1/*
2 * Copyright 2016 Google Inc.
3 *
4 * Use of this source code is governed by a BSD-style license that can be
5 * found in the LICENSE file.
6 */
7
8#ifndef SkScaleToSides_DEFINED
9#define SkScaleToSides_DEFINED
10
11#include <cmath>
12#include "SkScalar.h"
13#include "SkTypes.h"
14
15class SkScaleToSides {
16public:
17    // This code assumes that a and b fit in a float, and therefore the resulting smaller value
18    // of a and b will fit in a float. The side of the rectangle may be larger than a float.
19    // Scale must be less than or equal to the ratio limit / (*a + *b).
20    // This code assumes that NaN and Inf are never passed in.
21    static void AdjustRadii(double limit, double scale, SkScalar* a, SkScalar* b) {
22        SkASSERTF(scale < 1.0 && scale > 0.0, "scale: %g", scale);
23
24        *a = (float)((double)*a * scale);
25        *b = (float)((double)*b * scale);
26
27        if (*a + *b > limit) {
28            float* minRadius = a;
29            float* maxRadius = b;
30
31            // Force minRadius to be the smaller of the two.
32            if (*minRadius > *maxRadius) {
33                SkTSwap(minRadius, maxRadius);
34            }
35
36            // newMinRadius must be float in order to give the actual value of the radius.
37            // The newMinRadius will always be smaller than limit. The largest that minRadius can be
38            // is 1/2 the ratio of minRadius : (minRadius + maxRadius), therefore in the resulting
39            // division, minRadius can be no larger than 1/2 limit + ULP. The newMinRadius can be
40            // 1/2 a ULP off at this point.
41            float newMinRadius = *minRadius;
42
43            // Because newMaxRadius is the result of a double to float conversion, it can be larger
44            // than limit, but only by one ULP.
45            float newMaxRadius = (float)(limit - newMinRadius);
46
47            // The total sum of newMinRadius and newMaxRadius can be upto 1.5 ULPs off. If the
48            // sum is greater than the limit then newMaxRadius may have to be reduced twice.
49            // Note: nextafterf is a c99 call and should be std::nextafter, but this is not
50            // implemented in the GCC ARM compiler.
51            if (newMaxRadius + newMinRadius > limit) {
52                newMaxRadius = nextafterf(newMaxRadius, 0.0f);
53                if (newMaxRadius + newMinRadius > limit) {
54                    newMaxRadius = nextafterf(newMaxRadius, 0.0f);
55                }
56            }
57            *maxRadius = newMaxRadius;
58        }
59
60        SkASSERTF(*a >= 0.0f && *b >= 0.0f, "a: %g, b: %g, limit: %g, scale: %g", *a, *b, limit,
61                  scale);
62
63        SkASSERTF(*a + *b <= limit,
64                  "\nlimit: %.17f, sum: %.17f, a: %.10f, b: %.10f, scale: %.20f",
65                  limit, *a + *b, *a, *b, scale);
66    }
67};
68#endif // ScaleToSides_DEFINED
69