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/* Simple regex program
* Uses NFA trees, and follows multiple possible paths
*
* This is part of the tutorial in https://swtch.com/~rsc/regexp/regexp1.html
*
* '|' choice
* '?' optional
* '*' multiple
* '+' one or more
*
*/
#include <stdio.h>
#include <assert.h>
#include <stdlib.h>
#include <stdbool.h>
#define STATE_SPLIT 256
#define STATE_MATCH 257
#define STACK_SIZE 100
#define VIZ_ENABLE 1
typedef struct state {
// c < 256: character matching, c >= 256 Split or Match
unsigned short c;
struct state *n1;
struct state *n2;
#if VIZ_ENABLE == 1
// Used for vizualization
int v;
#endif
} state_t;
typedef struct stlist {
state_t **st;
struct stlist *next;
} stlist_t;
// Used when compiling for intermediate state machines
typedef struct {
state_t *start;
// Possible output states of the state machine
stlist_t *out;
} frag_t;
state_t *st_crt(unsigned short c, state_t *n1, state_t *n2) {
state_t *s = (state_t *) malloc(sizeof(state_t));
assert(s);
s->c = c;
s->n1 = n1;
s->n2 = n2;
#if VIZ_ENABLE == 1
s->v = -1;
#endif
return s;
}
stlist_t *list_crt(state_t **init) {
stlist_t *l = (stlist_t *) malloc(sizeof(stlist_t));
assert(l);
l->st = init;
l->next = NULL;
return l;
}
// This will append b to a and return a
stlist_t *list_append(stlist_t *a, stlist_t *b) {
stlist_t *end;
for (end = a; end->next; end++);
end->next = b;
return a;
}
// Sets all pointers in list to s
void list_patch(stlist_t *l, state_t *s) {
for ( ; l; l = l->next) {
*l->st = s;
}
}
void frag_init(frag_t *f, state_t *start, stlist_t *out) {
f->start = start;
f->out = out;
}
/*
* Compiles a regex in the postfix notation.
*
* '.' for concat is added.
*
* a(bb)+a becomes abb.+.a.
*/
state_t *compile(char *postfix) {
char *c;
frag_t stack[STACK_SIZE];
state_t *s;
frag_t *cur = stack, *e1, *e2;
#define PUSH (cur++)
#define POP (--cur)
for (c = postfix; *c; c++) {
switch(*c) {
case '.':
e2 = POP;
e1 = POP;
//patch e2 on e1
list_patch(e1->out, e2->start);
frag_init(PUSH, e1->start, e2->out);
break;
case '|':
e2 = POP;
e1 = POP;
s = st_crt(STATE_SPLIT, e1->start, e2->start);
frag_init(PUSH, s, list_append(e1->out, e2->out));
break;
case '?':
e1 = POP;
s = st_crt(STATE_SPLIT, e1->start, NULL);
frag_init(PUSH, s, list_append(e1->out, list_crt(&s->n2)));
break;
case '*':
e1 = POP;
s = st_crt(STATE_SPLIT, e1->start, NULL);
list_patch(e1->out, s);
frag_init(PUSH, s, list_crt(&s->n2));
break;
case '+':
e1 = POP;
s = st_crt(STATE_SPLIT, e1->start, NULL);
list_patch(e1->out, s);
frag_init(PUSH, e1->start, list_crt(&s->n2));
break;
default:
s = st_crt(*c, NULL, NULL);
frag_init(PUSH, s, list_crt(&s->n1));
}
}
// Connect dangling ends to a matching state
cur--;
list_patch(cur->out, st_crt(STATE_MATCH, NULL, NULL));
return cur->start;
}
#if VIZ_ENABLE == 1
int viz(state_t *s, int *ip) {
if (ip == NULL) {
// First run, print begin and end of graph
int i = 0;
printf("digraph G {\n");
printf("0 [label=\"s\"]");
viz(s, &i);
printf("}\n");
return i;
}
if (s->v != -1) {
return s->v;
}
int this_i = (*ip)++;
s->v = this_i;
switch (s->c) {
case STATE_SPLIT:
printf("%d -> %d;\n", this_i, viz(s->n1, ip));
printf("%d -> %d;\n", this_i, viz(s->n2, ip));
break;
case STATE_MATCH:
printf("%d [label=\"m\"];\n", this_i);
break;
default:
printf("%d -> %d [label=\"%c\"];\n", this_i, viz(s->n1, ip), s->c);
}
return this_i;
}
#endif
int main() {
state_t *s = compile("c+a.de.|");
viz(s, 0);
}
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