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Linear Algebra utilities for Java
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package ch.akuhn.matrix;

/**
 * A dense vector.
 * 
 * @author Adrian Kuhn
 */
public class DenseVector extends Vector {

    private double unit = 0.0;

    /* package */double[] values;

    protected DenseVector(double[] values) {
        this.values = values;
    }

    protected DenseVector(int size) {
        values = new double[size];
    }

    /**
     * Cosine of angle between this and another vector
     * 
     * @param other
     * @return the cosine
     */
    public double cosine(DenseVector other) {
        if (other.size() != size()) {
            throw new IllegalArgumentException("other.size() != this.size()");
        }
        double sum = 0.0;
        for (int n = 0; n < values.length; n++) {
            sum += values[n] * other.values[n];
        }
        return sum / (this.norm() * other.norm());
    }

    @Override
    public double get(int index) {
        return values[index];
    }

    @Override
    public double norm() {
        if (unit != 0.0) {
            return unit; // FIXME should purge cache on edit
        }
        double qsum = 0.0;
        for (double value : values) {
            qsum += value * value;
        }
        if (qsum == 0.0) {
            qsum = 1.0;
        }
        return unit = Math.sqrt(qsum);
    }

    @Override
    public double put(int index, double value) {
        return values[index] = value;
    }

    @Override
    public int size() {
        return values.length;
    }

    @Override
    public Vector times(double scalar) {
        double[] times = new double[values.length];
        for (int n = 0; n < values.length; n++) {
            times[n] = values[n] * scalar;
        }
        return new DenseVector(times);
    }

    @Override
    public Vector timesEquals(double scalar) {
        for (int n = 0; n < values.length; n++) {
            values[n] *= scalar;
        }
        return this;
    }

    @Override
    public boolean equals(Vector v, double epsilon) {
        if (!(v instanceof DenseVector)) {
            return false;
        }
        if (size() != v.size()) {
            return false;
        }
        DenseVector d = (DenseVector) v;
        for (int i = 0; i < values.length; i++) {
            if ((values[i] - d.values[i]) > epsilon) {
                return false;
            }
        }
        return true;
    }

    @Override
    public double[] unwrap() {
        return values;
    }
}