ziglings/exercises/046_optionals2.zig

//
// Now that we have optional types, we can apply them to structs.
// The last time we checked in with our elephants, we had to link
// all three of them together in a "circle" so that the last tail
// linked to the first elephant. This is because we had NO CONCEPT
// of a tail that didn't point to another elephant!
//
// We also introduce the handy `.?` shortcut:
//
//     const foo = bar.?;
//
// is the same as
//
//     const foo = bar orelse unreachable;
//
// Check out where we use this shortcut below to change control flow
// based on if an optional value exists.
//
// Now let's make those elephant tails optional!
//
const std = @import("std");

const Elephant = struct{
  letter: u8,
  tail: ?*Elephant = null, // Hmm... tail needs something...
  visited: bool = false,
};

pub fn main() void{
  var elephantA = Elephant{ .letter = 'A' };
  var elephantB = Elephant{ .letter = 'B' };
  var elephantC = Elephant{ .letter = 'C' };
  
  // Link the elephants so that each tail "points" to the next.
  linkElephants(&elephantA, &elephantB);
  linkElephants(&elephantB, &elephantC);
  linkElephants(&elephantC, &elephantA);
  
  // `linkElephants` will stop the program if you try and link an
  // elephant that doesn't exist! Uncomment and see what happens.
  // const missingElephant: ?*Elephant = null;
  // linkElephants(&elephantC, missingElephant);
  
  visitElephants(&elephantA);
  
  std.debug.print("\n", .{});
}

// If e1 and e2 are valid pointers to elephants,
// this function links the elephants so that e1's tail "points" to e2.
fn linkElephants(e1: ?*Elephant, e2: ?*Elephant) void{
  e1.?.tail = e2.?;
}

// This function visits all elephants once, starting with the
// first elephant and following the tails to the next elephant.
fn visitElephants(first_elephant: *Elephant) void{
  var e = first_elephant;
  
  while(!e.visited){
    std.debug.print("Elephant {u}. ", .{e.letter});
    e.visited = true;
    
    // We should stop once we encounter a tail that
    // does NOT point to another element. What can
    // we put here to make that happen?
    
    // HINT: We want something similar to what `.?` does,
    // but instead of ending the program, we want to exit the loop...
    e = e.tail orelse break;
  }
}