kas_core/layout/

size_rules.rs

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License in the LICENSE-APACHE file or at:
//     https://www.apache.org/licenses/LICENSE-2.0

//! [`SizeRules`] type

use smallvec::SmallVec;
use std::iter::Sum;

use super::Stretch;
use crate::cast::{Cast, Conv};

// for doc use
#[allow(unused)] use super::FrameRules;
#[allow(unused)] use crate::theme::SizeCx;

/// Widget sizing information
///
/// This is the return value of [`crate::Layout::size_rules`] and is used to
/// describe size and margin requirements for widgets. This type only concerns
/// size requirements along a *single* axis.
///
/// All units are in pixels. Sizes usually come directly from [`SizeCx`]
/// or from a fixed quantity multiplied by [`SizeCx::scale_factor`].
///
/// ### Sizes
///
/// The widget size model is simple: a rectangular box, plus a margin on each
/// side. The `SizeRules` type represents expectations along a single axis:
///
/// -   The minimum acceptable size (almost always met)
/// -   The ideal size (often the same size; this distinction is most useful for
///     scrollable regions which are *ideally* large enough not to require
///     scrolling, but can be much smaller)
/// -   A [`Stretch`] priority, used to prioritize allocation of excess space
///
/// Note that `Stretch::None` does not *prevent* stretching, but simply states
/// that it is undesired (lowest priority). Actually preventing stretching
/// requires alignment.
///
/// ### Margins
///
/// Required margin sizes are handled separately for each side of a widget.
/// Since [`SizeRules`] concerns only one axis, it stores only two margin sizes:
/// "pre" (left/top) and "post" (right/bottom). These are stored as `u16` values
/// on the assumption that no margin need exceed 65536.
///
/// When widgets are placed next to each other, their margins may be combined;
/// e.g. if a widget with margin of 6px is followed by another with margin 2px,
/// the required margin between the two is the maximum, 6px.
///
/// Only the layout engine and parent widgets need consider margins (beyond
/// their specification). For these cases, one needs to be aware that due to
/// margin-merging behaviour, one cannot simply "add" two `SizeRules`. Instead,
/// when placing one widget next to another, use [`SizeRules::append`] or
/// [`SizeRules::appended`]; when placing a widget within a frame, use
/// [`FrameRules::surround`].
/// When calculating the size of a sequence of
/// widgets, one may use the [`Sum`] implementation (this assumes that the
/// sequence is in left-to-right or top-to-bottom order).
///
/// ### Alignment
///
/// `SizeRules` concerns calculations of size requirements, which the layout
/// engine uses to assign each widget a [`Rect`]; it is up to the widget itself
/// to either fill this rect or align itself within the given space.
/// See [`crate::Layout::set_rect`] for more information.
///
/// For widgets with a stretch priority of [`Stretch::None`], it is still
/// possible for layout code to assign a size larger than the preference. It is
/// up to the widget to align itself within this space: see
/// [`crate::Layout::set_rect`] and [`crate::layout::AlignHints`].
///
/// [`Rect`]: crate::geom::Rect
#[derive(Copy, Clone, Debug, Default, PartialEq, Eq)]
pub struct SizeRules {
    // minimum good size
    a: i32,
    // ideal size; b >= a
    b: i32,
    // (pre, post) margins
    m: (u16, u16),
    stretch: Stretch,
}

impl SizeRules {
    /// Empty (zero size) widget
    ///
    /// Warning: appending another size to `EMPTY` *does* include margins
    /// even though `EMPTY` itself has zero size. However, `EMPTY` itself has
    /// zero-size margins, so this only affects appending an `EMPTY` with a
    /// non-empty `SizeRules`.
    pub const EMPTY: Self = SizeRules::empty(Stretch::None);

    /// Empty space with the given stretch priority
    ///
    /// See warning on [`SizeRules::EMPTY`].
    #[inline]
    pub const fn empty(stretch: Stretch) -> Self {
        SizeRules {
            a: 0,
            b: 0,
            m: (0, 0),
            stretch,
        }
    }

    /// Construct for a fixed size
    ///
    /// Margins are zero-sized by default; use [`Self::with_margin`] or
    /// [`Self::with_margins`] to set.
    #[inline]
    pub fn fixed(size: i32) -> Self {
        debug_assert!(size >= 0);
        SizeRules {
            a: size,
            b: size,
            m: (0, 0),
            stretch: Stretch::None,
        }
    }

    /// Construct with custom rules
    ///
    /// Region size should meet the given `min`-imum size and has a given
    /// `ideal` size, plus a given `stretch` priority.
    ///
    /// Expected: `ideal >= min` (if not, ideal is clamped to min).
    ///
    /// Margins are zero-sized by default; use [`Self::with_margin`] or
    /// [`Self::with_margins`] to set.
    #[inline]
    pub fn new(min: i32, ideal: i32, stretch: Stretch) -> Self {
        debug_assert!(0 <= min && 0 <= ideal);
        SizeRules {
            a: min,
            b: ideal.max(min),
            m: (0, 0),
            stretch,
        }
    }

    /// Set both margins (symmetric)
    ///
    /// Both margins are set to the same value. By default these are 0.
    #[inline]
    pub fn with_margin(mut self, margin: u16) -> Self {
        self.m = (margin, margin);
        self
    }

    /// Set both margins (top/left, bottom/right)
    ///
    /// By default these are 0.
    #[inline]
    pub fn with_margins(mut self, (first, second): (u16, u16)) -> Self {
        self.m = (first, second);
        self
    }

    /// Set stretch factor, inline
    #[inline]
    pub fn with_stretch(mut self, stretch: Stretch) -> Self {
        self.stretch = stretch;
        self
    }

    /// Get the minimum size
    #[inline]
    pub fn min_size(self) -> i32 {
        self.a
    }

    /// Get the ideal size
    #[inline]
    pub fn ideal_size(self) -> i32 {
        self.b
    }

    /// Get the `(pre, post)` margin sizes
    #[inline]
    pub fn margins(self) -> (u16, u16) {
        self.m
    }

    /// Get the `(pre, post)` margin sizes, cast to `i32`
    #[inline]
    pub fn margins_i32(self) -> (i32, i32) {
        (self.m.0.into(), self.m.1.into())
    }

    /// Get the stretch priority
    #[inline]
    pub fn stretch(self) -> Stretch {
        self.stretch
    }

    /// Set the stretch priority
    #[inline]
    pub fn set_stretch(&mut self, stretch: Stretch) {
        self.stretch = stretch;
    }

    /// Set margins
    #[inline]
    pub fn set_margins(&mut self, margins: (u16, u16)) {
        self.m = margins;
    }

    /// Use the maximum size of `self` and `rhs`.
    #[must_use = "method does not modify self but returns a new value"]
    pub fn max(self, rhs: Self) -> SizeRules {
        SizeRules {
            a: self.a.max(rhs.a),
            b: self.b.max(rhs.b),
            m: (self.m.0.max(rhs.m.0), self.m.1.max(rhs.m.1)),
            stretch: self.stretch.max(rhs.stretch),
        }
    }

    /// Set `self = self.max(rhs);`
    #[inline]
    pub fn max_with(&mut self, rhs: Self) {
        *self = self.max(rhs);
    }

    /// Multiply the `(min, ideal)` size, including internal margins
    ///
    /// E.g. given `margin = margins.0 + margins.1` and factors `(2, 5)`, the
    /// minimum size is set to `min * 2 + margin` and the ideal to
    /// `ideal * 5 + 4 * margin`.
    ///
    /// Panics if either factor is 0.
    pub fn multiply_with_margin(&mut self, min_factor: i32, ideal_factor: i32) {
        let margin = i32::from(self.m.0).max(i32::from(self.m.1));
        assert!(min_factor > 0);
        assert!(ideal_factor > 0);
        self.a = min_factor * self.a + (min_factor - 1) * margin;
        self.b = ideal_factor * self.b + (ideal_factor - 1) * margin;
    }

    /// Append the rules for `rhs` to self
    ///
    /// This implies that `rhs` rules concern an element to the right of or
    /// below self. Note that order matters since margins may be combined.
    ///
    /// Note also that appending [`SizeRules::EMPTY`] does include interior
    /// margins (those between `EMPTY` and the other rules) within the result.
    pub fn append(&mut self, rhs: SizeRules) {
        let c: i32 = self.m.1.max(rhs.m.0).into();
        self.a += rhs.a + c;
        self.b += rhs.b + c;
        self.m.1 = rhs.m.1;
        self.stretch = self.stretch.max(rhs.stretch);
    }

    /// Return the rules for self appended by `rhs`
    ///
    ///
    /// This implies that `rhs` rules concern an element to the right of or
    /// below self. Note that order matters since margins may be combined.
    ///
    /// Note also that appending [`SizeRules::EMPTY`] does include interior
    /// margins (those between `EMPTY` and the other rules) within the result.
    #[must_use = "method does not modify self but returns a new value"]
    pub fn appended(self, rhs: SizeRules) -> Self {
        let c: i32 = self.m.1.max(rhs.m.0).into();
        SizeRules {
            a: self.a + rhs.a + c,
            b: self.b + rhs.b + c,
            m: (self.m.0, rhs.m.1),
            stretch: self.stretch.max(rhs.stretch),
        }
    }

    /// Return the result of appending all given ranges
    pub fn sum(range: &[SizeRules]) -> SizeRules {
        range.iter().sum()
    }

    /// Return the result of appending all given ranges (min only)
    ///
    /// This is a specialised version of sum: only the minimum is calculated
    pub fn min_sum(range: &[SizeRules]) -> SizeRules {
        if range.is_empty() {
            return SizeRules::EMPTY;
        }

        let mut rules = range[0];
        for r in &range[1..] {
            rules.a += i32::from(rules.m.1.max(r.m.0)) + r.a;
        }
        rules.b = rules.a;
        rules.m.1 = range[range.len() - 1].m.1;
        rules
    }

    /// Set self to `self - x + y`, clamped to 0 or greater
    ///
    /// This is a specialised operation to join two spans, subtracing the
    /// common overlap (`x`), thus margins are `self.m.0` and `y.m.1`.
    pub fn sub_add(&mut self, x: Self, y: Self) {
        self.a = (self.a - x.a + y.a).max(0);
        self.b = (self.b - x.b + y.b).max(0);
        self.m.1 = y.m.1;
        self.stretch = self.stretch.max(y.stretch);
    }

    /// Reduce the minimum size
    ///
    /// If `min` is greater than the current minimum size, this has no effect.
    #[inline]
    pub fn reduce_min_to(&mut self, min: i32) {
        self.a = self.a.min(min);
    }

    /// Solve a sequence of rules with even distribution
    ///
    /// Given a sequence of width (or height) `rules` from children and a
    /// `target` size, find an appropriate size for each child.
    /// The method attempts to ensure the following, in order of priority:
    ///
    /// 1.  All widths are at least their minimum size requirement
    /// 2.  The sum of widths plus margins between items equals `target`
    /// 3.  No width exceeds its ideal size while other widths are below their
    ///     own ideal size
    /// 4.  No item with a [`Stretch`] priority less than the highest in `rules`
    ///     exceeds its ideal size
    /// 5.  When extra space is available and some widgets are below their ideal
    ///     size, extra space is divided evenly between these widgets until they
    ///     have reached their ideal size
    /// 6.  If all rules use `Stretch::None`, then widths are not increased over
    ///     their ideal size.
    /// 7.  Extra space (after all widths are at least their ideal size) is
    ///     shared equally between all widgets with the highest stretch priority
    ///
    /// Input requirements: `rules.len() == widths.len()`.
    ///
    /// This method is idempotent: if input widths already match the above
    /// requirements then they will not be modified.
    /// Moreover, this method attempts to ensure that if `target`
    /// is increased, then decreased back to the previous value, this will
    /// revert to the previous solution. (The reverse may not hold if widths
    /// had previously been affected by a different agent.)
    #[inline]
    pub fn solve_widths(widths: &mut [i32], rules: &[Self], target: i32) {
        let total = SizeRules::sum(rules);
        Self::solve_widths_with_total(widths, rules, total, target);
    }

    /// Solve a sequence of rules
    ///
    /// This is the same as [`SizeRules::solve_widths`] except that the rules' sum
    /// is passed explicitly.
    ///
    /// Input requirements:
    /// - `rules.len() == widths.len()`
    /// - `SizeRules::sum(rules) == total`
    pub fn solve_widths_with_total(widths: &mut [i32], rules: &[Self], total: Self, target: i32) {
        Self::solve_widths_impl(widths, rules, total, target, Even)
    }

    /// Solve a sequence of rules with priority distribution
    ///
    /// Given a sequence of width (or height) `rules` from children and a
    /// `target` size, find an appropriate size for each child.
    /// The method attempts to ensure that:
    ///
    /// 1.  All widths are at least their minimum size requirement
    /// 2.  The sum of widths plus margins between items equals `target`
    /// 3.  No width exceeds its ideal size while other widths are below their
    ///     own ideal size
    /// 4.  No item with a [`Stretch`] priority less than the highest in `rules`
    ///     exceeds its ideal size
    /// 5.  When extra space is available and some widgets are below their ideal
    ///     size, extra space is divided evenly between these widgets until they
    ///     have reached their ideal size
    /// 6.  If all rules use `Stretch::None`, then widths are not increased over
    ///     their ideal size.
    /// 7.  Extra space (after all widths are at least their ideal size) is
    ///     allocated to the first (or `last`) item with the highest stretch
    ///     priority
    ///
    /// Input requirements: `rules.len() == widths.len()`.
    ///
    /// This method is idempotent: given satisfactory input widths, these will
    /// be preserved. Moreover, this method attempts to ensure that if target
    /// is increased, then decreased back to the previous value, this will
    /// revert to the previous solution. (The reverse may not hold if widths
    /// had previously been affected by a different agent.)
    #[allow(clippy::collapsible_if, clippy::needless_return)]
    pub fn solve_widths_with_priority(widths: &mut [i32], rules: &[Self], target: i32, last: bool) {
        let total = SizeRules::sum(rules);
        Self::solve_widths_impl(widths, rules, total, target, Prioritize { last })
    }

    #[allow(
        clippy::comparison_chain,
        clippy::needless_range_loop,
        clippy::needless_return
    )]
    #[inline(always)]
    fn solve_widths_impl(
        out: &mut [i32],
        rules: &[Self],
        total: Self,
        target: i32,
        pri: impl SolvePriority,
    ) {
        #[allow(non_snake_case)]
        let N = out.len();
        assert_eq!(rules.len(), N);
        if N == 0 {
            return;
        }
        #[cfg(debug_assertions)]
        {
            assert!(out.iter().all(|w| *w >= 0));
            let mut sum = SizeRules::sum(rules);
            sum.m = total.m; // external margins are unimportant here
            assert_eq!(sum, total);
        }

        if target <= total.a {
            // Below minimum size: ignore target and use minimum sizes.
            for n in 0..N {
                out[n] = rules[n].a;
            }
            return;
        }
        let highest_stretch = total.stretch;

        // All minimum sizes can be met.
        out[0] = out[0].max(rules[0].a);
        let mut margin_sum = 0;
        let mut sum = out[0];
        let mut dist_under_b = (rules[0].b - out[0]).max(0);
        let mut dist_over_b = (out[0] - rules[0].b).max(0);
        let mut dist_over_b_lower_stretch = 0;
        if rules[0].stretch < highest_stretch {
            dist_over_b_lower_stretch = dist_over_b;
        }
        for i in 1..N {
            out[i] = out[i].max(rules[i].a);
            margin_sum += i32::from((rules[i - 1].m.1).max(rules[i].m.0));
            sum += out[i];
            dist_under_b += (rules[i].b - out[i]).max(0);
            let over = (out[i] - rules[i].b).max(0);
            dist_over_b += over;
            if rules[i].stretch < highest_stretch {
                dist_over_b_lower_stretch += over;
            }
        }
        let target = target - margin_sum;

        // Shrink: sum - target
        // + dist_under_b because this lets us increase targets under b
        let mut to_shrink = sum + dist_under_b - target;
        if dist_over_b <= to_shrink {
            // Ensure nothing exceeds the ideal and consider shrinking anything over min:
            let mut targets = Targets::new();
            sum = 0;
            for i in 0..N {
                out[i] = out[i].min(rules[i].b);
                sum += out[i];
                if out[i] > rules[i].a {
                    targets.push(i.cast());
                }
            }

            if sum > target {
                let avail = target + margin_sum - total.a;
                reduce_targets(out, &mut targets, |i| rules[i].a, avail);
                debug_assert_eq!(target, (0..N).fold(0, |x, i| x + out[i]));
                return;
            }
        } else if dist_over_b_lower_stretch >= to_shrink {
            // Ensure only highest-stretch-priority items exceed the ideal:
            sum = 0;
            for i in 0..N {
                if rules[i].stretch < highest_stretch {
                    out[i] = out[i].min(rules[i].b);
                }
                sum += out[i];
            }
        } else if to_shrink > 0 {
            // we do not go below ideal, and will keep at least one above
            if let Some(prioritize_last) = pri.priority() {
                if !prioritize_last {
                    for i in 0..N {
                        if out[i] > rules[i].b {
                            let decr = to_shrink.min((out[i] - rules[i].b).max(0));
                            to_shrink -= decr;
                            out[i] -= decr;
                        }
                    }
                } else {
                    for i in (0..N).rev() {
                        if out[i] > rules[i].b {
                            let decr = to_shrink.min((out[i] - rules[i].b).max(0));
                            to_shrink -= decr;
                            out[i] -= decr;
                        }
                    }
                }
                debug_assert_eq!(to_shrink, 0);
            } else {
                // calculate distance over for each stretch priority
                const MAX_STRETCH: usize = Stretch::Maximize as usize + 1;
                let mut dists = [0; MAX_STRETCH];
                for i in 0..N {
                    dists[rules[i].stretch as usize] += (out[i] - rules[i].b).max(0);
                }
                let mut accum = 0;
                let mut highest_affected = 0;
                for i in 0..MAX_STRETCH {
                    highest_affected = i;
                    dists[i] += accum;
                    accum = dists[i];
                    if accum >= to_shrink {
                        break;
                    }
                }

                sum = 0;
                let mut avail = 0;
                let mut targets = Targets::new();
                for i in 0..N {
                    let stretch = rules[i].stretch as usize;
                    if out[i] > rules[i].b {
                        let over = (out[i] - rules[i].b).max(0);
                        if stretch < highest_affected {
                            out[i] = rules[i].b;
                        } else if stretch == highest_affected {
                            avail += over;
                            targets.push(i.cast());
                        }
                    }
                    sum += out[i];
                }
                let to_shrink = sum + dist_under_b - target;
                if 0 < to_shrink && to_shrink <= avail {
                    avail -= to_shrink;
                    reduce_targets(out, &mut targets, |i| rules[i].b, avail);
                    sum -= to_shrink;
                }
            }
        }

        if sum < target {
            if target - sum >= dist_under_b {
                // We can increase all sizes to their ideal. Since this may
                // not be enough, we also count the number with highest
                // stretch factor and how far these are over their ideal.
                // If highest stretch is None, do not expand beyond ideal.
                sum = 0;
                if let Some(prioritize_last) = pri.priority() {
                    let mut pick = None;
                    for i in 0..N {
                        out[i] = out[i].max(rules[i].b);
                        sum += out[i];
                        if highest_stretch > Stretch::None && rules[i].stretch == highest_stretch {
                            if prioritize_last || pick.is_none() {
                                pick = Some(i);
                            }
                        }
                    }

                    if let Some(i) = pick {
                        out[i] += target - sum;
                    }
                } else {
                    let mut targets = Targets::new();
                    let mut over = 0;
                    for i in 0..N {
                        out[i] = out[i].max(rules[i].b);
                        sum += out[i];
                        if highest_stretch > Stretch::None && rules[i].stretch == highest_stretch {
                            over += out[i] - rules[i].b;
                            targets.push(i.cast());
                        }
                    }

                    let avail = target - sum + over;
                    increase_targets(out, &mut targets, |i| rules[i].b, avail);
                }

                debug_assert!(target >= (0..N).fold(0, |x, i| x + out[i]));
                return;
            } else {
                // We cannot increase sizes as far as their ideal: instead
                // increase over minimum size and under ideal
                let mut targets = Targets::new();
                let mut over = 0;
                for i in 0..N {
                    if out[i] < rules[i].b {
                        over += out[i] - rules[i].a;
                        targets.push(i.cast());
                    }
                }

                let avail = target - sum + over;
                increase_targets(out, &mut targets, |i| rules[i].a, avail);
            }
        }

        debug_assert_eq!(
            target,
            (0..N).fold(0, |x, i| x + out[i]),
            "widths {out:?} not equal to target {target}"
        );
    }

    /// Ensure at least one of `rules` has stretch priority at least as high as self
    ///
    /// The stretch policies are increased according to the highest `scores`.
    /// Required: `rules.len() == scores.len()`.
    pub(crate) fn distribute_stretch_over_by(self, rules: &mut [Self], scores: &[u32]) {
        assert_eq!(rules.len(), scores.len());
        if rules.iter().any(|r| r.stretch >= self.stretch) {
            return;
        }

        let highest = scores.iter().cloned().max().unwrap_or(0);
        for i in 0..rules.len() {
            if scores[i] == highest {
                rules[i].stretch = self.stretch;
            }
        }
    }

    /// Adjust a sequence of `rules` to ensure that the total is at least `self`
    ///
    /// This is a cheap hack used by grids to ensure that cell spans are sufficiently large.
    pub(crate) fn distribute_span_over(self, rules: &mut [Self]) {
        let len = rules.len();
        assert!(len > 0);
        let len1 = len - 1;
        let sum: SizeRules = rules.iter().sum();

        rules[0].m.0 = rules[0].m.0.max(self.m.0);
        rules[len1].m.1 = rules[len1].m.1.max(self.m.1);

        let excess_a = (self.a - sum.a).max(0);
        let excess_b = (self.b - sum.b).max(0);
        if excess_a == 0 && excess_b == 0 {
            return;
        }

        let highest_stretch = sum.stretch;
        let count = i32::conv(
            (0..len)
                .filter(|i| rules[*i].stretch == highest_stretch)
                .count(),
        );
        let a_per_elt = excess_a / count;
        let b_per_elt = excess_b / count;
        let mut extra_a = excess_a - count * a_per_elt;
        let mut extra_b = excess_b - count * b_per_elt;
        for rules in rules.iter_mut() {
            if rules.stretch == highest_stretch {
                rules.a += a_per_elt;
                rules.b += b_per_elt;
                if extra_a > 0 {
                    rules.a += 1;
                    extra_a -= 1;
                }
                if extra_b > 0 {
                    rules.b += 1;
                    extra_b -= 1;
                }
                if highest_stretch < self.stretch {
                    rules.stretch = self.stretch;
                }
            }
        }
    }
}

trait SolvePriority: std::fmt::Debug {
    /// Are we giving priority to some widget when increasing/decreasing size?
    ///
    /// - `None` means even distribution
    /// - `Some(true)` means give priority to the *last* item
    /// - `Some(false)` means give priority to the *first* item
    fn priority(&self) -> Option<bool>;
}

#[derive(Debug)]
struct Even;
impl SolvePriority for Even {
    #[inline]
    fn priority(&self) -> Option<bool> {
        None
    }
}

#[derive(Debug)]
struct Prioritize {
    last: bool,
}
impl SolvePriority for Prioritize {
    #[inline]
    fn priority(&self) -> Option<bool> {
        Some(self.last)
    }
}

/// Return the sum over a sequence of rules, assuming these are ordered
///
/// Uses [`SizeRules::appended`] on all rules in sequence.
impl Sum for SizeRules {
    fn sum<I: Iterator<Item = Self>>(mut iter: I) -> Self {
        if let Some(first) = iter.next() {
            iter.fold(first, |x, y| x.appended(y))
        } else {
            SizeRules::EMPTY
        }
    }
}

/// Return the sum over a sequence of rules, assuming these are ordered
///
/// Uses [`SizeRules::appended`] on all rules in sequence.
impl<'a> Sum<&'a Self> for SizeRules {
    fn sum<I: Iterator<Item = &'a Self>>(mut iter: I) -> Self {
        if let Some(first) = iter.next() {
            iter.fold(*first, |x, y| x.appended(*y))
        } else {
            SizeRules::EMPTY
        }
    }
}

type Targets = SmallVec<[i32; 16]>;

fn increase_targets(
    out: &mut [i32],
    targets: &mut Targets,
    base: impl Fn(usize) -> i32,
    mut avail: i32,
) {
    if targets.is_empty() {
        return;
    }

    // Calculate ceiling above which sizes will not be increased
    let mut any_removed = true;
    while any_removed {
        any_removed = false;
        let count = i32::conv(targets.len());
        let ceil = (avail + count - 1) / count; // round up
        let mut t = 0;
        while t < targets.len() {
            let i = usize::conv(targets[t]);
            if out[i] >= base(i) + ceil {
                avail -= out[i] - base(i);
                targets.remove(t);
                any_removed = true;
                continue;
            }
            t += 1;
        }
        if targets.is_empty() {
            return;
        }
    }

    // Since no more are removed by a ceiling, all remaining
    // targets will be (approx) equal. Arbitrarily distribute
    // rounding errors to the first ones.
    let count = i32::conv(targets.len());
    let per_elt = avail / count;
    let extra = usize::conv(avail - per_elt * count);
    assert!(extra < targets.len());
    for t in 0..extra {
        let i = usize::conv(targets[t]);
        out[i] = base(i) + per_elt + 1;
    }
    for t in extra..targets.len() {
        let i = usize::conv(targets[t]);
        out[i] = base(i) + per_elt;
    }
}

/// Reduce targets such that the sum over `i in targets` of `out[i] + base(i)` is `avail`.
///
/// Assumption: for `i in targets`, each `out[i] >= base(i)`.
fn reduce_targets<F: Fn(usize) -> i32>(
    out: &mut [i32],
    targets: &mut Targets,
    base: F,
    mut avail: i32,
) {
    debug_assert!(avail >= 0);
    // We can ignore everything below the floor
    let mut any_removed = true;
    while any_removed {
        any_removed = false;
        let floor = avail / i32::conv(targets.len());
        let mut t = 0;
        while t < targets.len() {
            let i = usize::conv(targets[t]);
            let base = base(i);
            debug_assert!(out[i] >= base);
            if out[i] <= base + floor {
                avail -= out[i] - base;
                targets.remove(t);
                any_removed = true;
                continue;
            }
            t += 1;
        }
    }

    // All targets remaining must be reduced to floor, bar rounding errors
    let floor = avail / i32::conv(targets.len());
    let extra = usize::conv(avail) - usize::conv(floor) * targets.len();
    assert!(extra < targets.len());
    for t in 0..extra {
        let i = usize::conv(targets[t]);
        out[i] = base(i) + floor + 1;
    }
    for t in extra..targets.len() {
        let i = usize::conv(targets[t]);
        out[i] = base(i) + floor;
    }
}