/* Custom string builder, comparable to functionality provided by SV*
 * functions, but with less magic and better inlineable. */

typedef struct {
    size_t len;
    size_t size;
    char *buf;
} fustr;

/* No need to call this, an empty fustr is already usable.
 * This allows setting a custom initial size. */
static void fustr_init(fustr *s, size_t prealloc) {
    s->len = 0;
    s->size = prealloc;
    s->buf = safemalloc(prealloc);
}

static void fustr_grow(fustr *s, size_t add) {
    if (s->size == 0) s->size = 512;
    while (s->size < s->len + add)
        s->size *= 2;
    s->buf = saferealloc(s->buf, s->size);
}

#define fustr_reserve(s, n) do {\
        if (UNLIKELY((s)->size < (s)->len + (n))) fustr_grow(s, n);\
    } while(0)

#define fustr_write(s, str, n) do {\
        fustr_reserve(s, n);\
        memcpy((s)->buf+(s)->len, str, (n));\
        (s)->len += (n);\
    } while(0) 

/* Move the string buffer into a new SV; fustr should be considered invalid after this call.
 * Does not set the UTF8 flag. */
static SV *fustr_sv(fustr *s) {
    SV *r = newSV(0);
    fustr_write(s, "", 1); // trailing nul
    sv_usepvn_flags(r, s->buf, s->len-1, SV_HAS_TRAILING_NUL);
    // TODO: SvPV_shrink_to_cur?
    return r;
}