Crate a binary tree

create_union_enum.zig

const std = @import("std");
const expect = std.testing.expect;

const cell = struct {
    v1: ?*Variant,
    v2: ?*Variant,
};

const Variant = union(enum) {
    int: i32,
    boolean: bool,
    cell: cell,

    // void can be omitted when inferring enum tag type.
    none,

    fn truthy(self: Variant) bool {
        return switch (self) {
            Variant.int => |x_int| x_int != 0,
            Variant.boolean => |x_bool| x_bool,
            Variant.cell => false,
            Variant.none => false,
        };
    }

    fn deinit(self: *Variant, allocator: std.mem.Allocator) void {
        switch (self.*) {
            .cell => |*c| {
                if (c.v1) |v1| v1.deinit(allocator);
                if (c.v2) |v2| v2.deinit(allocator);
            },
            else => {},
        }
        allocator.destroy(self);
    }
};

test "union method" {
    var v1 = Variant{ .int = 1 };
    var v2 = Variant{ .boolean = false };

    try expect(v1.truthy());
    try expect(!v2.truthy());
}

test "create union" {
    const allocator = std.testing.allocator;

    var v1 = try allocator.create(Variant);
    defer allocator.destroy(v1);

    v1.* = .{ .int = 1 };
    try expect(v1.truthy());
}

test "create union" {
    const allocator = std.testing.allocator;

    var top = try allocator.create(Variant);
    top.* = .{ .cell = cell{ .v1 = null, .v2 = null } };
    defer top.deinit(allocator);

    var p = top;
    var i: i32 = 0;
    while (i < 3) : (i += 1) {
        var left = try allocator.create(Variant);
        left.* = .{ .int = i + 1 };
        p.cell.v1 = left;

        var right = try allocator.create(Variant);
        right.* = .{ .cell = cell{ .v1 = null, .v2 = null } };
        p.cell.v2 = right;

        p = p.cell.v2.?;
    }

    try std.testing.expectEqual(@as(i32, 2), top.cell.v2.?.cell.v1.?.int);
    std.log.warn("{}", .{top.cell.v2.?.cell.v1}); // top から辿ると 2 であって欲しい
}

test.log

```
$ zig test create_union_enum.zig 
Test [3/3] test "create union"... [default] (warn): Variant{ .int = 2 }
All 3 tests passed.
$ zig version
0.10.0-dev.2989+4a28c1d5c
```