Insert space for additional async exercises

exercises/115_packed2.zig

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+//
+// We've already learned about switch statements in exercises 030, 031 and 108.
+// They also work with packed containers:
+
+const S = packed struct(u2) {
+    a: bool,
+    b: i1,
+};
+
+// Try to make it compile without adding an `else` prong!
+
+comptime {
+    const s: S = .{ .a = true, .b = -1 };
+    switch (s) {
+        .{ .a = true, .b = -1 } => {}, // ok!
+        .{ .a = true, .b = ??? },
+        .{ .a = ???, .b = 0 },
+        .{ .a = ???, .b = ??? },
+        => @compileError("We don't want to end up here!"),
+    }
+}
+
+// As we can see, switching on packed structs is pretty straightforward.
+// When switching on packed unions however, we'll realize that a packed
+// union never keeps track of its active tag, not even in debug mode! This
+// means that packed unions compare solely by their bit pattern (again, just
+// like integers).
+
+const U = packed union(u2) {
+    a: u2,
+    b: i2,
+};
+
+// Find and remove the duplicate case!
+
+comptime {
+    const u: U = .{ .a = 3 };
+    switch (u) {
+        .{ .a = 3 } => {}, // ok!
+        .{ .a = 2 },
+        .{ .b = 1 },
+        .{ .b = -1 },
+        .{ .a = 0 },
+        => @compileError("We don't want to end up here!"),
+    }
+}
+
+// Since packed unions don't have the concept of an active tag, it's always legal
+// to access any of their fields. This can be useful to view the same data from
+// different perspectives seamlessly.
+//
+// Try to make the float below negative:
+
+/// IEEE 754 half precision float
+const Float = packed union(u16) {
+    value: f16,
+    bits: packed struct(u16) {
+        mantissa: u10,
+        exponent: u5,
+        sign: u1,
+    },
+};
+
+pub fn main() void {
+    // Reminder: if the sign bit of a float is set, the number is negative!
+
+    var number: Float = .{ .value = 2.34 };
+    number.bits.??? = ???;
+    if (number.value != -2.34) {
+        std.debug.print("Make it negative!\n", .{});
+    }
+}
+
+// This concludes our introduction to packed containers. The next time you need
+// control over individual bits, keep them in mind as a potent alternative!
+//
+
+const std = @import("std");