/*
 * SPDX-License-Identifier: Apache-2.0
 *
 * The OpenSearch Contributors require contributions made to
 * this file be licensed under the Apache-2.0 license or a
 * compatible open source license.
 */

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/*
 * Modifications Copyright OpenSearch Contributors. See
 * GitHub history for details.
 */
package org.apache.lucene.util.packed;

import org.apache.lucene.store.DataInput;
import org.apache.lucene.util.RamUsageEstimator;

import java.io.IOException;
import java.util.Arrays;

/**
 * Forked from Lucene 8.x; removed in Lucene 9.0
 *
 * @todo further investigate a better alternative
 *
 * Space optimized random access capable array of values with a fixed number of bits/value. Values
 * are packed contiguously.
 *
 * <p>The implementation strives to perform as fast as possible under the constraint of contiguous
 * bits, by avoiding expensive operations. This comes at the cost of code clarity.
 *
 * <p>Technical details: This implementation is a refinement of a non-branching version. The
 * non-branching get and set methods meant that 2 or 4 atomics in the underlying array were always
 * accessed, even for the cases where only 1 or 2 were needed. Even with caching, this had a
 * detrimental effect on performance. Related to this issue, the old implementation used lookup
 * tables for shifts and masks, which also proved to be a bit slower than calculating the shifts and
 * masks on the fly. See https://issues.apache.org/jira/browse/LUCENE-4062 for details.
 */
class XPacked64 extends XPackedInts.MutableImpl {
    static final int BLOCK_SIZE = 64; // 32 = int, 64 = long
    static final int BLOCK_BITS = 6; // The #bits representing BLOCK_SIZE
    static final int MOD_MASK = BLOCK_SIZE - 1; // x % BLOCK_SIZE

    /** Values are stores contiguously in the blocks array. */
    private final long[] blocks;
    /** A right-aligned mask of width BitsPerValue used by {@link #get(int)}. */
    private final long maskRight;
    /** Optimization: Saves one lookup in {@link #get(int)}. */
    private final int bpvMinusBlockSize;

    /**
     * Creates an array with the internal structures adjusted for the given limits and initialized to
     * 0.
     *
     * @param valueCount the number of elements.
     * @param bitsPerValue the number of bits available for any given value.
     */
    public XPacked64(int valueCount, int bitsPerValue) {
        super(valueCount, bitsPerValue);
        final PackedInts.Format format = PackedInts.Format.PACKED;
        final int longCount = format.longCount(PackedInts.VERSION_CURRENT, valueCount, bitsPerValue);
        this.blocks = new long[longCount];
        maskRight = ~0L << (BLOCK_SIZE - bitsPerValue) >>> (BLOCK_SIZE - bitsPerValue);
        bpvMinusBlockSize = bitsPerValue - BLOCK_SIZE;
    }

    /**
     * Creates an array with content retrieved from the given DataInput.
     *
     * @param in a DataInput, positioned at the start of Packed64-content.
     * @param valueCount the number of elements.
     * @param bitsPerValue the number of bits available for any given value.
     * @throws java.io.IOException if the values for the backing array could not be retrieved.
     */
    public XPacked64(int packedIntsVersion, DataInput in, int valueCount, int bitsPerValue) throws IOException {
        super(valueCount, bitsPerValue);
        final PackedInts.Format format = PackedInts.Format.PACKED;
        final long byteCount = format.byteCount(packedIntsVersion, valueCount, bitsPerValue); // to know how much to read
        final int longCount = format.longCount(PackedInts.VERSION_CURRENT, valueCount, bitsPerValue); // to size the array
        blocks = new long[longCount];
        // read as many longs as we can
        for (int i = 0; i < byteCount / 8; ++i) {
            blocks[i] = in.readLong();
        }
        final int remaining = (int) (byteCount % 8);
        if (remaining != 0) {
            // read the last bytes
            long lastLong = 0;
            for (int i = 0; i < remaining; ++i) {
                lastLong |= (in.readByte() & 0xFFL) << (56 - i * 8);
            }
            blocks[blocks.length - 1] = lastLong;
        }
        maskRight = ~0L << (BLOCK_SIZE - bitsPerValue) >>> (BLOCK_SIZE - bitsPerValue);
        bpvMinusBlockSize = bitsPerValue - BLOCK_SIZE;
    }

    /**
     * @param index the position of the value.
     * @return the value at the given index.
     */
    @Override
    public long get(final int index) {
        // The abstract index in a bit stream
        final long majorBitPos = (long) index * bitsPerValue;
        // The index in the backing long-array
        final int elementPos = (int) (majorBitPos >>> BLOCK_BITS);
        // The number of value-bits in the second long
        final long endBits = (majorBitPos & MOD_MASK) + bpvMinusBlockSize;

        if (endBits <= 0) { // Single block
            return (blocks[elementPos] >>> -endBits) & maskRight;
        }
        // Two blocks
        return ((blocks[elementPos] << endBits) | (blocks[elementPos + 1] >>> (BLOCK_SIZE - endBits))) & maskRight;
    }

    @Override
    public int get(int index, long[] arr, int off, int len) {
        assert len > 0 : "len must be > 0 (got " + len + ")";
        assert index >= 0 && index < valueCount;
        len = Math.min(len, valueCount - index);
        assert off + len <= arr.length;

        final int originalIndex = index;
        final PackedInts.Decoder decoder = BulkOperation.of(PackedInts.Format.PACKED, bitsPerValue);

        // go to the next block where the value does not span across two blocks
        final int offsetInBlocks = index % decoder.longValueCount();
        if (offsetInBlocks != 0) {
            for (int i = offsetInBlocks; i < decoder.longValueCount() && len > 0; ++i) {
                arr[off++] = get(index++);
                --len;
            }
            if (len == 0) {
                return index - originalIndex;
            }
        }

        // bulk get
        assert index % decoder.longValueCount() == 0;
        int blockIndex = (int) (((long) index * bitsPerValue) >>> BLOCK_BITS);
        assert (((long) index * bitsPerValue) & MOD_MASK) == 0;
        final int iterations = len / decoder.longValueCount();
        decoder.decode(blocks, blockIndex, arr, off, iterations);
        final int gotValues = iterations * decoder.longValueCount();
        index += gotValues;
        len -= gotValues;
        assert len >= 0;

        if (index > originalIndex) {
            // stay at the block boundary
            return index - originalIndex;
        } else {
            // no progress so far => already at a block boundary but no full block to get
            assert index == originalIndex;
            return super.get(index, arr, off, len);
        }
    }

    @Override
    public void set(final int index, final long value) {
        // The abstract index in a contiguous bit stream
        final long majorBitPos = (long) index * bitsPerValue;
        // The index in the backing long-array
        final int elementPos = (int) (majorBitPos >>> BLOCK_BITS); // / BLOCK_SIZE
        // The number of value-bits in the second long
        final long endBits = (majorBitPos & MOD_MASK) + bpvMinusBlockSize;

        if (endBits <= 0) { // Single block
            blocks[elementPos] = blocks[elementPos] & ~(maskRight << -endBits) | (value << -endBits);
            return;
        }
        // Two blocks
        blocks[elementPos] = blocks[elementPos] & ~(maskRight >>> endBits) | (value >>> endBits);
        blocks[elementPos + 1] = blocks[elementPos + 1] & (~0L >>> endBits) | (value << (BLOCK_SIZE - endBits));
    }

    @Override
    public int set(int index, long[] arr, int off, int len) {
        assert len > 0 : "len must be > 0 (got " + len + ")";
        assert index >= 0 && index < valueCount;
        len = Math.min(len, valueCount - index);
        assert off + len <= arr.length;

        final int originalIndex = index;
        final PackedInts.Encoder encoder = BulkOperation.of(PackedInts.Format.PACKED, bitsPerValue);

        // go to the next block where the value does not span across two blocks
        final int offsetInBlocks = index % encoder.longValueCount();
        if (offsetInBlocks != 0) {
            for (int i = offsetInBlocks; i < encoder.longValueCount() && len > 0; ++i) {
                set(index++, arr[off++]);
                --len;
            }
            if (len == 0) {
                return index - originalIndex;
            }
        }

        // bulk set
        assert index % encoder.longValueCount() == 0;
        int blockIndex = (int) (((long) index * bitsPerValue) >>> BLOCK_BITS);
        assert (((long) index * bitsPerValue) & MOD_MASK) == 0;
        final int iterations = len / encoder.longValueCount();
        encoder.encode(arr, off, blocks, blockIndex, iterations);
        final int setValues = iterations * encoder.longValueCount();
        index += setValues;
        len -= setValues;
        assert len >= 0;

        if (index > originalIndex) {
            // stay at the block boundary
            return index - originalIndex;
        } else {
            // no progress so far => already at a block boundary but no full block to get
            assert index == originalIndex;
            return super.set(index, arr, off, len);
        }
    }

    @Override
    public String toString() {
        return "Packed64(bitsPerValue=" + bitsPerValue + ",size=" + size() + ",blocks=" + blocks.length + ")";
    }

    @Override
    public long ramBytesUsed() {
        return RamUsageEstimator.alignObjectSize(
            RamUsageEstimator.NUM_BYTES_OBJECT_HEADER + 3 * Integer.BYTES // bpvMinusBlockSize,valueCount,bitsPerValue
                + Long.BYTES // maskRight
                + RamUsageEstimator.NUM_BYTES_OBJECT_REF
        ) // blocks ref
            + RamUsageEstimator.sizeOf(blocks);
    }

    @Override
    public void fill(int fromIndex, int toIndex, long val) {
        assert PackedInts.unsignedBitsRequired(val) <= getBitsPerValue();
        assert fromIndex <= toIndex;

        // minimum number of values that use an exact number of full blocks
        final int nAlignedValues = 64 / gcd(64, bitsPerValue);
        final int span = toIndex - fromIndex;
        if (span <= 3 * nAlignedValues) {
            // there needs be at least 2 * nAlignedValues aligned values for the
            // block approach to be worth trying
            super.fill(fromIndex, toIndex, val);
            return;
        }

        // fill the first values naively until the next block start
        final int fromIndexModNAlignedValues = fromIndex % nAlignedValues;
        if (fromIndexModNAlignedValues != 0) {
            for (int i = fromIndexModNAlignedValues; i < nAlignedValues; ++i) {
                set(fromIndex++, val);
            }
        }
        assert fromIndex % nAlignedValues == 0;

        // compute the long[] blocks for nAlignedValues consecutive values and
        // use them to set as many values as possible without applying any mask
        // or shift
        final int nAlignedBlocks = (nAlignedValues * bitsPerValue) >> 6;
        final long[] nAlignedValuesBlocks;
        {
            XPacked64 values = new XPacked64(nAlignedValues, bitsPerValue);
            for (int i = 0; i < nAlignedValues; ++i) {
                values.set(i, val);
            }
            nAlignedValuesBlocks = values.blocks;
            assert nAlignedBlocks <= nAlignedValuesBlocks.length;
        }
        final int startBlock = (int) (((long) fromIndex * bitsPerValue) >>> 6);
        final int endBlock = (int) (((long) toIndex * bitsPerValue) >>> 6);
        for (int block = startBlock; block < endBlock; ++block) {
            final long blockValue = nAlignedValuesBlocks[block % nAlignedBlocks];
            blocks[block] = blockValue;
        }

        // fill the gap
        for (int i = (int) (((long) endBlock << 6) / bitsPerValue); i < toIndex; ++i) {
            set(i, val);
        }
    }

    private static int gcd(int a, int b) {
        if (a < b) {
            return gcd(b, a);
        } else if (b == 0) {
            return a;
        } else {
            return gcd(b, a % b);
        }
    }

    @Override
    public void clear() {
        Arrays.fill(blocks, 0L);
    }
}
