority.query(qname, qtype) {
            return Ok(qr);
        }

        if !recursive || !context.allow_recursive {
            let mut packet = DnsPacket::new();
            packet.header.rescode = ResultCode::REFUSED;
            return Ok(packet);
        }

        if let Some(qr) = context.cache.lookup(qname, qtype) {
            return Ok(qr);
        }

        if qtype == QueryType::A || qtype == QueryType::AAAA {
            if let Some(qr) = context.cache.lookup(qname, QueryType::CNAME) {
                return Ok(qr);
            }
        }

        for filter in context.filters.iter() {
            if let Some(packet) = filter.lookup(qname, qtype, recursive) {
                context.cache.store(&packet.answers)?;
                return Ok(packet);
            }
        }

        self.perform(qname, qtype)
    }

    fn perform(&mut self, qname: &str, qtype: QueryType) -> Result<DnsPacket>;
}

/// A Forwarding DNS Resolver
///
/// This resolver uses an external DNS server to service a query
pub struct ForwardingDnsResolver {
    context: Arc<ServerContext>,
    upstreams: Vec<String>
}

impl ForwardingDnsResolver {
    pub fn new(context: Arc<ServerContext>, upstreams: Vec<String>) -> ForwardingDnsResolver {
        ForwardingDnsResolver { context, upstreams }
    }
}

impl DnsResolver for ForwardingDnsResolver {
    fn get_context(&self) -> Arc<ServerContext> {
        Arc::clone(&self.context)
    }

    fn perform(&mut self, qname: &str, qtype: QueryType) -> Result<DnsPacket> {
        if let Some(packet) = self.context.cache.lookup(qname, qtype) {
            return Ok(packet);
        }

        let ordered = self.context.forwarder_tracker.select_ordered(&self.upstreams);
        let mut last_err = ResolveError::NoServerFound;

        for upstream in &ordered {
            let start = Instant::now();
            let query_result = if is_url(upstream) {
                if let Some(client) = &self.context.doh_client {
                    client.send_query(qname, qtype, upstream, true)
                } else {
                    log::error!("This build doesn't support DoH");
                    continue;
                }
            } else {
                self.context.old_client.send_query(qname, qtype, upstream, true)
            };

            match query_result {
                Ok(mut result) => {
                    let elapsed = start.elapsed().as_secs_f64() * 1000.0;
                    self.context.forwarder_tracker.record_success(upstream, elapsed);
                    self.context.cache.store(&result.answers)?;

                    // Fix domain names in answers to match original query case
                    let qname_lower = qname.to_lowercase();
                    for answer in &mut result.answers {
                        if let Some(domain) = answer.get_domain() {
                            if domain.to_lowercase() == qname_lower {
                                answer.set_domain(qname.to_string());
                            }
                        }
                    }
                    return Ok(result);
                }
                Err(e) => {
                    self.context.forwarder_tracker.record_failure(upstream);
                    last_err = e.into();
                }
            }
        }

        Err(last_err)
    }
}

/// A Recursive DNS resolver
///
/// This resolver can answer any request using the root servers of the internet
pub struct RecursiveDnsResolver {
    context: Arc<ServerContext>
}

impl RecursiveDnsResolver {
    pub fn new(context: Arc<ServerContext>) -> RecursiveDnsResolver {
        RecursiveDnsResolver { context }
    }
}

impl DnsResolver for RecursiveDnsResolver {
    fn get_context(&self) -> Arc<ServerContext> {
        Arc::clone(&self.context)
    }

    fn perform(&mut self, qname: &str, qtype: QueryType) -> Result<DnsPacket> {
        // Find the closest name server by splitting the label and progessively
        // moving towards the root servers. I.e. check "google.com", then "com",
        // and finally "".
        let mut tentative_ns = None;

        let labels = qname.split('.').collect::<Vec<&str>>();
        for lbl_idx in 0..labels.len() + 1 {
            let domain = labels[lbl_idx..].join(".");

            match self
                .context
                .cache
                .lookup(&domain, QueryType::NS)
                .and_then(|qr| qr.get_unresolved_ns(&domain))
                .and_then(|ns| self.context.cache.lookup(&ns, QueryType::A))
                .and_then(|qr| qr.get_random_a())
            {
                Some(addr) => {
                    tentative_ns = Some(addr);
                    break;
                }
                None => continue
            }
        }

        let mut ns = tentative_ns.ok_or(ResolveError::NoServerFound)?;

        // Start querying name servers
        loop {
            println!("attempting lookup of {:?} {} with ns {}", qtype, qname, ns);

            let ns_copy = ns.clone();

            let server = format!("{}:{}", ns_copy.as_str(), 53);
            let response = self.context.old_client.send_query(qname, qtype, &server, false)?;

            // If we've got an actual answer, we're done!
            if !response.answers.is_empty() && response.header.rescode == ResultCode::NOERROR {
                let _ = self.context.cache.store(&response.answers);
                let _ = self.context.cache.store(&response.authorities);
                let _ = self.context.cache.store(&response.resources);

                // Fix domain names in answers to match original query case
                let qname_lower = qname.to_lowercase();
                let mut fixed_response = response;
                for answer in &mut fixed_response.answers {
                    if let Some(domain) = answer.get_domain() {
                        if domain.to_lowercase() == qname_lower {
                            answer.set_domain(qname.to_string());
                        }
                    }
                }
                return Ok(fixed_response);
            }

            if response.header.rescode == ResultCode::NXDOMAIN {
                if let Some(ttl) = response.get_ttl_from_soa() {
                    let _ = self.context.cache.store_nxdomain(qname, qtype, ttl);
                }
                return Ok(response);
            }

            // Otherwise, try to find a new nameserver based on NS and a
            // corresponding A record in the additional section
            if let Some(new_ns) = response.get_resolved_ns(qname) {
                // If there is such a record, we can retry the loop with that NS
                ns = new_ns.clone();
                let _ = self.context.cache.store(&response.answers);
                let _ = self.context.cache.store(&response.authorities);
                let _ = self.context.cache.store(&response.resources);

                continue;
            }

            // If not, we'll have to resolve the ip of a NS record
            let new_ns_name = match response.get_unresolved_ns(qname) {
                Some(x) => x,
                None => {
                    // Fix domain names before returning
                    let qname_lower = qname.to_lowercase();
                    let mut fixed_response = response;
                    for answer in &mut fixed_response.answers {
                        if let Some(domain) = answer.get_domain() {
                            if domain.to_lowercase() == qname_lower {
                                answer.set_domain(qname.to_string());
                            }
                        }
                    }
                    return Ok(fixed_response);
                }
            };

            // Recursively resolve the NS
            let recursive_response = self.resolve(&new_ns_name, QueryType::A, true)?;

            // Pick a random IP and restart
            if let Some(new_ns) = recursive_response.get_random_a() {
                ns = new_ns.clone();
            } else {
                // Fix domain names before returning
                let qname_lower = qname.to_lowercase();
                let mut fixed_response = response;
                for answer in &mut fixed_response.answers {
                    if let Some(domain) = answer.get_domain() {
                        if domain.to_lowercase() == qname_lower {
                            answer.set_domain(qname.to_string());
                        }
                    }
                }
                return Ok(fixed_response);
            }
        }
    }
}

fn is_url(url: &str) -> bool {
    url.starts_with("https://")
}

#[cfg(test)]
mod tests {

    use std::sync::Arc;

    use super::*;
    use crate::dns::context::tests::create_test_context;
    use crate::dns::context::ResolveStrategy;
    use crate::dns::protocol::{DnsPacket, DnsRecord, QueryType, ResultCode, TransientTtl};

    #[test]
    fn test_forwarding_resolver() {
        let mut context = create_test_context(Box::new(|qname, _, _, _| {
            let mut packet = DnsPacket::new();

            if qname == "google.com" {
                packet.answers.push(DnsRecord::A {
                    domain: "google.com".to_string(),
                    addr: "127.0.0.1".parse().unwrap(),
                    ttl: TransientTtl(3600)
                });
            } else {
                packet.header.rescode = ResultCode::NXDOMAIN;
            }

            Ok(packet)
        }));

        match Arc::get_mut(&mut context) {
            Some(ctx) => {
                ctx.resolve_strategy = ResolveStrategy::Forward { upstreams: vec![String::from("127.0.0.1:53")] };
            }
            None => panic!()
        }

        let mut resolver = context.create_resolver(Arc::clone(&context));

        // First, verify that we get a match back
        {
            let res = match resolver.resolve("google.com", QueryType::A, true) {
                Ok(x) => x,
                Err(_) => panic!()
            };

            assert_eq!(1, res.answers.len());

            match res.answers[0] {
                DnsRecord::A { ref domain, .. } => {
                    assert_eq!("google.com", domain);
                }
                _ => panic!()
            }
        };

        // Do the same lookup again and verify that it's present in the cache
        // and that the counter has been updated
        {
            let res = match resolver.resolve("google.com", QueryType::A, true) {
                Ok(x) => x,
                Err(_) => panic!()
            };

            assert_eq!(1, res.answers.len());

            let list = match context.cache.list() {
                Ok(x) => x,
                Err(_) => panic!()
            };

            assert_eq!(1, list.len());

            assert_eq!("google.com", list[0].domain);
            assert_eq!(1, list[0].record_types.len());
            assert_eq!(1, list[0].hits);
        };

        // Do a failed lookup
        {
            let res = match resolver.resolve("yahoo.com", QueryType::A, true) {
                Ok(x) => x,
                Err(_) => panic!()
            };

            assert_eq!(0, res.answers.len());
            assert_eq!(ResultCode::NXDOMAIN, res.header.rescode);
        };
    }

    #[test]
    fn test_recursive_resolver_with_no_nameserver() {
        let context = create_test_context(Box::new(|_, _, _, _| {
            let mut packet = DnsPacket::new();
            packet.header.rescode = ResultCode::NXDOMAIN;
            Ok(packet)
        }));

        let mut resolver = context.create_resolver(Arc::clone(&context));

        // Expect failure when no name servers are available
        if let Ok(_) = resolver.resolve("google.com", QueryType::A, true) {
            panic!();
        }
    }

    #[test]
    fn test_recursive_resolver_with_missing_a_record() {
        let context = create_test_context(Box::new(|_, _, _, _| {
            let mut packet = DnsPacket::new();
            packet.header.rescode = ResultCode::NXDOMAIN;
            Ok(packet)
        }));

        let mut resolver = context.create_resolver(Arc::clone(&context));

        // Expect failure when no name servers are available
        if let Ok(_) = resolver.resolve("google.com", QueryType::A, true) {
            panic!();
        }

        // Insert name server, but no corresponding A record
        let mut nameservers = Vec::new();
        nameservers.push(DnsRecord::NS { domain: "".to_string(), host: "a.myroot.net".to_string(), ttl: TransientTtl(3600) });

        let _ = context.cache.store(&nameservers);

        if let Ok(_) = resolver.resolve("google.com", QueryType::A, true) {
            panic!();
        }
    }

    #[test]
    fn test_recursive_resolver_match_order() {
        let context = create_test_context(Box::new(|_, _, server, _| {
            let mut packet = DnsPacket::new();

            if server.starts_with("127.0.0.1") {
                packet.header.id = 1;

                packet.answers.push(DnsRecord::A {
                    domain: "a.google.com".to_string(),
                    addr: "127.0.0.1".parse().unwrap(),
                    ttl: TransientTtl(3600)
                });

                return Ok(packet);
            } else if server.starts_with("127.0.0.2") {
                packet.header.id = 2;

                packet.answers.push(DnsRecord::A {
                    domain: "b.google.com".to_string(),
                    addr: "127.0.0.1".parse().unwrap(),
                    ttl: TransientTtl(3600)
                });

                return Ok(packet);
            } else if server.starts_with("127.0.0.3") {
                packet.header.id = 3;

                packet.answers.push(DnsRecord::A {
                    domain: "c.google.com".to_string(),
                    addr: "127.0.0.1".parse().unwrap(),
                    ttl: TransientTtl(3600)
                });

                return Ok(packet);
            }

            packet.header.id = 999;
            packet.header.rescode = ResultCode::NXDOMAIN;
            Ok(packet)
        }));

        let mut resolver = context.create_resolver(Arc::clone(&context));

        // Expect failure when no name servers are available
        if let Ok(_) = resolver.resolve("google.com", QueryType::A, true) {
            panic!();
        }

        // Insert root servers
        {
            let mut nameservers = Vec::new();
            nameservers.push(DnsRecord::NS { domain: "".to_string(), host: "a.myroot.net".to_string(), ttl: TransientTtl(3600) });
            nameservers.push(DnsRecord::A {
                domain: "a.myroot.net".to_string(),
                addr: "127.0.0.1".parse().unwrap(),
                ttl: TransientTtl(3600)
            });

            let _ = context.cache.store(&nameservers);
        }

        match resolver.resolve("google.com", QueryType::A, true) {
            Ok(packet) => {
                assert_eq!(1, packet.header.id);
            }
            Err(_) => panic!()
        }

        // Insert TLD servers
        {
            let mut nameservers = Vec::new();
            nameservers.push(DnsRecord::NS { domain: "com".to_string(), host: "a.mytld.net".to_string(), ttl: TransientTtl(3600) });
            nameservers.push(DnsRecord::A {
                domain: "a.mytld.net".to_string(),
                addr: "127.0.0.2".parse().unwrap(),
                ttl: TransientTtl(3600)
            });

            let _ = context.cache.store(&nameservers);
        }

        match resolver.resolve("google.com", QueryType::A, true) {
            Ok(packet) => {
                assert_eq!(2, packet.header.id);
            }
            Err(_) => panic!()
        }

        // Insert authoritative servers
        {
            let mut nameservers = Vec::new();
            nameservers.push(DnsRecord::NS {
                domain: "google.com".to_string(),
                host: "ns1.google.com".to_string(),
                ttl: TransientTtl(3600)
            });
            nameservers.push(DnsRecord::A {
                domain: "ns1.google.com".to_string(),
                addr: "127.0.0.3".parse().unwrap(),
                ttl: TransientTtl(3600)
            });

            let _ = context.cache.store(&nameservers);
        }

        match resolver.resolve("google.com", QueryType::A, true) {
            Ok(packet) => {
                assert_eq!(3, packet.header.id);
            }
            Err(_) => panic!()
        }
    }

    #[test]
    fn test_recursive_resolver_successfully() {
        let context = create_test_context(Box::new(|qname, _, _, _| {
            let mut packet = DnsPacket::new();

            if qname == "google.com" {
                packet.answers.push(DnsRecord::A {
                    domain: "google.com".to_string(),
                    addr: "127.0.0.1".parse().unwrap(),
                    ttl: TransientTtl(3600)
                });
            } else {
                packet.header.rescode = ResultCode::NXDOMAIN;

                packet.authorities.push(DnsRecord::SOA {
                    domain: "google.com".to_string(),
                    r_name: "google.com".to_string(),
                    m_name: "google.com".to_string(),
                    serial: 0,
                    refresh: 3600,
                    retry: 3600,
                    expire: 3600,
                    minimum: 3600,
                    ttl: TransientTtl(3600)
                });
            }

            Ok(packet)
        }));

        let mut resolver = context.create_resolver(Arc::clone(&context));

        // Insert name servers
        let mut nameservers = Vec::new();
        nameservers.push(DnsRecord::NS { domain: "google.com".to_string(), host: "ns1.google.com".to_string(), ttl: TransientTtl(3600) });
        nameservers.push(DnsRecord::A {
            domain: "ns1.google.com".to_string(),
            addr: "127.0.0.1".parse().unwrap(),
            ttl: TransientTtl(3600)
        });

        let _ = context.cache.store(&nameservers);

        // Check that we can successfully resolve
        {
            let res = match resolver.resolve("google.com", QueryType::A, true) {
                Ok(x) => x,
                Err(_) => panic!()
            };

            assert_eq!(1, res.answers.len());

            match res.answers[0] {
                DnsRecord::A { ref domain, .. } => {
                    assert_eq!("google.com", domain);
                }
                _ => panic!()
            }
        };

        // And that we won't find anything for a domain that isn't present
        {
            let res = match resolver.resolve("foobar.google.com", QueryType::A, true) {
                Ok(x) => x,
                Err(_) => panic!()
            };

            assert_eq!(ResultCode::NXDOMAIN, res.header.rescode);
            assert_eq!(0, res.answers.len());
        };

        // Perform another successful query, that should hit the cache
        {
            let res = match resolver.resolve("google.com", QueryType::A, true) {
                Ok(x) => x,
                Err(_) => panic!()
            };

            assert_eq!(1, res.answers.len());
        };

        // Now check that the cache is used, and that the statistics is correct
        {
            let list = match context.cache.list() {
                Ok(x) => x,
                Err(_) => panic!()
            };

            assert_eq!(3, list.len());

            // Find entries by domain name (LRU order may vary)
            let google_entry = list.iter().find(|e| e.domain == "google.com").expect("google.com entry");
            let ns1_entry = list.iter().find(|e| e.domain == "ns1.google.com").expect("ns1.google.com entry");
            let foobar_entry = list.iter().find(|e| e.domain == "foobar.google.com").expect("foobar.google.com NXDOMAIN entry");

            // google.com should have a NS record and an A record for a total of 2 record types
            assert_eq!(2, google_entry.record_types.len());

            // Should have been hit two times for NS google.com and once for
            // A google.com
            assert_eq!(3, google_entry.hits);

            assert_eq!(1, ns1_entry.record_types.len());
            assert_eq!(2, ns1_entry.hits);

            // foobar.google.com should be a cached NXDOMAIN with 0 hits
            assert_eq!(0, foobar_entry.hits);
        };
    }
}