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The last two examples do not overflow, because the result is small enough. This was probably just a typing error in the original explanation.
74 lines
2.9 KiB
Zig
74 lines
2.9 KiB
Zig
//
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// The Zig compiler provides "builtin" functions. You've already
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// gotten used to seeing an @import() at the top of every
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// Ziglings exercise.
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//
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// We've also seen @intCast() in "016_for2.zig", "058_quiz7.zig";
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// and @enumToInt() in "036_enums2.zig".
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//
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// Builtins are special because they are intrinsic to the Zig
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// language itself (as opposed to being provided in the standard
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// library). They are also special because they can provide
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// functionality that is only possible with help from the
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// compiler, such as type introspection (the ability to examine
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// type properties from within a program).
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//
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// Zig currently contains 101 builtin functions. We're certainly
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// not going to cover them all, but we can look at some
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// interesting ones.
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//
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// Before we begin, know that many builtin functions have
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// parameters marked as "comptime". It's probably fairly clear
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// what we mean when we say that these parameters need to be
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// "known at compile time." But rest assured we'll be doing the
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// "comptime" subject real justice soon.
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//
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const print = @import("std").debug.print;
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pub fn main() void {
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// The first builtin, alphabetically, is:
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//
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// @addWithOverflow(comptime T: type, a: T, b: T, result: *T) bool
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// * 'T' will be the type of the other parameters.
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// * 'a' and 'b' are numbers of the type T.
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// * 'result' is a pointer to space you're providing of type T.
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// * The return value is true if the addition resulted in a
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// value over or under the capacity of type T.
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//
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// Let's try it with a tiny 4-bit integer size to make it clear:
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const a: u4 = 0b1101;
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const b: u4 = 0b0101;
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var my_result: u4 = undefined;
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var overflowed: bool = undefined;
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overflowed = @addWithOverflow(u4, a, b, &my_result);
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//
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// The print() below will produce: "1101 + 0101 = 0010 (true)".
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// Let's make sense of this answer by counting up from 1101:
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//
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// Overflowed?
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// 1101 + 1 = 1110 No.
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// 1110 + 1 = 1111 No.
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// 1111 + 1 = 0000 Yes! (Real answer is 10000)
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// 0000 + 1 = 0001 No.
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// 0001 + 1 = 0010 No.
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//
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// Also, check out our fancy formatting! b:0>4 means, "print
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// as a binary number, zero-pad right-aligned four digits."
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print("{b:0>4} + {b:0>4} = {b:0>4} ({})", .{ a, b, my_result, overflowed });
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print(". Furthermore, ", .{});
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// Here's a fun one:
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//
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// @bitReverse(comptime T: type, integer: T) T
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// * 'T' will be the type of the input and output.
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// * 'integer' is the value to reverse.
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// * The return value will be the same type with the
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// value's bits reversed!
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//
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// Now it's your turn. See if you can fix this attempt to use
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// this builtin to reverse the bits of a u8 integer.
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const input: u8 = 0b11110000;
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const tupni: u8 = @bitReverse(input);
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print("{b:0>8} backwards is {b:0>8}.\n", .{ input, tupni });
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}
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