Joshua Bloch Wtf.java break-in

WtfCreator.java

import org.apache.commons.math3.analysis.interpolation.NevilleInterpolator;

public class WtfCreator {
    public static void main(String[] args) {
        var text = "Hello, world!\n";

        double[] x = new double[text.length() + 1];
        double[] y = new double[text.length() + 1];
        for(var i = 0; i < text.length(); i++) {
            x[i] = i;
            y[i] = (int) text.charAt(i);
        }
        x[text.length()] = text.length();
        y[text.length()] = 0;

        var interpolator = new NevilleInterpolator();
        var function = interpolator.interpolate(x, y);
        double[] coeff = function.getCoefficients();
        System.out.println("static char p(int i) {");
        System.out.println("    return (char) (" + coeff[0] + " + 0.5");
        for(int i = 1; i < coeff.length; i++) {
            System.out.println("            + i * (" + coeff[i]);
        }
        System.out.println("        " + ")".repeat(coeff.length) + ";");
        System.out.println("}");

        // Use new function like this:
        // for(var i = 0; p(i) != 0; i++)
        //     System.out.print(p(i));
    }
}

WtfCreatorAndInterpreter.java

import jdk.jshell.JShell;
import org.apache.commons.math3.analysis.interpolation.NevilleInterpolator;

import java.io.ByteArrayOutputStream;
import java.io.PrintStream;

public class WtfCreator {
    public static void main(String[] args) {
        var text = "Hello, world!\n";

        double[] x = new double[text.length() + 1];
        double[] y = new double[text.length() + 1];
        for(var i = 0; i < text.length(); i++) {
            x[i] = i;
            y[i] = (int) text.charAt(i);
        }
        x[text.length()] = text.length();
        y[text.length()] = 0;

        var interpolator = new NevilleInterpolator();
        var function = interpolator.interpolate(x, y);
        double[] coeff = function.getCoefficients();

        var pMethod = new StringBuilder();
        pMethod.append("char p(int i) {\n");
        pMethod.append("    return (char) (" + coeff[0] + " + 0.5\n");
        for(int i = 1; i < coeff.length; i++) {
            pMethod.append("            + i * (" + coeff[i] + "\n");
        }
        pMethod.append(")".repeat(coeff.length) + ";\n");
        pMethod.append("}\n");

        var result = new ByteArrayOutputStream();
        var shell = JShell.builder().out(new PrintStream(result)).build();
        shell.eval(pMethod.toString());
        shell.eval("for(var i = 0; p(i) != 0; i++) System.out.print(p(i));");
        shell.close();
        System.out.println(result.toString());
    }
}