Sunday 10 March 2019

Mod player in Rust - part 5. Rust modules

For this post I am going to change my the style slightly. Instead of going through all the new code I am going to focus on the bits that were more interesting or challenging.
The mod player is now able to play most of the files that I throw at if although there are still some files that sound wrong or are missing some effects. Below a clip of the player playing an old school 90s demo tune.

Rust module basics

Getting my head around Rust's module system was a bit trickier than I thought. I am so used to the C-style model where each file is a translation unit that is compiled into its own .obj file which are then linked together that anything else seemed just wrong.
My initial mistake was to think that the key word mod was used to import other modules into a Rust file. This appeared to work as long as I was using it to 'import' modules into the file. As soon as I started using it in other .rs. files I got loads of unexpected errors.
My key to understanding the module system was to realize that modules form a tree which is rooted in the file. The key word mod is used to declare other sub-modules that are used in the current module. I split my project into three modules;
  • main the root module for the starting point and orchestrating all the calls into the library
  • mod_player for the code that parses and streams the audio mod file. This module has one sub-module of it's own;
    • textout that can be used to print info about the module and the players current stat
When the file has the following statement;
mod mod_player
it tells the compiler that the root module has a sub module mod_player. The compiler will locate this by looking for a file called in the root folder or by looking for a file in the folder /mod_player.
Because text_out is a sub-module of mod_player I need to declare it inside the file mod_player.rswith the line.
pub mod textout
In this case I have added the key word pub because I want anyone using mod_player to also be able to access textout. Without the pub keyword the textout module would be a private module only visible to mod_player.
I can now use public functions and structures from mod_player and textout by specifying the path to the items I want to use. To load an audio mod file using the functionality in mod_player I use,
let song = mod_player::read_mod_file("song_name.MOD");
and to print out some info about it using the textout sub-module I can write
mod_player::textout::print_song_info( &song );
Both functions read_mod_file and print_song_info have to be declared public with a pub modifier to visible. It is not enough fro the containing modules to be public.

Extending impl in sub-modules

For my library I wanted all the printing related functionality to be in the texout module while still preserving the clear relationship between data and the methods. Ideally, I could just extend the existing impl blocks.
Fortunately Rust lets me do exactly that by declaring a new impl block and adding the new methods. The only slight complications is in how to identify the structure that the impl block refers to. There are three equivalent ways of declaring the extension;
  • declare the full path from the crate root using the crate path specifier
impl crate::mod_player::Sample{
  • declare the the relative path using super path specifier
impl super::Sample{
  • use use to bring the structure into the current module
use super::Sample;      // at top of the file
impl Sample{
The three versions do the same thing; bring the Sample from the parent module into the scope of the current module.
I ended up using the last method as it brings the Sample structure into scope everywhere within the sub-module and means I don't need to use path specifiers elsewhere. The nice thing about the using use is that one statement can bring multiple items into the module. The line
use super::{Note,Song,Effect};
brings NoteSong and Effect into the current module.

Exporting to WAV

One of the challenges with debugging an audio player is that debugging consist of listening to the playback and trying to spot anything that doesn't sound right and then trying to figure out where exactly the anomaly took place. Writing the sound out into a wav file makes it easier to work out the exact play time.
The hound library provides exactly the functionality I needed for writing out WAV files. I am still impressed by how easy it is to add crates to a project. Once I decided I wanted the hound crate it took comfortably less than half an hour to get WAV export working ( and most of that was spent on modifying my code ).
Because exporting to WAV is not a part of mod_playing functionality I put the functionality into the root module in

Searching tuple arrays

I changed the way notes were printed from periods into actual notes. That meant having a function that takes a period value and converts it into a string. My first instinct was to use the match syntax but this becomes very ugly and unnecessarily verbose.
fn note_string( period : u32 ) -> &str{
    return match val{
        113 => "B-5", 
        120 => "A-5", 
With 60 notes this becomes a very ugly function. There is also the nagging feeling that the performance of this code will be pretty suboptimal. ( Granted, performance is not really an issue in this instance but the code is representative of situations where the performance would matter so I think its worth investigating)
What I really want is a map that is statically initialized. There doesn't seem to be a Rust native way of initializing a map. There is the lazy_static crate which would let me create the a static map but I want to learn to use vanilla Rust efficiently before starting to use too many other crates.
While Rust doesn't let me declare static maps I can declare static arrays of tuples and do a binary search on it. I can declare a static array of all of the (period, note) pairs as;
static NOTE_FREQUENCY_STRINGS : [ (u32, &str ); 60 ]= [
( 57,  "B-6" ), ( 60,  "A#6" ), ( 64,  "A-6" ),( 67,  "G#6" ), ( 71,  "G-6" ), ( 76,  "F#6" ), ( 80,  "F-6" ), ( 85 , "E-6" ), ( 90,  "D#6" ), ( 95 , "D-6" ), ( 101, "C#6" ), ( 107, "C-6"), 
I can use the slice function binary_search_by to search through the tuple array.
    fn note_string( &self ) -> &str{
        let idx = NOTE_FREQUENCY_STRINGS.binary_search_by( | val | val.0.cmp( &self.period) );
        if idx.is_ok() {
            return  NOTE_FREQUENCY_STRINGS[ idx.unwrap() ].1;
        } else { "..." }
Because I am searching through tuples I need to provide the search function with my own comparator function. That is what |val|val.0.cmp( &self.period) declares. For each tuples the comparator function is called, it takes the first part and compares it to the period value. I am using the standard cmp function that returns an Ordering enum that the comparator expects.
I am finding that it is really worth spending time understanding slice and all of its methods.

Casting bug

This was the first time in a long time that I had to deal with a casting bug. One of the effects in a mod file is tone portamento which will change the currently playing tone into the target tone at the specified speed. Handling the effect consists of changing the period counter which controls time between subsequent samples. Increasing or decreasing the period moves the tone lower or higher.
The original code which worked most of the time is below. The two checks are used to constrain the period to a range that is supported by the mod format.
fn change_note( current_period : u32, change : i32 ) -> u32 {
    let mut result : u32 = ( current_period as i32 + change ) as u32;
    if result > 856 {  result = 856;  }
    if result < 113 { result = 113; }
The current_period must always a positive number but change can be negative because the period can increase or decrease. The current_period is cast to i32 to make it compatible with change and the result is cast to u32 because that is the type of current_period and also the return type. This worked perfectly for all the mod files I initially tested with but then I came across one that had weird popping sounds.
The code breaks down when change is negative and larger than current_period. So if period is 120 and change is -200 the value of ( current_period as i32 + change ) is -80. When -80 is cast to u32 the result is a very large positive number.
The simple fix was to change the code to;
fn change_note( current_period : u32, change : i32 ) -> u32 {
    let mut result = current_period as i32 + change;
    if result > 856 {  result = 856;  }
    if result < 113 { result = 113; }
    result as u32
This lets Rust decide the type of result and only does the casting at the very end. This is not really a Rust specific bug but highlights the fact that by giving you more control, the compiler also give you more ways to shoot yourself in the foot.

Next Steps

All the code so far has been uploaded into
The next step is to improve the effects coverage as there are still some files that sound wrong ( or won't play at all ). The second step is to turn this into a crate that can be used by other projects.

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