denseranges.cpp   [plain text]

/*
*******************************************************************************
*   Copyright (C) 2010, International Business Machines
*   Corporation and others.  All Rights Reserved.
*******************************************************************************
*   file name:  denseranges.cpp
*   encoding:   US-ASCII
*   tab size:   8 (not used)
*   indentation:4
*
*   created on: 2010sep25
*   created by: Markus W. Scherer
*
* Helper code for finding a small number of dense ranges.
*/

#include "unicode/utypes.h"
#include "denseranges.h"

// Definitions in the anonymous namespace are invisible outside this file.
namespace {

/**
 * Collect up to 15 range gaps and sort them by ascending gap size.
 */
class LargestGaps {
public:
    LargestGaps(int32_t max) : maxLength(max<=kCapacity ? max : kCapacity), length(0) {}

    void add(int32_t gapStart, int64_t gapLength) {
        int32_t i=length;
        while(i>0 && gapLength>gapLengths[i-1]) {
            --i;
        }
        if(i<maxLength) {
            // The new gap is now one of the maxLength largest.
            // Insert the new gap, moving up smaller ones of the previous
            // length largest.
            int32_t j= length<maxLength ? length++ : maxLength-1;
            while(j>i) {
                gapStarts[j]=gapStarts[j-1];
                gapLengths[j]=gapLengths[j-1];
                --j;
            }
            gapStarts[i]=gapStart;
            gapLengths[i]=gapLength;
        }
    }

    void truncate(int32_t newLength) {
        if(newLength<length) {
            length=newLength;
        }
    }

    int32_t count() const { return length; }
    int32_t gapStart(int32_t i) const { return gapStarts[i]; }
    int64_t gapLength(int32_t i) const { return gapLengths[i]; }

    int32_t firstAfter(int32_t value) const {
        if(length==0) {
            return -1;
        }
        int32_t minValue=0;
        int32_t minIndex=-1;
        for(int32_t i=0; i<length; ++i) {
            if(value<gapStarts[i] && (minIndex<0 || gapStarts[i]<minValue)) {
                minValue=gapStarts[i];
                minIndex=i;
            }
        }
        return minIndex;
    }

private:
    static const int32_t kCapacity=15;

    int32_t maxLength;
    int32_t length;
    int32_t gapStarts[kCapacity];
    int64_t gapLengths[kCapacity];
};

}  // namespace

/**
 * Does it make sense to write 1..capacity ranges?
 * Returns 0 if not, otherwise the number of ranges.
 * @param values Sorted array of signed-integer values.
 * @param length Number of values.
 * @param density Minimum average range density, in 256th. (0x100=100%=perfectly dense.)
 *                Should be 0x80..0x100, must be 1..0x100.
 * @param ranges Output ranges array.
 * @param capacity Maximum number of ranges.
 * @return Minimum number of ranges (at most capacity) that have the desired density,
 *         or 0 if that density cannot be achieved.
 */
U_CAPI int32_t U_EXPORT2
uprv_makeDenseRanges(const int32_t values[], int32_t length,
                     int32_t density,
                     int32_t ranges[][2], int32_t capacity) {
    if(length<=2) {
        return 0;
    }
    int32_t minValue=values[0];
    int32_t maxValue=values[length-1];  // Assume minValue<=maxValue.
    // Use int64_t variables for intermediate-value precision and to avoid
    // signed-int32_t overflow of maxValue-minValue.
    int64_t maxLength=(int64_t)maxValue-(int64_t)minValue+1;
    if(length>=(density*maxLength)/0x100) {
        // Use one range.
        ranges[0][0]=minValue;
        ranges[0][1]=maxValue;
        return 1;
    }
    if(length<=4) {
        return 0;
    }
    // See if we can split [minValue, maxValue] into 2..capacity ranges,
    // divided by the 1..(capacity-1) largest gaps.
    LargestGaps gaps(capacity-1);
    int32_t i;
    int32_t expectedValue=minValue;
    for(i=1; i<length; ++i) {
        ++expectedValue;
        int32_t actualValue=values[i];
        if(expectedValue!=actualValue) {
            gaps.add(expectedValue, (int64_t)actualValue-(int64_t)expectedValue);
            expectedValue=actualValue;
        }
    }
    // We know gaps.count()>=1 because we have fewer values (length) than
    // the length of the [minValue..maxValue] range (maxLength).
    // (Otherwise we would have returned with the one range above.)
    int32_t num;
    for(i=0, num=2;; ++i, ++num) {
        if(i>=gaps.count()) {
            // The values are too sparse for capacity or fewer ranges
            // of the requested density.
            return 0;
        }
        maxLength-=gaps.gapLength(i);
        if(length>num*2 && length>=(density*maxLength)/0x100) {
            break;
        }
    }
    // Use the num ranges with the num-1 largest gaps.
    gaps.truncate(num-1);
    ranges[0][0]=minValue;
    for(i=0; i<=num-2; ++i) {
        int32_t gapIndex=gaps.firstAfter(minValue);
        int32_t gapStart=gaps.gapStart(gapIndex);
        ranges[i][1]=gapStart-1;
        ranges[i+1][0]=minValue=(int32_t)(gapStart+gaps.gapLength(gapIndex));
    }
    ranges[num-1][1]=maxValue;
    return num;
}