mruby-sprintf: support dots expression of negative integer format

For specifiers assume unsigned integers, negative numbers show dots (..)
to indicate virtual infinite 1s at the MSB side of 2's compliment.

mrbgems/mruby-bigint/core/bigint.c

@@ -1626,3 +1626,41 @@ mrb_bint_hash(mrb_state *mrb, mrb_value x)
   hash = mrb_byte_hash_step((uint8_t*)&b->mp.sn, sizeof(b->mp.sn), hash);
   return mrb_int_value(mrb, hash);
 }
+
+mrb_value
+mrb_bint_2comp(mrb_state *mrb, mrb_value x, mrb_int base)
+{
+  struct RBigint *b = RBIGINT(x);
+  mrb_int i;
+
+  for (i=b->mp.sz-1; i>=0; i--) {
+    if (b->mp.p[i] > 0) break;
+  }
+
+#ifdef __GNUC__
+  mrb_int dbits = __builtin_ctz(base);
+#else
+  mrb_int dbits;
+  switch (base) {
+  case 16: dbits = 4; break;
+  case  8: dbits = 3; break;
+  case  2: dbits = 1; break;
+  }
+#endif
+  mrb_int topbit = DIG_SIZE * (i+1) - lzb(b->mp.p[i]);
+  mrb_int nbits = (topbit / dbits + 1) * dbits;
+
+  mpz_t one;
+  mpz_t up;
+  struct RBigint *b2 = bint_new(mrb);
+
+  /* rounding up to nearest power of 2 */
+  mpz_init_set_int(mrb, &one, 1);
+  mpz_init(mrb, &up);
+  mpz_mul_2exp(mrb, &up, &one, nbits);
+  mpz_clear(mrb, &one);
+  mpz_add(mrb, &b2->mp, &up, &b->mp);
+  mpz_clear(mrb, &up);
+
+  return mrb_obj_value(b2);
+}

mrbgems/mruby-sprintf/src/sprintf.c

@@ -17,6 +17,8 @@
 #define BITSPERDIG MRB_INT_BIT
 #define EXTENDSIGN(n, l) (((~0U << (n)) >> (((n)*(l)) % BITSPERDIG)) & ~(~0U << (n)))
 
+mrb_value mrb_bint_2comp(mrb_state *mrb, mrb_value x, mrb_int base);
+
 static char*
 remove_sign_bits(char *str, int base)
 {
@@ -594,6 +596,13 @@ retry:
 #ifdef MRB_USE_BIGINT
           case MRB_TT_BIGINT:
             {
+              mrb_int n = (mrb_bint_cmp(mrb, val, mrb_fixnum_value(0)));
+              mrb_bool need_dots = ((flags & FPLUS) == 0) && (base == 16 || base == 8 || base == 2) && n < 0;
+              if (need_dots) {
+                val = mrb_bint_2comp(mrb, val, base);
+                dots = 1;
+                v = -1;
+              }
               mrb_value str = mrb_bint_to_s(mrb, val, base);
               s = RSTRING_PTR(str);
               len = RSTRING_LEN(str);
@@ -644,7 +653,9 @@ retry:
           len = (int)size;
         }
 
+#ifdef MRB_USE_BIGINT
       str_skip:
+#endif
         switch (base) {
         case 16:
           fc = 'f'; break;