mirror of
https://codeberg.org/ziglings/exercises.git
synced 2025-11-03 20:25:37 +00:00
472 lines
18 KiB
Zig
472 lines
18 KiB
Zig
//
|
|
// We've absorbed a lot of information about the variations of types
|
|
// we can use in Zig. Roughly, in order we have:
|
|
//
|
|
// u8 single item
|
|
// *u8 single-item pointer
|
|
// []u8 slice (size known at runtime)
|
|
// [5]u8 array of 5 u8s
|
|
// [*]u8 many-item pointer (zero or more)
|
|
// enum {a, b} set of unique values a and b
|
|
// error {e, f} set of unique error values e and f
|
|
// struct {y: u8, z: i32} group of values y and z
|
|
// union(enum) {a: u8, b: i32} single value either u8 or i32
|
|
//
|
|
// Values of any of the above types can be assigned as "var" or "const"
|
|
// to allow or disallow changes (mutability) via the assigned name:
|
|
//
|
|
// const a: u8 = 5; // immutable
|
|
// var b: u8 = 5; // mutable
|
|
//
|
|
// We can also make error unions or optional types from any of
|
|
// the above:
|
|
//
|
|
// var a: E!u8 = 5; // can be u8 or error from set E
|
|
// var b: ?u8 = 5; // can be u8 or null
|
|
//
|
|
// Knowing all of this, maybe we can help out a local hermit. He made
|
|
// a little Zig program to help him plan his trips through the woods,
|
|
// but it has some mistakes.
|
|
//
|
|
// *************************************************************
|
|
// * A NOTE ABOUT THIS EXERCISE *
|
|
// * *
|
|
// * You do NOT have to read and understand every bit of this *
|
|
// * program. This is a very big example. Feel free to skim *
|
|
// * through it and then just focus on the few parts that are *
|
|
// * actually broken! *
|
|
// * *
|
|
// *************************************************************
|
|
//
|
|
const print = @import("std").debug.print;
|
|
|
|
// The grue is a nod to Zork.
|
|
const TripError = error{ Unreachable, EatenByAGrue };
|
|
|
|
// Let's start with the Places on the map. Each has a name and a
|
|
// distance or difficulty of travel (as judged by the hermit).
|
|
//
|
|
// Note that we declare the places as mutable (var) because we need to
|
|
// assign the paths later. And why is that? Because paths contain
|
|
// pointers to places and assigning them now would create a dependency
|
|
// loop!
|
|
const Place = struct {
|
|
name: []const u8,
|
|
paths: []const Path = undefined,
|
|
};
|
|
|
|
var a = Place{ .name = "Archer's Point" };
|
|
var b = Place{ .name = "Bridge" };
|
|
var c = Place{ .name = "Cottage" };
|
|
var d = Place{ .name = "Dogwood Grove" };
|
|
var e = Place{ .name = "East Pond" };
|
|
var f = Place{ .name = "Fox Pond" };
|
|
|
|
// The hermit's hand-drawn ASCII map
|
|
// +---------------------------------------------------+
|
|
// | * Archer's Point ~~~~ |
|
|
// | ~~~ ~~~~~~~~ |
|
|
// | ~~~| |~~~~~~~~~~~~ ~~~~~~~ |
|
|
// | Bridge ~~~~~~~~ |
|
|
// | ^ ^ ^ |
|
|
// | ^ ^ / \ |
|
|
// | ^ ^ ^ ^ |_| Cottage |
|
|
// | Dogwood Grove |
|
|
// | ^ <boat> |
|
|
// | ^ ^ ^ ^ ~~~~~~~~~~~~~ ^ ^ |
|
|
// | ^ ~~ East Pond ~~~ |
|
|
// | ^ ^ ^ ~~~~~~~~~~~~~~ |
|
|
// | ~~ ^ |
|
|
// | ^ ~~~ <-- short waterfall |
|
|
// | ^ ~~~~~ |
|
|
// | ~~~~~~~~~~~~~~~~~ |
|
|
// | ~~~~ Fox Pond ~~~~~~~ ^ ^ |
|
|
// | ^ ~~~~~~~~~~~~~~~ ^ ^ |
|
|
// | ~~~~~ |
|
|
// +---------------------------------------------------+
|
|
//
|
|
// We'll be reserving memory in our program based on the number of
|
|
// places on the map. Note that we do not have to specify the type of
|
|
// this value because we don't actually use it in our program once
|
|
// it's compiled! (Don't worry if this doesn't make sense yet.)
|
|
const place_count = 6;
|
|
|
|
// Now let's create all of the paths between sites. A path goes from
|
|
// one place to another and has a distance.
|
|
const Path = struct {
|
|
from: *const Place,
|
|
to: *const Place,
|
|
dist: u8,
|
|
};
|
|
|
|
// By the way, if the following code seems like a lot of tedious
|
|
// manual labor, you're right! One of Zig's killer features is letting
|
|
// us write code that runs at compile time to "automate" repetitive
|
|
// code (much like macros in other languages), but we haven't learned
|
|
// how to do that yet!
|
|
const a_paths = [_]Path{
|
|
Path{
|
|
.from = &a, // from: Archer's Point
|
|
.to = &b, // to: Bridge
|
|
.dist = 2,
|
|
},
|
|
};
|
|
|
|
const b_paths = [_]Path{
|
|
Path{
|
|
.from = &b, // from: Bridge
|
|
.to = &a, // to: Archer's Point
|
|
.dist = 2,
|
|
},
|
|
Path{
|
|
.from = &b, // from: Bridge
|
|
.to = &d, // to: Dogwood Grove
|
|
.dist = 1,
|
|
},
|
|
};
|
|
|
|
const c_paths = [_]Path{
|
|
Path{
|
|
.from = &c, // from: Cottage
|
|
.to = &d, // to: Dogwood Grove
|
|
.dist = 3,
|
|
},
|
|
Path{
|
|
.from = &c, // from: Cottage
|
|
.to = &e, // to: East Pond
|
|
.dist = 2,
|
|
},
|
|
};
|
|
|
|
const d_paths = [_]Path{
|
|
Path{
|
|
.from = &d, // from: Dogwood Grove
|
|
.to = &b, // to: Bridge
|
|
.dist = 1,
|
|
},
|
|
Path{
|
|
.from = &d, // from: Dogwood Grove
|
|
.to = &c, // to: Cottage
|
|
.dist = 3,
|
|
},
|
|
Path{
|
|
.from = &d, // from: Dogwood Grove
|
|
.to = &f, // to: Fox Pond
|
|
.dist = 7,
|
|
},
|
|
};
|
|
|
|
const e_paths = [_]Path{
|
|
Path{
|
|
.from = &e, // from: East Pond
|
|
.to = &c, // to: Cottage
|
|
.dist = 2,
|
|
},
|
|
Path{
|
|
.from = &e, // from: East Pond
|
|
.to = &f, // to: Fox Pond
|
|
.dist = 1, // (one-way down a short waterfall!)
|
|
},
|
|
};
|
|
|
|
const f_paths = [_]Path{
|
|
Path{
|
|
.from = &f, // from: Fox Pond
|
|
.to = &d, // to: Dogwood Grove
|
|
.dist = 7,
|
|
},
|
|
};
|
|
|
|
// Once we've plotted the best course through the woods, we'll make a
|
|
// "trip" out of it. A trip is a series of Places connected by Paths.
|
|
// We use a TripItem union to allow both Places and Paths to be in the
|
|
// same array.
|
|
const TripItem = union(enum) {
|
|
place: *const Place,
|
|
path: *const Path,
|
|
|
|
// This is a little helper function to print the two different
|
|
// types of item correctly.
|
|
fn printMe(self: TripItem) void {
|
|
switch (self) {
|
|
// Oops! The hermit forgot how to capture the union values
|
|
// in a switch statement. Please capture both values as
|
|
// 'p' so the print statements work!
|
|
.place => print("{s}", .{p.name}),
|
|
.path => print("--{}->", .{p.dist}),
|
|
}
|
|
}
|
|
};
|
|
|
|
// The Hermit's Notebook is where all the magic happens. A notebook
|
|
// entry is a Place discovered on the map along with the Path taken to
|
|
// get there and the distance to reach it from the start point. If we
|
|
// find a better Path to reach a Place (shorter distance), we update the
|
|
// entry. Entries also serve as a "todo" list which is how we keep
|
|
// track of which paths to explore next.
|
|
const NotebookEntry = struct {
|
|
place: *const Place,
|
|
coming_from: ?*const Place,
|
|
via_path: ?*const Path,
|
|
dist_to_reach: u16,
|
|
};
|
|
|
|
// +------------------------------------------------+
|
|
// | ~ Hermit's Notebook ~ |
|
|
// +---+----------------+----------------+----------+
|
|
// | | Place | From | Distance |
|
|
// +---+----------------+----------------+----------+
|
|
// | 0 | Archer's Point | null | 0 |
|
|
// | 1 | Bridge | Archer's Point | 2 | < next_entry
|
|
// | 2 | Dogwood Grove | Bridge | 1 |
|
|
// | 3 | | | | < end_of_entries
|
|
// | ... |
|
|
// +---+----------------+----------------+----------+
|
|
//
|
|
const HermitsNotebook = struct {
|
|
// Remember the array repetition operator `**`? It is no mere
|
|
// novelty, it's also a great way to assign multiple items in an
|
|
// array without having to list them one by one. Here we use it to
|
|
// initialize an array with null values.
|
|
entries: [place_count]?NotebookEntry = .{null} ** place_count,
|
|
|
|
// The next entry keeps track of where we are in our "todo" list.
|
|
next_entry: u8 = 0,
|
|
|
|
// Mark the start of empty space in the notebook.
|
|
end_of_entries: u8 = 0,
|
|
|
|
// We'll often want to find an entry by Place. If one is not
|
|
// found, we return null.
|
|
fn getEntry(self: *HermitsNotebook, place: *const Place) ?*NotebookEntry {
|
|
for (&self.entries, 0..) |*entry, i| {
|
|
if (i >= self.end_of_entries) break;
|
|
|
|
// Here's where the hermit got stuck. We need to return
|
|
// an optional pointer to a NotebookEntry.
|
|
//
|
|
// What we have with "entry" is the opposite: a pointer to
|
|
// an optional NotebookEntry!
|
|
//
|
|
// To get one from the other, we need to dereference
|
|
// "entry" (with .*) and get the non-null value from the
|
|
// optional (with .?) and return the address of that. The
|
|
// if statement provides some clues about how the
|
|
// dereference and optional value "unwrapping" look
|
|
// together. Remember that you return the address with the
|
|
// "&" operator.
|
|
if (place == entry.*.?.place) return entry;
|
|
// Try to make your answer this long:__________;
|
|
}
|
|
return null;
|
|
}
|
|
|
|
// The checkNote() method is the beating heart of the magical
|
|
// notebook. Given a new note in the form of a NotebookEntry
|
|
// struct, we check to see if we already have an entry for the
|
|
// note's Place.
|
|
//
|
|
// If we DON'T, we'll add the entry to the end of the notebook
|
|
// along with the Path taken and distance.
|
|
//
|
|
// If we DO, we check to see if the path is "better" (shorter
|
|
// distance) than the one we'd noted before. If it is, we
|
|
// overwrite the old entry with the new one.
|
|
fn checkNote(self: *HermitsNotebook, note: NotebookEntry) void {
|
|
var existing_entry = self.getEntry(note.place);
|
|
|
|
if (existing_entry == null) {
|
|
self.entries[self.end_of_entries] = note;
|
|
self.end_of_entries += 1;
|
|
} else if (note.dist_to_reach < existing_entry.?.dist_to_reach) {
|
|
existing_entry.?.* = note;
|
|
}
|
|
}
|
|
|
|
// The next two methods allow us to use the notebook as a "todo"
|
|
// list.
|
|
fn hasNextEntry(self: *HermitsNotebook) bool {
|
|
return self.next_entry < self.end_of_entries;
|
|
}
|
|
|
|
fn getNextEntry(self: *HermitsNotebook) *const NotebookEntry {
|
|
defer self.next_entry += 1; // Increment after getting entry
|
|
return &self.entries[self.next_entry].?;
|
|
}
|
|
|
|
// After we've completed our search of the map, we'll have
|
|
// computed the shortest Path to every Place. To collect the
|
|
// complete trip from the start to the destination, we need to
|
|
// walk backwards from the destination's notebook entry, following
|
|
// the coming_from pointers back to the start. What we end up with
|
|
// is an array of TripItems with our trip in reverse order.
|
|
//
|
|
// We need to take the trip array as a parameter because we want
|
|
// the main() function to "own" the array memory. What do you
|
|
// suppose could happen if we allocated the array in this
|
|
// function's stack frame (the space allocated for a function's
|
|
// "local" data) and returned a pointer or slice to it?
|
|
//
|
|
// Looks like the hermit forgot something in the return value of
|
|
// this function. What could that be?
|
|
fn getTripTo(self: *HermitsNotebook, trip: []?TripItem, dest: *Place) void {
|
|
// We start at the destination entry.
|
|
const destination_entry = self.getEntry(dest);
|
|
|
|
// This function needs to return an error if the requested
|
|
// destination was never reached. (This can't actually happen
|
|
// in our map since every Place is reachable by every other
|
|
// Place.)
|
|
if (destination_entry == null) {
|
|
return TripError.Unreachable;
|
|
}
|
|
|
|
// Variables hold the entry we're currently examining and an
|
|
// index to keep track of where we're appending trip items.
|
|
var current_entry = destination_entry.?;
|
|
var i: u8 = 0;
|
|
|
|
// At the end of each looping, a continue expression increments
|
|
// our index. Can you see why we need to increment by two?
|
|
while (true) : (i += 2) {
|
|
trip[i] = TripItem{ .place = current_entry.place };
|
|
|
|
// An entry "coming from" nowhere means we've reached the
|
|
// start, so we're done.
|
|
if (current_entry.coming_from == null) break;
|
|
|
|
// Otherwise, entries have a path.
|
|
trip[i + 1] = TripItem{ .path = current_entry.via_path.? };
|
|
|
|
// Now we follow the entry we're "coming from". If we
|
|
// aren't able to find the entry we're "coming from" by
|
|
// Place, something has gone horribly wrong with our
|
|
// program! (This really shouldn't ever happen. Have you
|
|
// checked for grues?)
|
|
// Note: you do not need to fix anything here.
|
|
const previous_entry = self.getEntry(current_entry.coming_from.?);
|
|
if (previous_entry == null) return TripError.EatenByAGrue;
|
|
current_entry = previous_entry.?;
|
|
}
|
|
}
|
|
};
|
|
|
|
pub fn main() void {
|
|
// Here's where the hermit decides where he would like to go. Once
|
|
// you get the program working, try some different Places on the
|
|
// map!
|
|
const start = &a; // Archer's Point
|
|
const destination = &f; // Fox Pond
|
|
|
|
// Store each Path array as a slice in each Place. As mentioned
|
|
// above, we needed to delay making these references to avoid
|
|
// creating a dependency loop when the compiler is trying to
|
|
// figure out how to allocate space for each item.
|
|
a.paths = a_paths[0..];
|
|
b.paths = b_paths[0..];
|
|
c.paths = c_paths[0..];
|
|
d.paths = d_paths[0..];
|
|
e.paths = e_paths[0..];
|
|
f.paths = f_paths[0..];
|
|
|
|
// Now we create an instance of the notebook and add the first
|
|
// "start" entry. Note the null values. Read the comments for the
|
|
// checkNote() method above to see how this entry gets added to
|
|
// the notebook.
|
|
var notebook = HermitsNotebook{};
|
|
var working_note = NotebookEntry{
|
|
.place = start,
|
|
.coming_from = null,
|
|
.via_path = null,
|
|
.dist_to_reach = 0,
|
|
};
|
|
notebook.checkNote(working_note);
|
|
|
|
// Get the next entry from the notebook (the first being the
|
|
// "start" entry we just added) until we run out, at which point
|
|
// we'll have checked every reachable Place.
|
|
while (notebook.hasNextEntry()) {
|
|
var place_entry = notebook.getNextEntry();
|
|
|
|
// For every Path that leads FROM the current Place, create a
|
|
// new note (in the form of a NotebookEntry) with the
|
|
// destination Place and the total distance from the start to
|
|
// reach that place. Again, read the comments for the
|
|
// checkNote() method to see how this works.
|
|
for (place_entry.place.paths) |*path| {
|
|
working_note = NotebookEntry{
|
|
.place = path.to,
|
|
.coming_from = place_entry.place,
|
|
.via_path = path,
|
|
.dist_to_reach = place_entry.dist_to_reach + path.dist,
|
|
};
|
|
notebook.checkNote(working_note);
|
|
}
|
|
}
|
|
|
|
// Once the loop above is complete, we've calculated the shortest
|
|
// path to every reachable Place! What we need to do now is set
|
|
// aside memory for the trip and have the hermit's notebook fill
|
|
// in the trip from the destination back to the path. Note that
|
|
// this is the first time we've actually used the destination!
|
|
var trip = [_]?TripItem{null} ** (place_count * 2);
|
|
|
|
notebook.getTripTo(trip[0..], destination) catch |err| {
|
|
print("Oh no! {}\n", .{err});
|
|
return;
|
|
};
|
|
|
|
// Print the trip with a little helper function below.
|
|
printTrip(trip[0..]);
|
|
}
|
|
|
|
// Remember that trips will be a series of alternating TripItems
|
|
// containing a Place or Path from the destination back to the start.
|
|
// The remaining space in the trip array will contain null values, so
|
|
// we need to loop through the items in reverse, skipping nulls, until
|
|
// we reach the destination at the front of the array.
|
|
fn printTrip(trip: []?TripItem) void {
|
|
// We convert the usize length to a u8 with @intCast(), a
|
|
// builtin function just like @import(). We'll learn about
|
|
// these properly in a later exercise.
|
|
var i: u8 = @intCast(u8, trip.len);
|
|
|
|
while (i > 0) {
|
|
i -= 1;
|
|
if (trip[i] == null) continue;
|
|
trip[i].?.printMe();
|
|
}
|
|
|
|
print("\n", .{});
|
|
}
|
|
|
|
// Going deeper:
|
|
//
|
|
// In computer science terms, our map places are "nodes" or "vertices" and
|
|
// the paths are "edges". Together, they form a "weighted, directed
|
|
// graph". It is "weighted" because each path has a distance (also
|
|
// known as a "cost"). It is "directed" because each path goes FROM
|
|
// one place TO another place (undirected graphs allow you to travel
|
|
// on an edge in either direction).
|
|
//
|
|
// Since we append new notebook entries at the end of the list and
|
|
// then explore each sequentially from the beginning (like a "todo"
|
|
// list), we are treating the notebook as a "First In, First Out"
|
|
// (FIFO) queue.
|
|
//
|
|
// Since we examine all closest paths first before trying further ones
|
|
// (thanks to the "todo" queue), we are performing a "Breadth-First
|
|
// Search" (BFS).
|
|
//
|
|
// By tracking "lowest cost" paths, we can also say that we're
|
|
// performing a "least-cost search".
|
|
//
|
|
// Even more specifically, the Hermit's Notebook most closely
|
|
// resembles the Shortest Path Faster Algorithm (SPFA), attributed to
|
|
// Edward F. Moore. By replacing our simple FIFO queue with a
|
|
// "priority queue", we would basically have Dijkstra's algorithm. A
|
|
// priority queue retrieves items sorted by "weight" (in our case, it
|
|
// would keep the paths with the shortest distance at the front of the
|
|
// queue). Dijkstra's algorithm is more efficient because longer paths
|
|
// can be eliminated more quickly. (Work it out on paper to see why!)
|