Binary search routines : Sort Search « Collections Data Structure « Java
Binary search routines
/*
* Copyright (c) 1998-2002 Carnegie Mellon University. All rights
* reserved.
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*
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* 2. Redistributions in binary form must reproduce the above copyright
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* distribution.
*
* THIS SOFTWARE IS PROVIDED BY CARNEGIE MELLON UNIVERSITY ``AS IS'' AND
* ANY EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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/**
* Binary search routines.
*/
public abstract class BinarySearch {
/**
* Search a sorted array of integers.
* @param array Array of integers
* @param offset Starting offset of subarray to search
* @param length Length of subarray to search
* @param x Value to search for
* @return largest index i in subarray (offset <= i <= offset+length)
* such that all elements below i in the subarray are strictly less
* than x. If x is found in the subarray, then array[i] == x (and i is
* the first occurence of x in the subarray). If x is not found,
* then array[i] is where x should be inserted in the sort order.
*/
public static int search (int[] array, int offset, int length, int x) {
// handle 0-length subarray case right away
if (length <= 0)
return offset;
int low = offset;
int high = offset+length-1;
// since length > 0, array[low] and array[high] are valid indices
if (x <= array[low])
return low;
if (x > array[high])
return high+1;
while (low+1 < high) {
// loop invariant: array[low] < x <= array[high],
// offset <= low < high < offset+length
int mid = (low + high)/2;
if (x <= array[mid])
high = mid;
else
low = mid;
}
// now we have array[low] < x <= array[high]
// && (low+1 == high || low == high)
// implies low+1 == high
//debug.assertion (low+1 == high);
return high;
}
}
