playpen/src/Layout.zig

//! The split tree behind a view.
//!
//! A view's panes form a binary tree: every interior node is a split with an
//! orientation and two children, and every leaf is a pane. That is what lets
//! a view hold layouts a flat list can't express — three panes in a row where
//! one is then dragged to the bottom becomes a vertical split whose top half
//! is the remaining row, so the moved pane spans the full width beneath them.
//!
//! Each split node owns a `GtkPaned`, which gives draggable dividers for
//! free. Split nodes hold a strong reference to their paned and panes hold one
//! to their own widget, so detaching during a rearrangement never destroys
//! anything — widgets are only finalized when their node is.

const std = @import("std");
const gobject = @import("gobject");
const gtk = @import("gtk");

const Pane = @import("Pane.zig");

const Layout = @This();

/// Which side of a target a pane is placed on. The side also determines the
/// orientation of the split created to hold them.
pub const Side = enum {
    left,
    right,
    top,
    bottom,

    pub fn orientation(self: Side) gtk.Orientation {
        return switch (self) {
            .left, .right => .horizontal,
            .top, .bottom => .vertical,
        };
    }

    /// True if the placed pane comes first in the new split.
    pub fn isBefore(self: Side) bool {
        return self == .left or self == .top;
    }
};

pub const Node = struct {
    parent: ?*Node = null,
    kind: union(enum) {
        leaf: *Pane,
        split: Split,
    },

    pub fn widget(self: *Node) *gtk.Widget {
        return switch (self.kind) {
            .leaf => |pane| pane.widget(),
            .split => |s| s.paned.as(gtk.Widget),
        };
    }
};

pub const Split = struct {
    paned: *gtk.Paned,
    a: *Node,
    b: *Node,

    /// Divider position as a fraction of the available space. Kept as a ratio
    /// rather than pixels so it survives window resizes and rebuilds.
    ratio: f64 = 0.5,

    /// Set while we are pushing the ratio into the paned, so the resulting
    /// position change isn't mistaken for the user dragging the divider.
    applying: bool = false,
};

alloc: std.mem.Allocator,
root: ?*Node = null,

pub fn deinit(self: *Layout) void {
    if (self.root) |root| self.destroyTree(root);
    self.root = null;
}

/// Free a subtree's nodes. Panes are owned by the view, not by the tree, so
/// leaves are dropped without touching their pane.
fn destroyTree(self: *Layout, node: *Node) void {
    switch (node.kind) {
        .leaf => {},
        .split => |s| {
            self.destroyTree(s.a);
            self.destroyTree(s.b);
            s.paned.setStartChild(null);
            s.paned.setEndChild(null);
            s.paned.as(gobject.Object).unref();
        },
    }
    self.alloc.destroy(node);
}

pub fn newLeaf(self: *Layout, pane: *Pane) !*Node {
    const node = try self.alloc.create(Node);
    node.* = .{ .kind = .{ .leaf = pane } };
    return node;
}

/// Find the leaf holding `pane`.
pub fn find(self: *Layout, pane: *Pane) ?*Node {
    return findIn(self.root orelse return null, pane);
}

fn findIn(node: *Node, pane: *Pane) ?*Node {
    return switch (node.kind) {
        .leaf => |p| if (p == pane) node else null,
        .split => |s| findIn(s.a, pane) orelse findIn(s.b, pane),
    };
}

/// Place `node` beside `target`, on the given side, by replacing `target`
/// with a new split holding both.
pub fn insert(self: *Layout, node: *Node, target: *Node, side: Side, ratio: f64) !void {
    // Captured before the new split claims target as a child.
    const grandparent = target.parent;

    const first = if (side.isBefore()) node else target;
    const second = if (side.isBefore()) target else node;
    const split = try self.makeSplit(side.orientation(), first, second, ratio);

    split.parent = grandparent;
    if (grandparent) |g| {
        const s = &g.kind.split;
        if (s.a == target) s.a = split else s.b = split;
    } else {
        self.root = split;
    }
}

/// Detach `node` from the tree. Its parent split collapses, so the sibling
/// takes the parent's place and the layout never keeps a split with one child.
pub fn remove(self: *Layout, node: *Node) void {
    const parent = node.parent orelse {
        // Removing the only pane empties the tree.
        self.root = null;
        return;
    };

    const s = &parent.kind.split;
    const sibling = if (s.a == node) s.b else s.a;

    sibling.parent = parent.parent;
    if (parent.parent) |g| {
        const gs = &g.kind.split;
        if (gs.a == parent) gs.a = sibling else gs.b = sibling;
    } else {
        self.root = sibling;
    }

    node.parent = null;
    s.paned.setStartChild(null);
    s.paned.setEndChild(null);
    s.paned.as(gobject.Object).unref();
    self.alloc.destroy(parent);
}

/// Build a split holding two existing subtrees.
///
/// `insert` grows a tree one pane at a time, which is the right shape for
/// interactive splitting but can't express an arbitrary arrangement with
/// per-split ratios. Opening a saved layout needs exactly that, so it builds
/// the tree bottom-up through this instead.
pub fn newSplit(
    self: *Layout,
    orientation: gtk.Orientation,
    a: *Node,
    b: *Node,
    ratio: f64,
) !*Node {
    return self.makeSplit(orientation, a, b, ratio);
}

fn makeSplit(
    self: *Layout,
    orientation: gtk.Orientation,
    a: *Node,
    b: *Node,
    ratio: f64,
) !*Node {
    const node = try self.alloc.create(Node);
    errdefer self.alloc.destroy(node);

    const paned = gtk.Paned.new(orientation);
    // Take ownership of the floating reference so the paned survives being
    // unparented while the tree is reassembled.
    _ = paned.as(gobject.Object).refSink();

    // Both sides share newly available space, and neither may be squeezed to
    // nothing, so a terminal can't be collapsed out of existence by a resize.
    paned.setResizeStartChild(1);
    paned.setResizeEndChild(1);
    paned.setShrinkStartChild(0);
    paned.setShrinkEndChild(0);
    paned.setWideHandle(1);

    node.* = .{ .kind = .{ .split = .{
        .paned = paned,
        .a = a,
        .b = b,
        .ratio = ratio,
    } } };
    a.parent = node;
    b.parent = node;

    _ = gobject.Object.signals.notify.connect(
        paned,
        *Node,
        &onMaxPosition,
        node,
        .{ .detail = "max-position" },
    );
    _ = gobject.Object.signals.notify.connect(
        paned,
        *Node,
        &onPosition,
        node,
        .{ .detail = "position" },
    );

    return node;
}

/// The space available to the paned changed, so re-derive the divider
/// position from the ratio. Doing this on every change (rather than only the
/// first) is what keeps a split's proportions across window resizes.
fn onMaxPosition(paned: *gtk.Paned, _: *gobject.ParamSpec, node: *Node) callconv(.c) void {
    const s = &node.kind.split;
    const max = maxPosition(paned);
    if (max <= 0) return;

    s.applying = true;
    defer s.applying = false;
    paned.setPosition(@intFromFloat(@round(s.ratio * @as(f64, @floatFromInt(max)))));
}

/// Remember where the user dragged the divider to, as a fraction. The ratio
/// is the source of truth, so only user-driven changes update it.
fn onPosition(paned: *gtk.Paned, _: *gobject.ParamSpec, node: *Node) callconv(.c) void {
    const s = &node.kind.split;
    if (s.applying) return;

    const max = maxPosition(paned);
    if (max <= 0) return;

    s.ratio = @as(f64, @floatFromInt(paned.getPosition())) / @as(f64, @floatFromInt(max));
}

fn maxPosition(paned: *gtk.Paned) c_int {
    var value = gobject.ext.Value.new(c_int);
    defer value.unset();
    paned.as(gobject.Object).getProperty("max-position", &value);
    return gobject.ext.Value.get(&value, c_int);
}

/// Rebuild the widget hierarchy from the tree and mount it in `box`.
///
/// Everything is detached first so that reattaching never hits a widget that
/// still has a parent. This is cheap: a view holds a handful of panes, and
/// the terminals themselves are untouched by the reparenting.
pub fn materialize(self: *Layout, box: *gtk.Box) void {
    if (box.as(gtk.Widget).getFirstChild()) |child| box.remove(child);

    const root = self.root orelse return;
    detach(root);
    assemble(root);
    box.append(root.widget());
}

fn detach(node: *Node) void {
    switch (node.kind) {
        .leaf => {},
        .split => |s| {
            detach(s.a);
            detach(s.b);
            s.paned.setStartChild(null);
            s.paned.setEndChild(null);
        },
    }
}

fn assemble(node: *Node) void {
    switch (node.kind) {
        .leaf => {},
        .split => |s| {
            assemble(s.a);
            assemble(s.b);
            s.paned.setStartChild(s.a.widget());
            s.paned.setEndChild(s.b.widget());
        },
    }
}

/// The side `node` sits on within its parent split, and the parent's ratio.
/// Used to put a pane back where it started when a drag is cancelled.
pub fn positionOf(node: *Node) ?struct { sibling: *Node, side: Side, ratio: f64 } {
    const parent = node.parent orelse return null;
    const s = parent.kind.split;
    const first = s.a == node;
    const vertical = s.paned.as(gtk.Orientable).getOrientation() == .vertical;

    return .{
        .sibling = if (first) s.b else s.a,
        .side = if (vertical)
            (if (first) .top else .bottom)
        else
            (if (first) .left else .right),
        .ratio = s.ratio,
    };
}