use crate::{
    nd, Arc, BTreeMap, CompactFile, Data, HashMap, HashSet, Mutex, RwLock, SaveOp, Storage,
};
use std::ops::Bound::Included;

/// ```Arc<Mutex<PageInfo>>```
pub type PageInfoPtr = Arc<Mutex<PageInfo>>;

/// Cached information about a logical page.
pub struct PageInfo {
    pub current: Option<Data>,
    pub history: BTreeMap<u64, Data>,
}

impl PageInfo {
    /// Construct a new PageInfo.
    pub fn new() -> PageInfoPtr {
        Arc::new(Mutex::new(Self {
            current: None,
            history: BTreeMap::new(),
        }))
    }

    /// Get the Data for the page, checking history if not a writer.
    /// Reads Data from file if necessary.
    pub fn get(&mut self, lpnum: u64, a: &AccessPagedData) -> Data {
        if !a.writer {
            if let Some((_k, v)) = self
                .history
                .range((Included(&a.time), Included(&u64::MAX)))
                .next()
            {
                return v.clone();
            }
        }

        if let Some(p) = &self.current {
            return p.clone();
        }

        // Get data from file.
        let file = a.spd.file.read().unwrap();
        let data = file.get_page(lpnum);
        self.current = Some(data.clone());
        data
    }

    /// Set the page data, updating the history using the specified time and current data.
    pub fn set(&mut self, time: u64, data: Data) {
        if let Some(old) = self.current.take() {
            self.history.insert(time, old);
        }
        self.current = Some(data);
    }

    fn trim(&mut self, to: u64) {
        while let Some(&f) = self.history.keys().next() {
            if f >= to {
                break;
            }
            self.history.remove(&f);
        }
    }
}

/// Central store of data.
pub struct Stash {
    /// Write time - number of writes.
    pub time: u64,
    /// Page number -> page info.
    pub pages: HashMap<u64, PageInfoPtr>,
    /// Time -> reader count.
    pub readers: BTreeMap<u64, usize>,
    /// Time -> set of page numbers.
    pub updates: BTreeMap<u64, HashSet<u64>>,
}

impl Stash {
    /// Set the value of the specified page for the current time.
    pub fn set(&mut self, lpnum: u64, data: Data) {
        let time = self.time;
        let u = self.updates.entry(time).or_insert_with(HashSet::new);
        if u.insert(lpnum) {
            let p = self.pages.entry(lpnum).or_insert_with(PageInfo::new);
            p.lock().unwrap().set(time, data);
        }
    }

    /// Get the PageInfoPtr for the specified page.  
    pub fn get(&mut self, lpnum: u64) -> PageInfoPtr {
        let p = self.pages.entry(lpnum).or_insert_with(PageInfo::new);
        p.clone()
    }

    /// Register that there is a client reading the database. The result is the current time.
    pub fn begin_read(&mut self) -> u64 {
        let time = self.time;
        // println!("Stash begin read time={}", time);
        let n = self.readers.entry(time).or_insert(0);
        *n += 1;
        time
    }

    /// Register that the read at the specified time has ended. Stashed pages may be freed.
    pub fn end_read(&mut self, time: u64) {
        // println!("Stash end read time={}", time);
        let n = self.readers.get_mut(&time).unwrap();
        *n -= 1;
        if *n == 0 {
            self.readers.remove(&time);
            self.trim();
        }
    }

    /// Register that an update operation has completed. Time is incremented.
    /// Stashed pages may be freed.
    pub fn end_write(&mut self) -> usize {
        // println!("Stash tick time={}", self.time);
        let result = if let Some(u) = self.updates.get(&self.time) {
            u.len()
        } else {
            0
        };
        self.time += 1;
        self.trim();
        result
    }

    /// Trim due to a read or write ending.
    fn trim(&mut self) {
        // rt is time of first remaining reader.
        let rt = *self.readers.keys().next().unwrap_or(&self.time);
        // wt is time of first remaining update.
        while let Some(&wt) = self.updates.keys().next() {
            if wt >= rt {
                break;
            }
            for lpnum in self.updates.remove(&wt).unwrap() {
                let p = self.pages.get(&lpnum).unwrap();
                // println!("Stash trim page {}", lpnum);
                p.lock().unwrap().trim(rt);
            }
        }
    }
}

impl Default for Stash {
    fn default() -> Self {
        Self {
            time: 0,
            pages: HashMap::new(),
            readers: BTreeMap::new(),
            updates: BTreeMap::new(),
        }
    }
}

/// Allows logical database pages to be shared to allow concurrent readers.
pub struct SharedPagedData {
    pub stash: RwLock<Stash>,
    pub file: RwLock<CompactFile>,
    pub sp_size: usize,
    pub ep_size: usize,
}

impl SharedPagedData {
    /// Construct SharedPageData based on specified underlying storage.
    pub fn new(file: Box<dyn Storage>) -> Self {
        let file = CompactFile::new(file, 400, 1024);
        // Note : if it's not a new file, sp_size and ep_size are read from file header.
        let sp_size = file.sp_size;
        let ep_size = file.ep_size;
        Self {
            stash: RwLock::new(Stash::default()),
            file: RwLock::new(file),
            sp_size,
            ep_size,
        }
    }

    /// Access to a virtual read-only copy of the database logical pages.
    pub fn open_read(self: &Arc<SharedPagedData>) -> AccessPagedData {
        let mut stash = self.stash.write().unwrap();
        AccessPagedData {
            writer: false,
            time: stash.begin_read(),
            spd: self.clone(),
        }
    }

    /// Write access to the database logical pages.
    pub fn open_write(self: &Arc<SharedPagedData>) -> AccessPagedData {
        AccessPagedData {
            writer: true,
            time: 0,
            spd: self.clone(),
        }
    }
}

/// Access to shared paged data.
pub struct AccessPagedData {
    pub writer: bool,
    pub time: u64,
    pub spd: Arc<SharedPagedData>,
}

impl AccessPagedData {
    /// Get the Data for the specified page.
    pub fn get_page(&self, lpnum: u64) -> Data {
        // Get PageInfoPtr for the specified page.
        let pinfo = self.spd.stash.write().unwrap().get(lpnum);

        // Lock the Mutex for the page.
        let mut pinfo = pinfo.lock().unwrap();

        // Read the page data.
        pinfo.get(lpnum, self)
    }

    /// Set the data of the specified page.
    pub fn set_page(&self, lpnum: u64, data: Data) {
        debug_assert!(self.writer);

        // First update the stash ( ensures any readers will not attempt to read the file ).
        self.spd.stash.write().unwrap().set(lpnum, data.clone());

        // Write data to underlying file.
        self.spd.file.write().unwrap().set_page(lpnum, data);
    }

    /// Is the underlying file new (so needs to be initialised ).
    pub fn is_new(&self) -> bool {
        self.writer && self.spd.file.read().unwrap().is_new()
    }

    /// Check whether compressing a page is worthwhile.
    pub fn compress(&self, size: usize, saving: usize) -> bool {
        debug_assert!(self.writer);
        CompactFile::compress(self.spd.sp_size, self.spd.ep_size, size, saving)
    }

    /// Allocate a logical page.
    pub fn alloc_page(&self) -> u64 {
        debug_assert!(self.writer);
        self.spd.file.write().unwrap().alloc_page()
    }

    /// Free a logical page.
    pub fn free_page(&self, lpnum: u64) {
        debug_assert!(self.writer);
        self.spd.stash.write().unwrap().set(lpnum, nd());
        self.spd.file.write().unwrap().free_page(lpnum);
    }

    /// Commit changes to underlying file ( or rollback logical page allocations ).
    pub fn save(&self, op: SaveOp) -> usize {
        debug_assert!(self.writer);
        match op {
            SaveOp::Save => {
                self.spd.file.write().unwrap().save();
                self.spd.stash.write().unwrap().end_write()
            }
            SaveOp::RollBack => {
                // Note: rollback happens before any pages are updated.
                // However logical page allocations need to be rolled back.
                self.spd.file.write().unwrap().rollback();
                0
            }
        }
    }
}

impl Drop for AccessPagedData {
    fn drop(&mut self) {
        if !self.writer {
            self.spd.stash.write().unwrap().end_read(self.time);
        }
    }
}