pathops version two

src/pathops/SkQuarticRoot.cpp

-// from http://tog.acm.org/resources/GraphicsGems/gems/Roots3And4.c
-/*
- *  Roots3And4.c
- *
- *  Utility functions to find cubic and quartic roots,
- *  coefficients are passed like this:
- *
- *      c[0] + c[1]*x + c[2]*x^2 + c[3]*x^3 + c[4]*x^4 = 0
- *
- *  The functions return the number of non-complex roots and
- *  put the values into the s array.
- *
- *  Author:         Jochen Schwarze (schwarze@isa.de)
- *
- *  Jan 26, 1990    Version for Graphics Gems
- *  Oct 11, 1990    Fixed sign problem for negative q's in SolveQuartic
- *                  (reported by Mark Podlipec),
- *                  Old-style function definitions,
- *                  IsZero() as a macro
- *  Nov 23, 1990    Some systems do not declare acos() and cbrt() in
- *                  <math.h>, though the functions exist in the library.
- *                  If large coefficients are used, EQN_EPS should be
- *                  reduced considerably (e.g. to 1E-30), results will be
- *                  correct but multiple roots might be reported more
- *                  than once.
- */
-
-#include "SkPathOpsCubic.h"
-#include "SkPathOpsQuad.h"
-#include "SkQuarticRoot.h"
-
-int SkReducedQuarticRoots(const double t4, const double t3, const double t2, const double t1,
-        const double t0, const bool oneHint, double roots[4]) {
-#ifdef SK_DEBUG
-    // create a string mathematica understands
-    // GDB set print repe 15 # if repeated digits is a bother
-    //     set print elements 400 # if line doesn't fit
-    char str[1024];
-    sk_bzero(str, sizeof(str));
-    SK_SNPRINTF(str, sizeof(str),
-            "Solve[%1.19g x^4 + %1.19g x^3 + %1.19g x^2 + %1.19g x + %1.19g == 0, x]",
-            t4, t3, t2, t1, t0);
-    SkPathOpsDebug::MathematicaIze(str, sizeof(str));
-#if ONE_OFF_DEBUG && ONE_OFF_DEBUG_MATHEMATICA
-    SkDebugf("%s\n", str);
-#endif
-#endif
-    if (approximately_zero_when_compared_to(t4, t0)  // 0 is one root
-            && approximately_zero_when_compared_to(t4, t1)
-            && approximately_zero_when_compared_to(t4, t2)) {
-        if (approximately_zero_when_compared_to(t3, t0)
-            && approximately_zero_when_compared_to(t3, t1)
-            && approximately_zero_when_compared_to(t3, t2)) {
-            return SkDQuad::RootsReal(t2, t1, t0, roots);
-        }
-        if (approximately_zero_when_compared_to(t4, t3)) {
-            return SkDCubic::RootsReal(t3, t2, t1, t0, roots);
-        }
-    }
-    if ((approximately_zero_when_compared_to(t0, t1) || approximately_zero(t1))  // 0 is one root
-      //      && approximately_zero_when_compared_to(t0, t2)
-            && approximately_zero_when_compared_to(t0, t3)
-            && approximately_zero_when_compared_to(t0, t4)) {
-        int num = SkDCubic::RootsReal(t4, t3, t2, t1, roots);
-        for (int i = 0; i < num; ++i) {
-            if (approximately_zero(roots[i])) {
-                return num;
-            }
-        }
-        roots[num++] = 0;
-        return num;
-    }
-    if (oneHint) {
-        SkASSERT(approximately_zero_double(t4 + t3 + t2 + t1 + t0) ||
-                approximately_zero_when_compared_to(t4 + t3 + t2 + t1 + t0,  // 1 is one root
-                SkTMax(fabs(t4), SkTMax(fabs(t3), SkTMax(fabs(t2), SkTMax(fabs(t1), fabs(t0)))))));
-        // note that -C == A + B + D + E
-        int num = SkDCubic::RootsReal(t4, t4 + t3, -(t1 + t0), -t0, roots);
-        for (int i = 0; i < num; ++i) {
-            if (approximately_equal(roots[i], 1)) {
-                return num;
-            }
-        }
-        roots[num++] = 1;
-        return num;
-    }
-    return -1;
-}
-
-int SkQuarticRootsReal(int firstCubicRoot, const double A, const double B, const double C,
-        const double D, const double E, double s[4]) {
-    double  u, v;
-    /* normal form: x^4 + Ax^3 + Bx^2 + Cx + D = 0 */
-    const double invA = 1 / A;
-    const double a = B * invA;
-    const double b = C * invA;
-    const double c = D * invA;
-    const double d = E * invA;
-    /*  substitute x = y - a/4 to eliminate cubic term:
-    x^4 + px^2 + qx + r = 0 */
-    const double a2 = a * a;
-    const double p = -3 * a2 / 8 + b;
-    const double q = a2 * a / 8 - a * b / 2 + c;
-    const double r = -3 * a2 * a2 / 256 + a2 * b / 16 - a * c / 4 + d;
-    int num;
-    double largest = SkTMax(fabs(p), fabs(q));
-    if (approximately_zero_when_compared_to(r, largest)) {
-    /* no absolute term: y(y^3 + py + q) = 0 */
-        num = SkDCubic::RootsReal(1, 0, p, q, s);
-        s[num++] = 0;
-    } else {
-        /* solve the resolvent cubic ... */
-        double cubicRoots[3];
-        int roots = SkDCubic::RootsReal(1, -p / 2, -r, r * p / 2 - q * q / 8, cubicRoots);
-        int index;
-        /* ... and take one real solution ... */
-        double z;
-        num = 0;
-        int num2 = 0;
-        for (index = firstCubicRoot; index < roots; ++index) {
-            z = cubicRoots[index];
-            /* ... to build two quadric equations */
-            u = z * z - r;
-            v = 2 * z - p;
-            if (approximately_zero_squared(u)) {
-                u = 0;
-            } else if (u > 0) {
-                u = sqrt(u);
-            } else {
-                continue;
-            }
-            if (approximately_zero_squared(v)) {
-                v = 0;
-            } else if (v > 0) {
-                v = sqrt(v);
-            } else {
-                continue;
-            }
-            num = SkDQuad::RootsReal(1, q < 0 ? -v : v, z - u, s);
-            num2 = SkDQuad::RootsReal(1, q < 0 ? v : -v, z + u, s + num);
-            if (!((num | num2) & 1)) {
-                break;  // prefer solutions without single quad roots
-            }
-        }
-        num += num2;
-        if (!num) {
-            return 0;  // no valid cubic root
-        }
-    }
-    /* resubstitute */
-    const double sub = a / 4;
-    for (int i = 0; i < num; ++i) {
-        s[i] -= sub;
-    }
-    // eliminate duplicates
-    for (int i = 0; i < num - 1; ++i) {
-        for (int j = i + 1; j < num; ) {
-            if (AlmostDequalUlps(s[i], s[j])) {
-                if (j < --num) {
-                    s[j] = s[num];
-                }
-            } else {
-                ++j;
-            }
-        }
-    }
-    return num;
-}