1 // poly34.cpp : solution of cubic and quartic equation
2 // (c) Khashin S.I. http://math.ivanovo.ac.ru/dalgebra/Khashin/index.html
3 // khash2 (at) gmail.com
4 // Thanks to Alexandr Rakhmanin <rakhmanin (at) gmail.com>
9 #include "poly34.h" // solution of cubic and quartic equation
10 #define TwoPi 6.28318530717958648
11 const btScalar eps = SIMD_EPSILON;
13 //=============================================================================
14 // _root3, root3 from http://prografix.narod.ru
15 //=============================================================================
16 static SIMD_FORCE_INLINE btScalar _root3(btScalar x)
30 r -= 1. / 3. * (r - x / (r * r));
31 r -= 1. / 3. * (r - x / (r * r));
32 r -= 1. / 3. * (r - x / (r * r));
33 r -= 1. / 3. * (r - x / (r * r));
34 r -= 1. / 3. * (r - x / (r * r));
35 r -= 1. / 3. * (r - x / (r * r));
39 btScalar SIMD_FORCE_INLINE root3(btScalar x)
49 // x - array of size 2
50 // return 2: 2 real roots x[0], x[1]
51 // return 0: pair of complex roots: x[0]i*x[1]
52 int SolveP2(btScalar* x, btScalar a, btScalar b)
53 { // solve equation x^2 + a*x + b = 0
54 btScalar D = 0.25 * a * a - b;
66 //---------------------------------------------------------------------------
67 // x - array of size 3
68 // In case 3 real roots: => x[0], x[1], x[2], return 3
69 // 2 real roots: x[0], x[1], return 2
70 // 1 real root : x[0], x[1] i*x[2], return 1
71 int SolveP3(btScalar* x, btScalar a, btScalar b, btScalar c)
72 { // solve cubic equation x^3 + a*x^2 + b*x + c = 0
74 btScalar q = (a2 - 3 * b) / 9;
77 btScalar r = (a * (2 * a2 - 9 * b) + 27 * c) / 54;
78 // equation x^3 + q*x + r = 0
80 btScalar q3 = q * q * q;
84 btScalar t = r / sqrt(q3);
92 x[0] = q * cos(t / 3) - a;
93 x[1] = q * cos((t + TwoPi) / 3) - a;
94 x[2] = q * cos((t - TwoPi) / 3) - a;
99 //A =-pow(fabs(r)+sqrt(r2-q3),1./3);
100 A = -root3(fabs(r) + sqrt(r2 - q3));
103 B = (A == 0 ? 0 : q / A);
107 x[1] = -0.5 * (A + B) - a;
108 x[2] = 0.5 * sqrt(3.) * (A - B);
109 if (fabs(x[2]) < eps)
116 } // SolveP3(btScalar *x,btScalar a,btScalar b,btScalar c) {
117 //---------------------------------------------------------------------------
119 void CSqrt(btScalar x, btScalar y, btScalar& a, btScalar& b) // returns: a+i*s = sqrt(x+i*y)
121 btScalar r = sqrt(x * x + y * y);
138 a = sqrt(0.5 * (x + r));
142 //---------------------------------------------------------------------------
143 int SolveP4Bi(btScalar* x, btScalar b, btScalar d) // solve equation x^4 + b*x^2 + d = 0
145 btScalar D = b * b - 4 * d;
148 btScalar sD = sqrt(D);
149 btScalar x1 = (-b + sD) / 2;
150 btScalar x2 = (-b - sD) / 2; // x2 <= x1
151 if (x2 >= 0) // 0 <= x2 <= x1, 4 real roots
153 btScalar sx1 = sqrt(x1);
154 btScalar sx2 = sqrt(x2);
161 if (x1 < 0) // x2 <= x1 < 0, two pair of imaginary roots
163 btScalar sx1 = sqrt(-x1);
164 btScalar sx2 = sqrt(-x2);
171 // now x2 < 0 <= x1 , two real roots and one pair of imginary root
172 btScalar sx1 = sqrt(x1);
173 btScalar sx2 = sqrt(-x2);
181 { // if( D < 0 ), two pair of compex roots
182 btScalar sD2 = 0.5 * sqrt(-D);
183 CSqrt(-0.5 * b, sD2, x[0], x[1]);
184 CSqrt(-0.5 * b, -sD2, x[2], x[3]);
187 } // SolveP4Bi(btScalar *x, btScalar b, btScalar d) // solve equation x^4 + b*x^2 d
188 //---------------------------------------------------------------------------
195 static void dblSort3(btScalar& a, btScalar& b, btScalar& c) // make: a <= b <= c
199 SWAP(a, b); // now a<=b
202 SWAP(b, c); // now a<=b, b<=c
204 SWAP(a, b); // now a<=b
207 //---------------------------------------------------------------------------
208 int SolveP4De(btScalar* x, btScalar b, btScalar c, btScalar d) // solve equation x^4 + b*x^2 + c*x + d
210 //if( c==0 ) return SolveP4Bi(x,b,d); // After that, c!=0
211 if (fabs(c) < 1e-14 * (fabs(b) + fabs(d)))
212 return SolveP4Bi(x, b, d); // After that, c!=0
214 int res3 = SolveP3(x, 2 * b, b * b - 4 * d, -c * c); // solve resolvent
215 // by Viet theorem: x1*x2*x3=-c*c not equals to 0, so x1!=0, x2!=0, x3!=0
216 if (res3 > 1) // 3 real roots,
218 dblSort3(x[0], x[1], x[2]); // sort roots to x[0] <= x[1] <= x[2]
219 // Note: x[0]*x[1]*x[2]= c*c > 0
220 if (x[0] > 0) // all roots are positive
222 btScalar sz1 = sqrt(x[0]);
223 btScalar sz2 = sqrt(x[1]);
224 btScalar sz3 = sqrt(x[2]);
225 // Note: sz1*sz2*sz3= -c (and not equal to 0)
228 x[0] = (-sz1 - sz2 - sz3) / 2;
229 x[1] = (-sz1 + sz2 + sz3) / 2;
230 x[2] = (+sz1 - sz2 + sz3) / 2;
231 x[3] = (+sz1 + sz2 - sz3) / 2;
235 x[0] = (-sz1 - sz2 + sz3) / 2;
236 x[1] = (-sz1 + sz2 - sz3) / 2;
237 x[2] = (+sz1 - sz2 - sz3) / 2;
238 x[3] = (+sz1 + sz2 + sz3) / 2;
240 } // if( x[0] > 0) // all roots are positive
241 // now x[0] <= x[1] < 0, x[2] > 0
242 // two pair of comlex roots
243 btScalar sz1 = sqrt(-x[0]);
244 btScalar sz2 = sqrt(-x[1]);
245 btScalar sz3 = sqrt(x[2]);
247 if (c > 0) // sign = -1
250 x[1] = (sz1 - sz2) / 2; // x[0]i*x[1]
252 x[3] = (-sz1 - sz2) / 2; // x[2]i*x[3]
255 // now: c<0 , sign = +1
257 x[1] = (-sz1 + sz2) / 2;
259 x[3] = (sz1 + sz2) / 2;
261 } // if( res3>1 ) // 3 real roots,
262 // now resoventa have 1 real and pair of compex roots
263 // x[0] - real root, and x[0]>0,
264 // x[1]i*x[2] - complex roots,
265 // x[0] must be >=0. But one times x[0]=~ 1e-17, so:
268 btScalar sz1 = sqrt(x[0]);
270 CSqrt(x[1], x[2], szr, szi); // (szr+i*szi)^2 = x[1]+i*x[2]
271 if (c > 0) // sign = -1
273 x[0] = -sz1 / 2 - szr; // 1st real root
274 x[1] = -sz1 / 2 + szr; // 2nd real root
279 // now: c<0 , sign = +1
280 x[0] = sz1 / 2 - szr; // 1st real root
281 x[1] = sz1 / 2 + szr; // 2nd real root
285 } // SolveP4De(btScalar *x, btScalar b, btScalar c, btScalar d) // solve equation x^4 + b*x^2 + c*x + d
286 //-----------------------------------------------------------------------------
287 btScalar N4Step(btScalar x, btScalar a, btScalar b, btScalar c, btScalar d) // one Newton step for x^4 + a*x^3 + b*x^2 + c*x + d
289 btScalar fxs = ((4 * x + 3 * a) * x + 2 * b) * x + c; // f'(x)
291 return x; //return 1e99; <<-- FIXED!
292 btScalar fx = (((x + a) * x + b) * x + c) * x + d; // f(x)
295 //-----------------------------------------------------------------------------
296 // x - array of size 4
297 // return 4: 4 real roots x[0], x[1], x[2], x[3], possible multiple roots
298 // return 2: 2 real roots x[0], x[1] and complex x[2]i*x[3],
299 // return 0: two pair of complex roots: x[0]i*x[1], x[2]i*x[3],
300 int SolveP4(btScalar* x, btScalar a, btScalar b, btScalar c, btScalar d)
301 { // solve equation x^4 + a*x^3 + b*x^2 + c*x + d by Dekart-Euler method
303 btScalar d1 = d + 0.25 * a * (0.25 * b * a - 3. / 64 * a * a * a - c);
304 btScalar c1 = c + 0.5 * a * (0.25 * a * a - b);
305 btScalar b1 = b - 0.375 * a * a;
306 int res = SolveP4De(x, b1, c1, d1);
325 // one Newton step for each real root:
328 x[0] = N4Step(x[0], a, b, c, d);
329 x[1] = N4Step(x[1], a, b, c, d);
333 x[2] = N4Step(x[2], a, b, c, d);
334 x[3] = N4Step(x[3], a, b, c, d);
338 //-----------------------------------------------------------------------------
339 #define F5(t) (((((t + a) * t + b) * t + c) * t + d) * t + e)
340 //-----------------------------------------------------------------------------
341 btScalar SolveP5_1(btScalar a, btScalar b, btScalar c, btScalar d, btScalar e) // return real root of x^5 + a*x^4 + b*x^3 + c*x^2 + d*x + e = 0
347 btScalar brd = fabs(a); // brd - border of real roots
356 brd++; // brd - border of real roots
358 btScalar x0, f0; // less than root
359 btScalar x1, f1; // greater than root
360 btScalar x2, f2, f2s; // next values, f(x2), f'(x2)
385 // now x0<x1, f(x0)<0, f(x1)>0
386 // Firstly 10 bisections
387 for (cnt = 0; cnt < 10; cnt++)
389 x2 = (x0 + x1) / 2; // next point
390 //x2 = x0 - f0*(x1 - x0) / (f1 - f0); // next point
391 f2 = F5(x2); // f(x2)
407 // x0<x1, f(x0)<0, f(x1)>0.
409 // we hope that x0 < x2 < x1, but not necessarily
414 if (x2 <= x0 || x2 >= x1)
415 x2 = (x0 + x1) / 2; // now x0 < x2 < x1
416 f2 = F5(x2); // f(x2)
429 f2s = (((5 * x2 + 4 * a) * x2 + 3 * b) * x2 + 2 * c) * x2 + d; // f'(x2)
437 } while (fabs(dx) > eps);
439 } // SolveP5_1(btScalar a,btScalar b,btScalar c,btScalar d,btScalar e) // return real root of x^5 + a*x^4 + b*x^3 + c*x^2 + d*x + e = 0
440 //-----------------------------------------------------------------------------
441 int SolveP5(btScalar* x, btScalar a, btScalar b, btScalar c, btScalar d, btScalar e) // solve equation x^5 + a*x^4 + b*x^3 + c*x^2 + d*x + e = 0
443 btScalar r = x[0] = SolveP5_1(a, b, c, d, e);
444 btScalar a1 = a + r, b1 = b + r * a1, c1 = c + r * b1, d1 = d + r * c1;
445 return 1 + SolveP4(x + 1, a1, b1, c1, d1);
446 } // SolveP5(btScalar *x,btScalar a,btScalar b,btScalar c,btScalar d,btScalar e) // solve equation x^5 + a*x^4 + b*x^3 + c*x^2 + d*x + e = 0
447 //-----------------------------------------------------------------------------