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//
// The Zig compiler provides "builtin" functions. You've already
// gotten used to seeing an @import() at the top of every
// Ziglings exercise.
//
// We've also seen @intCast() in "016_for2.zig", "058_quiz7.zig";
// and @enumToInt() in "036_enums2.zig".
//
// Builtins are special because they are intrinsic to the Zig
// language itself (as opposed to being provided in the standard
// library). They are also special because they can provide
// functionality that is only possible with help from the
// compiler, such as type introspection (the ability to examine
// type properties from within a program).
//
// Zig currently contains 101 builtin functions. We're certainly
// not going to cover them all, but we can look at some
// interesting ones.
//
// Before we begin, know that many builtin functions have
// parameters marked as "comptime". It's probably fairly clear
// what we mean when we say that these parameters need to be
// "known at compile time." But rest assured we'll be doing the
// "comptime" subject real justice soon.
//
const print = @import("std").debug.print;
pub fn main() void {
// The first builtin, alphabetically, is:
//
// @addWithOverflow(comptime T: type, a: T, b: T, result: *T) bool
// * 'T' will be the type of the other parameters.
// * 'a' and 'b' are numbers of the type T.
// * 'result' is a pointer to space you're providing of type T.
// * The return value is true if the addition resulted in a
// value over or under the capacity of type T.
//
// Let's try it with a tiny 4-bit integer size to make it clear:
const a: u4 = 0b1101;
const b: u4 = 0b0101;
var my_result: u4 = undefined;
var overflowed: bool = undefined;
overflowed = @addWithOverflow(u4, a, b, &my_result);
// Check out our fancy formatting! b:0>4 means, "print
// as a binary number, zero-pad right-aligned four digits."
// The print() below will produce: "1101 + 0101 = 0010 (true)".
print("{b:0>4} + {b:0>4} = {b:0>4} ({})", .{ a, b, my_result, overflowed });
// Let's make sense of this answer. The value of 'b' in decimal is 5.
// Let's add 5 to 'a' but go one by one and see where it overflows:
//
// a | b | result | overflowed?
// ----------------------------------
// 1101 + 0001 = 1110 | false
// 1110 + 0001 = 1111 | false
// 1111 + 0001 = 0000 | true (the real answer is 10000)
// 0000 + 0001 = 0001 | false
// 0001 + 0001 = 0010 | false
//
// In the last two lines the value of 'a' is corrupted because there was
// an overflow in line 3, but the operations of lines 4 and 5 themselves
// do not overflow.
// There is a difference between
// - a value, that overflowed at some point and is now corrupted
// - a single operation that overflows and maybe causes subsequent errors
// In practise we usually notice the overflowed value first and have to work
// our way backwards to the operation that caused the overflow.
//
// If there was no overflow at all while adding 5 to a, what value would
// 'my_result' hold? Write the answer in into 'expected_result'.
const expected_result: u8 = ???;
print(". Without overflow: {b:0>8}. ", .{expected_result});
print("Furthermore, ", .{});
// Here's a fun one:
//
// @bitReverse(comptime T: type, integer: T) T
// * 'T' will be the type of the input and output.
// * 'integer' is the value to reverse.
// * The return value will be the same type with the
// value's bits reversed!
//
// Now it's your turn. See if you can fix this attempt to use
// this builtin to reverse the bits of a u8 integer.
const input: u8 = 0b11110000;
const tupni: u8 = @bitReverse(input);
print("{b:0>8} backwards is {b:0>8}.\n", .{ input, tupni });
}
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