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84 lines
3.5 KiB
Zig
84 lines
3.5 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 @intFromEnum() 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 contains over 100 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 second builtin, alphabetically, is:
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// @addWithOverflow(a: anytype, b: anytype) struct { @TypeOf(a, b), u1 }
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// * 'a' and 'b' are numbers of anytype.
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// * The return value is a tuple with the result and a possible overflow bit.
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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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const my_result = @addWithOverflow(a, b);
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// 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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// The print() below will produce: "1101 + 0101 = 0010 (true)".
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print("{b:0>4} + {b:0>4} = {b:0>4} ({s})", .{ a, b, my_result[0], if (my_result[1] == 1) "true" else "false" });
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// Let's make sense of this answer. The value of 'b' in decimal is 5.
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// Let's add 5 to 'a' but go one by one and see where it overflows:
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//
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// a | b | result | overflowed?
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// ----------------------------------
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// 1101 + 0001 = 1110 | false
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// 1110 + 0001 = 1111 | false
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// 1111 + 0001 = 0000 | true (the real answer is 10000)
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// 0000 + 0001 = 0001 | false
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// 0001 + 0001 = 0010 | false
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//
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// In the last two lines the value of 'a' is corrupted because there was
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// an overflow in line 3, but the operations of lines 4 and 5 themselves
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// do not overflow.
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// There is a difference between
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// - a value, that overflowed at some point and is now corrupted
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// - a single operation that overflows and maybe causes subsequent errors
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// In practice we usually notice the overflowed value first and have to work
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// our way backwards to the operation that caused the overflow.
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//
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// If there was no overflow at all while adding 5 to a, what value would
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// 'my_result' hold? Write the answer in into 'expected_result'.
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const expected_result: u8 = ???;
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print(". Without overflow: {b:0>8}. ", .{expected_result});
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print("Furthermore, ", .{});
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// Here's a fun one:
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//
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// @bitReverse(integer: anytype) T
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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, tupni);
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print("{b:0>8} backwards is {b:0>8}.\n", .{ input, tupni });
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}
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