concinnity-engine 0.19.23

// src/ui/scroll_layout.rs
//
// Pure vertical-layout solver for a scrollable panel. A scrollable settings
// page is a vertical stack of rows; some rows belong to a collapsible group and
// disappear when that group is collapsed; the visible stack may be taller than
// the band it shows through, in which case it scrolls.
//
// This module owns only the geometry: given each row's natural height, its
// group membership, the per-group collapsed state, the band height, and the
// current scroll offset, it returns where each row ends up (a delta from its
// build-time position), whether it is visible, the total content height, the
// clamped scroll, and the scrollbar thumb size + offset. It knows nothing about
// AssetIds or rendering; `UiInputSystem` maps these results onto concrete
// elements.

// One row in the stack. `group` is the index of the collapsible group whose
// collapsed state hides this row, or `None` for a row that is always shown
// (chrome-in-stack, e.g. a group header).
#[derive(Debug, Clone, Copy)]
pub(super) struct RowSpec {
    pub height: f32,
    pub group: Option<usize>,
}

// Where one row ends up after solving. `dy` is the vertical offset from the
// row's build-time (all-expanded, unscrolled) position, so a client adds it to
// the element's authored y. A hidden row's `dy` is unspecified.
#[derive(Debug, Clone, Copy, PartialEq)]
pub(super) struct RowPlacement {
    pub dy: f32,
    pub visible: bool,
}

// The scalar solution for one panel; the per-row placements land in the
// caller's buffer (see `solve_into`).
#[derive(Debug, Clone, Copy, PartialEq)]
pub(super) struct Solved {
    // Total height of the visible stack (sum of shown row heights).
    pub(crate) content_height: f32,
    // The scroll offset after clamping to `[0, max_scroll]`.
    pub scroll: f32,
    // Scrollbar thumb size as a fraction of the track, in `(0, 1]`. 1.0 means
    // the content fits and no scrolling is possible (the bar can be hidden).
    pub(crate) thumb_frac: f32,
    // Scrollbar thumb top as a fraction of the track, in `[0, 1 - thumb_frac]`.
    pub(crate) thumb_offset_frac: f32,
}

impl Solved {
    // Whether the content overflows the band (i.e. scrolling does anything and
    // the scrollbar is meaningful).
    pub(super) fn scrollable(&self) -> bool {
        self.thumb_frac < 1.0
    }
}

// Solve the vertical layout, writing each row's placement into `out` (cleared
// first, so a caller reuses its capacity frame over frame). `rows` are in
// top-to-bottom order with their build-time (all-expanded, unscrolled)
// positions implied by the running sum of heights. `collapsed[g]` hides every
// row whose `group == Some(g)`. `band_height` is the visible window; `scroll`
// is the requested offset (clamped here).
pub(super) fn solve_into(
    rows: &[RowSpec],
    collapsed: &[bool],
    band_height: f32,
    scroll: f32,
    out: &mut Vec<RowPlacement>,
) -> Solved {
    let is_hidden = |r: &RowSpec| {
        r.group
            .is_some_and(|g| collapsed.get(g).copied().unwrap_or(false))
    };

    // Total content height with the current collapsed state.
    let content_height: f32 = rows
        .iter()
        .filter(|r| !is_hidden(r))
        .map(|r| r.height)
        .sum();

    let max_scroll = (content_height - band_height).max(0.0);
    let scroll = scroll.clamp(0.0, max_scroll);

    // Walk the rows, accumulating both the build-time top (the all-expanded
    // running sum the build laid every row out with) and the laid-out top of
    // the visible stack, and emit each visible row's delta between the two
    // minus the scroll.
    out.clear();
    out.reserve(rows.len());
    let mut base_top = 0.0_f32;
    let mut laid_top = 0.0_f32;
    for r in rows {
        if is_hidden(r) {
            out.push(RowPlacement {
                dy: 0.0,
                visible: false,
            });
        } else {
            let dy = laid_top - base_top - scroll;
            out.push(RowPlacement { dy, visible: true });
            laid_top += r.height;
        }
        base_top += r.height;
    }

    // Thumb size = band / content (clamped to 1 when content fits); offset =
    // scroll / content. Guard a zero content height (empty panel).
    let (thumb_frac, thumb_offset_frac) = if content_height <= 0.0 {
        (1.0, 0.0)
    } else {
        let frac = (band_height / content_height).min(1.0);
        let offset = (scroll / content_height).clamp(0.0, 1.0 - frac);
        (frac, offset)
    };

    Solved {
        content_height,
        scroll,
        thumb_frac,
        thumb_offset_frac,
    }
}

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

    fn row(height: f32, group: Option<usize>) -> RowSpec {
        RowSpec { height, group }
    }

    // Test shim over `solve_into`: the scalars plus the placements in one value.
    struct FullSolve {
        rows: Vec<RowPlacement>,
        content_height: f32,
        scroll: f32,
        thumb_frac: f32,
        thumb_offset_frac: f32,
    }

    impl FullSolve {
        fn scrollable(&self) -> bool {
            self.thumb_frac < 1.0
        }
    }

    fn solve(rows: &[RowSpec], collapsed: &[bool], band_height: f32, scroll: f32) -> FullSolve {
        let mut placements = Vec::new();
        let s = solve_into(rows, collapsed, band_height, scroll, &mut placements);
        FullSolve {
            rows: placements,
            content_height: s.content_height,
            scroll: s.scroll,
            thumb_frac: s.thumb_frac,
            thumb_offset_frac: s.thumb_offset_frac,
        }
    }

    #[test]
    fn solve_into_clears_the_reused_buffer_before_placing() {
        let rows = [row(50.0, None), row(50.0, None)];
        let mut out = Vec::new();
        solve_into(&rows, &[], 300.0, 0.0, &mut out);
        solve_into(&rows, &[], 300.0, 0.0, &mut out);
        assert_eq!(out.len(), 2, "a reused buffer holds one solve, not two");
    }

    #[test]
    fn fits_in_band_no_scroll_no_offset() {
        // Three 50px rows = 150px content in a 300px band: nothing moves, the
        // thumb fills the track, and scrolling is a no-op.
        let rows = [row(50.0, None), row(50.0, None), row(50.0, None)];
        let s = solve(&rows, &[], 300.0, 0.0);
        assert_eq!(s.content_height, 150.0);
        assert!(!s.scrollable());
        assert_eq!(s.thumb_frac, 1.0);
        assert_eq!(s.scroll, 0.0);
        for p in &s.rows {
            assert!(p.visible);
            assert_eq!(p.dy, 0.0);
        }
    }

    #[test]
    fn overflow_clamps_scroll_and_shifts_rows_up() {
        // Ten 50px rows = 500px content in a 200px band: max scroll = 300.
        let rows: Vec<RowSpec> = (0..10).map(|_| row(50.0, None)).collect();
        let s = solve(&rows, &[], 200.0, 1000.0);
        assert_eq!(s.content_height, 500.0);
        assert_eq!(s.scroll, 300.0, "scroll clamps to content - band");
        assert!(s.scrollable());
        // Every visible row shifts up by the scroll amount.
        for p in &s.rows {
            assert!(p.visible);
            assert_eq!(p.dy, -300.0);
        }
        // Thumb = 200/500 = 0.4, offset = 300/500 = 0.6 (pinned to the bottom).
        assert!((s.thumb_frac - 0.4).abs() < 1e-5);
        assert!((s.thumb_offset_frac - 0.6).abs() < 1e-5);
    }

    #[test]
    fn collapsed_group_hides_body_and_pulls_rows_below_up() {
        // Layout: header(None), body0(g0), body1(g0), footer(None). All 40px.
        let rows = [
            row(40.0, None),
            row(40.0, Some(0)),
            row(40.0, Some(0)),
            row(40.0, None),
        ];
        // Expanded: nothing hidden, nothing moves.
        let open = solve(&rows, &[false], 1000.0, 0.0);
        assert_eq!(open.content_height, 160.0);
        for p in &open.rows {
            assert!(p.visible);
            assert_eq!(p.dy, 0.0);
        }
        // Collapsed: the two body rows hide and the footer rises by 80px (their
        // combined height); the header is unmoved.
        let shut = solve(&rows, &[true], 1000.0, 0.0);
        assert_eq!(shut.content_height, 80.0);
        assert!(shut.rows[0].visible && shut.rows[0].dy == 0.0); // header
        assert!(!shut.rows[1].visible); // body0
        assert!(!shut.rows[2].visible); // body1
        assert!(shut.rows[3].visible);
        assert_eq!(
            shut.rows[3].dy, -80.0,
            "footer rises by the collapsed height"
        );
    }

    #[test]
    fn collapse_then_scroll_compose() {
        // header + 6 body rows (g0) + footer, all 50px = 400px content.
        let mut rows = vec![row(50.0, None)];
        rows.extend((0..6).map(|_| row(50.0, Some(0))));
        rows.push(row(50.0, None));
        // Collapsed body: content = header + footer = 100px, fits a 200px band,
        // so no scroll regardless of the requested offset.
        let s = solve(&rows, &[true], 200.0, 500.0);
        assert_eq!(s.content_height, 100.0);
        assert_eq!(s.scroll, 0.0);
        assert!(!s.scrollable());
        // The footer rises by the 6 collapsed bodies (300px), unscrolled.
        assert_eq!(s.rows.last().unwrap().dy, -300.0);
    }

    #[test]
    fn empty_panel_is_stable() {
        let s = solve(&[], &[], 200.0, 10.0);
        assert_eq!(s.content_height, 0.0);
        assert_eq!(s.scroll, 0.0);
        assert_eq!(s.thumb_frac, 1.0);
        assert!(s.rows.is_empty());
    }

    #[test]
    fn out_of_range_group_index_treated_as_expanded() {
        // A row referencing a group with no collapsed entry stays visible.
        let rows = [row(50.0, Some(5))];
        let s = solve(&rows, &[false], 200.0, 0.0);
        assert!(s.rows[0].visible);
    }
}