Horrible code to solve a puzzle brutally

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

static double
evalop(int op, double x, double y)
{
        switch (op) {
        case 0:
                return x + y;
        case 1:
                return x - y;
        case 2:
                return x * y;
        case 3:
                return x / y;
        }
        abort();
        return 0.0;
}

static void
enum_expr(double *val, int al, int ar, int bl, int br, int cl, int cr)
{
        int o1, o2, o3;

        static char *opname[4] = { "+", "-", "*", "/" };

        for (o1 = 0; o1 < 4; o1 ++) {
                for (o2 = 0; o2 < 4; o2 ++) {
                        for (o3 = 0; o3 < 4; o3 ++) {
                                double tmp[7];

                                memcpy(tmp, val, 4 * sizeof(double));
                                tmp[4] = evalop(o1, tmp[al], tmp[ar]);
                                tmp[5] = evalop(o2, tmp[bl], tmp[br]);
                                tmp[6] = evalop(o3, tmp[cl], tmp[cr]);
                                if (fabs(tmp[6] - 21.0) < 0.02) {
                                        printf("-----------------\n");
                                        printf("%f %s %f -> %f\n",
                                                tmp[al], opname[o1],
                                                tmp[ar], tmp[4]);
                                        printf("%f %s %f -> %f\n",
                                                tmp[bl], opname[o2],
                                                tmp[br], tmp[5]);
                                        printf("%f %s %f -> %f\n",
                                                tmp[cl], opname[o3],
                                                tmp[cr], tmp[6]);
                                }
                        }
                }
        }
}

static void
enum_expr2(double *val)
{
        enum_expr(val, 0, 1, 4, 2, 5, 3);
        enum_expr(val, 1, 2, 0, 4, 5, 3);
        enum_expr(val, 1, 2, 4, 3, 0, 5);
        enum_expr(val, 2, 3, 1, 4, 0, 5);
        enum_expr(val, 0, 1, 2, 3, 4, 5);
}

#define SWAP(so1, so2)   do { \
                double sotmp = (so1); \
                (so1) = (so2); \
                (so2) = sotmp; \
        } while (0)

int
main(void)
{
        int i, j, k;

        for (i = 0; i < 4; i ++) {
                for (j = 1; j < 4; j ++) {
                        for (k = 2; k < 4; k ++) {
                                double val[4];

                                val[0] = 1.0;
                                val[1] = 5.0;
                                val[2] = 6.0;
                                val[3] = 7.0;
                                SWAP(val[0], val[i]);
                                SWAP(val[1], val[j]);
                                SWAP(val[2], val[k]);
                                enum_expr2(val);
                        }
                }
        }
        return 0;
}