create distinct interned string type

Interned strings are used pervasively, so it's a good idea to add a
layer of type safety to differentiate them from general cstrs and avoid
potential bugs from comparing non-interned and interned strings. Not
that that's happened so far that I can remember, but it could.

I'm 90% sure it's legal to alias `struct {char c;}` pointers with `char`
pointers. This specific typedef gives type safety but with a simple
one-way `internstr -> const char *` typecast (with `&istr->c`).
Converting the other way around is more intentional: a straight up cast
`(internstr)cstr` which sticks out as unchecked and probably wrong, or
calling the intern(cstr) function, which is the right way.

1 files changed, 3 insertions, 3 deletions

diff --git a/mem.c b/mem.c
index 6866216..eefe96a 100644
--- a/mem.c
+++ b/mem.c
@@ -37,12 +37,12 @@ void *
 }
 
 /** string interning **/
-const char *
+internstr
 intern(const char *s)
 {
    static uint N, n;
    static struct ht {
-      const char *s;
+      internstr s;
       size_t h;
    } *ht;
    static struct { char m[sizeof(struct arena) + (2<<10)]; struct arena *_a; } amem;
@@ -75,7 +75,7 @@ intern(const char *s)
          N = nnew;
          i = h;
          continue;
-      } else if (h == ht[i].h && !strcmp(s, ht[i].s)) {
+      } else if (h == ht[i].h && !strcmp(s, &ht[i].s->c)) {
          return ht[i].s;
       }
       ++i;