/*
 * SPDX-FileCopyrightText: 2020 Stalwart Labs LLC <hello@stalw.art>
 *
 * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-SEL
 */

use std::{
    sync::atomic::{AtomicU64, Ordering},
    time::{Duration, SystemTime},
};

#[derive(Debug)]
pub struct SnowflakeIdGenerator {
    epoch: SystemTime,
    sequence: AtomicU64,
}

const SEQUENCE_LEN: u64 = 12;
const NODE_ID_LEN: u64 = 9;

const SEQUENCE_MASK: u64 = (1 << SEQUENCE_LEN) - 1;
const NODE_ID_MASK: u64 = (1 << NODE_ID_LEN) - 1;

const DEFAULT_EPOCH: u64 = 1632280000; // 52 years after UNIX_EPOCH

static mut NODE_ID: u64 = 1;
static SEQUENCE_ID: AtomicU64 = AtomicU64::new(0);

/*

ID characteristics:

- 43 bits for milliseconds since January 1st, 2022: 2^43 / (1000 * 60 * 60 * 24 * 365) = 278.92 years (from year 2022 until 2300)
- 9 bits for a node id: 2^9 = 512 nodes
- 12 bits for a sequence number: 2^12 = 4096 ids per millisecond

*/

#[inline(always)]
fn node_id() -> u64 {
    unsafe { std::ptr::read_volatile(&raw const NODE_ID) }
}

impl SnowflakeIdGenerator {
    pub fn new() -> Self {
        Self {
            epoch: SystemTime::UNIX_EPOCH + Duration::from_secs(DEFAULT_EPOCH), // 52 years after UNIX_EPOCH
            sequence: 0.into(),
        }
    }

    pub fn set_node_id(set_node_id: u64) {
        let set_node_id = set_node_id & NODE_ID_MASK;

        if set_node_id != node_id() {
            unsafe {
                NODE_ID = set_node_id;
            }
        }
    }

    pub fn from_duration(period: Duration) -> Option<u64> {
        (SystemTime::UNIX_EPOCH + Duration::from_secs(DEFAULT_EPOCH))
            .elapsed()
            .ok()
            .and_then(|elapsed| elapsed.checked_sub(period))
            .map(|elapsed| (elapsed.as_millis() as u64) << (SEQUENCE_LEN + NODE_ID_LEN))
    }

    pub fn from_timestamp(timestamp: u64) -> Option<u64> {
        SystemTime::now()
            .duration_since(SystemTime::UNIX_EPOCH)
            .ok()
            .and_then(|now| now.as_secs().checked_sub(timestamp))
            .and_then(|diff| Self::from_duration(Duration::from_secs(diff)))
    }

    pub fn global_id_from_timestamp(timestamp: u64) -> Option<u64> {
        let sequence = SEQUENCE_ID.fetch_add(1, Ordering::Relaxed) & SEQUENCE_MASK;
        Self::from_timestamp(timestamp).map(|id| id | (sequence << NODE_ID_LEN) | node_id())
    }

    pub fn global_id() -> Option<u64> {
        let sequence = SEQUENCE_ID.fetch_add(1, Ordering::Relaxed) & SEQUENCE_MASK;

        (SystemTime::UNIX_EPOCH + Duration::from_secs(DEFAULT_EPOCH))
            .elapsed()
            .ok()
            .map(|elapsed| {
                ((elapsed.as_millis() as u64) << (SEQUENCE_LEN + NODE_ID_LEN))
                    | (sequence << NODE_ID_LEN)
                    | node_id()
            })
    }

    pub fn to_timestamp(id: u64) -> u64 {
        (id >> (SEQUENCE_LEN + NODE_ID_LEN)) / 1000 + DEFAULT_EPOCH
    }

    #[inline(always)]
    pub fn past_id(&self, period: Duration) -> Option<u64> {
        self.epoch
            .elapsed()
            .ok()
            .and_then(|elapsed| elapsed.checked_sub(period))
            .map(|elapsed| (elapsed.as_millis() as u64) << (SEQUENCE_LEN + NODE_ID_LEN))
    }

    pub fn is_valid(&self) -> bool {
        self.epoch.elapsed().is_ok()
    }

    #[inline(always)]
    pub fn generate(&self) -> u64 {
        let elapsed = self
            .epoch
            .elapsed()
            .map(|e| e.as_millis())
            .unwrap_or_default() as u64;
        let sequence = self.sequence.fetch_add(1, Ordering::Relaxed) & SEQUENCE_MASK;

        (elapsed << (SEQUENCE_LEN + NODE_ID_LEN)) | (sequence << NODE_ID_LEN) | node_id()
    }
}

impl Default for SnowflakeIdGenerator {
    fn default() -> Self {
        Self::new()
    }
}

impl Clone for SnowflakeIdGenerator {
    fn clone(&self) -> Self {
        Self {
            epoch: self.epoch,
            sequence: 0.into(),
        }
    }
}
