[Dev Diaries] Todotxt

For reasons I took an interest in playing with Todo.txt. Naturally, I went and looked for a swift package capable of importing and exporting this format. It is surprisingly hard to find something that is somewhat recent and full-featured.

Anyways, I went with the intention of contributing to this package, both because I like its simplicity and the fact that it uses the new DSL semantics for the regexps. And boy, oh boy, it tested me.

The trouble with my brain

We are all products of our history. Regular expressions don't faze me one bit. I used to write parts of compilers, lexers and yaccers are old friends, and the whole lambda reduction way regexps work isn't new.

BUT I know that they are a major hindrance for a lot of newcomers. Their syntax is unlike anything they have ever encountered (well, except for when they use * to mean "zero or more characters"), and don't get me started on the whole capture groups ( especially in Javascript ).

So I came into this project thinking it would be half an hour of clever regexps, and that's it.

Oh noes, there's Builders

In theory, I love builders. SwiftUI may not be my favorite way to build UIs, but it's fun and it works. Ditto for Ktor's HTML/CSS DSLs.

For regular expressions though... to me they fail to capture (haha) the mechanics, and will work only for relatively simple expressions.

let regex = Regex {
    Capture {
      Repeat(.digit, count: 2)

This is equivalent to \$([0-9]+\.[0-9][0-9])$ I guess, and it's actually an example of why the DSL system is easier: $ and . have meaning in regexps and have to be escaped. It makes that line totally undecipherable for most newcomers.

Let's set aside the fact that nesting too many builders leads to incredibly long type inference (when it doesn't fail) and compile time. It comes with the territory, SwiftUI afficionados will tell you.

Let's also set aside the compactness of the regexps versus the DSL type because it's irrelevant to the point. In any language, you can write your code as compact and illegible as you like, it doesn't mean it's better. The tradeoffs you choose for your project in terms of inclusiveness (towards more junior members, or at least with regard to the tech stack you are using), and maintainability (look at you being all clever and writing some l33tk0d3 that you won't be able to debug in 6 months time... I learned that lesson a while ago and have decided that in most cases, readability is better than cleverness).

The problem (for me) with DSLs as applied to regular expressions is that it fails to capture the mechanics of backtracking and cursor position.

How regular expressions kind of work

As anyone who's taken an algorithmics class will tell you, strings are a pain. They can mostly only been read linearly (going from one character to the next or the previous), they have encodings and special cases (pun intended), and what they contain mean something different to the programmer than the actual text it contains (looking at you JSON).

"17.51" kind of is a number, and most definitely a string. If an error creeps in, it becomes something really really tedious to deal with. Like "17.,51". What the hell does THAT mean? 17.0 followed by 51? Or is the comma used in the French way to separate the decimal part, and . for multiplication? 17 x 0.51 ? And don't get me started on dates...

And so we have to define rules and use pattern matching. You can say that the string contains elements separated by commas, and each element is a number. And if it isn't, raise an error.

  • Look for every combination of n digits, followed by an optional point, followed by m optional digits, followed by a comma OR the end of the line
  • You should end up with a list of n∂.m∂ strings, that you can now parse to floating point
  • That should give an expression that looks like this:

There's a lot of problems to unpack here, like the fact that we kind of have to capture the group matching the separator/end of line to keep the expression simple, the weird dance of square and curly brackets and parenthesiseseseses, the whys and the hows of + and *, and so much more. But this will suffice for a brief explanation of how it works under the hood.

See, the point of it all is state machines and backtracking. When you use the regexp on a string, it keeps track of what it's done, what it's trying to do and where to go next. And if the current attempt fails, it reverts back to a previous state. The experts in regexps will groan and moan because I explain things too casually, and they should, but you don't need the nitty gritty details of how it actually works to grasp what I'm getting at.

So let's look at "17.,.6,51", and try to apply the regular expression.

  • We are at character 0,  and we are looking for 1 or more digits.
  • We find 17, so we look for 0 or 1 dot.
  • We find one, so we have 17., and we look for 0 or more digits
  • We find none, so we're still good, and we look for either a comma or the end of the line
  • We find a comma, so we have a complete match: 17.,
  • We push the result in the list of "found things", and advance the character to the end of the match, 3
  • We look again for a digit or more
  • We find none, so it can't be a match, so we advance by one character, and try again
  • Ditto for the dot
  • We're now at character 5, and try again
  • We have a digit, so we start a new match: 6
  • 0 or more dots, check. 0 or more digits, check. Comma, check. So we have a new match: 6
  • Start again, and we find 51
  • In the end we have 3 matches: 17., 6, and 51 (plus the comma or end of line, that is captured in a separate group)

Hold on. It was .6 not 6! Well... yes, but not according to our rules. We have to have a digit before the dot.

Never you mind, you say, we'll say we can have 0 or more instead of one or more:

Yes, and it almost works the same:

  • Up to 51, we get almost identical results, except we do capture .6 instead of 6
  • BUT we're not done. We try the regexp again at the end of the string, because we have to look at it all, and we get:
  • There is 0 digit, followed by 0 dot, followed by 0 digit, followed by the end of the line. So there's an extra match: ""...

The two important parts to grasp from all this are that starting at the character we are at, we try to match the pattern to the rest of the string, and if it fails, we advance by one character, and try again. Current state, and backtracking to try again.

The weeds: lookaheads and negations

We need to talk about a couple of special cases:

  • What if we need to not match something (reject some characters, for example)?
  • What if we need to match things only if up ahead in the string there's something else (or not something else)?

Like, say, the dot should appear only if there are digits afterwards (we say that 17. isn't a valid number but 17.0, and .6, are). Or if we are dealing with dates (😖), the rules for matching depend on whether or not we have a time afterwards.

Using "normal" regexps, negation it is kind of easy: [^abc] will match anything but a, b or c. Note that the caret that is used to usually indicate "beginning of a line" is used for negation within the match... Yea. Oh and you have to list every character you do not wish to match.

So kind of a solution is to use "lookaheads". They are basically patterns that match (or not) things before or after the current character, but do not move the current character cursor. They kind of start their own internal state, disregarding the global state of the parent expression.

.*(?=,) means trying to find a comma, and matching anything ( .* ) up till then.

"17.,.6,51" and this pattern will match "17.,.6". Why is it stopping at the last comma instead of the first? Because .* is greedy, it tries to get as many characters as possible. Comma is a character, so it matches, until it can't anymore because it's the last comma.

We can amend the .* to a .*? which will act as a lazy matcher, stopping at the first occurence, and giving us 17., .6, and two empty strings. Because, yes, after we found 17. we try again starting there, and it does match "anything with a comma at the end".

We can also look backwards and see if anything before the current character matches something, with .*(?<=,), and both negations of these with... !. Because yea, caret was really a weird idea. It'd look like this: .*(?!,) and .*(?!<,).

That was a long weeds expedition.

And the DSLs?

Ok, so the regexp mechanics is messy and requires a lot of concentration, so why the hell not make it prettier and/or more legible? After all I did say that I value maintainability over compactness most of the time.

Here's a regexp to match the title of a todo.txt file:

        let reference = Reference(String.self)
        let titlematch = OneOrMore {
          ChoiceOf {
          NegativeLookahead {
            OneOrMore {
        let regex = Regex {
          ZeroOrMore {
            ChoiceOf {
              One("x ")
              OneOrMore {
                (try? Regex("[A-Z]"))!
                ") "
              One(.iso8601Date(timeZone: .gmt))
          NegativeLookahead {
            ChoiceOf {
              One(.iso8601Date(timeZone: .gmt))
              OneOrMore {
          Capture(titlematch, as: reference,
                  transform: { word -> String in String(word) })
        let match = input.firstMatch(of: regex)
        if let match {
          return match[reference].trimmingCharacters(in: CharacterSet.whitespacesAndNewlines) // because we capture the last space
        } else {
          return nil

reference is the variable that should hold the mattern once it's matched. It's weird, but not any weirder than the numbered capture groups, so there.

Because of the Builder's very complex type inference, I had to split the "two" regexps, otherwise, the compiler failed to build. That's a legibility problem because that thing I store in titlematch is really a part of the bigger regexp, and the call site is now murkier. Plus it's a variable, so does it mean I can use it multiple times? In capture groups? Unclear, so probably not.

Then there is .word... it doesn't mean match a word. It means match a character that could be part of a word. Yes. A single character. When you're not paying attention, it can bite you really bad.

I'll set aside the fact that I capture an extra space every now and again. I'm sure that there is a way to avoid doing it, but the regexp was complicated enough as it was and I spent way too much time on it that I care to admit. It's a peculiarity of todo.txt rather than of the regex system, but a space is used to separate the elemets of the format, except when it's in the title.

Also, why is there an actual regular expression in there? Because insofar as I can understand it, you can't express [A-Z] (any capital latin letter) with this system. It's either .word (any character that can be part of a word, uppercase or not), or something else that's not a character.

Also, also, One doesn't accept a closure like OneOrMore does, so I'm forced to accept multiple priorities ( (LETTER) ) instead of only one, for some reason.

Let's dig into that one, shall we? So we're looking for some text that is after the start of the line, after the optional x that marks completion of the task, the (LETTER) marking the priority, and after the optional two dates (creation and optionally completion), but before any special character ( + and @ denote project and context, respectively) or key:value pair.

If you got that by looking at the code instantly, congratulations. I didn't. And I wrote it.

For reference, the regexp would look something like that (I don't include the date formatting, because I'm lazy and would probably do it after I extracted the matches):
^(x |\([A-Z]\) |[0-9-]| )+([\w ]+)(?! \w+:\w+)

OK, sure, this expression is a bit cryptic, and captures an extra space as well for the title. Because I didn't go into details for the date, it also doesn't quite work, because it lets dates with less or more than 3 components pass, instead of only valid ones.

But these 40-ish lines really aren't that clear to me in the things they will and will not match. Especially with the callsite being split into variables, but that's on Swift, not on the DSL thing. To my mind, the regexp is something that you "slide along" your string, stopping when it matches. Yes it's made of blocks and captures and weird characters, but it's kind of the same "thing" as a string, whereas that visual block syntax seems kind of "orthogonal" to the string, not of the same type, and therefore makes it harder for me to think about them.

But it's probably me and my old habit-encrusted brain. Drop me a line if you find an error, or if you disagree with decent arguments!

[AoC 2022] Recap


I'm not as rusty as I thought I'd be. And YES that kind of challenge has a place in the coding world (see conclusion)

Just like every year, I had a blast banging my head on the Advent of Code calendar. It so happens that this year, I had a lot less brain power to focus on them due to the exam season at school and the ensuing panicky students, but it could also be because my brain isn't up to spec.

Some of my students are/were doing the challenges, so I didn't want to post anything that would help them, but now that the year is almost over, I wanted to go over the puzzles and give out impressions (and maybe hints).

Easing in

Days 1 to 7 were mostly about setting up the stage, getting into the habit of parsing the input and using the right kind of structure to store the data.

Nothing super hard, it was "just" lists, hashmaps, and trees, until day 4. Day 4 was especially funny to me, because I wrote HoledRange / Domain just for that purpose (disjointed ranges and operations on them). Except I decided to do this year's calendar in Julia, and the library I wrote is for Swift. Just for kicks, I rewrote parts of the library, and I might even publish it.

Days 5, 6 and 7 highlighted the use of stacks, strings, and trees again. Nothing too hard.

Getting harder

My next favorite is day 9. It's about a piece of rope you drag the head of, and have to figure out what the tail does. If you've ever played zig-zaging a shoelace you'll know what I mean. String physics are fun, especially in inelastic cases.

Many ways to do that, but once you realize how the tail catches up to the head when the latter is moved, multi-segmented chains are just a recursive application of the same.

I was waiting for a day 10-like puzzle, as there tends to be one every year, and I majored in compilers all those years ago. State machines, yuuuuuusssssssss.

A lot of puzzles involve path finding after that, which isn't my strong suit for some reason. But since the algorithms are already out there (it was really funny to see the spike in google searches for Dijkstra and A*), it's "just" a matter of encoding the nodes and the edges in a way that works.

Day 13 is fun, if only because it can be instantly solved in some language with eval, which will treat the input as a program. I still wrote my own comparison functions, because I like manipulating numbers, lists and inequalities.

Day 14 is "sand simulation", that is grains of sand that settle in a conical shape that keeps expanding laterally. Once you find the landing point on each ledge and the maximum width of the pile, there's a calculable result. Otherwise, running the simulation works too, there aren't that many grains. For part 2, I just counted the holes rather than the grains.

Day 15 is about union and intersections of disjointed ranges again, except in 2D. Which, with the Manhattan distance approximation, gets back to 1D fairly quickly.

Day 16 stumped quite a few people, because of the explosive nature of path searching. Combinatorics are pretty hard to wrap your head around. Personally, I went for "reachability" within the remaining time, constructed my graph, and explored. It was probably non optimal.

Day 17 make me inordinately proud. Nuff said.

Catching up

Because of the aforementioned  workload, I was late by that point, so I decided to take my time and not complete the challenge by Xmas. Puzzles were getting hard, work was time-consuming, so the pressure needed to go down.

Because of the 3D background that I had, I tackled day 18 with raytracing, which is way over-engineered, but reminded me of the good ole times. Part 2 was trickier with that method, because suddenly I had 2 kinds of "inside".

Day 19 was path finding again, the trick being how to prune paths that didn't lead in a good direction. Probably the one that used up the most memory, and therefore the one I failed the most.

Because of my relative newness to Julia, I had to go through many hoops to solve day 20. As it turns out, screw_dog over on mastodon gave me the bit I lacked to solve it simply, although way after I solved it using other means.

Day 21 goes back to my compiler roots and tree optimizations, and Julia makes the huge integer manipulation relatively easy, so, there. Pretty proud of my solution:

Part 1:   3.709 ms (31291 allocations: 1.94 MiB)
Part 2:   4.086 ms (31544 allocations: 1.95 MiB)

Which, on my relatively slow mac mini is not bad at all! Symbolic linear equation solving (degree one, okay) is a fun thing to think about. I even think that the algorithm I devised would work on trees where the unknown appears on both sides of the tree. Maybe I'll test it some day.

Day 22. Aaaaaaaah day 22. Linked lists get you all the way if and only if you know how to fold a cube from a 2D pattern. I don't so, as many of the other participants, I hardcoded the folding. A general solution just eluded me. It's on my todo list of reading for later.

Day 23 is an interesting variant of Conway's Game of Life, and I don't believe there is a way to simplify a straight up simulation, but I fully accept I could be wrong. So I used no trick, and let the thing run for 40s to get the result.

Day 24 was especially interesting for me, for all the wrong reasons. As I mentioned, graph traversal isn't my forte. But the problem was setup in a way that "worked" for me: pruning useless paths was relatively easy, so the problem space didn't explode too quickly. I guess I should use the same method on previous puzzles that I was super clumsy with.

Finally day 25 is a straight up algorithmic base conversion problem that's a lot of fun for my brain. If you remember how carry works when adding or subtracting numbers, it's not a big challenge, but thinking in base 5 can trip you up.


I honestly didn't believe I could hack it this year. I don't routinely do that kind of problem anymore, I have a lot of things going on at school, on top of dealing with the long tail of Covid and its effects on education. Family life was a bit busy with health issues (nothing life threatening, but still time consuming), and the precious little free time that I had was sure to be insufficient for AoC.

I'm glad I persevered, even if it took me longer than I wished it had. I'm glad I learned how to use Julia better. And I'm happy I can still hack it.

I see here and there grumblings about formal computer science. During and after AoC, I see posts, tweets, toots, etc, saying that the "l33t c0d3" is useless in practical, day-to-day, professional development. Big O notation, formal analysis, made up puzzles that take you into voluntarily difficult territories, all these things aren't a reflection of the skills that are needed nowadays to write good apps, to make good websites, and so on.

It's true. Ish.

You can write code that works without any kind of formal training. Today's computing power and memory availability makes optimization largely irrelevant unless you are working with games or embedded systems, or maybe data science. I mean, we can use 4GB of temporary memory for like 1/4 of a second to parse and apply that 100kB json file, and it has close to no impact on the perceived speed of our app, right? Right. And most of the clever algorithms are part of the standard library anyway, or easily findable.

The problem, as usual, is at scale. The proof-of-concept, prototype, or even 1.0 version, of the program may very well work just fine with the first 100 users, or 1000 or whatever the metric is for success. Once everything takes longer than it should, there are only 3 solutions:

  • rely on bigger machines, which may work for a time, but ultimately does not address the problem
  • scale things horizontally, which poses huge synchronization issues between the shards
  • reduce the technical debt, which is really hard

The first two rely on compute power being relatively cheap. And most of us know about the perils of infrastructure costs. That meme regularly makes the rounds.

It's not about whether you personally can solve some artificially hard problem using smart techniques, so that's ok if you can't do every puzzle in AoC or other coding challenges. It's not about flexing with your big brain capable of intuiting the bigO complexity of a piece of code. It's about being able to think about these problems in a way that challenges how you would normally do it. It's about expanding your intuition and your knowledge about the field you decided to work in.

It's perfectly OK for an architect to build only 1 or 2 level houses, there's no shame in it. But if that architect ever wants to build a 20+ stories building, the way to approach the problem is different.

Same deal with computer stuff. Learning is part of the experience.

[Dev Diaries] Advent of Code

I've been really interested in Julia for a while now, tinkering here and there with its quirks and capabilities.

This year, I've decided to try and do the whole of Advent of Code using that language.

First impressions are pretty good:
- map, reduce, and list/array management in general are really nice, being first-class citizens. I might even get over the fact that indices start at 1
- automatic multithreading when iterating over collections means that some of these operations are pretty speedy
- it's included in standard jupyterhub images, meaning that my server install gives me access to a Julia environment if I am not at my computer for some reason

Now it's kind of hard to teach old dogs new tricks, so I'm sure I misuse some of the features by thinking in "other languages". We'll see, but 4 days in, I'm still fairly confident.

[Dev Diary] Vanilla Is The Best Flavor

I have a weird thing with the multiplication of command-line tools and gizmos: I forget them.

Do I want to run supercool gitlab commands? Hell yea! Do I need to install 12 utilities (or code a new one) to archive every project older than a year? I hope not...

The setup

I am a sucker for well documented fully linted code. But the thing is, all the gizmos that help me do that have to be installed in the system or in my ~/bin and I have to remember to update them, and I have to install them on my CD machine, and on every new environment I setup, and make sure they are still compatible with the toolchain, and it freaks me out, ok?

Plus,watching the students try to do it is painful.

So, given a 100% vanilla swift-capable environment, can I manage to run documentation and linting?

The idea

We have Swift Package Manager, which is now a first-class citizen in XCode, but it can't run shell script phases without some nasty hacks.

What if some targets were (wait for it) built to do the documentation and the linting?


One of the most popular linters out there is swiftlint, and it supports SPM. It can also build a library instead of an executable, which means one of my targets could just run the linting and output it in the terminal.

In the Package.swift file, all I needed to do was add the right dependency, and the right product and voila!

let package = Package(
	name: "WonderfulPackage",
    products: [
    	// ...
         .executable(name: "Lint", targets: ["Lint"])
    dependencies: [
        // Dependencies declare other packages that this package depends on.
        // .package(url: /* package url */, from: "1.0.0"),
		// ... normal dependencies
        .package(url: "https://github.com/realm/SwiftLint", from: "0.39.0")
    targets: [
    	// ... normal targets
            name: "Lint",
            dependencies: ["SwiftLintFramework"]),

Now, SPM is very strict with paths, so I had to put a file named main.swift in the Sources/<target>/ directory, in this case Sources/Lint.

Running the linter is fairly straightforward, and goes in the main.swift file:

// Lint command main
// runs SourceDocs
import Foundation
import SwiftLintFramework

let config = Configuration(path: FileManager.default.currentDirectoryPath+"/.swiftlint.yml",
                           rootPath: FileManager.default.currentDirectoryPath,
                           optional: true,
                           quiet: true,
                           enableAllRules: false,
                           cachePath: nil,
                           customRulesIdentifiers: [])

for lintable in config.lintableFiles(inPath: FileManager.default.currentDirectoryPath, forceExclude: false) {
    let linter = Linter(file: lintable, configuration: config)
    let storage = RuleStorage()
    let collected = linter.collect(into: storage)
    let violations = collected.styleViolations(using: storage)
    if !violations.isEmpty {

print("🎉 All done!")

Setup the .swiftlint file as usual, and run the command via swift run Lint

⛔️ Line 15: Variable name should be between 3 and 40 characters long: 'f'
⚠️ Line 13: Arguments can be omitted when matching enums with associated types if they are not used.
⚠️ Line 12: Line should be 120 characters or less: currently 143 characters


Documentation is actually trickier, because most documentation tools out there aren't built in swift, or compatible with SPM. Doxygen and jazzy are great, but they don't fit my needs.

I found a project that was extremely promising called SourceDocs by Eneko Alonso, but it isn't a library, so I had to fork it and make it into one (while providing a second target to generate the executable if needed). One weird issue is that SPM doesn't like subtargets to bear the same name so I had to rename a couple of them to avoid conflict with Swift Argument Parser (long story).

I finally found myself in the same spot than with the linter. All I needed to do was create another target, and Bob's you're uncle. Well actually he was mine. I digress.

let package = Package(
	name: "WonderfulPackage",
    products: [
    	// ...
         .executable(name: "Docs", targets: ["Docs"])
    dependencies: [
        // Dependencies declare other packages that this package depends on.
        // .package(url: /* package url */, from: "1.0.0"),
		// ... normal dependencies
        .package(url: "https://github.com/krugazor/SourceDocs", from: "0.7.0")
    targets: [
    	// ... normal targets
            name: "Docs",
            dependencies: ["sourcedocslib"])

Another well-placed main file:

// Docs command main
// runs SourceDocs
import Foundation
import SourceDocs

do {
    switch try SourceDocs().runOnSPM(moduleName: "WonderfulPackage",
                                     outputDirectory: FileManager.default.currentDirectoryPath+"/Documentation") {
    case .success:
        print("Successful run of the documentation phase")
    case .failure(let failure):
} catch {

Now, the command swift run Docs generates the markdown documentation in the Documentation directory.

Parsing main.swift (1/1)
Removing reference documentation at 'WonderfulPackage/Documentation/KituraStarter'... ✔
Generating Markdown documentation...
  Writing documentation file: WonderfulPackage/Documentation/WonderfulPackage/structs/WonderfulPackage.md ✔
  Writing documentation file: WonderfulPackage/Documentation/WonderfulPackage/README.md ✔
Done 🎉
Successful run of the documentation phase


✅ Vanilla swift environment
✅ No install needed
✅ Works on Linux and MacOS
✅ Integrated into SPM
⚠️ When running in XCode, the current directory is always wonky for packages

[Utilities] Time Tracking Structure

Every now and again (especially when training a model), I need to have a guesstimate as to how long a "step" takes, and how long the process will take, so I wrote myself a little piece of code that does that. Because I've had the question multiple times (and because I think everyone codes their own after a while), here's mine. Feel free to use it

/// Structure that keeps track of the time it takes to complete steps, to average or estimate the remaining time
public struct TimeRecord {
    /// The number of steps to keep for averaging. 5 is a decent default, increase or decrease as needed
    /// Minimum for average is 2, obvioulsy
    public var smoothing: Int = 5 {
        didSet {
            smoothing = max(smoothing, 2) // minimum 2 values
    /// dates for the steps
    private var dates : [Date] = []
    /// formatter for debug print and/or display
    private var formatter = DateComponentsFormatter()
    public var formatterStyle : DateComponentsFormatter.UnitsStyle {
        didSet {
            formatter.allowedUnits = [.hour, .minute, .second, .nanosecond]
            formatter.unitsStyle = formatterStyle
    public init(smoothing s: Int = 5, style fs: DateComponentsFormatter.UnitsStyle = .positional) {
        smoothing = max(s, 2)
        formatterStyle = fs
        formatter = DateComponentsFormatter()
        // not available everywhere
        // formatter.allowedUnits = [.hour, .minute, .second, .nanosecond]
        formatter.allowedUnits = [.hour, .minute, .second]
        formatter.zeroFormattingBehavior = .pad
        formatter.unitsStyle = fs
    /// adds the record for a step
    /// - param d: the date of the step. If unspecified, current date is taken
    mutating func addRecord(_ d: Date? = nil) {
        if let d = d { dates.append(d) }
        else { dates.append(Date()) }
        while(dates.count > smoothing) { dates.remove(at: 0) }
    /// gives the average delta between two steps (in seconds)
    var averageDelta : Double {
        if dates.count <= 1 { return 0.0 }
        var totalTime = 0.0
        for i in 1..<dates.count {
            totalTime += dates[i].timeIntervalSince(dates[i-1])
        return totalTime/Double(dates.count)
    /// gives the average delta between two steps in human readable form
    /// - see formatterStyle for options, default is "02:46:40"
    var averageDeltaHumanReadable : String {
        let delta = averageDelta
        return formatter.string(from: delta) ?? ""
    /// given a number of remaining steps, gives an estimate of the time left on the process (in s)
    func estimatedTimeRemaining(_ steps: Int) -> Double {
        return Double(steps) * averageDelta
    /// given a number of remaining steps, gives an estimate of the time left on the process in human readable form
    /// - see formatterStyle for options, default is "02:46:40"
    func estimatedTimeRemainingHumanReadable(_ steps: Int) -> String {
        let delta = estimatedTimeRemaining(steps)
         return formatter.string(from: delta) ?? ""

When I train a model, I tend to use it that way:

// prepare model
var tt = TimeRecord()

while currentEpoch < maxEpochs {
  // train the model
  if currentEpoch > 0 && currentEpoch % 5 == 0 {
  	print(tt.averageDeltaHumanReadable + " per epoch, " 
    	+ tt.(estimatedTimeRemainingHumanReadable(maxEpochs - currentEpoch) + " remaining"