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Show HN: I made a spreadsheet where formulas also update backwards

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Hello HN! I'm happy to release this project today. It's a bidirectional calculator (hence the name bidicalc).

I've been obsessed with the idea of making a spreadsheet where you can update both inputs and outputs, instead of regular spreadsheets where you can only update inputs.

Please let me know what you think! Especially if you find bugs or good example use cases.

The interesting thing here isn’t “spreadsheet, but backwards” so much as “spreadsheet as a constraint system”. Classic spreadsheets are basically DAGs: data flows one way and a lot of UX assumptions (and people’s intuition) rely on that. As soon as you allow arbitrary cells to be solved for, you’re in “which variables are free?” land, and most of the confusion in this thread is really about degrees of freedom, not about the math.

One way to make this less surprising might be to flip the default: treat all cells as fixed unless explicitly marked as solver variables, and give a lightweight visualization of “these are the cells that will move if you edit this one.” That keeps the power of a general constraint solver while preserving the mental model spreadsheet users already have, and it opens the door to more serious use cases (financial models, physics, scheduling) without feeling like spooky action at a distance.

1 month agoby pedrozieg

The first example on the main page has a formula with two variables being updated from changing one value. The immediate question I have is if I change the output, where does the extra degree of freedom come from on the inputs? Does one stay locked in place? Unclear.

I am a huge fan of the concept though. It's been bugging me for years that my spreadsheet doesn't allow editing text fields after filtering and sorting them down to the subset I want. I have to go all the way back to the mess of unsorted input rows to actually edit them.

1 month agoby willrshansen

I have wanted one general application of this idea in a spreadsheet. Specifically, I track some of my running, including speed (pace), distance, and time. Under different circumstances, I have exactly two of the three available and I want the third to be computed, but it varies which. I have found it fairly difficult to implement this kind of data entry in Google Spreadsheets and Excel, even know conceptually it's a very simple constraint "a*b=c" where I know some two variables.

As a more substantive comment: You may find the thesis "Propagation networks : a flexible and expressive substrate for computation" by Alexey Radul interesting. https://dspace.mit.edu/handle/1721.1/54635

1 month agoby areyousure

Can you enter an RSA key and have it produce two prime numbers?

1 month agoby SoftTalker

In Excel you have goal seek for this functionality. I believe it does some form of numerical solving of the equation system.

Good for every situation when you need to solve equations!

In the context of using spreadsheets I think about solving simple financial or maybe construction/mechanical design problems where you don’t want to solve it manually or program it and a spreadsheet is a quick and useful interface.

1 month agoby b-karl

This is really cool! It's like Excel's goal seek but can also handle the case of arbitrary input cells. Goal seeek can only handle one input and one output cell.

But how do you handle the case where multiple variables can be changed? If multiple input cells is the key difference from Goal seek, i think some more rigor should be placed into the algorithm here

e.g. setting A1 + B1 and wanting the result to be 5. Currently it bumps both A1 and B1 equally. What's the thought process behind this?

1 month agoby thomastay

Cool!

Constraint propagation from SICP is a great reference here:

https://sicp.sourceacademy.org/chapters/3.3.5.html

1 month agoby amirhirsch

“Formulas that update backwards” is the main idea behind neural networks such as LLMs: the computation network produces a value, the error in this value is computed, and then the error quantity is pushed backward through the network; this relies on the differentiability of the function computed at each node in the network.

1 month agoby adonovan

Wow! See the classic https://en.wikipedia.org/wiki/TK_Solver

1 month agoby PaulHoule

This is pretty cool.

This functionality is called ‘break back’ in a lot of enterprise modelling software. See [IBM TM1](https://www.ibm.com/docs/en/cognos-planning/10.2.1?topic=bre...) and [Anaplan](https://help.anaplan.com/breakback-1b7aa87d-aa13-49f6-8f7d-d...). They generally work in terms of scaling the inputs to match the new output though, which is a bit more basic than this approach.

1 month agoby jsax

Somebody did this back in the DOS era. The program was sometimes called "the crooked accountant's spreadsheet", because you could start with the outputs you wanted and get the input numbers adjusted to fit.

Anyone remember?

1 month agoby Animats

I think the concept is solid. I’ve only had a few minutes of playing with it, but I have the opinion is that from a UX perspective constants are more common than variables. So perhaps a cell containing a constant should not have a #, but a variable should.

1 month agoby kccqzy

Ah, two way data binding. If you've used any frameworks before React (and a couple earlier ones with the same philosophy) you'll understand how it becomes a spaghetti mess over time.

1 month agoby satvikpendem

A bidirectional formula is also known as an integrity constraint in databases (plus some triggers for restoring the constraint upon updates)!

1 month agoby remywang

This is great.

Since you've asked about bugs, I tried pushing the limits and found the following:

    A1: 100          B1: =100-A1
    A2: =A1*(100-A1)
    A3: =A1*B1

A2 can be successfully set to anything reasonable (up to 2500)

However, setting A3 to exactly 100 doesn't work, even though setting it to 101 or 99 (or even 100.000001) does work.

Another limit:

    A1: 100          B1: 100
    A2: =A1+B1
    A3: =A1*B1
    A4: =abs(A2-100) + abs(A3-100)

Setting A4 to zero (or anything below 80) doesn't work. This doesn't improve if the constants in the A4 formula are moved a short distance away from 100.

In case you can't tell from that last example, I think being able to fix the intended values of multiple outputs simultaneously would be interesting. If you were to give more details about the solver's internals, I'd be keen to hear them.

1 month agoby penteract

The idea is very interesting. As a default strategy you could preserve the ratio of inputs by scaling them to match the scaling of the output, instead of making them equal (for addition). Similarly, for multiplication, you could preserve the ratio of inputs as well, by scaling them by nth root of the scaling factor of the output.

1 month agoby zkmon

The examples are great and these bidirectional calculators are something that people would love to have in traditional spreadsheets.

So much so that Credit Suisse, which basically was running everything on heavily modded Excel, created a full language whose outputs were Excel spreadsheets capable of doing that. That thing called “paradise” was a total monstrosity but showed how much people wanted this.

That said, you really need a way to set which cells are fixed and which cells are allowed to move if you want to move past basic examples.

Most times you know what you want to do. like => if the user modifies that cell, find a solution for those specific ones.

If you can enter that info, then you have a lot more constrains for your solver and will avoid a lot of edge cases where everything goes to 0, and you can check that the calculation entered is indeed reversible or not, or if it could have multiple solutions, and so on.

1 month agoby d--b

Could you build an inverse kinematics solver with this? (I recently watched a youtube video of someone iteratively working out the solutions for a robotic arm, by alternating modifying the inputs and the results)

1 month agoby nico

Spredsheet but no VLOOKUP?

1 month agoby rvba

you might like https://omrelli.ug/g9/ which is a similar concept but for graphics

1 month agoby jy14898

Sympy can (often) solve under constrained systems in terms of the free variables. The problem I run into is discrete constraints that make solving less closed form and more combinatorial search. When textbook amplifier formulas significantly diverge from physical reality I model the errors as linear correction factors and use gradient descent to correct it in a few experiments, but I’m curious if there is software that has solved this problem.

1 month agoby deckar01

Reminds me of functional logical programming languages like Verse. when you specify the output and ask for the inputs, you get all possible inputs.

1 month agoby iamwil

Sounds a bit like Prolog with a Spreadsheet UI?

1 month agoby 1718627440

Hm? I don't get it.

What's the point of calculating backwards non-invertible operations such as addition? Isn't the result just arbitrary?

1 month agoby throw310822

Phenomenal! This is a solid prototype, clean execution. I've had exactly this idea, too: I've already given the spreadsheet the relationships between these values, why can't it just work backwards when values change? That premise hides a ton of complexity, though, I'm sure. Lots of scary matrices.

1 month agoby hallole

Very cool!

I'd love to see a version where cells are "torn off" and named as they were in Lotus Improv and one had a "formula pane" where one could see all the formulae for a spreadsheet.

Would it be possible to create this in Python so that it could be a part of pyspread?

1 month agoby WillAdams

LOL! Gemini suggested to implement this to me literally yesterday: bidirectional computations. The example was that given a temperature in Celsius and Fahrenheit, modifying either of them should update their counterpart. In angular that would be two linked signals for instance, but even that is a bit fringe. Gemini was going for something even more elaborated.

I told Gemini that spreadsheets were actually not doing that and that I had ways to implement that behavior without the complexity.

Just writing that to show the rabbit hole people are going to fall into if they let their llms go brrr. ;D

In any case, the problem is interesting. The point was to include bi-directionality inside a graph of computations so that we didn't get bogged down by cycles. The benefit being that it would handle float precision issues.

My more manual solution expect that floats precision issues are handled explicitly. I think that this level of explicitness is needed anyway for proper floating point error mitigation.

1 month agoby aatd86

A 2d sketcher with constraints is kind of similar. For example the equation

A = B + C

Where A, B, C are the lengths of 3 parallel lines. Within the sketcher you can drag the length of any one of those lines and the other two will adjust to keep the constraints.

1 month agoby injidup

Could this easily represent a Kalman filter and other typically complex control problems?

Makes me imagine plotting a inverted pendulum and other real time systems. Could a cell variable be set to Time?

1 month agoby koolala

Super cool! Well done. Now take it down and never let Microsoft get their hands on the code, or the entire economic system will go down in flames.

1 month agoby digdugdirk

Thanks for sharing. It’s an alluring idea. I think you’d find the concept of statistical identifiability interesting.

1 month agoby RA_Fisher

Excellent (sorry accidental pun)

This is a nice exploration.

1 month agoby nrhrjrjrjtntbt

interesting. like Excel Solver? or OpenSolver, Gurobi, other optimizers? or different objective?