稀疏矩陣可以使用 try 高效實現,它僅使用兩個數組索引操作計算某個元素是否存在於表中,從而提供對特定矩陣元素的快速訪問。
Tries 的主要特點:
優點:
範例實作:
<code class="java">public class DoubleTrie { // Matrix options private static final int SIZE_I = 1024; private static final int SIZE_J = 1024; private static final double DEFAULT_VALUE = 0.0; // Internal splitting options private static final int SUBRANGEBITS_I = 4; private static final int SUBRANGEBITS_J = 4; // Internal splitting constants private static final int SUBRANGE_I = 1 << SUBRANGEBITS_I; private static final int SUBRANGE_J = 1 << SUBRANGEBITS_J; private static final int SUBRANGEMASK_I = SUBRANGE_I - 1; private static final int SUBRANGEMASK_J = SUBRANGE_J - 1; // Internal data private double[] values; private int[] subrangePositions; // Fast subrange and position computation methods private static int subrangeOf(int i, int j) { return (i >> SUBRANGEBITS_I) * SUBRANGE_J + (j >> SUBRANGEBITS_J); } private static int positionOffsetOf(int i, int j) { return (i & SUBRANGEMASK_I) * SUBRANGE_J + (j & SUBRANGEMASK_J); } // Fast indexed getter public double getAt(int i, int j) { return values[subrangePositions[subrangeOf(i, j)] + positionOffsetOf(i, j)]; } // Fast indexed setter public double setAt(int i, int j, double value) { final int subrange = subrangeOf(i, j); final int positionOffset = positionOffsetOf(i, j); // Check if the assignment will change something int subrangePosition, valuePosition; if (Double.compare( values[valuePosition = (subrangePosition = subrangePositions[subrange]) + positionOffset], value) != 0) { // Perform the assignment in values if (isSharedValues) { values = values.clone(); isSharedValues = false; } // Scan other subranges to check if the value is shared by another subrange for (int otherSubrange = subrangePositions.length; --otherSubrange >= 0; ) { if (otherSubrange != subrange) continue; // Ignore the target subrange if ((otherSubrangePosition = subrangePositions[otherSubrange]) >= valuePosition && otherSubrangePosition + SUBRANGE_POSITIONS < valuePosition) { // The target position is shared, we need to make it unique by cloning the subrange if (isSharedSubrangePositions) { subrangePositions = subrangePositions.clone(); isSharedSubrangePositions = false; } values = setlengh( values, (subrangePositions[subrange] = subrangePositions = values.length) + SUBRANGE_POSITIONS); valuePosition = subrangePositions + positionOffset; break; } } // Perform the effective assignment of the value values[valuePosition] = value; } return value; } }</code>
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