jruby · headius · Mar 28, 2026 · Mar 28, 2026
diff --git a/core/src/main/java/org/jruby/RubyBignum.java b/core/src/main/java/org/jruby/RubyBignum.java
@@ -1017,6 +1017,20 @@ private IRubyObject float_cmp(ThreadContext context, RubyFloat y) {
         return RubyFixnum.minus_one(runtime);
     }
 
+    // mri : rb_integer_float_eq
+    private boolean float_eq(ThreadContext context, RubyFloat y) {
+        double yd = y.value;
+
+        if (Double.isNaN(yd) || Double.isInfinite(yd)) return false;
+
+        // if yd has a fractional part, it can't equal an integer
+        if (Math.floor(yd) != yd) return false;
+
+        // compare as integers to avoid precision loss
+        IRubyObject rel = op_cmp(context, newBignorm(context.runtime, yd));
+        return rel instanceof RubyFixnum fix && fix.getLongValue() == 0;
+    }
+
     @Override
     public final int compareTo(IRubyObject other) {
         if (other instanceof RubyBignum bignum) return value.compareTo(bignum.value);
@@ -1045,9 +1059,7 @@ public IRubyObject op_cmp(ThreadContext context, IRubyObject other) {
         } else if (other instanceof RubyBignum big) {
             otherValue = big.value;
         } else if (other instanceof RubyFloat flt) {
-            return flt.isInfinite() ?
-                    asFixnum(context, flt.value > 0.0 ? -1 : 1) :
-                    dbl_cmp(context.runtime, big2dbl(this), flt.value);
+            return float_cmp(context, flt);
         } else {
             return coerceCmp(context, sites(context).op_cmp, other);
         }
@@ -1067,9 +1079,7 @@ public IRubyObject op_equal(ThreadContext context, IRubyObject other) {
         } else if (other instanceof RubyBignum bignum) {
             otherValue = bignum.value;
         } else if (other instanceof RubyFloat flote) {
-            double a = flote.value;
-            if (Double.isNaN(a) || Double.isInfinite(a)) return context.fals;
-            return asBoolean(context, a == big2dbl(this));
+            return asBoolean(context, float_eq(context, flote));
         } else {
             return other.op_eqq(context, this);
         }

diff --git a/spec/regression/bignum_float_comparison_precision_spec.rb b/spec/regression/bignum_float_comparison_precision_spec.rb
@@ -0,0 +1,54 @@
+require 'rspec'
+
+# Regression test for Bignum <=> and == with Float precision.
+#
+# When comparing a large Bignum against a Float, the comparison must
+# not lose precision by converting the Bignum to double. For example,
+# 2**64 + 1 cannot be exactly represented as a double (it rounds to
+# 2**64), so (2**64 + 1) <=> (2**64).to_f must return 1, not 0.
+#
+# The bug was that op_cmp used dbl_cmp(big2dbl(this), flt.value)
+# which lost precision, and op_equal used big2dbl comparison which
+# had the same problem.
+
+describe "Bignum comparison with Float precision" do
+  context "<=>" do
+    it "returns 1 when Bignum is greater than Float despite double rounding" do
+      expect((2**64 + 1) <=> (2**64).to_f).to eq 1
+      expect((2**100 + 1) <=> (2**100).to_f).to eq 1
+    end
+
+    it "returns 0 when Bignum exactly equals Float" do
+      expect((2**64) <=> (2**64).to_f).to eq 0
+    end
+
+    it "returns -1 when Bignum is less than Float" do
+      expect((2**64 - 1) <=> (2**64).to_f).to eq(-1)
+    end
+
+    it "returns nil for NaN" do
+      expect((2**100) <=> Float::NAN).to be_nil
+    end
+
+    it "handles Infinity correctly" do
+      expect((2**100) <=> Float::INFINITY).to eq(-1)
+      expect((2**100) <=> -Float::INFINITY).to eq 1
+    end
+  end
+
+  context "==" do
+    it "returns false when Bignum differs from Float due to precision" do
+      expect((2**64 + 1) == (2**64).to_f).to be false
+      expect((2**100 + 1) == (2**100).to_f).to be false
+    end
+
+    it "returns true when Bignum exactly equals Float" do
+      expect((2**64) == (2**64).to_f).to be true
+    end
+
+    it "returns false for NaN and Infinity" do
+      expect((2**100) == Float::NAN).to be false
+      expect((2**100) == Float::INFINITY).to be false
+    end
+  end
+end