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95 lines
3.6 KiB
Zig
95 lines
3.6 KiB
Zig
//
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// Bit manipulations is a very powerful tool just also from Zig.
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// Since the dawn of the computer age, numerous algorithms have been
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// developed that solve tasks solely by moving, setting, or logically
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// combining bits.
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//
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// Zig also uses direct bit manipulation in its standard library for
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// functions where possible. And it is often possible with calculations
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// based on integers.
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//
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// Often it is not easy to understand at first glance what exactly these
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// algorithms do when only "numbers" in memory areas change outwardly.
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// But it must never be forgotten that the numbers only represent the
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// interpretation of the bit sequences.
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//
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// Quasi the reversed case we have otherwise, namely that we represent
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// numbers in bit sequences.
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//
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// We remember: 1 byte = 8 bits = 11111111 = 255 decimal = FF hex.
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//
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// Zig provides all the necessary functions to change the bits inside
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// a variable. It is distinguished whether the bit change leads to an
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// overflow or not.The details are in the Zig documentation in section
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// 10.1 "Table of Operators".
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//
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// Here are some examples of how the bits of variables can be changed:
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//
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// const numOne: u8 = 15; // = 0000 1111
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// const numTwo: u8 = 245; // = 1111 0101
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//
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// const res1 = numOne >> 4; // = 0000 0000 - shift right
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// const res2 = numOne << 4; // = 1111 0000 - shift left
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// const res3 = numOne & numTwo; // = 0000 0101 - and
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// const res4 = numOne | numTwo; // = 1111 1111 - or
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// const res5 = numOne ^ numTwo; // = 1111 1010 - xor
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//
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//
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// To familiarize ourselves with bit manipulation, we start with a simple
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// but often underestimated function and then add other exercises in
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// loose order.
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//
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// The following text contains excerpts from Wikipedia.
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//
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// Swap
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// In computer programming, the act of swapping two variables refers to
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// mutually exchanging the values of the variables. Usually, this is
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// done with the data in memory. For example, in a program, two variables
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// may be defined thus (in pseudocode):
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//
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// data_item x := 1
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// data_item y := 0
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//
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// swap (x, y);
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//
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// After swap() is performed, x will contain the value 0 and y will
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// contain 1; their values have been exchanged. The simplest and probably
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// most widely used method to swap two variables is to use a third temporary
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// variable:
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//
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// define swap (x, y)
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// temp := x
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// x := y
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// y := temp
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//
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// However, with integers we can also achieve the swap function simply by
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// bit manipulation. To do this, the variables are mutually linked with xor
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// and the result is the same.
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const std = @import("std");
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const print = std.debug.print;
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pub fn main() !void {
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// As in the example above, we use 1 and 0 as values for x and y
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var x: u8 = 1;
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var y: u8 = 0;
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// Now we swap the values of the two variables by doing xor on them
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x ^= y;
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y ^= x;
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// What must be written here?
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???;
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print("x = {d}; y = {d}\n", .{ x, y });
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}
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// This variable swap takes advantage of the fact that the value resulting
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// from the xor of two values contains both of these values.
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// This circumstance was (and still is) sometimes used for encryption.
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// Value XOR Key = Crypto. => Crypto XOR Key = Value.
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// Since this can be swapped arbitrarily, you can swap two variables in this way.
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//
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// For Crypto it is better not to use this, but in sorting algorithms like
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// Bubble Sort it works very well.
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