//! A WebDAV filesystem provider for Memona, speaking only the host's HTTP.

mod dav;
mod sdk;

use sdk::host;
use sdk::{
  Capabilities, ChangePage, Entry, EntryKind, ErrorCode, FileInfo, Guest,
  GuestReader, GuestWriter, ListPage, Locator, Reader, Setting, WriteCondition,
  Writer,
};
use std::cell::{Cell, RefCell};

/// The PROPFIND body. Asking for named properties rather than allprop keeps
/// replies small on servers that store a great many of them.
const PROPS: &str = concat!(
  r#"<?xml version="1.0" encoding="utf-8"?>"#,
  r#"<d:propfind xmlns:d="DAV:"><d:prop>"#,
  "<d:resourcetype/><d:getcontentlength/><d:getetag/>",
  "</d:prop></d:propfind>",
);
const MAX_CHUNK: u32 = 1024 * 1024;

struct Session {
  root: String,
  authorization: Option<String>,
}

impl Session {
  fn from_config(config: &[(String, Setting)]) -> Result<Self, ErrorCode> {
    let lookup = |wanted: &str| {
      config
        .iter()
        .find(|(key, _)| key == wanted)
        .and_then(|(_, value)| text(Some(value.clone())))
    };
    let root = lookup("url").ok_or(ErrorCode::InvalidInput)?;
    if !root.starts_with("https://") && !root.starts_with("http://") {
      return Err(ErrorCode::InvalidInput);
    }
    let authorization = lookup("username").map(|username| {
      dav::basic(&username, &lookup("password").unwrap_or_default())
    });
    Ok(Self {
      root,
      authorization,
    })
  }
}

thread_local! {
  static SESSION: RefCell<Option<Session>> = const { RefCell::new(None) };
}

struct Webdav;

fn text(setting: Option<Setting>) -> Option<String> {
  match setting {
    Some(Setting::Text(value)) => {
      let value = value.trim().to_owned();
      (!value.is_empty()).then_some(value)
    }
    Some(Setting::Secret(value)) => Some(value),
    Some(Setting::Toggle(_)) | None => None,
  }
}

fn transport(error: host::HostError) -> ErrorCode {
  match error {
    host::HostError::Denied => ErrorCode::Denied,
    host::HostError::Cancelled => ErrorCode::Cancelled,
    host::HostError::InvalidInput => ErrorCode::InvalidInput,
    host::HostError::Internal => ErrorCode::Internal,
    host::HostError::Unavailable => ErrorCode::Unavailable,
  }
}

/// The path a locator names, the only way the contract addresses a resource.
fn route(target: &Locator) -> String {
  let Locator::Path(path) = target;
  path.clone()
}

fn session<T>(
  read: impl FnOnce(&Session) -> Result<T, ErrorCode>,
) -> Result<T, ErrorCode> {
  SESSION.with_borrow(|session| match session {
    Some(session) => read(session),
    None => Err(ErrorCode::Internal),
  })
}

/// Starts one request with this connection's credentials attached.
fn begin(
  method: host::Method,
  path: &str,
  extra: Vec<host::Header>,
) -> Result<host::Exchange, ErrorCode> {
  let (url, authorization) = session(|session| {
    Ok((dav::url(&session.root, path), session.authorization.clone()))
  })?;
  let mut headers = extra;
  if let Some(value) = authorization {
    headers.push(host::Header {
      name: "authorization".to_owned(),
      value,
    });
  }
  host::http(&host::Request {
    method,
    url,
    headers,
  })
  .map_err(transport)
}

/// Runs one request that carries a small body and answers a small body.
fn exchange(
  method: host::Method,
  path: &str,
  extra: Vec<host::Header>,
  body: &[u8],
) -> Result<Vec<u8>, ErrorCode> {
  let call = begin(method, path, extra)?;
  if !body.is_empty() {
    call.push(body).map_err(transport)?;
  }
  let head = call.respond().map_err(transport)?;
  dav::outcome(head.status)?;
  let mut data = Vec::new();
  loop {
    let chunk = call.pull(MAX_CHUNK).map_err(transport)?;
    if chunk.is_empty() {
      return Ok(data);
    }
    data.extend_from_slice(&chunk);
    if data.len() > 8 * 1024 * 1024 {
      return Err(ErrorCode::Unavailable);
    }
  }
}

fn depth(value: &str) -> host::Header {
  host::Header {
    name: "depth".to_owned(),
    value: value.to_owned(),
  }
}

fn content_type() -> host::Header {
  host::Header {
    name: "content-type".to_owned(),
    value: "application/xml; charset=utf-8".to_owned(),
  }
}

/// Asks the server about one path, and its children when `depth` is 1.
fn describe(
  path: &str,
  depth_value: &str,
) -> Result<Vec<dav::Found>, ErrorCode> {
  let body = exchange(
    host::Method::Propfind,
    path,
    vec![depth(depth_value), content_type()],
    PROPS.as_bytes(),
  )?;
  dav::multistatus(&body)
}

fn info_of(found: &dav::Found) -> FileInfo {
  FileInfo {
    kind: if found.collection {
      EntryKind::Directory
    } else {
      EntryKind::File
    },
    size: found.length,
    revision: found.etag.clone(),
  }
}

/// Reads one file through a GET that started at an offset.
struct DavReader {
  call: host::Exchange,
  revision: Option<String>,
  body: RefCell<dav::Body>,
}

impl GuestReader for DavReader {
  fn pull(&self, limit: u32) -> Result<Vec<u8>, ErrorCode> {
    self
      .body
      .borrow_mut()
      .pull(limit.min(MAX_CHUNK) as usize, |wanted| {
        self.call.pull(wanted).map_err(transport)
      })
  }
  fn revision(&self) -> Option<String> {
    self.revision.clone()
  }
}

/// Writes one file through a PUT whose body is pushed in pieces.
struct DavWriter {
  call: host::Exchange,
  done: RefCell<bool>,
  written: Cell<u64>,
}

impl GuestWriter for DavWriter {
  fn push(&self, data: Vec<u8>) -> Result<(), ErrorCode> {
    if *self.done.borrow() {
      return Err(ErrorCode::InvalidInput);
    }
    self.call.push(&data).map_err(transport)?;
    self.written.set(self.written.get() + data.len() as u64);
    Ok(())
  }
  /// One call is enough: the request body is already complete, so the response
  /// head settles the write. The etag it returns, when the server sends one,
  /// is the revision a later conditional write compares against.
  fn finish(&self) -> Result<Option<FileInfo>, ErrorCode> {
    if *self.done.borrow() {
      return Err(ErrorCode::InvalidInput);
    }
    *self.done.borrow_mut() = true;
    let head = self.call.respond().map_err(transport)?;
    dav::outcome(head.status)?;
    let revision = head
      .headers
      .iter()
      .find(|header| header.name.eq_ignore_ascii_case("etag"))
      .map(|header| header.value.clone());
    Ok(Some(FileInfo {
      kind: EntryKind::File,
      size: self.written.get(),
      revision,
    }))
  }
}

impl Guest for Webdav {
  type Reader = DavReader;
  type Writer = DavWriter;

  fn open(
    _provider_id: String,
    config: Vec<(String, Setting)>,
  ) -> Result<Capabilities, ErrorCode> {
    SESSION.set(None);
    SESSION.set(Some(Session::from_config(&config)?));
    let probe = Self::stat(Locator::Path("/".to_owned())).and_then(|info| {
      if matches!(info.kind, EntryKind::Directory) {
        Ok(())
      } else {
        Err(ErrorCode::InvalidInput)
      }
    });
    if let Err(error) = probe {
      SESSION.set(None);
      return Err(error);
    }
    Ok(Capabilities {
      write: true,
      mkdir: true,
      rename: true,
      remove: true,
      // A server that returns etags supports If-Match, and one that does not
      // will refuse the header rather than silently overwrite.
      conditional_write: true,
      watch: false,
      virtual_directories: false,
    })
  }

  fn close() {
    SESSION.set(None);
  }

  fn stat(target: Locator) -> Result<FileInfo, ErrorCode> {
    let found = describe(&route(&target), "0")?;
    let entry = found.first().ok_or(ErrorCode::NotFound)?;
    Ok(info_of(entry))
  }

  /// WebDAV has no cursor of its own, so one PROPFIND returns the whole
  /// directory and the page is taken from the front of it.
  fn list(
    target: Locator,
    cursor: Option<String>,
    limit: u32,
  ) -> Result<ListPage, ErrorCode> {
    let path = route(&target);
    let start: usize = match cursor {
      Some(value) => value.parse().map_err(|_| ErrorCode::InvalidInput)?,
      None => 0,
    };
    let found = describe(&path, "1")?;
    let url = session(|session| Ok(dav::url(&session.root, &path)))?;
    let children: Vec<_> = dav::children(&found, &url)?
      .iter()
      .map(|entry| Entry {
        name: dav::name_of(&entry.href),
        info: info_of(entry),
      })
      .collect();
    let end = start.saturating_add(limit as usize).min(children.len());
    let entries = children.get(start..end).unwrap_or_default().to_vec();
    Ok(ListPage {
      entries,
      cursor: (end < children.len()).then(|| end.to_string()),
    })
  }

  /// A GET on a collection answers with whatever page the server likes to
  /// show, so the path is checked to be a file before any byte is read.
  fn open_read(target: Locator, offset: u64) -> Result<Reader, ErrorCode> {
    let path = route(&target);
    let found = describe(&path, "0")?;
    let entry = found.first().ok_or(ErrorCode::NotFound)?;
    if entry.collection {
      return Err(ErrorCode::InvalidInput);
    }
    let call = begin(
      host::Method::Get,
      &path,
      vec![host::Header {
        name: "range".to_owned(),
        value: format!("bytes={offset}-"),
      }],
    )?;
    let head = call.respond().map_err(transport)?;
    let body = if dav::range_outcome(head.status)? {
      dav::Body::default()
    } else {
      dav::Body::empty()
    };
    // The listing's etag is the one stat and list report, and the strong form
    // a later conditional write must send; the GET's may be a weak variant.
    let revision = entry.etag.clone().or_else(|| {
      head
        .headers
        .iter()
        .find(|header| header.name.eq_ignore_ascii_case("etag"))
        .map(|header| header.value.clone())
    });
    Ok(Reader::new(DavReader {
      call,
      revision,
      body: RefCell::new(body),
    }))
  }

  fn open_write(
    target: Locator,
    condition: WriteCondition,
    _size_hint: Option<u64>,
  ) -> Result<Writer, ErrorCode> {
    let path = route(&target);
    let guard = match condition {
      WriteCondition::CreateOnly => Some(host::Header {
        name: "if-none-match".to_owned(),
        value: "*".to_owned(),
      }),
      WriteCondition::IfMatch(revision) => Some(host::Header {
        name: "if-match".to_owned(),
        value: revision,
      }),
      WriteCondition::Overwrite => None,
    };
    let call = begin(host::Method::Put, &path, guard.into_iter().collect())?;
    Ok(Writer::new(DavWriter {
      call,
      done: RefCell::new(false),
      written: Cell::new(0),
    }))
  }

  fn mkdir(target: Locator) -> Result<FileInfo, ErrorCode> {
    let path = route(&target);
    exchange(host::Method::Mkcol, &path, Vec::new(), &[])?;
    Ok(FileInfo {
      kind: EntryKind::Directory,
      size: 0,
      revision: None,
    })
  }

  fn rename(
    target: Locator,
    destination: String,
  ) -> Result<FileInfo, ErrorCode> {
    let path = route(&target);
    let to = session(|session| Ok(dav::url(&session.root, &destination)))?;
    exchange(
      host::Method::Move,
      &path,
      vec![
        host::Header {
          name: "destination".to_owned(),
          value: to,
        },
        // Never clobber an existing target: a rename that would replace one is
        // a conflict for the caller to resolve.
        host::Header {
          name: "overwrite".to_owned(),
          value: "F".to_owned(),
        },
      ],
      &[],
    )?;
    Self::stat(Locator::Path(destination))
  }

  /// DELETE takes a whole collection at once, so a directory is looked at
  /// first and refused unless it is empty.
  fn remove(target: Locator) -> Result<(), ErrorCode> {
    let path = route(&target);
    let url = session(|session| Ok(dav::url(&session.root, &path)))?;
    dav::removable(&describe(&path, "1")?, &url)?;
    exchange(host::Method::Delete, &path, Vec::new(), &[])?;
    Ok(())
  }

  fn poll(_cursor: Option<String>) -> Result<ChangePage, ErrorCode> {
    Err(ErrorCode::Unsupported)
  }
}

sdk::export!(Webdav with_types_in sdk);

#[cfg(test)]
mod tests {
  use super::{SESSION, Session, Webdav};
  use crate::sdk::{ErrorCode, Guest, Setting};

  #[tokio::test]
  async fn passwords_keep_leading_and_trailing_whitespace()
  -> Result<(), ErrorCode> {
    let config = vec![
      (
        "url".to_owned(),
        Setting::Text("https://dav.example/".to_owned()),
      ),
      ("username".to_owned(), Setting::Text("user".to_owned())),
      (
        "password".to_owned(),
        Setting::Secret(" pass word \t".to_owned()),
      ),
    ];
    assert_eq!(
      Session::from_config(&config)?.authorization.as_deref(),
      Some("Basic dXNlcjogcGFzcyB3b3JkIAk=")
    );
    Ok(())
  }

  #[tokio::test]
  async fn a_username_without_a_password_setting_sends_basic_auth()
  -> Result<(), ErrorCode> {
    let config = vec![
      (
        "url".to_owned(),
        Setting::Text("https://dav.example/".to_owned()),
      ),
      ("username".to_owned(), Setting::Text("user".to_owned())),
    ];
    assert_eq!(
      Session::from_config(&config)?.authorization.as_deref(),
      Some("Basic dXNlcjo=")
    );
    Ok(())
  }

  #[tokio::test]
  async fn no_username_keeps_the_connection_anonymous() -> Result<(), ErrorCode>
  {
    let config = vec![
      (
        "url".to_owned(),
        Setting::Text("https://dav.example/".to_owned()),
      ),
      ("password".to_owned(), Setting::Secret("unused".to_owned())),
    ];
    assert!(Session::from_config(&config)?.authorization.is_none());
    Ok(())
  }

  #[tokio::test]
  async fn a_failed_reopen_discards_the_previous_connection() {
    SESSION.set(Some(Session {
      root: "https://dav.example/files/".to_owned(),
      authorization: None,
    }));
    let result = Webdav::open("webdav".to_owned(), Vec::new());
    assert_eq!(result.err(), Some(ErrorCode::InvalidInput));
    assert!(SESSION.with_borrow(Option::is_none));
  }
}