I've been waiting decades for anyone to be interested in a divisible-by-7 test that works right-to-left. Only 7 states required: #include <stdio.h> int x

I've been waiting decades for anyone to be interested in a divisible-by-7 test that works right-to-left. Only 7 states required: #include <stdio.h> int x[7][10] = { /* 0 */ {0, 2, 4, 6, 1, 3, 5, 0, 2, 4}, /* 1 */ {5, 0, 2, 4, 6, 1, 3, 5, 0, 2}, /* 2 */ {3, 5, 0, 2, 4, 6, 1, 3, 5, 0}, /* 3 */ {1, 3, 5, 0, 2, 4, 6, 1, 3, 5}, /* 4 */ {6, 1, 3, 5, 0, 2, 4, 6, 1, 3}, /* 5 */ {4, 6, 1, 3, 5, 0, 2, 4, 6, 1}, /* 6 */ {2, 4, 6, 1, 3, 5, 0, 2, 4, 6} }; int divisible_by_7(char *b) { char *p = b; while (*p) p++; // skip to NUL int state = 0; while (--p >= b) state = x[state][*p-'0']; return !state; } int main(int argc, char *argv[]) { int i; for (i = 1; i<argc; i++) printf("%s is%s divisible by 7\n", argv[i], divisible_by_7(argv[i]) ? "" : "n't"); return 0; }