/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

/*
 * Source:
 * http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166e/Striped64.java?revision=1.9
 */

package com.google.common.cache;

import com.google.common.annotations.GwtIncompatible;
import java.util.Random;

/**
 * A package-local class holding common representation and mechanics
 * for classes supporting dynamic striping on 64bit values. The class
 * extends Number so that concrete subclasses must publicly do so.
 */


@GwtIncompatible
abstract class Striped64 extends Number {
    /*
     * This class maintains a lazily-initialized table of atomically
     * updated variables, plus an extra "base" field. The table size
     * is a power of two. Indexing uses masked per-thread hash codes.
     * Nearly all declarations in this class are package-private,
     * accessed directly by subclasses.
     *
     * Table entries are of class Cell; a variant of AtomicLong padded
     * to reduce cache contention on most processors. Padding is
     * overkill for most Atomics because they are usually irregularly
     * scattered in memory and thus don't interfere much with each
     * other. But Atomic objects residing in arrays will tend to be
     * placed adjacent to each other, and so will most often share
     * cache lines (with a huge negative performance impact) without
     * this precaution.
     *
     * In part because Cells are relatively large, we avoid creating
     * them until they are needed.  When there is no contention, all
     * updates are made to the base field.  Upon first contention (a
     * failed CAS on base update), the table is initialized to size 2.
     * The table size is doubled upon further contention until
     * reaching the nearest power of two greater than or equal to the
     * number of CPUS. Table slots remain empty (null) until they are
     * needed.
     *
     * A single spinlock ("busy") is used for initializing and
     * resizing the table, as well as populating slots with new Cells.
     * There is no need for a blocking lock; when the lock is not
     * available, threads try other slots (or the base).  During these
     * retries, there is increased contention and reduced locality,
     * which is still better than alternatives.
     *
     * Per-thread hash codes are initialized to random values.
     * Contention and/or table collisions are indicated by failed
     * CASes when performing an update operation (see method
     * retryUpdate). Upon a collision, if the table size is less than
     * the capacity, it is doubled in size unless some other thread
     * holds the lock. If a hashed slot is empty, and lock is
     * available, a new Cell is created. Otherwise, if the slot
     * exists, a CAS is tried.  Retries proceed by "double hashing",
     * using a secondary hash (Marsaglia XorShift) to try to find a
     * free slot.
     *
     * The table size is capped because, when there are more threads
     * than CPUs, supposing that each thread were bound to a CPU,
     * there would exist a perfect hash function mapping threads to
     * slots that eliminates collisions. When we reach capacity, we
     * search for this mapping by randomly varying the hash codes of
     * colliding threads.  Because search is random, and collisions
     * only become known via CAS failures, convergence can be slow,
     * and because threads are typically not bound to CPUS forever,
     * may not occur at all. However, despite these limitations,
     * observed contention rates are typically low in these cases.
     *
     * It is possible for a Cell to become unused when threads that
     * once hashed to it terminate, as well as in the case where
     * doubling the table causes no thread to hash to it under
     * expanded mask.  We do not try to detect or remove such cells,
     * under the assumption that for long-running instances, observed
     * contention levels will recur, so the cells will eventually be
     * needed again; and for short-lived ones, it does not matter.
     */

    /**
     * Padded variant of AtomicLong supporting only raw accesses plus CAS.
     * The value field is placed between pads, hoping that the JVM doesn't
     * reorder them.
     *
     * JVM intrinsics note: It would be possible to use a release-only
     * form of CAS here, if it were provided.
     */
  static final class Cell {
    volatile long p0, p1, p2, p3, p4, p5, p6;
    volatile long value;
    volatile long q0, q1, q2, q3, q4, q5, q6;

    Cell(long x) {
      value = x;
    }

    final boolean cas(long cmp, long val) {
      return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val);
    }

        // Unsafe mechanics

    private static final sun.misc.Unsafe UNSAFE;
    private static final long valueOffset;

    static {
      try {
        UNSAFE = getUnsafe();
        Class<?> ak = Cell.class;
        valueOffset = UNSAFE.objectFieldOffset(ak.getDeclaredField("value"));
      } catch (Exception e) {
        throw new Error(e);
      }
    }
  }

    /**
     * ThreadLocal holding a single-slot int array holding hash code.
     * Unlike the JDK8 version of this class, we use a suboptimal
     * int[] representation to avoid introducing a new type that can
     * impede class-unloading when ThreadLocals are not removed.
     */


  static final ThreadLocal<int[]> threadHashCode = new ThreadLocal<int[]>();

    /**
     * Generator of new random hash codes
     */

  static final Random rng = new Random();

    /** Number of CPUS, to place bound on table size */

  static final int NCPU = Runtime.getRuntime().availableProcessors();

    /**
     * Table of cells. When non-null, size is a power of 2.
     */
  transient volatile Cell[] cells;

    /**
     * Base value, used mainly when there is no contention, but also as
     * a fallback during table initialization races. Updated via CAS.
     */
  transient volatile long base;

    /**
     * Spinlock (locked via CAS) used when resizing and/or creating Cells.
     */
  transient volatile int busy;

    /**
     * Package-private default constructor
     */

  Striped64() {}

    /**
     * CASes the base field.
     */


  final boolean casBase(long cmp, long val) {
    return UNSAFE.compareAndSwapLong(this, baseOffset, cmp, val);
        }

    /**
     * CASes the busy field from 0 to 1 to acquire lock.
     */

  final boolean casBusy() {
    return UNSAFE.compareAndSwapInt(this, busyOffset, 0, 1);
        }

    /**
     * Computes the function of current and new value. Subclasses
     * should open-code this update function for most uses, but the
     * virtualized form is needed within retryUpdate.
     *
     * @param currentValue the current value (of either base or a cell)
     * @param newValue the argument from a user update call
     * @return result of the update function
     */

  abstract long fn(long currentValue, long newValue);

    /**
     * Handles cases of updates involving initialization, resizing,
     * creating new Cells, and/or contention. See above for
     * explanation. This method suffers the usual non-modularity
     * problems of optimistic retry code, relying on rechecked sets of
     * reads.
     *
     * @param x the value
     * @param hc the hash code holder
     * @param wasUncontended false if CAS failed before call
     */


  final void retryUpdate(long x, int[] hc, boolean wasUncontended) {
    int h;
    if (hc == null) {
      threadHashCode.set(hc = new int[1]); // Initialize randomly
      int r = rng.nextInt(); // Avoid zero to allow xorShift rehash
      h = hc[0] = (r == 0) ? 1 : r;
    } else h = hc[0];
    boolean collide = false;                // True if last slot nonempty
    for (; ; ) {
      Cell[] as;
      Cell a;
      int n;
      long v;
      if ((as = cells) != null && (n = as.length) > 0) {
        if ((a = as[(n - 1) & h]) == null) {
          if (busy == 0) {            // Try to attach new Cell
            Cell r = new Cell(x);   // Optimistically create
            if (busy == 0 && casBusy()) {
              boolean created = false;
              try {               // Recheck under lock
                Cell[] rs;
                int m, j;
                if ((rs = cells) != null && (m = rs.length) > 0 && rs[j = (m - 1) & h] == null) {
                  rs[j] = r;
                  created = true;
                }
              } finally {
                busy = 0;
              }
              if (created) break;
              continue;           // Slot is now non-empty
            }
          }
          collide = false;
        } else if (!wasUncontended)       // CAS already known to fail
                  wasUncontended = true;      // Continue after rehash
else if (a.cas(v = a.value, fn(v, x))) break; else if (n >= NCPU || cells != as)
collide = false;            // At max size or stale
else if (!collide)
        collide = true; else if (busy == 0 && casBusy()) {
                        try {
                          if (cells == as) {      // Expand table unless stale
                            Cell[] rs = new Cell[n << 1];
                            for (int i = 0; i < n; ++i)rs[i] = as[i];
                            cells = rs;
                          }
                        } finally {
                          busy = 0;
                        }
                        collide = false;
                        continue;                   // Retry with expanded table
     }
        h ^= h << 13;                   // Rehash
        h ^= h >>> 17;
        h ^= h << 5;
        hc[0] = h;                      // Record index for next time
      } else if (busy == 0 && cells == as && casBusy()) {
        boolean init = false;
        try {                           // Initialize table
          if (cells == as) {
            Cell[] rs = new Cell[2];
            rs[h & 1] = new Cell(x);
            cells = rs;
            init = true;
          }
        } finally {
          busy = 0;
        }
        if (init) break;
      } else if (casBase(v = base, fn(v, x))) break;                          // Fall back on using base
    }
  }

    /**
     * Sets base and all cells to the given value.
     */

  final void internalReset(long initialValue) {
    Cell[] as = cells;
    base = initialValue;
    if (as != null) {
      int n = as.length;
      for (int i = 0; i < n; ++i) {
        Cell a = as[i];
        if (a != null)
          a.value = initialValue;
      }
    }
  }

    // Unsafe mechanics

  private static final sun.misc.Unsafe UNSAFE;
  private static final long baseOffset;
  private static final long busyOffset;

  static {
    try {
      UNSAFE = getUnsafe();
      Class<?> sk = Striped64.class;
      baseOffset = UNSAFE.objectFieldOffset(sk.getDeclaredField("base"));
      busyOffset = UNSAFE.objectFieldOffset(sk.getDeclaredField("busy"));
    } catch (Exception e) {
      throw new Error(e);
    }
  }

    /**
     * Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
     * Replace with a simple call to Unsafe.getUnsafe when integrating
     * into a jdk.
     *
     * @return a sun.misc.Unsafe
     */

  private static sun.misc.Unsafe getUnsafe() {
    try {
      return sun.misc.Unsafe.getUnsafe();
    } catch (SecurityException tryReflectionInstead) {}
    try {
      return java.security.AccessController.doPrivileged(new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {

                                                           public sun.misc.Unsafe run() throws Exception {
                                                             Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
                                                             for (java.lang.reflect.Field f : k.getDeclaredFields()) {
                                                               f.setAccessible(true);
                                                               Object x = f.get(null);
                                                               if (k.isInstance(x)) return k.cast(x);
                                                             }
                                                             throw new NoSuchFieldError("the Unsafe");
                                                           }
                                                         });
    } catch (java.security.PrivilegedActionException e) {
      throw new RuntimeException("Could not initialize intrinsics", e.getCause());
    }
  }
}