//! Shared pagination helpers for the public page handlers.

use crate::constants;

/// Build a sliding window of page numbers for pagination controls.
///
/// One definition for every public list page (feed, discover, landing), they
/// previously each carried a byte-identical copy, so a tweak to the window
/// behavior had to be made in lockstep or silently drift.
pub(super) fn build_pagination_range(current_page: u32, total_pages: u32) -> Vec<u32> {
    if total_pages <= constants::PAGINATION_WINDOW_SIZE {
        (1..=total_pages).collect()
    } else {
        let start = current_page.saturating_sub(2).max(1);
        let end = (start + 4).min(total_pages);
        let start = end.saturating_sub(4).max(1);
        (start..=end).collect()
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn pagination_small_total() {
        assert_eq!(build_pagination_range(1, 3), vec![1, 2, 3]);
        assert_eq!(build_pagination_range(2, 5), vec![1, 2, 3, 4, 5]);
    }

    #[test]
    fn pagination_large_at_start() {
        assert_eq!(build_pagination_range(1, 20), vec![1, 2, 3, 4, 5]);
        assert_eq!(build_pagination_range(2, 20), vec![1, 2, 3, 4, 5]);
    }

    #[test]
    fn pagination_large_at_middle() {
        assert_eq!(build_pagination_range(10, 20), vec![8, 9, 10, 11, 12]);
    }

    #[test]
    fn pagination_large_at_end() {
        assert_eq!(build_pagination_range(20, 20), vec![16, 17, 18, 19, 20]);
        assert_eq!(build_pagination_range(19, 20), vec![16, 17, 18, 19, 20]);
    }

    #[test]
    fn pagination_zero_pages() {
        assert_eq!(build_pagination_range(1, 0), Vec::<u32>::new());
    }

    #[test]
    fn pagination_single_page() {
        assert_eq!(build_pagination_range(1, 1), vec![1]);
    }
}