use {
    std::{
        ffi::OsStr,
        fmt::{Display, Formatter},
        path::Path,
        sync::Arc,
    },
    tokio_rustls::rustls::{
        self,
        crypto::ring::sign::any_supported_type,
        pki_types::{self, CertificateDer, PrivateKeyDer},
        server::{ClientHello, ResolvesServerCert},
        sign::{CertifiedKey, SigningKey},
    },
};

/// A struct that holds all loaded certificates and the respective domain
/// names.
#[derive(Debug)]
pub(crate) struct CertStore {
    /// Stores the certificates and the domains they apply to, sorted by domain
    /// names, longest matches first
    certs: Vec<(String, Arc<CertifiedKey>)>,
}

pub static CERT_FILE_NAME: &str = "cert.der";
pub static KEY_FILE_NAME: &str = "key.der";

#[derive(Debug)]
pub enum CertLoadError {
    /// could not access the certificate root directory
    NoReadCertDir,
    /// no certificates or keys were found
    Empty,
    /// the key file for the specified domain is bad (e.g. does not contain a
    /// key or is invalid)
    BadKey(String, rustls::Error),
    /// the key file for the specified domain is missing (but a certificate
    /// file was present)
    MissingKey(String),
    /// the certificate file for the specified domain is missing (but a key
    /// file was present)
    MissingCert(String),
    /// neither a key file nor a certificate file were present for the given
    /// domain (but a folder was present)
    EmptyDomain(String),
}

impl Display for CertLoadError {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::NoReadCertDir => write!(f, "Could not read from certificate directory."),
            Self::Empty => write!(
                f,
                "No keys or certificates were found in the given directory.\nSpecify the --hostname option to generate these automatically."
            ),
            Self::BadKey(domain, err) => {
                write!(f, "The key file for {domain} is malformed: {err:?}")
            }
            Self::MissingKey(domain) => write!(f, "The key file for {domain} is missing."),
            Self::MissingCert(domain) => {
                write!(f, "The certificate file for {domain} is missing.")
            }
            Self::EmptyDomain(domain) => write!(
                f,
                "A folder for {domain} exists, but there is no certificate or key file."
            ),
        }
    }
}

impl std::error::Error for CertLoadError {}

fn load_domain(certs_dir: &Path, domain: String) -> Result<CertifiedKey, CertLoadError> {
    let mut path = certs_dir.to_path_buf();
    path.push(&domain);
    // load certificate from file
    path.push(CERT_FILE_NAME);
    if !path.is_file() {
        return Err(if !path.with_file_name(KEY_FILE_NAME).is_file() {
            CertLoadError::EmptyDomain(domain)
        } else {
            CertLoadError::MissingCert(domain)
        });
    }
    let cert = CertificateDer::from(
        std::fs::read(&path).map_err(|_| CertLoadError::MissingCert(domain.clone()))?,
    );

    // load key from file
    path.set_file_name(KEY_FILE_NAME);
    let Ok(der) = std::fs::read(&path) else {
        return Err(CertLoadError::MissingKey(domain));
    };

    // transform key to correct format
    let key = der_to_private_key(&der).map_err(|e| CertLoadError::BadKey(domain.clone(), e))?;

    Ok(CertifiedKey::new(vec![cert], key))
}

/// We don't know the key type of the private key DER file, so try each
/// possible type until we find one that works.
///
/// We should probably stop doing this and use a PEM file instead:
/// <https://github.com/rustls/rustls/issues/1661>
fn der_to_private_key(der: &[u8]) -> Result<Arc<dyn SigningKey>, rustls::Error> {
    let keys = [
        PrivateKeyDer::Pkcs1(pki_types::PrivatePkcs1KeyDer::from(der)),
        PrivateKeyDer::Sec1(pki_types::PrivateSec1KeyDer::from(der)),
        PrivateKeyDer::Pkcs8(pki_types::PrivatePkcs8KeyDer::from(der)),
    ];

    let mut err = None;
    for key in keys {
        match any_supported_type(&key) {
            Ok(key) => return Ok(key),
            Err(e) => err = Some(e),
        }
    }
    Err(err.unwrap())
}

impl CertStore {
    /// Load certificates from a certificate directory.
    /// Certificates should be stored in a folder for each hostname, for example
    /// the certificate and key for `example.com` should be in the files
    /// `certs_dir/example.com/{cert.der,key.der}` respectively.
    ///
    /// If there are `cert.der` and `key.der` directly in `certs_dir`, these
    /// will be loaded as default certificates.
    pub fn load_from(certs_dir: &Path) -> Result<Self, CertLoadError> {
        // load all certificates from directories
        let mut certs = vec![];

        // Try to load fallback certificate and key directly from the top level
        // certificate directory.
        match load_domain(certs_dir, String::new()) {
            Err(CertLoadError::EmptyDomain(_)) => { /* there are no fallback keys */ }
            Err(CertLoadError::Empty) | Err(CertLoadError::NoReadCertDir) => unreachable!(),
            Err(CertLoadError::BadKey(_, e)) => {
                return Err(CertLoadError::BadKey("fallback".to_string(), e));
            }
            Err(CertLoadError::MissingKey(_)) => {
                return Err(CertLoadError::MissingKey("fallback".to_string()));
            }
            Err(CertLoadError::MissingCert(_)) => {
                return Err(CertLoadError::MissingCert("fallback".to_string()));
            }
            // For the fallback keys there is no domain name to verify them
            // against, so we can skip that step and only have to do it for the
            // other keys below.
            Ok(key) => certs.push((String::new(), Arc::new(key))),
        }

        for file in certs_dir
            .read_dir()
            .or(Err(CertLoadError::NoReadCertDir))?
            .filter_map(Result::ok)
            .filter(|x| x.path().is_dir())
        {
            let path = file.path();

            // the filename should be the domain name
            let filename = path
                .file_name()
                .and_then(OsStr::to_str)
                .unwrap()
                .to_string();

            let key = load_domain(certs_dir, filename.clone())?;

            certs.push((filename, Arc::new(key)));
        }

        if certs.is_empty() {
            return Err(CertLoadError::Empty);
        }

        certs.sort_unstable_by(|(a, _), (b, _)| {
            // Try to match as many domain segments as possible. If one is a
            // substring of the other, the `zip` will only compare the smaller
            // length of either a or b and the for loop will not decide.
            for (a_part, b_part) in a.split('.').rev().zip(b.split('.').rev()) {
                if a_part != b_part {
                    // Here we have to make sure that the empty string will
                    // always be sorted to the end, so we reverse the usual
                    // ordering of str.
                    return a_part.cmp(b_part).reverse();
                }
            }
            // Sort longer domains first.
            a.len().cmp(&b.len()).reverse()
        });

        log::debug!(
            "certs loaded for {:?}",
            certs.iter().map(|t| &t.0).collect::<Vec<_>>()
        );

        Ok(Self { certs })
    }

    /// Checks if a certificate fitting a specific domain has been loaded.
    /// The same rules about using a certificate at the level above apply.
    pub fn has_domain(&self, domain: &str) -> bool {
        self.certs.iter().any(|(s, _)| domain.ends_with(s))
    }
}

impl ResolvesServerCert for CertStore {
    fn resolve(&self, client_hello: ClientHello<'_>) -> Option<Arc<CertifiedKey>> {
        if let Some(name) = client_hello.server_name() {
            let name: &str = name;
            // The certificate list is sorted so the longest match will always
            // appear first. We have to find the first that is either this
            // domain or a parent domain of the current one.
            self.certs
                .iter()
                .find(|(s, _)| name.ends_with(s))
                // only the key is interesting
                .map(|(_, k)| k)
                .cloned()
        } else {
            // Fallback to default cert.
            // * must exist in the `.certificates` root
            // * CN value can be any
            self.certs
                .iter()
                .find(|(domain, _)| domain.is_empty())
                .map(|(_, key)| Arc::clone(key))
        }
    }
}