mruby-random: support bigint in rand method

To achieve this, the following changes were made:

- Exported `mrb_bint_size`, `mrb_bint_from_bytes`, and `mrb_bint_sign`
  functions from `mruby-bigint` to be used in other mrbgems.
- Modified `mruby-random` to use these new functions to handle Bigint
  arguments in the `rand` method.

Co-authored-by: Gemini <gemini@google.com>

include/mruby/internal.h

@@ -258,6 +258,9 @@ void mrb_bint_copy(mrb_state *mrb, mrb_value x, mrb_value y);
 size_t mrb_bint_memsize(mrb_value x);
 mrb_value mrb_bint_hash(mrb_state *mrb, mrb_value x);
 mrb_value mrb_bint_sqrt(mrb_state *mrb, mrb_value x);
+mrb_int mrb_bint_size(mrb_state *mrb, mrb_value bint);
+mrb_value mrb_bint_from_bytes(mrb_state *mrb, const uint8_t *bytes, mrb_int len);
+mrb_int mrb_bint_sign(mrb_state *mrb, mrb_value bint);
 #endif
 
 #endif  /* MRUBY_INTERNAL_H */

mrbgems/mruby-bigint/core/bigint.c

@@ -2756,6 +2756,34 @@ mrb_bint_sqrt(mrb_state *mrb, mrb_value x)
   return bint_norm(mrb, bint_new(mrb, &z));
 }
 
+mrb_int
+mrb_bint_sign(mrb_state *mrb, mrb_value bint)
+{
+  return RBIGINT_SIGN(RBIGINT(bint));
+}
+
+mrb_int
+mrb_bint_size(mrb_state *mrb, mrb_value bint)
+{
+  mpz_t z;
+  bint_as_mpz(RBIGINT(bint), &z);
+  return z.sz * sizeof(mp_limb);
+}
+
+mrb_value
+mrb_bint_from_bytes(mrb_state *mrb, const uint8_t *bytes, mrb_int len)
+{
+  mpz_t z;
+  mpz_init(mrb, &z);
+  size_t limb_len = (len + sizeof(mp_limb) - 1) / sizeof(mp_limb);
+  mpz_realloc(mrb, &z, limb_len);
+  memcpy(z.p, bytes, len);
+  z.sn = (len > 0) ? 1 : 0;
+  z.sz = limb_len;
+  trim(&z);
+  return bint_norm(mrb, bint_new(mrb, &z));
+}
+
 mrb_value
 mrb_bint_hash(mrb_state *mrb, mrb_value x)
 {

mrbgems/mruby-random/src/random.c

@@ -13,6 +13,7 @@
 #include <mruby/presym.h>
 #include <mruby/range.h>
 #include <mruby/string.h>
+#include <mruby/internal.h>
 
 #include <time.h>
 
@@ -210,6 +211,22 @@ random_rand_impl(mrb_state *mrb, rand_state *t, mrb_value self)
     return random_range(mrb, t, arg);
   }
 
+#ifdef MRB_USE_BIGINT
+  if (mrb_bigint_p(arg)) {
+    if (mrb_bint_sign(mrb, arg) < 0) {
+      mrb_raise(mrb, E_ARGUMENT_ERROR, "negative value as random limit");
+    }
+    mrb_int size = mrb_bint_size(mrb, arg);
+    mrb_value bytes = mrb_str_new(mrb, NULL, size);
+    uint8_t *p = (uint8_t*)RSTRING_PTR(bytes);
+    for (mrb_int i = 0; i < size; i++) {
+      p[i] = (uint8_t)rand_uint32(t);
+    }
+    mrb_value rand_bint = mrb_bint_from_bytes(mrb, p, size);
+    return mrb_bint_mod(mrb, rand_bint, arg);
+  }
+#endif
+
   range_error(mrb, arg);
 }