src/core/SkClampRange.cpp

/*
    Copyright 2011 Google Inc.

    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
    You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

    Unless required by applicable law or agreed to in writing, software
    distributed under the License is distributed on an "AS IS" BASIS,
    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    See the License for the specific language governing permissions and
    limitations under the License.
 */

#include "SkClampRange.h"

/*
 *  returns [0..count] for the number of steps (<= count) for which x0 <= edge
 *  given each step is followed by x0 += dx
 */
static int chop(int64_t x0, SkFixed edge, int64_t x1, int64_t dx, int count) {
    SkASSERT(dx > 0);
    SkASSERT(count >= 0);

    if (x0 >= edge) {
        return 0;
    }
    if (x1 <= edge) {
        return count;
    }
    int64_t n = (edge - x0 + dx - 1) / dx;
    SkASSERT(n >= 0);
    SkASSERT(n <= count);
    return (int)n;
}

static bool overflows_fixed(int64_t x) {
    return x < -SK_FixedMax || x > SK_FixedMax;
}

void SkClampRange::initFor1(SkFixed fx) {
    fCount0 = fCount1 = fCount2 = 0;
    if (fx <= 0) {
        fCount0 = 1;
    } else if (fx < 0xFFFF) {
        fCount1 = 1;
        fFx1 = fx;
    } else {
        fCount2 = 1;
    }
}

void SkClampRange::init(SkFixed fx0, SkFixed dx0, int count, int v0, int v1) {
    SkASSERT(count > 0);

    fV0 = v0;
    fV1 = v1;
    fOverflowed = false;

    // special case 1 == count, as it is slightly common for skia
    // and avoids us ever calling divide or 64bit multiply
    if (1 == count) {
        this->initFor1(fx0);
        return;
    }

    int64_t fx = fx0;
    int64_t dx = dx0;
    // start with ex equal to the last computed value
    int64_t ex = fx + (count - 1) * dx;
    fOverflowed = overflows_fixed(ex);

    if ((uint64_t)(fx | ex) <= 0xFFFF) {
        fCount0 = fCount2 = 0;
        fCount1 = count;
        fFx1 = fx0;
        return;
    }
    if (fx <= 0 && ex <= 0) {
        fCount1 = fCount2 = 0;
        fCount0 = count;
        return;
    }
    if (fx >= 0xFFFF && ex >= 0xFFFF) {
        fCount0 = fCount1 = 0;
        fCount2 = count;
        return;
    }

    int extraCount = 0;

    // now make ex be 1 past the last computed value
    ex += dx;
    fOverflowed = overflows_fixed(ex);
    // now check for over/under flow
    if (fOverflowed) {
        int originalCount = count;
        int64_t ccount;
        bool swap = dx < 0;
        if (swap) {
            dx = -dx;
            fx = -fx;
        }
        ccount = (SK_FixedMax - fx + dx - 1) / dx;
        if (swap) {
            dx = -dx;
            fx = -fx;
        }
        SkASSERT(ccount > 0 && ccount <= SK_FixedMax);

        count = (int)ccount;
        if (0 == count) {
            this->initFor1(fx0);
            if (dx > 0) {
                fCount2 += originalCount - 1;
            } else {
                fCount0 += originalCount - 1;
            }
            return;
        }
        extraCount = originalCount - count;
        ex = fx + dx * count;
    }
    
    bool doSwap = dx < 0;

    if (doSwap) {
        ex -= dx;
        fx -= dx;
        SkTSwap(fx, ex);
        dx = -dx;
    }


    fCount0 = chop(fx, 0, ex, dx, count);
    count -= fCount0;
    fx += fCount0 * dx;
    SkASSERT(fx >= 0);
    SkASSERT(fCount0 == 0 || (fx - dx) < 0);
    fCount1 = chop(fx, 0xFFFF, ex, dx, count);
    count -= fCount1;
    fCount2 = count;

#ifdef SK_DEBUG
    fx += fCount1 * dx;
    SkASSERT(fx <= ex);
    if (fCount2 > 0) {
        SkASSERT(fx >= 0xFFFF);
        if (fCount1 > 0) {
            SkASSERT(fx - dx < 0xFFFF);
        }
    }
#endif

    if (doSwap) {
        SkTSwap(fCount0, fCount2);
        SkTSwap(fV0, fV1);
        dx = -dx;
    }

    if (fCount1 > 0) {
        fFx1 = fx0 + fCount0 * (int)dx;
    }

    if (dx > 0) {
        fCount2 += extraCount;
    } else {
        fCount0 += extraCount;
    }
}