src/core/SkScaleToSides.h

/*
 * Copyright 2016 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#ifndef SkScaleToSides_DEFINED
#define SkScaleToSides_DEFINED

#include "include/core/SkScalar.h"
#include "include/core/SkTypes.h"

#include <cmath>
#include <utility>

class SkScaleToSides {
public:
    // This code assumes that a and b fit in a float, and therefore the resulting smaller value
    // of a and b will fit in a float. The side of the rectangle may be larger than a float.
    // Scale must be less than or equal to the ratio limit / (*a + *b).
    // This code assumes that NaN and Inf are never passed in.
    static void AdjustRadii(double limit, double scale, SkScalar* a, SkScalar* b) {
        SkASSERTF(scale < 1.0 && scale > 0.0, "scale: %g", scale);

        *a = (float)((double)*a * scale);
        *b = (float)((double)*b * scale);

        if (*a + *b > limit) {
            float* minRadius = a;
            float* maxRadius = b;

            // Force minRadius to be the smaller of the two.
            if (*minRadius > *maxRadius) {
                using std::swap;
                swap(minRadius, maxRadius);
            }

            // newMinRadius must be float in order to give the actual value of the radius.
            // The newMinRadius will always be smaller than limit. The largest that minRadius can be
            // is 1/2 the ratio of minRadius : (minRadius + maxRadius), therefore in the resulting
            // division, minRadius can be no larger than 1/2 limit + ULP. The newMinRadius can be
            // 1/2 a ULP off at this point.
            float newMinRadius = *minRadius;

            // Because newMaxRadius is the result of a double to float conversion, it can be larger
            // than limit, but only by one ULP.
            float newMaxRadius = (float)(limit - newMinRadius);

            // The total sum of newMinRadius and newMaxRadius can be upto 1.5 ULPs off. If the
            // sum is greater than the limit then newMaxRadius may have to be reduced twice.
            // Note: nextafterf is a c99 call and should be std::nextafter, but this is not
            // implemented in the GCC ARM compiler.
            if (newMaxRadius + newMinRadius > limit) {
                newMaxRadius = nextafterf(newMaxRadius, 0.0f);
                if (newMaxRadius + newMinRadius > limit) {
                    newMaxRadius = nextafterf(newMaxRadius, 0.0f);
                }
            }
            *maxRadius = newMaxRadius;
        }

        SkASSERTF(*a >= 0.0f && *b >= 0.0f, "a: %g, b: %g, limit: %g, scale: %g", *a, *b, limit,
                  scale);

        SkASSERTF(*a + *b <= limit,
                  "\nlimit: %.17f, sum: %.17f, a: %.10f, b: %.10f, scale: %.20f",
                  limit, *a + *b, *a, *b, scale);
    }
};
#endif // ScaleToSides_DEFINED