use std::{collections::{BTreeMap, HashMap}, error::Error, io::ErrorKind};
use std::borrow::Cow;
use std::collections::HashSet;
use std::fmt::Display;
use std::path::{Path, PathBuf};

use log::{debug, info, trace, warn};
use sha1_smol::Digest;
use tokio::{fs, io};
use crate::util;
use crate::version::{*, manifest::*};

use super::constants::*;

struct RemoteVersionList {
    versions: HashMap<String, VersionManifestVersion>,
    latest: LatestVersions
}

impl RemoteVersionList {
    async fn new() -> Result<RemoteVersionList, Box<dyn Error>> {
        debug!("Looking up remote version manifest.");
        let text = reqwest::get(URL_VERSION_MANIFEST).await?.error_for_status()?.text().await?;
        debug!("Parsing version manifest.");
        let manifest: VersionManifest = serde_json::from_str(text.as_str())?;

        let mut versions = HashMap::new();
        for v in manifest.versions {
            versions.insert(v.id.clone(), v);
        }

        debug!("Done loading remote versions!");
        Ok(RemoteVersionList {
            versions,
            latest: manifest.latest
        })
    }

    async fn download_version(&self, ver: &VersionManifestVersion, path: &Path) -> Result<CompleteVersion, Box<dyn Error>> {
        // ensure parent directory exists
        info!("Downloading version {}.", ver.id);
        match tokio::fs::create_dir_all(path.parent().expect("version .json has no parent (impossible)")).await {
            Err(e) => {
                if e.kind() != ErrorKind::AlreadyExists {
                    warn!("failed to create {} parent dirs: {e}", path.to_string_lossy());
                    return Err(e.into());
                }
            },
            Ok(()) => {}
        }

        // download it
        let ver_text = reqwest::get(ver.url.as_str()).await?.error_for_status()?.text().await?;
        
        debug!("Validating downloaded {}...", ver.id);
        // make sure it's valid
        util::verify_sha1(ver.sha1, ver_text.as_str())
            .map_err::<Box<dyn Error>, _>(|e| format!("downloaded version {} has wrong hash! (expect {}, got {})", ver.id.as_str(), &ver.sha1, e).as_str().into())?;
        
        // make sure it's well-formed
        let cver: CompleteVersion = serde_json::from_str(ver_text.as_str())?;

        debug!("Saving version {}...", ver.id);

        // write it out
        tokio::fs::write(path, ver_text).await.map_err(|e| {
            warn!("Failed to save version {}: {}", ver.id, e);
            e
        })?;

        info!("Done downloading and verifying {}!", ver.id);

        Ok(cver)
    }
}

struct LocalVersionList {
    versions: BTreeMap<String, CompleteVersion>
}

#[derive(Debug)]
enum LocalVersionError {
    Sha1Mismatch { exp: Digest, got: Digest },
    VersionMismatch { fname: String, json: String },
    Unknown(Box<dyn Error>)
}

impl Display for LocalVersionError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            LocalVersionError::Sha1Mismatch { exp, got } => {
                write!(f, "sha1 mismatch (exp {exp}, got {got})")
            },
            LocalVersionError::VersionMismatch { fname, json } => {
                write!(f, "version ID mismatch (filename {fname}, json {json})")
            },
            LocalVersionError::Unknown(err) => {
                write!(f, "unknown version error: {err}")
            }
        }
    }
}

impl Error for LocalVersionError {}

impl LocalVersionList {
    async fn load_version(path: &Path, sha1: Option<Digest>) -> Result<CompleteVersion, LocalVersionError> {
        // grumble grumble I don't like reading in the whole file at once
        info!("Loading local version at {}.", path.display());
        let ver = tokio::fs::read_to_string(path).await.map_err(|e| LocalVersionError::Unknown(Box::new(e)))?;
        if let Some(digest_exp) = sha1 {
            debug!("Verifying local version {}.", path.display());
            util::verify_sha1(digest_exp, ver.as_str())
                .map_err(|got| {
                    warn!("Local version sha1 mismatch: {} (exp: {}, got: {})", path.display(), digest_exp, got);
                    LocalVersionError::Sha1Mismatch { exp: digest_exp.to_owned(), got }
                })?;
        }

        let ver: CompleteVersion = serde_json::from_str(ver.as_str()).map_err(|e| {
            warn!("Invalid version JSON {}: {}", path.display(), e);
            LocalVersionError::Unknown(Box::new(e))
        })?;

        let fname_id = path.file_stem()
            .expect("tried to load a local version with no path") // should be impossible
            .to_str()
            .expect("tried to load a local version with invalid UTF-8 filename"); // we already checked if the filename is valid UTF-8 at this point

        if fname_id == ver.id.as_str() {
            info!("Loaded local version {}.", ver.id);
            Ok(ver)
        } else {
            warn!("Local version {} has a version ID conflict (filename: {}, json: {})!", path.display(), fname_id, ver.id);
            Err(LocalVersionError::VersionMismatch { fname: fname_id.to_owned(), json: ver.id })
        }
    }

    async fn load_versions(home: &Path, skip: impl Fn(&str) -> bool) -> Result<LocalVersionList, Box<dyn Error>> {
        info!("Loading local versions.");
        let mut rd = tokio::fs::read_dir(home).await?;
        let mut versions = BTreeMap::new();

        while let Some(ent) = rd.next_entry().await? {
            if !ent.file_type().await?.is_dir() { continue; }

            // when the code is fugly
            let path = match ent.file_name().to_str() {
                Some(s) => {
                    if skip(s) {
                        debug!("Skipping local version {s} because (I assume) it is remotely tracked.");
                        continue
                    }

                    /* FIXME: once https://github.com/rust-lang/rust/issues/127292 is closed,
                     *  use add_extension to avoid extra heap allocations (they hurt my feelings) */
                    let mut path = ent.path();

                    // can't use set_extension since s might contain a . (like 1.8.9)
                    path.push(format!("{s}.json"));
                    path
                },

                /* We just ignore directories with names that contain invalid unicode. Unfortunately, the laucher
                 * will not be supporting such custom versions. Name your version something sensible please. */
                None => {
                    warn!("Ignoring a local version {} because its id contains invalid unicode.", ent.file_name().to_string_lossy());
                    continue
                }
            };

            match Self::load_version(&path, None).await {
                Ok(v) => {
                    versions.insert(v.id.clone(), v);
                },
                Err(e) => {
                    // FIXME: just display the filename without to_string_lossy when https://github.com/rust-lang/rust/issues/120048 is closed
                    warn!("Ignoring local version {}: {e}", ent.file_name().to_string_lossy());
                }
            }
        }

        info!("Loaded {} local version(s).", versions.len());
        Ok(LocalVersionList { versions })
    }
}

pub struct VersionList {
    remote: Option<RemoteVersionList>,
    local: LocalVersionList,
    home: PathBuf
}

pub enum VersionResult<'a> {
    Complete(&'a CompleteVersion),
    Remote(&'a VersionManifestVersion),
    None
}

impl<'a> From<&'a CompleteVersion> for VersionResult<'a> {
    fn from(value: &'a CompleteVersion) -> Self {
        Self::Complete(value)
    }
}

impl<'a> From<&'a VersionManifestVersion> for VersionResult<'a> {
    fn from(value: &'a VersionManifestVersion) -> Self {
        Self::Remote(value)
    }
}

impl<'a, T: Into<VersionResult<'a>>> From<Option<T>> for VersionResult<'a> {
    fn from(value: Option<T>) -> Self {
        value.map_or(VersionResult::None, |v| v.into())
    }
}

#[derive(Debug)]
pub enum VersionResolveError {
    InheritanceLoop(String),
    MissingVersion(String),
    Unknown(Box<dyn Error>)
}

impl Display for VersionResolveError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            VersionResolveError::InheritanceLoop(s) => write!(f, "inheritance loop (saw {s} twice)"),
            VersionResolveError::MissingVersion(s) => write!(f, "unknown version {s}"),
            VersionResolveError::Unknown(err) => write!(f, "unknown error: {err}")
        }
    }
}

impl Error for VersionResolveError {}



impl VersionList {
    async fn create_dir_for(home: &Path) -> Result<(), io::Error> {
        debug!("Creating versions directory.");
        match fs::create_dir(home).await {
            Ok(_) => Ok(()),
            Err(e) => match e.kind() {
                ErrorKind::AlreadyExists => Ok(()),
                _ => {
                    debug!("failed to create version home: {}", e);
                    Err(e)
                }
            }
        }
    }

    pub async fn online(home: &Path) -> Result<VersionList, Box<dyn Error>> {
        Self::create_dir_for(home).await?;

        let remote = RemoteVersionList::new().await?;
        let local = LocalVersionList::load_versions(home.as_ref(), |s| remote.versions.contains_key(s)).await?;

        Ok(VersionList {
            remote: Some(remote),
            local,
            home: home.to_path_buf()
        })
    }

    pub async fn offline(home: &Path) -> Result<VersionList, Box<dyn Error>> {
        Self::create_dir_for(home).await?;

        let local = LocalVersionList::load_versions(home, |_| false).await?;

        Ok(VersionList {
            remote: None,
            local,
            home: home.to_path_buf()
        })
    }

    pub fn is_online(&self) -> bool {
        self.remote.is_some()
    }

    pub fn get_version_lazy(&self, id: &str) -> VersionResult {
        self.remote.as_ref()
            .map_or_else(|| self.local.versions.get(id).into(), |r| r.versions.get(id).into())
    }

    pub fn get_remote_version(&self, id: &str) -> Option<&VersionManifestVersion> {
        let remote = self.remote.as_ref().expect("get_remote_version called in offline mode!");

        remote.versions.get(id)
    }

    pub async fn load_remote_version(&self, ver: &VersionManifestVersion) -> Result<CompleteVersion, Box<dyn Error>> {
        let remote = self.remote.as_ref().expect("load_remote_version called in offline mode!");

        let id = ver.id.as_str();
        let mut ver_path = self.home.join(id);
        ver_path.push(format!("{id}.json"));

        debug!("Loading local copy of remote version {}", ver.id);

        match LocalVersionList::load_version(ver_path.as_path(), Some(ver.sha1)).await {
            Ok(v) => return Ok(v),
            Err(e) => {
                info!("Redownloading {id}, since the local copy could not be loaded: {e}");
            }
        }

        remote.download_version(ver, ver_path.as_path()).await
    }

    pub async fn resolve_version<'v>(&self, ver: &'v CompleteVersion) -> Result<Cow<'v, CompleteVersion>, VersionResolveError> {
        let mut seen: HashSet<String> = HashSet::new();
        seen.insert(ver.id.clone());

        let Some(inherit) = ver.inherits_from.as_ref() else {
            return Ok(Cow::Borrowed(ver));
        };

        if *inherit == ver.id {
            warn!("Version {} directly inherits from itself!", ver.id);
            return Err(VersionResolveError::InheritanceLoop(ver.id.clone()));
        }

        debug!("Resolving version inheritance: {} (inherits from {})", ver.id, inherit);

        let mut ver = ver.clone();
        let mut inherit = inherit.clone();

        loop {
            if seen.insert(inherit.clone()) {
                warn!("Version inheritance loop detected in {}: {} transitively inherits from itself.", ver.id, inherit);
                return Err(VersionResolveError::InheritanceLoop(inherit));
            }
            
            let inherited_ver = match self.get_version_lazy(inherit.as_str()) {
                VersionResult::Complete(v) => Cow::Borrowed(v),
                VersionResult::Remote(v) =>
                    Cow::Owned(self.load_remote_version(v).await.map_err(|e| VersionResolveError::Unknown(e))?),
                VersionResult::None => {
                    warn!("Cannot resolve version {}, it inherits an unknown version {inherit}", ver.id);
                    return Err(VersionResolveError::MissingVersion(inherit));
                }
            };

            ver.apply_child(inherited_ver.as_ref());
            
            let Some(new_inherit) = inherited_ver.inherits_from.as_ref() else {
                break
            };
            
            inherit.replace_range(.., new_inherit.as_str());
        }

        Ok(Cow::Owned(ver))
    }
}