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