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//
// You can also put 'comptime' before a function parameter to
// enforce that the argument passed to the function must be known
// at compile time. We've actually been using a function like
// this the entire time, std.debug.print():
//
// fn print(comptime fmt: []const u8, args: anytype) void
//
// Notice that the format string parameter 'fmt' is marked as
// 'comptime'. One of the neat benefits of this is that the
// format string can be checked for errors at compile time rather
// than crashing at runtime.
//
// (The actual formatting is done by std.fmt.format() and it
// contains a complete format string parser that runs entirely at
// compile time!)
//
const print = @import("std").debug.print;
// This struct is the model of a model boat. We can transform it
// to any scale we would like: 1:2 is half-size, 1:32 is
// thirty-two times smaller than the real thing, and so forth.
const Schooner = struct {
name: []const u8,
scale: u32 = 1,
hull_length: u32 = 143,
bowsprit_length: u32 = 34,
mainmast_height: u32 = 95,
fn scaleMe(self: *Schooner, comptime scale: u32) void {
var my_scale = scale;
// We did something neat here: we've anticipated the
// possibility of accidentally attempting to create a
// scale of 1:0. Rather than having this result in a
// divide-by-zero error at runtime, we've turned this
// into a compile error.
//
// This is probably the correct solution most of the
// time. But our model boat model program is very casual
// and we just want it to "do what I mean" and keep
// working.
//
// Please change this so that it sets a 0 scale to 1
// instead.
if (my_scale == 0) @compileError("Scale 1:0 is not valid!");
self.scale = my_scale;
self.hull_length /= my_scale;
self.bowsprit_length /= my_scale;
self.mainmast_height /= my_scale;
}
fn printMe(self: Schooner) void {
print("{s} (1:{}, {} x {})\n", .{
self.name,
self.scale,
self.hull_length,
self.mainmast_height,
});
}
};
pub fn main() void {
var whale = Schooner{ .name = "Whale" };
var shark = Schooner{ .name = "Shark" };
var minnow = Schooner{ .name = "Minnow" };
// Hey, we can't just pass this runtime variable as an
// argument to the scaleMe() method. What would let us do
// that?
var scale: u32 = undefined;
scale = 32; // 1:32 scale
minnow.scaleMe(scale);
minnow.printMe();
scale -= 16; // 1:16 scale
shark.scaleMe(scale);
shark.printMe();
scale -= 16; // 1:0 scale (oops, but DON'T FIX THIS!)
whale.scaleMe(scale);
whale.printMe();
}
//
// Going deeper:
//
// What would happen if you DID attempt to build a model in the
// scale of 1:0?
//
// A) You're already done!
// B) You would suffer a mental divide-by-zero error.
// C) You would construct a singularity and destroy the
// planet.
//
// And how about a model in the scale of 0:1?
//
// A) You're already done!
// B) You'd arrange nothing carefully into the form of the
// original nothing but infinitely larger.
// C) You would construct a singularity and destroy the
// planet.
//
// Answers can be found on the back of the Ziglings packaging.
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