ziglings/exercises/110_quiz9.zig

// ----------------------------------------------------------------------------
// Toggling, Setting, and Clearing Bits
// ----------------------------------------------------------------------------
//
// Another exciting thing about Zig is its suitability for embedded
// programming. Your Zig code doesn't have to remain on your laptop. You can
// also deploy your code to microcontrollers! This means you can write Zig to
// drive your next robot or greenhouse climate control system! Ready to enter
// the exciting world of embedded programming? This exercise is designed to
// give you a taste of what it's like to control registers in a
// microcontroller. Let's get started!
//
// A common activity in microcontroller programming is setting and clearing
// bits on input and output pins. This lets you control LEDs, sensors, motors
// and more! In a previous exercise (097_bit_manipulation.zig) you learned how
// to swap two bytes using the ^ (XOR - exclusive or) operator. In this
// exercise, we'll take a closer look at bit manipulation and how we can write
// code that sets and clears specific bits as we would if we were programming
// the pins on a real microcontroller. Included at the end of this exercise are
// some helper functions that demonstrate how we might make our code a little
// more readable.
//
// Below is a pinout diagram for the famous ATmega328 AVR microcontroller used
// as the primary microchip on popular microcontroller platforms like the
// Arduino UNO.
//
//  ============ PINOUT DIAGRAM FOR ATMEGA328 MICROCONTROLLER ============
//                                _____ _____
//                               |     U     |
//                 (RESET) PC6 --|  1     28 |-- PC5
//                         PD0 --|  2     27 |-- PC4
//                         PD1 --|  3     26 |-- PC3
//                         PD2 --|  4     25 |-- PC2
//                         PD3 --|  5     24 |-- PC1
//                         PD4 --|  6     23 |-- PC0
//                         VCC --|  7     22 |-- GND
//                         GND --|  8     21 |-- AREF
//                     |-- PB6 --|  9     20 |-- AVCC
//                     |-- PB7 --| 10     19 |-- PB5 --|
//                     |   PD5 --| 11     18 |-- PB4 --|
//                     |   PD6 --| 12     17 |-- PB3 --|
//                     |   PD7 --| 13     16 |-- PB2 --|
//                     |-- PB0 --| 14     15 |-- PB1 --|
//                     |         |___________|         |
//                     \_______________________________/
//                                    |
//                                  PORTB
//
// Drawing inspiration from this diagram, we'll use the pins for PORTB as our
// mental model for this exercise in bit manipulation. It should be noted that
// in the following examples we are using ordinary variables, one of which we
// have named PORTB, to simulate modifying the bits of real hardware registers.
// But in actual microcontroller code, PORTB would be defined something like
// this:
//          pub const PORTB = @as(*volatile u8, @ptrFromInt(0x25));
//
// This lets the compiler know not to make any optimizations to PORTB so that
// the IO pins are properly mapped to our code.
//
// NOTE : To keep things simple, the following examples are given using type
// u4, so applying the output to PORTB would only affect the lower four pins
// PB0..PB3. Of course, there is nothing to prevent you from swapping the u4
// with a u8 so you can control all 8 of PORTB's IO pins.

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

pub fn main() !void {
    var PORTB: u4 = 0b0000; // only 4 bits wide for simplicity
    //
    // Let's first take a look at toggling bits.
    //
    // ------------------------------------------------------------------------
    // Toggling bits with XOR:
    // ------------------------------------------------------------------------
    // XOR stands for "exclusive or". We can toggle bits with the ^ (XOR)
    // bitwise operator, like so:
    //
    //
    // In order to output a 1, the logic of an XOR operation requires that the
    // two input bits are of different values. Therefore, 0 ^ 1 and 1 ^ 0 will
    // both yield a 1 but 0 ^ 0 and 1 ^ 1 will output 0. XOR's unique behavior
    // of outputing a 0 when both inputs are 1s is what makes it different from
    // the OR operator; it also gives us the ability to toggle bits by putting
    // 1s into our bitmask.
    //
    // - 1s in our bitmask operand, can be thought of as causing the
    //   corresponding bits in the other operand to flip to the opposite value.
    // - 0s cause no change.
    //
    //                            The 0s in our bitmask preserve these values
    // -XOR op- ---expanded---    in the output.
    //            _______________/
    //           /       /
    //   0110   1   1   0   0
    // ^ 1111   0   1   0   1 (bitmask)
    // ------   -   -   -   -
    // = 1001   1   0   0   1 <- This bit was already cleared.
    //              \_______\
    //                       \
    //                         We can think of these bits having flipped
    //                         because of the presence of 1s in those columns
    //                         of our bitmask.

    print("Toggle pins with XOR on PORTB\n", .{});
    print("-----------------------------\n", .{});
    PORTB = 0b1100;
    print("  {b:0>4} // (initial state of PORTB)\n", .{PORTB});
    print("^ {b:0>4} // (bitmask)\n", .{0b0101});
    PORTB ^= (1 << 1) | (1 << 0); // What's wrong here?
    checkAnswer(0b1001, PORTB);

    newline();

    PORTB = 0b1100;
    print("  {b:0>4} // (initial state of PORTB)\n", .{PORTB});
    print("^ {b:0>4} // (bitmask)\n", .{0b0011});
    PORTB ^= (1 << 1) & (1 << 0); // What's wrong here?
    checkAnswer(0b1111, PORTB);

    newline();

    // Now let's take a look at setting bits with the | operator.
    //
    // ------------------------------------------------------------------------
    // Setting bits with OR:
    // ------------------------------------------------------------------------
    // We can set bits on PORTB with the | (OR) operator, like so:
    //
    // var PORTB: u4 = 0b1001;
    // PORTB = PORTB | 0b0010;
    // print("PORTB: {b:0>4}\n", .{PORTB}); // output: 1011
    //
    // -OR op-  ---expanded---
    //                    _ Set only this bit.
    //                   /
    //   1001   1   0   0   1
    // | 0010   0   0   1   0 (bit mask)
    // ------   -   -   -   -
    // = 1011   1   0   1   1
    //           \___\_______\
    //                        \
    //                          These bits remain untouched because OR-ing with
    //                          a 0 effects no change.
    //
    // ------------------------------------------------------------------------
    // To create a bit mask like 0b0010 used above:
    //
    // 1. First, shift the value 1 over one place with the bitwise << (shift
    // left) operator as indicated below:
    //           1 << 0 -> 0001
    //           1 << 1 -> 0010  <-- Shift 1 one place to the left
    //           1 << 2 -> 0100
    //           1 << 3 -> 1000
    //
    // This allows us to rewrite the above code like this:
    //
    // var PORTB: u4 = 0b1001;
    // PORTB = PORTB | (1 << 1);
    // print("PORTB: {b:0>4}\n", .{PORTB}); // output: 1011
    //
    // Finally, as in the C language, Zig allows us to use the |= operator, so
    // we can rewrite our code again in an even more compact and idiomatic
    // form: PORTB |= (1 << 1)

    print("Set pins with OR on PORTB\n", .{});
    print("-------------------------\n", .{});

    PORTB = 0b1001; // reset PORTB
    print("  {b:0>4} // (initial state of PORTB)\n", .{PORTB});
    print("| {b:0>4} // (bitmask)\n", .{0b0100});
    PORTB = PORTB ??? (1 << 2); // What's missing here?
    checkAnswer(0b1101, PORTB);

    newline();

    PORTB = 0b1001; // reset PORTB
    print("  {b:0>4} // (reset state)\n", .{PORTB});
    print("| {b:0>4} // (bitmask)\n", .{0b0100});
    PORTB ??? (1 << 2); // What's missing here?
    checkAnswer(0b1101, PORTB);

    newline();

    // So now we've covered how to toggle and set bits. What about clearing
    // them? Well, this is where Zig throws us a curve ball. Don't worry we'll
    // go through it step by step.

    // ------------------------------------------------------------------------
    // Clearing bits with AND and NOT:
    // ------------------------------------------------------------------------
    // We can clear bits with the & (AND) bitwise operator, like so:

    // PORTB = 0b1110; // reset PORTB
    // PORTB = PORTB & 0b1011;
    // print("PORTB: {b:0>4}\n", .{PORTB}); // output -> 1010
    //
    // - 0s clear bits when used in conjuction with a bitwise AND.
    // - 1s do nothing, thus preserving the original bits.
    //
    // -AND op- ---expanded---
    //                __________ Clear only this bit.
    //               /
    //   1110   1   1   1   0
    // & 1011   1   0   1   1 (bit mask)
    // ------   -   -   -   -
    // = 1010   1   0   1   0 <- This bit was already cleared.
    //           \_______\
    //                    \
    //                      These bits remain untouched because AND-ing with a
    //                      1 preserves the original bit value whether 0 or 1.
    //
    // ------------------------------------------------------------------------
    // We can use the ~ (NOT) operator to easily create a bit mask like 1011:
    //
    //  1. First, shift the value 1 over two places with the bit-wise << (shift
    //     left) operator as indicated below:
    //          1 << 0 -> 0001
    //          1 << 1 -> 0010
    //          1 << 2 -> 0100 <- The 1 has been shifted two places to the left
    //          1 << 3 -> 1000
    //
    //  2. The second step in creating our bit mask is to invert the bits
    //          ~0100 -> 1011
    //     in C we would write this as:
    //          ~(1 << 2) -> 1011
    //
    //     But if we try to compile ~(1 << 2) in Zig, we'll get an error:
    //          unable to perform binary not operation on type 'comptime_int'
    //
    //     Before Zig can invert our bits, it needs to know the number of
    //     bits it's being asked to invert.
    //
    //     We do this with the @as (cast as) built-in like this:
    //          @as(u4, 1 << 2) -> 0100
    //
    //     Finally, we can invert our new mask by placing the NOT ~ operator
    //     before our expression, like this:
    //          ~@as(u4, 1 << 2) -> 1011
    //
    //     If you are offput by the fact that you can't simply invert bits like
    //     you can in languages such as C without casting to a particular size
    //     of integer, you're not alone. However, this is actually another
    //     instance where Zig is really helpful because it protects you from
    //     difficult to debug integer overflow bugs that can have you tearing
    //     your hair out. In the interest of keeping things sane, Zig requires
    //     you simply to tell it the size of number you are inverting. In the
    //     words of Andrew Kelley, "If you want to invert the bits of an
    //     integer, zig has to know how many bits there are."
    //
    //     For more insight into the Zig team's position on why the language
    //     takes the approach it does with the ~ operator, take a look at
    //     Andrew's comments on the following github issue:
    //     https://github.com/ziglang/zig/issues/1382#issuecomment-414459529
    //
    // Whew, so after all that what we end up with is:
    //          PORTB = PORTB & ~@as(u4, 1 << 2);
    //
    // We can shorten this with the &= combined AND and assignment operator,
    // which applies the AND operator on PORTB and then reassigns PORTB. Here's
    // what that looks like:
    //          PORTB &= ~@as(u4, 1 << 2);
    //

    print("Clear pins with AND and NOT on PORTB\n", .{});
    print("------------------------------------\n", .{});

    PORTB = 0b1110; // reset PORTB
    print("  {b:0>4} // (initial state of PORTB)\n", .{PORTB});
    print("& {b:0>4} // (bitmask)\n", .{0b1011});
    PORTB = PORTB & ???@as(u4, 1 << 2); // What character is missing here?
    checkAnswer(0b1010, PORTB);

    newline();

    PORTB = 0b0111; // reset PORTB
    print("  {b:0>4} // (reset state)\n", .{PORTB});
    print("& {b:0>4} // (bitmask)\n", .{0b1110});
    PORTB &= ~(1 << 0); // What's missing here?
    checkAnswer(0b0110, PORTB);

    newline();
    newline();

    // ------------------------------------------------------------------------
    // Conclusion
    // ------------------------------------------------------------------------
    //
    // While the examples in this exercise have used only 4-bit wide variables,
    // working with 8 bits is no different. Here's a an example where we set
    // every other bit beginning with the two's place:

    // var PORTD: u8 = 0b0000_0000;
    // print("PORTD: {b:0>8}\n", .{PORTD});
    // PORTD |= (1 << 1);
    // PORTD = setBit(u8, PORTD, 3);
    // PORTD |= (1 << 5) | (1 << 7);
    // print("PORTD: {b:0>8} // set every other bit\n", .{PORTD});
    // PORTD = ~PORTD;
    // print("PORTD: {b:0>8} // bits flipped with NOT (~)\n", .{PORTD});
    // newline();
    //
    // // Here we clear every other bit beginning with the two's place.
    //
    // PORTD = 0b1111_1111;
    // print("PORTD: {b:0>8}\n", .{PORTD});
    // PORTD &= ~@as(u8, 1 << 1);
    // PORTD = clearBit(u8, PORTD, 3);
    // PORTD &= ~@as(u8, (1 << 5) | (1 << 7));
    // print("PORTD: {b:0>8} // clear every other bit\n", .{PORTD});
    // PORTD = ~PORTD;
    // print("PORTD: {b:0>8} // bits flipped with NOT (~)\n", .{PORTD});
    // newline();
}

// ----------------------------------------------------------------------------
// Here are some helper functions for manipulating bits
// ----------------------------------------------------------------------------

// Functions for setting, clearing, and toggling a single bit
fn setBit(comptime T: type, byte: T, comptime bit_pos: T) !T {
    return byte | (1 << bit_pos);
}

test "setBit" {
    try testing.expectEqual(setBit(u8, 0b0000_0000, 3), 0b0000_1000);
}

fn clearBit(comptime T: type, byte: T, comptime bit_pos: T) T {
    return byte & ~@as(T, (1 << bit_pos));
}

test "clearBit" {
    try testing.expectEqual(clearBit(u8, 0b1111_1111, 0), 0b1111_1110);
}

fn toggleBit(comptime T: type, byte: T, comptime bit_pos: T) T {
    return byte ^ (1 << bit_pos);
}

test "toggleBit" {
    var byte = toggleBit(u8, 0b0000_0000, 0);
    try testing.expectEqual(byte, 0b0000_0001);
    byte = toggleBit(u8, byte, 0);
    try testing.expectEqual(byte, 0b0000_0000);
}

// ----------------------------------------------------------------------------
// Some additional functions for setting, clearing, and toggling multiple bits
// at once with a tuple because, hey, why not?
// ----------------------------------------------------------------------------
//

fn createBitmask(comptime T: type, comptime bits: anytype) !T {
    comptime var bitmask: T = 0;
    inline for (bits) |bit| {
        if (bit >= @bitSizeOf(T)) return error.BitPosTooLarge;
        if (bit < 0) return error.BitPosTooSmall;

        bitmask |= (1 << bit);
    }
    return bitmask;
}

test "creating bitmasks from a tuple" {
    try testing.expectEqual(createBitmask(u8, .{0}), 0b0000_0001);
    try testing.expectEqual(createBitmask(u8, .{1}), 0b0000_0010);
    try testing.expectEqual(createBitmask(u8, .{2}), 0b0000_0100);
    try testing.expectEqual(createBitmask(u8, .{3}), 0b0000_1000);
    //
    try testing.expectEqual(createBitmask(u8, .{ 0, 4 }), 0b0001_0001);
    try testing.expectEqual(createBitmask(u8, .{ 1, 5 }), 0b0010_0010);
    try testing.expectEqual(createBitmask(u8, .{ 2, 6 }), 0b0100_0100);
    try testing.expectEqual(createBitmask(u8, .{ 3, 7 }), 0b1000_1000);

    try testing.expectError(error.BitPosTooLarge, createBitmask(u4, .{4}));
}

fn setBits(byte: u8, bits: anytype) !u8 {
    const bitmask = try createBitmask(u8, bits);
    return byte | bitmask;
}

test "setBits" {
    try testing.expectEqual(setBits(0b0000_0000, .{0}), 0b0000_0001);
    try testing.expectEqual(setBits(0b0000_0000, .{7}), 0b1000_0000);

    try testing.expectEqual(setBits(0b0000_0000, .{ 0, 1, 2, 3, 4, 5, 6, 7 }), 0b1111_1111);
    try testing.expectEqual(setBits(0b1111_1111, .{ 0, 1, 2, 3, 4, 5, 6, 7 }), 0b1111_1111);

    try testing.expectEqual(setBits(0b0000_0000, .{ 2, 3, 4, 5 }), 0b0011_1100);

    try testing.expectError(error.BitPosTooLarge, setBits(0b1111_1111, .{8}));
    try testing.expectError(error.BitPosTooSmall, setBits(0b1111_1111, .{-1}));
}

fn clearBits(comptime byte: u8, comptime bits: anytype) !u8 {
    const bitmask: u8 = try createBitmask(u8, bits);
    return byte & ~@as(u8, bitmask);
}

test "clearBits" {
    try testing.expectEqual(clearBits(0b1111_1111, .{0}), 0b1111_1110);
    try testing.expectEqual(clearBits(0b1111_1111, .{7}), 0b0111_1111);

    try testing.expectEqual(clearBits(0b1111_1111, .{ 0, 1, 2, 3, 4, 5, 6, 7 }), 0b000_0000);
    try testing.expectEqual(clearBits(0b0000_0000, .{ 0, 1, 2, 3, 4, 5, 6, 7 }), 0b000_0000);

    try testing.expectEqual(clearBits(0b1111_1111, .{ 0, 1, 6, 7 }), 0b0011_1100);

    try testing.expectError(error.BitPosTooLarge, clearBits(0b1111_1111, .{8}));
    try testing.expectError(error.BitPosTooSmall, clearBits(0b1111_1111, .{-1}));
}

fn toggleBits(comptime byte: u8, comptime bits: anytype) !u8 {
    const bitmask = try createBitmask(u8, bits);
    return byte ^ bitmask;
}

test "toggleBits" {
    try testing.expectEqual(toggleBits(0b0000_0000, .{0}), 0b0000_0001);
    try testing.expectEqual(toggleBits(0b0000_0000, .{7}), 0b1000_0000);

    try testing.expectEqual(toggleBits(0b1111_1111, .{ 0, 1, 2, 3, 4, 5, 6, 7 }), 0b000_0000);
    try testing.expectEqual(toggleBits(0b0000_0000, .{ 0, 1, 2, 3, 4, 5, 6, 7 }), 0b1111_1111);

    try testing.expectEqual(toggleBits(0b0000_1111, .{ 0, 1, 2, 3, 4, 5, 6, 7 }), 0b1111_0000);
    try testing.expectEqual(toggleBits(0b0000_1111, .{ 0, 1, 2, 3 }), 0b0000_0000);

    try testing.expectEqual(toggleBits(0b0000_0000, .{ 0, 2, 4, 6 }), 0b0101_0101);

    try testing.expectError(error.BitPosTooLarge, toggleBits(0b1111_1111, .{8}));
    try testing.expectError(error.BitPosTooSmall, toggleBits(0b1111_1111, .{-1}));
}

// ----------------------------------------------------------------------------
// Utility functions
// ----------------------------------------------------------------------------

fn newline() void {
    print("\n", .{});
}

fn checkAnswer(expected: u4, answer: u4) void {
    if (expected != answer) {
        print("*************************************************************\n", .{});
        print("= {b:0>4} <- INCORRECT! THE EXPECTED OUTPUT IS {b:0>4}\n", .{ answer, expected });
        print("*************************************************************\n", .{});
    } else {
        print("= {b:0>4}", .{answer});
    }
    newline();
}