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Before yesterdayNews from the Ada programming language world

The Three Lakes Problem

24 May 2020 at 16:56
Played with a old numerics code of mine rescued from a backup CD, cleaned up, and put now on SourceForge and GitHub (spot the file "three_lakes.adb" in the diff_eq/ode/ directory)...

The problem is to predict the evolution of levels of three lakes connected by two channels, given some knowledge of the inflows and outflows.
Thanks to the magic of differential equations, it is possible!
The variation of each level is a function of the in- and out-flows, and of the flow from / to neighboring lakes.
If lake X is connected to Y, there is a flow from X to Y if X's level is higher than Y's.
When X's level is lower than Y's, the flow reverses.
Obvious, isn't it?

So, the program solves a vectorial differential equation, with the famous Runge-Kutta method.
The unknown x is a vector containing the levels of three lakes (Bienne, Neuchâtel, Morat in Switzerland).
The lakes are connected by two channels (marked with "in out" on the map).
Boundary conditions take the form of natural inflows (marked "in") into the lakes,
and a single, controlled outflow (marked "out") out of one of the lakes (Bienne).
The variation of all levels is x', a function of x and of the boundary conditions.

Even with static boundary conditions, we can have situations where one of the lakes' level goes up, then down, then up again. Finally (due to the artificial static conditions) the flows find an equilibrium.

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A change in level direction (up/down) of one of the lakes (Neuchâtel) happens at the moment when two connected lakes have the same level, and the connecting channel changes its direction (points marked with arrows). You see also a change of direction of lake of Bienne's level, a bit later.

Of course in reality, the boundary conditions vary all the time, so we can have much more complex situations if these conditions are injected into the simulation.

Related publication:
Évolution simulée des niveaux dans le système des Trois-Lacs,
F. & G. de Montmollin,
Bulletin de la Société vaudoise des sciences naturelles.
88.2: 121-129, ISSN 0037-9603, 2002

Here is a wonderful picture of the lake of Bienne taken a few days ago (courtesy of Stéphane Perret).

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• 24 May 2020 at 16:56

Advent of Code 2020 with HAC and LEA

2 December 2020 at 20:27

First use of HAC, via the LEA editor, for the famous Advent of Code contest.

 LEA screenshot: a parser for Day 2 Advent of Code's puzzle

In my (of course biased) opinion, LEA is perfect (among other tasks...) for developing quickly Ada solutions to the Advent of Code problems.

After the rush, I tidy up the source code with GNAT's style checks. Hence the presence of a GNAT project file, aoc_2020.gpr .

Then I post my solutions as HAC examples, here and here

• 2 December 2020 at 20:27

HAC at work with shiny gold bags (Advent of Code, Day 7)

7 December 2020 at 14:13

Day 7 of the Advent of Code, titled "Handy Haversacks" is nice pair of puzzles involving recursion.

The data is a set of rules, beginning with

wavy green bags contain 1 posh black bag, 1 faded green bag, 4 wavy red bags.

dotted chartreuse bags contain 1 light beige bag.

dark white bags contain 2 dotted white bags.

...

There are hundreds of such rules.

The puzzles involve recursion, which makes them fun.

You can see below HAC at work on the problem, from the LEA editor.

There is also a "full Ada" version. I began with that one, because the current limitations of HAC would have been too time-consuming (in terms of development time) for submitting a solution quickly enough. The limitations are not around recursion, that HAC masters like a big one, but mostly around the enumeration type I/O which is currently non-existent in HAC (v.0.081).

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Solutions will be soon be posted on the HAC repositories.

• 7 December 2020 at 14:13

Christmas 2021

25 December 2021 at 09:51

Time for the family to celebrate... and to recover from the advent puzzles epidemic 😉

Merry Christmas to everybody!
• 25 December 2021 at 09:51

Advent of Code 2022 in pictures

14 January 2023 at 09:25

Note for subscribers: if you are interested in my Ada programming articles only, you can use this RSS feed link.

Just as a teaser for the next Advent of Code in 10 1/2 months, we would like to show a few pictures related to last edition. The 25 puzzles, data and solutions can be found here and here. They are programmed with HAC (the HAC Ada Compiler), thus in a small subset of the Ada language. The HAC compiler is very fast, so you run your program without noticing it was ever compiled, which is perfect for completing a programming puzzle.

 Day 22 run with HAC (here, embedded in the LEA environment)

However, the program will run substantially slower than compiled with a native, optimizing compiler like GNAT.
This is not an issue for most Advent of Code puzzles, but for some, it is, especially on the later days. Fortunately, changing from HAC to GNAT is trivial (just switch compilers), unlike the traditional reprogramming of Python prototypes in C++, for instance.

The pictures

Day 8's data is a map representing the height of trees. Once the puzzle was solved, I was curious how the forest looked like. Note that different users get different data, so you are unlikely to find a visualisation of exactly your data on the internet.

Day 12's puzzle is a shortest path problem with specific rules and a nice data - a terrain with a hill - which seems designed to trap depth-first-search algorithms into an almost infinite search. The yellowish path is the shortest from the green dot, elevation 'a', to blue dot, elevation 'z'.
The pinkish path is the shortest from the blue dot to any dot with elevation 'a'. Fortunately Dijkstra's algorithm (and perhaps others) allows for such a special criterion regarding the end point.

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For day 22’s puzzle, a walk on a cube’s surface is involved, so it is helpful to sketch it on a piece of paper, cut it and glue the faces. A banana skin of that puzzle is that the example’s layout may be different from the data’s layout. We slipped on that one and ended up gluing and programming the face-to-face transitions for two layouts…

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Other Adaists’ solutions, discussions and pictures can be found here and in the "2022 Day x" threads in the same forum.

• 14 January 2023 at 09:25

Ceci n'est pas une quine

22 July 2023 at 07:50
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These are not really quines, but funny self-references around computer programs...

1) LEA:

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2) HAC:

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• 22 July 2023 at 07:50