PolyFit

A lightweight C library for fitting polynomials to data—fast, simple, and no external dependencies.

What is it?

PolyFit is a small, embeddable C library for polynomial regression. It helps you fit a curve through a set of data points so you can interpolate or extrapolate with minimal effort.

Use PolyFit for signal smoothing, approximating system behavior, or forming simple transfer-function polynomials. Include the two source files and call the API; no extra setup required.

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Where It’s Useful

Here's a few real-world uses:

Signal Processing

Useful in cleaning up noisy data e.g., smoothing biomedical signals like EEGs.

Robotics & Navigation

Model sensor noise, trajectory paths, or estimate system motion with smooth, continuous polynomials.

Control Systems

Originally built to model dynamic systems using polynomial-based transfer functions. Great for:

• Root locus
• Frequency response
• Pole-zero plots

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Features

• Polynomial curve fitting with configurable order
• Supports both interpolation and extrapolation
• Single-file: just drop in polyfit.c and polyfit.h
• No dependencies beyond the standard library

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Quickstart

Clone the Repo

git clone https://github.com/FinOrr/polyfit.git

Then just copy polyfit.c and polyfit.h into your project.

Include It

#include "polyfit.h"

Basic Usage

Example 1: Simple linear fit (recommended)

#include "polyfit.h"
#include <stdio.h>

int main(void) {
    // Your data points
    float x[] = {1.0, 2.0, 3.0, 4.0, 5.0};
    float y[] = {2.1, 3.9, 6.1, 8.0, 9.9};

    // Fit a linear polynomial (degree 1)
    Polynomial *poly = polyfit(x, y, 5, 1, NULL);
    if (poly) {
        float result;
        polyfit_evaluate(poly, 6.0f, &result);
        printf("f(6.0) = %f\n", result);  // Predict at x=6
        polyfit_free(poly);
    }

    return 0;
}

Example 2: One-shot evaluation (no memory management)

// Fit and evaluate in one call - perfect for quick predictions
float x[] = {0.0, 1.0, 2.0, 3.0};
float y[] = {1.0, 2.0, 5.0, 10.0};
float result;

if (polyfit_eval_at(x, y, 4, 2, 4.0f, &result) == POLYFIT_SUCCESS) {
    printf("f(4.0) = %f\n", result);
}

Example 3: Extract coefficients

Polynomial *poly = polyfit(x, y, 5, 2, NULL);
if (poly) {
    float coeffs[3];  // degree + 1
    if (polyfit_get_coefficients(poly, coeffs, 3) == POLYFIT_SUCCESS) {
        printf("y = %.2f + %.2f*x + %.2f*x^2\n",
               coeffs[0], coeffs[1], coeffs[2]);
    }
    polyfit_free(poly);
}

Example 4: Error handling

polyfit_error_t err;
Polynomial *poly = polyfit(x, y, num_points, degree, &err);
if (!poly) {
    fprintf(stderr, "Error: %s\n", polyfit_error_string(err));
    return -1;
}
// Use polynomial...
polyfit_free(poly);

Integration

Just copy these two files into your project:

• polyfit.c
• polyfit.h

Then compile with your source code:

gcc -o myapp main.c polyfit.c -lm

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Documentation

More detailed info and examples are available on the Wiki.

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Contributing

Found a bug? Want to improve performance or add features? PRs welcome, just read the contributing guide first.

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License

MIT. See LICENSE for full details.

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Questions?

Open an issue if you get stuck or want to ask anything.