test/guide/language/comptime.zig

const expect = @import("std").testing.expect;
const print = @import("std").debug.print;

// Blocks of code may be forcibly executed at compile time using the `comptime` keyword.
// In this example, the variables x and y are equivalent.
fn fibonacci(n: u16) u16 {
    if (n == 0 or n == 1) return n;
    return fibonacci(n - 1) + fibonacci(n - 2);
}

test "comptime blocks" {
    const x = comptime fibonacci(10);
    const y = comptime blk: {
        break :blk fibonacci(10);
    };
    try expect(y == 55);
    try expect(x == 55);
}

// Integer literals are of the type `comptime_int`.
// These are special in a way that they have no size (they cannot be used at runtime!), and they have arbitrary precision.
// `comptime_int` values coerce to any integer type that can hold them. They also coerce to floats.
// Character literals are of this type.
test "comptime_int" {
    const a = 12;
    const b = a + 10;

    const c: u4 = a;
    const d: f32 = b;

    try expect(c == 12);
    try expect(d == 22);
}

// Types in Zig are values of the type type.
// These are available at compile time.
// We have previously encountered them by checking `@TypeOf` and comparing with other types, but we can do more.
test "branching on types" {
    const a = 5;
    const b: if (a < 10) f32 else i32 = 5;
    print("b: {}, {any}\n", .{ b, @TypeOf(b) });
    try expect(b == 5);
    try expect(@TypeOf(b) == f32);
}

// Function parameters in Zig can be tagged as being `comptime`.
// This means that the value passed to that function parameter must be known at compile time.
// Let's make a function that returns a type.
// Notice how this function is PascalCase, as it returns a type.
fn Matrix(
    comptime T: type,
    comptime width: comptime_int,
    comptime height: comptime_int,
) type {
    return [height][width]T;
}

test "returning a type" {
    try expect(Matrix(f32, 4, 4) == [4][4]f32);
}

// We can reflect upon types using the built-in `@typeInfo`, which takes in a type and returns a tagged union.
// This tagged union type can be found in `std.builtin.Type` (info on how to make use of imports and `std` later).
fn addSmallInts(comptime T: type, a: T, b: T) T {
    return switch (@typeInfo(T)) {
        // .ComptimeInt => a + b,
        // .Int => |info| if (info.bits <= 16) a + b else @compileError("ints too large"),
        // FIXME: To pass the test, we need to use the `int` tag instead of the `Int` tag and no `.Comptime` field.
        // - There should be no fields `.Int` and `.ComptimeInt` in `@typeinfo(u16 or i16)` on `zig-1.15.2` version.
        .int => |info| if (info.bits <= 16) a + b else @compileError("ints too large"),
        else => @compileError("only ints accepted"),
    };
}

test "typeinfo switch" {
    const x = addSmallInts(u7, 20, 30);
    try expect(@TypeOf(x) == u7);
    try expect(x == 50);
}

// We can use the `@Type` function to create a type from a `@typeInfo`.
// `@Type` is implemented for most types but is notably unimplemented for enums, unions, functions, and structs.
// Here anonymous struct syntax is used with
// `.{}`, because the `T` in `T{}` can be inferred. Anonymous structs will be covered in detail later. In this example we will get a compile error if the Int tag isn't set.
fn GetBiggerInt(comptime T: type) type {
    return @Type(.{
        // .Int = .{
        //     .bits = @typeInfo(T).Int.bits + 1,
        //     .signedness = @typeInfo(T).Int.signedness,
        // FIXME: To pass the test, we need to use the `Int` tag instead of the `int` tag and no `.Comptime` field.
        .int = .{
            .bits = @typeInfo(T).int.bits + 1,
            .signedness = @typeInfo(T).int.signedness,
        },
    });
}

test "@Type" {
    try expect(GetBiggerInt(u8) == u9);
    try expect(GetBiggerInt(i31) == i32);
}

// Returning a struct type is how you make generic data structures in Zig.
// The usage of `@This` is required here, which gets the type of the innermost struct, union, or enum.
// Here `std.mem.eql` is also used which compares two slices.
fn Vec(
    comptime count: comptime_int,
    comptime T: type,
) type {
    return struct {
        data: [count]T,
        const Self = @This();

        fn abs(self: Self) Self {
            var tmp = Self{ .data = undefined };
            for (self.data, 0..) |elem, i| {
                tmp.data[i] = if (elem < 0)
                    -elem
                else
                    elem;
            }
            return tmp;
        }

        fn init(data: [count]T) Self {
            return Self{ .data = data };
        }
    };
}

const eql = @import("std").mem.eql;

test "generic vector" {
    const x = Vec(3, f32).init([_]f32{ 10, -10, 5 });
    const y = x.abs();
    try expect(eql(f32, &y.data, &[_]f32{ 10, 10, 5 }));
}

// The types of function parameters can also be inferred by using `anytype` in place of a type.
// `@TypeOf` can then be used on the parameter.
fn plusOne(x: anytype) @TypeOf(x) {
    return x + 1;
}

test "inferred function parameter" {
    try expect(plusOne(@as(u32, 1)) == 2);
}

// `comptime` also introduces the operators `++` and `**` for concatenating and repeating arrays and slices.
// These operators do not work at runtime.
test "++" {
    const x: [4]u8 = undefined;
    const y = x[0..];

    const a: [6]u8 = undefined;
    const b = a[0..];

    const new = y ++ b;
    try expect(new.len == 10);
}

test "**" {
    const pattern = [_]u8{ 0xCC, 0xAA };
    const memory = pattern ** 3;
    try expect(eql(u8, &memory, &[_]u8{ 0xCC, 0xAA, 0xCC, 0xAA, 0xCC, 0xAA }));
}