Recursive brute-force cracking

The C language source code is as follows. It is compiled and run with GCC on the Linux platform. Attach Sudoku. dat to the data file and calculate the result.

8----------36------7--9-2---5---7-------457-----1---3---1----68--85---1--9----4--812753649943682175675491283154237896369845721287169534521974368438526917796318452

Compilation and running methods:

GCC Sudoku. C-o Sudoku

./Sudoku <Sudoku. dat

#include <stdio.h>#include <string.h>#define ROWS    9#define COLS    9#define BLKS    9#define NAN     '-'typedef enum { false = 0, true = !false } bool;struct step {    char num[ROWS][COLS];    int cell[ROWS][COLS];};static int ctz(int x) { return __builtin_ctz(x); }static int rowcol2blk(int row, int col) { return row / 3 * 3 + col / 3; }static int blk2row(int blk) { return blk / 3 * 3; }static int blk2col(int blk) { return blk % 3 * 3; }bool setnum(struct step *curr, int row, int col, int num);void init(struct step *curr){    int row, col;    for (row = 0; row < ROWS; ++row) {        for (col = 0; col < COLS; ++col) {            curr->cell[row][col] = 0x3FE;            curr->num[row][col] = NAN;        }    }}bool exclude(struct step *curr, int row, int col, int num){    int x = curr->cell[row][col] & ~(1 << num);    if (0 == x) return false;    curr->cell[row][col] = x;    if (NAN != curr->num[row][col] || 0 != (x & (x - 1)))        return true;    return setnum(curr, row, col, ctz(x));}bool setnum(struct step *curr, int row, int col, int num){    int mask = 1 << num;    int blk = rowcol2blk(row, col);    int r, c, r0, c0;    if (0 == (curr->cell[row][col] & mask))        return false;    curr->num[row][col] = '0' + num;    curr->cell[row][col] = mask;    for (r = 0; r < ROWS; ++r) {        if (r == row) continue;        if (!exclude(curr, r, col, num))            return false;    }    for (c = 0; c < COLS; ++col) {        if (c == col) continue;        if (!exclude(curr, row, c, num))            return false;    }    r0 = blk2row(blk);    c0 = blk2col(blk);    for (r = r0; r < r0 + 3; ++r) {        for (c= c0; c < c0 + 3; ++c) {            if (r == row && c == col) continue;            if (!exclude(curr, r, c, num))                return false;        }    }    return true;}bool try(struct step *curr){    struct step next;    int row, col, num;    int cell;    for (row = 0; row < ROWS; ++row) {        for (col = 0; col < COLS; ++col) {            if (NAN == curr->num[row][col])                goto outer;        }    }outer:    if (row > ROWS) return true;    cell = curr->cell[row][col];    while (0 != cell) {        memcpy(&next, curr, sizeof next);        num = ctz(cell);        if (setnum(&next, row, col, num) && try(&next)) {            memcpy(curr, &next, sizeof *curr);            break;        }        cell &= ~(1 << num);    }    return (0 != cell);}int main(){    struct step step;    int row, col, num;    init(&step);    for (row = 0; row < ROWS; ++row) {        for (col = 0; col < COLS; ++col) {            if (NAN == (num = getchar())) continue;            if (!setnum(&step, row, col, num - '0')) {                puts("no answer\n");                return 1;            }        }        num = getchar(); /* new line */    }    if (!try(&step)) {        puts("no answer\n");        return 1;    }    for (row = 0; row < ROWS; ++row) {        for (col = 0; col < COLS; ++col) {            putchar(step.num[row][col];        }        putchar('\n');    }    return 0;}