use crate::page::Page;
use serde::Deserialize;

/// Abstracts away the `next_page` logic into a single stream of items
#[derive(Debug, Clone)]
pub struct ItemsIter<'a, T: Clone + for<'de> Deserialize<'de>> {
    page: Page<'a, T>,
    buffer: Vec<T>,
    cur_idx: usize,
    cur_page: usize,
    use_initial: bool,
}

impl<'a, T: Clone + for<'de> Deserialize<'de>> ItemsIter<'a, T> {
    pub(crate) fn new(page: Page<'a, T>) -> ItemsIter<'a, T> {
        ItemsIter {
            page,
            buffer: vec![],
            cur_idx: 0,
            cur_page: 0,
            use_initial: true,
        }
    }

    fn need_next_page(&self) -> bool {
        self.buffer.is_empty() || self.cur_idx == self.buffer.len()
    }

    async fn fill_next_page(&mut self) -> Option<()> {
        let items = if let Ok(items) = self.page.next_page().await {
            items
        } else {
            return None;
        };
        if let Some(items) = items {
            if items.is_empty() {
                return None;
            }
            self.buffer = items;
            self.cur_idx = 0;
            self.cur_page += 1;
            Some(())
        } else {
            None
        }
    }

    pub fn page_num(&self) -> usize {
        self.cur_page
    }

    pub async fn next_item(&mut self) -> Option<T> {
        if self.use_initial {
            if self.page.initial_items.is_empty() || self.cur_idx == self.page.initial_items.len() {
                return None;
            }
            let idx = self.cur_idx;
            if self.cur_idx == self.page.initial_items.len() - 1 {
                self.cur_idx = 0;
                self.use_initial = false;
            } else {
                self.cur_idx += 1;
            }
            Some(self.page.initial_items[idx].clone())
        } else {
            if self.need_next_page() && self.fill_next_page().await.is_none() {
                return None;
            }
            let idx = self.cur_idx;
            self.cur_idx += 1;
            Some(self.buffer[idx].clone())
        }
    }
}