源码的魅力 - ArrayMap的工作原理

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int[] mHashes;
Object[] mArray;
public ArrayMap(int capacity, boolean identityHashCode) {
mIdentityHashCode = identityHashCode;
if (capacity < 0) {
mHashes = EMPTY_IMMUTABLE_INTS;
mArray = EmptyArray.OBJECT;
} else if (capacity == 0) {
mHashes = EmptyArray.INT;
mArray = EmptyArray.OBJECT;
} else {
allocArrays(capacity);
}
mSize = 0;
}复制代码

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@Override
public V put(K key, V value) {
final int hash;
int index;
if (key == null) {
hash = 0;
index = indexOfNull();
} else {
hash = mIdentityHashCode ? System.identityHashCode(key) : key.hashCode();
index = indexOf(key, hash);
}
if (index >= 0) {
index = (index<<1) + 1;
final V old = (V)mArray[index];
mArray[index] = value;
return old;
}
index = ~index;
if (mSize >= mHashes.length) {
final int n = mSize >= (BASE_SIZE*2) ? (mSize+(mSize>>1))
: (mSize >= BASE_SIZE ? (BASE_SIZE*2) : BASE_SIZE);
if (DEBUG) Log.d(TAG, "put: grow from " + mHashes.length + " to " + n);
final int[] ohashes = mHashes;
final Object[] oarray = mArray;
allocArrays(n);
if (mHashes.length > 0) {
if (DEBUG) Log.d(TAG, "put: copy 0-" + mSize + " to 0");
System.arraycopy(ohashes, 0, mHashes, 0, ohashes.length);
System.arraycopy(oarray, 0, mArray, 0, oarray.length);
}
freeArrays(ohashes, oarray, mSize);
}
if (index < mSize) {
if (DEBUG) Log.d(TAG, "put: move " + index + "-" + (mSize-index)
+ " to " + (index+1));
System.arraycopy(mHashes, index, mHashes, index + 1, mSize - index);
System.arraycopy(mArray, index << 1, mArray, (index + 1) << 1, (mSize - index) << 1);
}
mHashes[index] = hash;
mArray[index<<1] = key;
mArray[(index<<1)+1] = value;
mSize++;
return null;
}复制代码

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//获取索引值
int indexOf(Object key, int hash) {
final int N = mSize;
// Important fast case: if nothing is in here, nothing to look for.
if (N == 0) {
return ~0;
}
int index = ContainerHelpers.binarySearch(mHashes, N, hash);
// If the hash code wasn't found, then we have no entry for this key.
if (index < 0) {
return index;
}
// If the key at the returned index matches, that's what we want.
if (key.equals(mArray[index<<1])) {
return index;
}
// Search for a matching key after the index.
int end;
for (end = index + 1; end < N && mHashes[end] == hash; end++) {
if (key.equals(mArray[end << 1])) return end;
}
// Search for a matching key before the index.
for (int i = index - 1; i >= 0 && mHashes[i] == hash; i--) {
if (key.equals(mArray[i << 1])) return i;
}
// Key not found -- return negative value indicating where a
// new entry for this key should go.
We use the end of the
// hash chain to reduce the number of array entries that will
// need to be copied when inserting.
return ~end;
}
static int binarySearch(int[] array, int size, int value) {
int lo = 0;
int hi = size - 1;
while (lo <= hi) {
final int mid = (lo + hi) >>> 1;
final int midVal = array[mid];
if (midVal < value) {
lo = mid + 1;
} else if (midVal > value) {
hi = mid - 1;
} else {
return mid;
// value found
}
}
return ~lo;
// value not present
}复制代码

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private void allocArrays(final int size) {
if (mHashes == EMPTY_IMMUTABLE_INTS) {
throw new UnsupportedOperationException("ArrayMap is immutable");
}
if (size == (BASE_SIZE*2)) {
synchronized (ArrayMap.class) {
if (mTwiceBaseCache != null) {
final Object[] array = mTwiceBaseCache;
mArray = array;
mTwiceBaseCache = (Object[])array[0];
mHashes = (int[])array[1];
array[0] = array[1] = null;
mTwiceBaseCacheSize--;
if (DEBUG) Log.d(TAG, "Retrieving 2x cache " + mHashes
+ " now have " + mTwiceBaseCacheSize + " entries");
return;
}
}
} else if (size == BASE_SIZE) {
synchronized (ArrayMap.class) {
if (mBaseCache != null) {
final Object[] array = mBaseCache;
mArray = array;
mBaseCache = (Object[])array[0];
mHashes = (int[])array[1];
array[0] = array[1] = null;
mBaseCacheSize--;
if (DEBUG) Log.d(TAG, "Retrieving 1x cache " + mHashes
+ " now have " + mBaseCacheSize + " entries");
return;
}
}
}
mHashes = new int[size];
mArray = new Object[size<<1];
}
private static void freeArrays(final int[] hashes, final Object[] array, final int size) {
if (hashes.length == (BASE_SIZE*2)) {
synchronized (ArrayMap.class) {
if (mTwiceBaseCacheSize < CACHE_SIZE) {
array[0] = mTwiceBaseCache;
array[1] = hashes;
for (int i=(size<<1)-1; i>=2; i--) {
array[i] = null;
}
mTwiceBaseCache = array;
mTwiceBaseCacheSize++;
if (DEBUG) Log.d(TAG, "Storing 2x cache " + array
+ " now have " + mTwiceBaseCacheSize + " entries");
}
}
} else if (hashes.length == BASE_SIZE) {
synchronized (ArrayMap.class) {
if (mBaseCacheSize < CACHE_SIZE) {
array[0] = mBaseCache;
array[1] = hashes;
for (int i=(size<<1)-1; i>=2; i--) {
array[i] = null;
}
mBaseCache = array;
mBaseCacheSize++;
if (DEBUG) Log.d(TAG, "Storing 1x cache " + array
+ " now have " + mBaseCacheSize + " entries");
}
}
}
}复制代码

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final int n = mSize >= (BASE_SIZE*2) ? (mSize+(mSize>>1))
: (mSize >= BASE_SIZE ? (BASE_SIZE*2) : BASE_SIZE);复制代码