private static FastLoadedDiceRollerDiscreteSampler createSampler()

in commons-rng-sampling/src/main/java/org/apache/commons/rng/sampling/distribution/FastLoadedDiceRollerDiscreteSampler.java [723:771]

29 lines of code
5 McCabe index (conditional complexity)


    private static FastLoadedDiceRollerDiscreteSampler createSampler(UniformRandomProvider rng,
                                                                     long[] frequencies,
                                                                     int[] offsets,
                                                                     int[] indices,
                                                                     BigInteger m) {
        // Repeat the logic from createSampler(...) using extended arithmetic to test the bits

        // ALGORITHM 5: PREPROCESS
        // a == frequencies
        // m = sum(a)
        // h = leaf node count
        // H = leaf node label (lH)

        final int n = frequencies.length;

        // k = ceil(log2(m))
        final int k = m.subtract(BigInteger.ONE).bitLength();
        // r = a(n+1) = 2^k - m
        final BigInteger r = BigInteger.ONE.shiftLeft(k).subtract(m);

        final int[] h = new int[k];
        final int[] lH = new int[checkArraySize((n + 1L) * k)];
        Arrays.fill(lH, NO_LABEL);

        int d;
        for (int j = 0; j < k; j++) {
            final int shift = (k - 1) - j;

            d = 0;
            for (final int i : indices) {
                // bool w ← (a[i] >> (k − 1) − j)) & 1
                // h[j] = h[j] + w
                // if w then:
                if (testBit(frequencies[i], offsets[i], shift)) {
                    h[j]++;
                    // H[d][j] = i
                    lH[d * k + j] = i;
                    d++;
                }
            }
            // process a(n+1) without extending the input frequencies array by 1
            if (r.testBit(shift)) {
                h[j]++;
                lH[d * k + j] = n;
            }
        }

        return new FLDRSampler(rng, n, k, h, lH);
    }