AndroidシステムにおけるSparseArrayのソースコードを深く分析する
Use new SparseArray (...) instead for better performance
この警告は、より良いパフォーマンスを得るためにSparseArrayを使用することを意味します.
ソースコードはSparseArray全体のコードが簡単なので、まずソースコードを示してから、なぜSparseArrayを使うのがHashMapを使うより性能が良いのかを分析します.
public class SparseArray implements Cloneable {
private static final Object DELETED = new Object();
private boolean mGarbage = false;
private int[] mKeys;
private Object[] mValues;
private int mSize;
/**
* Creates a new SparseArray containing no mappings.
*/
public SparseArray() {
this(10);
}
/**
* Creates a new SparseArray containing no mappings that will not
* require any additional memory allocation to store the specified
* number of mappings. If you supply an initial capacity of 0, the
* sparse array will be initialized with a light-weight representation
* not requiring any additional array allocations.
*/
public SparseArray(int initialCapacity) {
if (initialCapacity == 0) {
mKeys = ContainerHelpers.EMPTY_INTS;
mValues = ContainerHelpers.EMPTY_OBJECTS;
} else {
initialCapacity = ArrayUtils.idealIntArraySize(initialCapacity);
mKeys = new int[initialCapacity];
mValues = new Object[initialCapacity];
}
mSize = 0;
}
@Override
@SuppressWarnings("unchecked")
public SparseArray clone() {
SparseArray clone = null;
try {
clone = (SparseArray) super.clone();
clone.mKeys = mKeys.clone();
clone.mValues = mValues.clone();
} catch (CloneNotSupportedException cnse) {
/* ignore */
}
return clone;
}
/**
* Gets the Object mapped from the specified key, or null
* if no such mapping has been made.
*/
public E get(int key) {
return get(key, null);
}
/**
* Gets the Object mapped from the specified key, or the specified Object
* if no such mapping has been made.
*/
@SuppressWarnings("unchecked")
public E get(int key, E valueIfKeyNotFound) {
int i = ContainerHelpers.binarySearch(mKeys, mSize, key);
if (i < 0 || mValues[i] == DELETED) {
return valueIfKeyNotFound;
} else {
return (E) mValues[i];
}
}
/**
* Removes the mapping from the specified key, if there was any.
*/
public void delete(int key) {
int i = ContainerHelpers.binarySearch(mKeys, mSize, key);
if (i >= 0) {
if (mValues[i] != DELETED) {
mValues[i] = DELETED;
mGarbage = true;
}
}
}
/**
* Alias for {@link #delete(int)}.
*/
public void remove(int key) {
delete(key);
}
/**
* Removes the mapping at the specified index.
*/
public void removeAt(int index) {
if (mValues[index] != DELETED) {
mValues[index] = DELETED;
mGarbage = true;
}
}
/**
* Remove a range of mappings as a batch.
*
* @param index Index to begin at
* @param size Number of mappings to remove
*/
public void removeAtRange(int index, int size) {
final int end = Math.min(mSize, index + size);
for (int i = index; i < end; i++) {
removeAt(i);
}
}
private void gc() {
// Log.e("SparseArray", "gc start with " + mSize);
int n = mSize;
int o = 0;
int[] keys = mKeys;
Object[] values = mValues;
for (int i = 0; i < n; i++) {
Object val = values[i];
if (val != DELETED) {
if (i != o) {
keys[o] = keys[i];
values[o] = val;
values[i] = null;
}
o++;
}
}
mGarbage = false;
mSize = o;
// Log.e("SparseArray", "gc end with " + mSize);
}
/**
* Adds a mapping from the specified key to the specified value,
* replacing the previous mapping from the specified key if there
* was one.
*/
public void put(int key, E value) {
int i = ContainerHelpers.binarySearch(mKeys, mSize, key);
if (i >= 0) {
mValues[i] = value;
} else {
i = ~i;
if (i < mSize && mValues[i] == DELETED) {
mKeys[i] = key;
mValues[i] = value;
return;
}
if (mGarbage && mSize >= mKeys.length) {
gc();
// Search again because indices may have changed.
i = ~ContainerHelpers.binarySearch(mKeys, mSize, key);
}
if (mSize >= mKeys.length) {
int n = ArrayUtils.idealIntArraySize(mSize + 1);
int[] nkeys = new int[n];
Object[] nvalues = new Object[n];
// Log.e("SparseArray", "grow " + mKeys.length + " to " + n);
System.arraycopy(mKeys, 0, nkeys, 0, mKeys.length);
System.arraycopy(mValues, 0, nvalues, 0, mValues.length);
mKeys = nkeys;
mValues = nvalues;
}
if (mSize - i != 0) {
// Log.e("SparseArray", "move " + (mSize - i));
System.arraycopy(mKeys, i, mKeys, i + 1, mSize - i);
System.arraycopy(mValues, i, mValues, i + 1, mSize - i);
}
mKeys[i] = key;
mValues[i] = value;
mSize++;
}
}
/**
* Returns the number of key-value mappings that this SparseArray
* currently stores.
*/
public int size() {
if (mGarbage) {
gc();
}
return mSize;
}
/**
* Given an index in the range 0...size()-1, returns
* the key from the indexth key-value mapping that this
* SparseArray stores.
*
* The keys corresponding to indices in ascending order are guaranteed to
* be in ascending order, e.g., keyAt(0) will return the
* smallest key and keyAt(size()-1) will return the largest
* key.
*/
public int keyAt(int index) {
if (mGarbage) {
gc();
}
return mKeys[index];
}
/**
* Given an index in the range 0...size()-1, returns
* the value from the indexth key-value mapping that this
* SparseArray stores.
*
* The values corresponding to indices in ascending order are guaranteed
* to be associated with keys in ascending order, e.g.,
* valueAt(0) will return the value associated with the
* smallest key and valueAt(size()-1) will return the value
* associated with the largest key.
*/
@SuppressWarnings("unchecked")
public E valueAt(int index) {
if (mGarbage) {
gc();
}
return (E) mValues[index];
}
/**
* Given an index in the range 0...size()-1, sets a new
* value for the indexth key-value mapping that this
* SparseArray stores.
*/
public void setValueAt(int index, E value) {
if (mGarbage) {
gc();
}
mValues[index] = value;
}
/**
* Returns the index for which {@link #keyAt} would return the
* specified key, or a negative number if the specified
* key is not mapped.
*/
public int indexOfKey(int key) {
if (mGarbage) {
gc();
}
return ContainerHelpers.binarySearch(mKeys, mSize, key);
}
/**
* Returns an index for which {@link #valueAt} would return the
* specified key, or a negative number if no keys map to the
* specified value.
* Beware that this is a linear search, unlike lookups by key,
* and that multiple keys can map to the same value and this will
* find only one of them.
*
Note also that unlike most collections' {@code indexOf} methods,
* this method compares values using {@code ==} rather than {@code equals}.
*/
public int indexOfValue(E value) {
if (mGarbage) {
gc();
}
for (int i = 0; i < mSize; i++)
if (mValues[i] == value)
return i;
return -1;
}
/**
* Removes all key-value mappings from this SparseArray.
*/
public void clear() {
int n = mSize;
Object[] values = mValues;
for (int i = 0; i < n; i++) {
values[i] = null;
}
mSize = 0;
mGarbage = false;
}
/**
* Puts a key/value pair into the array, optimizing for the case where
* the key is greater than all existing keys in the array.
*/
public void append(int key, E value) {
if (mSize != 0 && key <= mKeys[mSize - 1]) {
put(key, value);
return;
}
if (mGarbage && mSize >= mKeys.length) {
gc();
}
int pos = mSize;
if (pos >= mKeys.length) {
int n = ArrayUtils.idealIntArraySize(pos + 1);
int[] nkeys = new int[n];
Object[] nvalues = new Object[n];
// Log.e("SparseArray", "grow " + mKeys.length + " to " + n);
System.arraycopy(mKeys, 0, nkeys, 0, mKeys.length);
System.arraycopy(mValues, 0, nvalues, 0, mValues.length);
mKeys = nkeys;
mValues = nvalues;
}
mKeys[pos] = key;
mValues[pos] = value;
mSize = pos + 1;
}
/**
* {@inheritDoc}
*
*
This implementation composes a string by iterating over its mappings. If
* this map contains itself as a value, the string "(this Map)"
* will appear in its place.
*/
@Override
public String toString() {
if (size() <= 0) {
return "{}";
}
StringBuilder buffer = new StringBuilder(mSize * 28);
buffer.append('{');
for (int i=0; i 0) {
buffer.append(", ");
}
int key = keyAt(i);
buffer.append(key);
buffer.append('=');
Object value = valueAt(i);
if (value != this) {
buffer.append(value);
} else {
buffer.append("(this Map)");
}
}
buffer.append('}');
return buffer.toString();
}
}
まず、SparseArrayのコンストラクション関数を見てみましょう.
/**
* Creates a new SparseArray containing no mappings.
*/
public SparseArray() {
this(10);
}
/**
* Creates a new SparseArray containing no mappings that will not
* require any additional memory allocation to store the specified
* number of mappings. If you supply an initial capacity of 0, the
* sparse array will be initialized with a light-weight representation
* not requiring any additional array allocations.
*/
public SparseArray(int initialCapacity) {
if (initialCapacity == 0) {
mKeys = ContainerHelpers.EMPTY_INTS;
mValues = ContainerHelpers.EMPTY_OBJECTS;
} else {
initialCapacity = ArrayUtils.idealIntArraySize(initialCapacity);
mKeys = new int[initialCapacity];
mValues = new Object[initialCapacity];
}
mSize = 0;
}
構造方法からも分かるように、ここでも予め容器の大きさが設定されており、デフォルトの大きさは10である.
データの追加操作を見てみましょう.
/**
* Adds a mapping from the specified key to the specified value,
* replacing the previous mapping from the specified key if there
* was one.
*/
public void put(int key, E value) {
int i = ContainerHelpers.binarySearch(mKeys, mSize, key);
if (i >= 0) {
mValues[i] = value;
} else {
i = ~i;
if (i < mSize && mValues[i] == DELETED) {
mKeys[i] = key;
mValues[i] = value;
return;
}
if (mGarbage && mSize >= mKeys.length) {
gc();
// Search again because indices may have changed.
i = ~ContainerHelpers.binarySearch(mKeys, mSize, key);
}
if (mSize >= mKeys.length) {
int n = ArrayUtils.idealIntArraySize(mSize + 1);
int[] nkeys = new int[n];
Object[] nvalues = new Object[n];
// Log.e("SparseArray", "grow " + mKeys.length + " to " + n);
System.arraycopy(mKeys, 0, nkeys, 0, mKeys.length);
System.arraycopy(mValues, 0, nvalues, 0, mValues.length);
mKeys = nkeys;
mValues = nvalues;
}
if (mSize - i != 0) {
// Log.e("SparseArray", "move " + (mSize - i));
System.arraycopy(mKeys, i, mKeys, i + 1, mSize - i);
System.arraycopy(mValues, i, mValues, i + 1, mSize - i);
}
mKeys[i] = key;
mValues[i] = value;
mSize++;
}
}
データを調べる方法を見てみましょう.
/**
* Gets the Object mapped from the specified key, or null
* if no such mapping has been made.
*/
public E get(int key) {
return get(key, null);
}
/**
* Gets the Object mapped from the specified key, or the specified Object
* if no such mapping has been made.
*/
@SuppressWarnings("unchecked")
public E get(int key, E valueIfKeyNotFound) {
int i = ContainerHelpers.binarySearch(mKeys, mSize, key);
if (i < 0 || mValues[i] == DELETED) {
return valueIfKeyNotFound;
} else {
return (E) mValues[i];
}
}
putデータとgetデータの過程で,二分ルックアップアルゴリズムが統一的に呼び出されていることが分かるが,これはSparseArrayが効率を向上させる核心である.
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
}
个人的には(lo+hi)>>1の方法は少し変で、直接lo+(hi-lo)/2を使うほうがいいと思います.