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| #include <stdio.h> | |
| #include <stdint.h> | |
| static int palindromic(uint64_t x) | |
| { | |
| /* C version for reference | |
| uint64_t rev = 0; | |
| for (uint64_t tmp = x; tmp; tmp /= 10) { | |
| rev *= 10; | |
| rev += tmp % 10; | |
| } | |
| return x == rev; | |
| */ | |
| __asm__ goto ( | |
| // initialise | |
| " mov $10, %%r13 \n" // need 10 in a reg for mul/div | |
| " mov $0, %%r14 \n" // build reversed x into r14 [rev] | |
| " mov %0, %%r15 \n" // copy x into a temp [tmp] | |
| "Lrun: \n" | |
| // break when r15 becomes zero | |
| " test %%r15, %%r15 \n" | |
| " jz Ldone \n" | |
| // rev *= 10 | |
| " xchg %%rax, %%r14 \n" | |
| " mul %%r13 \n" | |
| " xchg %%rax, %%r14 \n" | |
| // rev += tmp%10; tmp /= 10 | |
| " xchg %%rax, %%r15 \n" | |
| " mov $0, %%rdx \n" | |
| " div %%r13 \n" | |
| " xchg %%rax, %%r15 \n" | |
| " add %%rdx, %%r14 \n" | |
| // if r14 doesn't become nonzero immediately, | |
| // x has a trailing zero & is therefore not palindromic | |
| " jz %l2 \n" // no | |
| // round the loop again | |
| " jmp Lrun \n" | |
| "Ldone: \n" | |
| " cmp %%r14, %0 \n" | |
| " je %l1 \n" // yes | |
| : | |
| : "r" (x) | |
| : "rax", "rdx", "r13", "r14", "r15", "cc" | |
| : yes, no | |
| ); | |
| no: | |
| return 0; | |
| yes: | |
| return 1; | |
| } | |
| int main() | |
| { | |
| uint64_t max = 0; | |
| uint64_t f_a, f_b, f_c, f_d; | |
| for (uint64_t r = 10000; r > 0; --r) | |
| for (uint64_t a = 99; a > 10; --a) { | |
| for (uint64_t b = a; b > 10; --b) { | |
| uint64_t ab = a*b; | |
| for (uint64_t c = b; c > 10; --c) { | |
| uint64_t abc = ab*c; | |
| for (uint64_t d = c; d > 10; --d) { | |
| uint64_t curr = abc*d; | |
| if (curr < max) break; | |
| if (!palindromic(curr)) continue; | |
| max = curr; | |
| f_a = a; | |
| f_b = b; | |
| f_c = c; | |
| f_d = d; | |
| } | |
| } | |
| } | |
| } | |
| printf("%lu * %lu * %lu * %lu = %lu\n", f_a, f_b, f_c, f_d, max); | |
| return 0; | |
| } |
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