//
//    "Trunks and tails
//     Are handy things"

//     from Holding Hands
//       by Lenore M. Link
//
// Now that we have tails all figured out, can you implement trunks?
//
const std = @import("std");

const Elephant = struct{
  letter: u8,
  tail: ?*Elephant = null,
  trunk: ?*Elephant = null,
  visited: bool = false,
  
  // Elephant tail methods!
  pub fn getTail(self: *Elephant) *Elephant{
    return self.tail.?; // Remember, this means "orelse unreachable"
  }
  
  pub fn hasTail(self: *Elephant) bool{
    return (self.tail != null);
  }
  
  // Your Elephant trunk methods go here!
  // ---------------------------------------------------
  
  pub fn getTrunk(self: *Elephant) *Elephant{
    return self.trunk.?; // Remember, this means "orelse unreachable"
  }
  
  pub fn hasTrunk(self: *Elephant) bool{
    return (self.trunk != null);
  }
  
  // ---------------------------------------------------
  
  pub fn visit(self: *Elephant) void{
    self.visited = true;
  }
  
  pub fn print(self: *Elephant) void{
    // Prints elephant letter and [v]isited
    const v: u8 = if(self.visited) 'v' else ' ';
    std.debug.print("{u}{u} ", .{ self.letter, v });
  }
};

pub fn main() void{
  var elephantA = Elephant{ .letter = 'A' };
  var elephantB = Elephant{ .letter = 'B' };
  var elephantC = Elephant{ .letter = 'C' };
  
  // We link the elephants so that each tail "points" to the next.
  elephantA.tail = &elephantB;
  elephantB.tail = &elephantC;
  
  // And link the elephants so that each trunk "points" to the previous.
  elephantB.trunk = &elephantA;
  elephantC.trunk = &elephantB;
  
  visitElephants(&elephantA);
  
  std.debug.print("\n", .{});
}

// This function visits all elephants twice, tails to trunks.
fn visitElephants(first_elephant: *Elephant) void{
  var e = first_elephant;
  
  // We follow the tails!
  while(true){
    e.print();
    e.visit();
    
    // This gets the next elephant or stops.
    if(e.hasTail()){
      e = e.getTail();
    }else{
      break;
    }
  }
  
  // We follow the trunks!
  while(true){
    e.print();
    
    // This gets the previous elephant or stops.
    if(e.hasTrunk()){
      e = e.getTrunk();
    }else{
      break;
    }
  }
}