Add Spectrum class with an implementation

.idea/libraries/google_guava.xml

@@ -2,13 +2,13 @@
   <library name="google.guava" type="repository">
     <properties maven-id="com.google.guava:guava:RELEASE" />
     <CLASSES>
-      <root url="jar://$PROJECT_DIR$/lib/guava-31.1-jre.jar!/" />
+      <root url="jar://$PROJECT_DIR$/lib/guava-32.1.2-jre.jar!/" />
       <root url="jar://$PROJECT_DIR$/lib/failureaccess-1.0.1.jar!/" />
       <root url="jar://$PROJECT_DIR$/lib/listenablefuture-9999.0-empty-to-avoid-conflict-with-guava.jar!/" />
       <root url="jar://$PROJECT_DIR$/lib/jsr305-3.0.2.jar!/" />
-      <root url="jar://$PROJECT_DIR$/lib/checker-qual-3.12.0.jar!/" />
-      <root url="jar://$PROJECT_DIR$/lib/error_prone_annotations-2.11.0.jar!/" />
-      <root url="jar://$PROJECT_DIR$/lib/j2objc-annotations-1.3.jar!/" />
+      <root url="jar://$PROJECT_DIR$/lib/checker-qual-3.33.0.jar!/" />
+      <root url="jar://$PROJECT_DIR$/lib/error_prone_annotations-2.18.0.jar!/" />
+      <root url="jar://$PROJECT_DIR$/lib/j2objc-annotations-2.8.jar!/" />
     </CLASSES>
     <JAVADOC />
     <SOURCES />

.idea/misc.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <project version="4">
-  <component name="ProjectRootManager" version="2" languageLevel="JDK_19" default="true" project-jdk-name="liberica-19" project-jdk-type="JavaSDK">
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_20" default="true" project-jdk-name="liberica-20" project-jdk-type="JavaSDK">
     <output url="file://$PROJECT_DIR$/out" />
   </component>
 </project>

src/xyz/marsavic/gfxlab/Spectrum.java

@@ -0,0 +1,93 @@
+package xyz.marsavic.gfxlab;
+
+import javafx.util.Pair;
+
+import java.util.Arrays;
+import java.util.Comparator;
+
+@FunctionalInterface
+public interface Spectrum {
+    static final double minWavelength = 380;
+    static final double maxWavelength = 750;
+    public double at(double wavelength);
+}
+
+class DoublePairKeyComparator implements Comparator<Pair<Double, Double>> {
+    @Override
+    public int compare(Pair<Double, Double> o1, Pair<Double, Double> o2) {
+        double k1 = o1.getKey(), k2 = o2.getKey();
+        if (k1 > k2)
+            return 1;
+        else if (k1 < k2) {
+            return -1;
+        } else {
+            return 0;
+        }
+    }
+}
+
+class SplineSpectrum implements Spectrum {
+    Pair<Double, Double>[] samples;
+    double[] m;
+
+    public SplineSpectrum(Pair<Double, Double>[] samples) {
+        if (samples == null) {
+            throw new NullPointerException();
+        }
+        this.samples = samples;
+        int n = samples.length;
+
+        double[] d = new double[n - 1];
+        m = new double[n];
+
+
+        for (int i = 1; i < samples.length; i++) {
+            double h = samples[i].getKey() - samples[i - 1].getKey();
+            if (h <= 0.0)
+                throw new IllegalArgumentException("Samples must have strictly increasing x coordinates.");
+            d[i] = (samples[i].getValue() - samples[i - 1].getValue());
+        }
+
+        m[0] = d[0];
+        for (int i = 1; i < n - 1; i++) {
+            if (d[i] == 0.0) {
+                m[i] = 0.0;
+                m[i + 1] = 0.0;
+            } else {
+                double a = m[i] / d[i];
+                double b = m[i + 1] / d[i];
+                double h = a*a+b*b;
+                if (h > 9.0) {
+                    double t = 3.0 / h;
+                    m[i] = t * a * d[i];
+                    m[i + 1] = t * b * d[i];
+                }
+            }
+        }
+    }
+
+    @Override
+    public double at(double wavelength) {
+        final int n = samples.length;
+
+        if (wavelength <= samples[0].getKey()) {
+            return  samples[0].getValue();
+        }
+        if (wavelength >= samples[n-1].getKey()) {
+            return samples[n-1].getValue();
+        }
+
+        Double.valueOf(wavelength);
+        int i = Arrays.binarySearch(samples,
+                                    new Pair<Double, Double>(Double.valueOf(wavelength), null),
+                                    new DoublePairKeyComparator());
+        if (i >= 0) {
+            return samples[i].getValue();
+        }
+        i = -(i+1);
+        double h = samples[i+1].getKey() - samples[i].getKey();
+        double t = (wavelength - samples[i].getKey()) / h;
+        return (samples[i].getValue() * (1 + 2*t) + h*m[i]*t) * (1 - t) * (1 - t)
+                + (samples[i+1].getValue() * (3 - 2*t) + h * m[i+1] * (t - 1)) * t * t;
+    }
+}