mruby-bigint, mruby-numeric-ext: fix Integer#pow with negative modulus

Support negative modulus in Integer#pow(exp, mod) with proper Ruby
semantics. Previously, negative modulus caused an infinite loop in
Barrett reduction. Now:

- Use absolute value of modulus for computation
- Apply signed modulo adjustment (result + m for non-zero result
  when m is negative)
- Add early return for zero base with positive exponent (0^n = 0)

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-bigint/core/bigint.c

@@ -3442,12 +3442,17 @@ mrb_value
 mrb_bint_powm(mrb_state *mrb, mrb_value x, mrb_value exp, mrb_value mod)
 {
   mpz_t a, b, c, z;
+  mrb_bool neg_mod = FALSE;
   MPZ_CTX_INIT(mrb, ctx, pool);
 
   bint_as_mpz(RBIGINT(x), &a);
   if (mrb_integer_p(mod)) {
     mrb_int m = mrb_integer(mod);
     if (m == 0) mrb_int_zerodiv(mrb);
+    if (m < 0) {
+      neg_mod = TRUE;
+      m = -m;
+    }
     mpz_init_set_int(ctx, &c, m);
   }
   else {
@@ -3456,7 +3461,29 @@ mrb_bint_powm(mrb_state *mrb, mrb_value x, mrb_value exp, mrb_value mod)
     if (zero_p(&c) || uzero_p(&c)) {
       mrb_int_zerodiv(mrb);
     }
+    if (c.sn < 0) {
+      neg_mod = TRUE;
+      c.sn = 1;  /* use absolute value */
+    }
   }
+
+  /* Check for zero base case: 0^n = 0 for n > 0 */
+  if (zero_p(&a) || uzero_p(&a)) {
+    mrb_bool exp_positive;
+    if (mrb_bigint_p(exp)) {
+      bint_as_mpz(RBIGINT(exp), &b);
+      exp_positive = (b.sn > 0) && !uzero_p(&b);
+    }
+    else {
+      exp_positive = mrb_integer(exp) > 0;
+    }
+    if (exp_positive) {
+      /* 0^n mod m = 0 for n > 0 */
+      if (mrb_integer_p(mod)) mpz_clear(ctx, &c);
+      return mrb_fixnum_value(0);
+    }
+  }
+
   mpz_init(ctx, &z);
   if (mrb_bigint_p(exp)) {
     bint_as_mpz(RBIGINT(exp), &b);
@@ -3468,6 +3495,13 @@ mrb_bint_powm(mrb_state *mrb, mrb_value x, mrb_value exp, mrb_value mod)
     if (e < 0) goto raise;
     mpz_powm_i(ctx, &z, &a, e, &c);
   }
+
+  /* Apply signed modulo adjustment for negative modulus */
+  /* Ruby: result + m for non-zero result when m is negative */
+  if (neg_mod && !zero_p(&z) && !uzero_p(&z)) {
+    mpz_sub(ctx, &z, &z, &c);  /* z = z - |m| = z + m (since m is negative) */
+  }
+
   if (mrb_integer_p(mod)) mpz_clear(ctx, &c);
   return bint_norm(mrb, bint_new(ctx, &z));
 

mrbgems/mruby-numeric-ext/src/numeric_ext.c

@@ -180,6 +180,7 @@ int_powm(mrb_state *mrb, mrb_value x)
 {
   mrb_value m, e;
   mrb_int exp, mod, result = 1;
+  mrb_bool neg_mod = FALSE;
 
   if (mrb_get_argc(mrb) == 1) {
     return mrb_int_pow(mrb, x, mrb_get_arg1(mrb));
@@ -204,10 +205,19 @@ int_powm(mrb_state *mrb, mrb_value x)
   if (exp < 0) mrb_raise(mrb, E_ARGUMENT_ERROR, "int.pow(n,m): n must be positive");
   if (!mrb_integer_p(m)) mrb_raise(mrb, E_TYPE_ERROR, "int.pow(n,m): m must be integer");
   mod = mrb_integer(m);
-  if (mod < 0) mrb_raise(mrb, E_ARGUMENT_ERROR, "int.pow(n,m): m must be positive when 2nd argument specified");
   if (mod == 0) mrb_int_zerodiv(mrb);
+  if (mod < 0) {
+    neg_mod = TRUE;
+    mod = -mod;
+  }
   if (mod == 1) return mrb_fixnum_value(0);
+
+  /* Early return for zero base with positive exponent */
   mrb_int base = mrb_integer(x);
+  if (base == 0 && exp > 0) {
+    return mrb_fixnum_value(0);
+  }
+
   for (;;) {
     mrb_int tmp;
     if (exp & 1) {
@@ -237,6 +247,12 @@ int_powm(mrb_state *mrb, mrb_value x)
     }
     base = tmp % mod;
   }
+
+  /* Apply signed modulo adjustment for negative modulus */
+  /* Ruby: result + m for non-zero result when m is negative */
+  if (neg_mod && result != 0) {
+    result = result - mod;  /* result - |m| = result + m (since m is negative) */
+  }
   return mrb_int_value(mrb, result);
 }