From 8d2aa424ff3de8601b0c051c824eb7638a60185b Mon Sep 17 00:00:00 2001
From: Julian T <julian@jtle.dk>
Date: Fri, 16 Oct 2020 14:36:29 +0200
Subject: Added regex parser and vizualiser

---
 lose/regex/simple_nfa.c | 192 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 192 insertions(+)
 create mode 100644 lose/regex/simple_nfa.c

(limited to 'lose/regex/simple_nfa.c')

diff --git a/lose/regex/simple_nfa.c b/lose/regex/simple_nfa.c
new file mode 100644
index 0000000..9d5ffcf
--- /dev/null
+++ b/lose/regex/simple_nfa.c
@@ -0,0 +1,192 @@
+/* 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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