src/core/SkClampRange.cpp

/*
 * Copyright 2011 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */


#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;
    }
}