diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..4d48419
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "reference/decimal"]
+	path = reference/decimal
+	url = https://github.com/cppalliance/decimal.git
diff --git a/d64/math_test.go b/d64/math_test.go
index 70912ab..8123c5a 100644
--- a/d64/math_test.go
+++ b/d64/math_test.go
@@ -99,6 +99,13 @@ func TestDecimalAddInf(t *testing.T) {
 	equal(t, QNaN, NegInf.Add(Inf))
 }
 
+func TestAddAdhoc(t *testing.T) {
+	t.Parallel()
+
+	// add := testBinop(Decimal.Add)
+	// t.Run("1", add("0.4164333216995441", "0.1628118190198403", "0.2536215026797038"))
+}
+
 func TestDecimalCmp(t *testing.T) {
 	t.Parallel()
 
diff --git a/d64/util_test.go b/d64/util_test.go
index e708fc4..f194b1e 100644
--- a/d64/util_test.go
+++ b/d64/util_test.go
@@ -58,6 +58,20 @@ func equalD64(t *testing.T, expected, actual Decimal) pass {
 	return equal(t, expected.String(), actual.String())
 }
 
+func testBinop(op func(a, b Decimal) Decimal) func(expected, a, b string) func(*testing.T) {
+	return func(expected, a, b string) func(*testing.T) {
+		return func(t *testing.T) {
+			t.Helper()
+
+			e := MustParse(expected)
+			x := MustParse(a)
+			y := MustParse(b)
+			z := op(x, y)
+			equalD64(t, e, z)
+		}
+	}
+}
+
 func isnil(t *testing.T, a any) pass {
 	t.Helper()
 	if a != nil {
diff --git a/reference/decimal b/reference/decimal
new file mode 160000
index 0000000..f34b02b
--- /dev/null
+++ b/reference/decimal
@@ -0,0 +1 @@
+Subproject commit f34b02b80dc098baa1f065200b40b3b2e3f05775
diff --git a/reference/decref/decref.cpp b/reference/decref/decref.cpp
new file mode 100644
index 0000000..c281476
--- /dev/null
+++ b/reference/decref/decref.cpp
@@ -0,0 +1,44 @@
+#include "decref.h"
+
+#include <stdio.h>
+#include <string.h>
+
+#include <boost/decimal.hpp>
+#include <iostream>
+#include <sstream>
+
+using namespace boost::decimal;
+
+// Inline function to convert decimal64 to Dec64
+inline Dec64 f64(decimal64 d) {
+  return *(Dec64*)&d;
+}
+
+// Inline function to convert Dec64 to decimal64
+inline decimal64 t64(Dec64 d) {
+  return *(decimal64*)(&d);
+}
+
+Dec64 parse64(const char* str) {
+  decimal64 d;
+  std::istringstream(str) >> d;
+  return f64(d);
+}
+
+char* string64(Dec64 wrapper) {
+  std::ostringstream oss;
+  oss << std::scientific
+      << std::setprecision(std::numeric_limits<double>::max_digits10)
+      << t64(wrapper) << std::flush;
+  return strdup(oss.str().c_str());
+}
+
+Dec64 frombid64(uint64_t b) { return f64(from_bid<decimal64>(b)); }
+uint64_t tobid64(Dec64 a) { return to_bid(t64(a)); }
+
+int isNaN64(Dec64 a) { return isnan(t64(a)); }
+
+Dec64 add64(Dec64 a, Dec64 b) { return f64(t64(a) + t64(b)); }
+Dec64 sub64(Dec64 a, Dec64 b) { return f64(t64(a) - t64(b)); }
+Dec64 mul64(Dec64 a, Dec64 b) { return f64(t64(a) * t64(b)); }
+Dec64 quo64(Dec64 a, Dec64 b) { return f64(t64(a) / t64(b)); }
diff --git a/reference/decref/decref.go b/reference/decref/decref.go
new file mode 100644
index 0000000..9cf4537
--- /dev/null
+++ b/reference/decref/decref.go
@@ -0,0 +1,34 @@
+package decref
+
+/*
+#cgo CXXFLAGS: -std=c++17 -I../decimal/include
+#include "decref.h"
+#include <stdlib.h>
+*/
+import "C"
+
+import "unsafe"
+
+type Dec64 C.Dec64
+
+func Parse64(s string) Dec64 {
+	cstr := C.CString(s)
+	defer C.free(unsafe.Pointer(cstr))
+	return Dec64(C.parse64(cstr))
+}
+
+func (d Dec64) String() string {
+	cstr := C.string64(C.Dec64(d))
+	defer C.free(unsafe.Pointer(cstr))
+	return C.GoString(cstr)
+}
+
+func FromBid64(b uint64) Dec64 { return Dec64(C.frombid64(C.uint64_t(b))) }
+func (d Dec64) ToBid() uint64  { return uint64(C.tobid64(C.Dec64(d))) }
+
+func (d Dec64) IsNaN() bool { return C.isNaN64(C.Dec64(d)) != 0 }
+
+func (d Dec64) Add(e Dec64) Dec64 { return Dec64(C.add64(C.Dec64(d), C.Dec64(e))) }
+func (d Dec64) Sub(e Dec64) Dec64 { return Dec64(C.sub64(C.Dec64(d), C.Dec64(e))) }
+func (d Dec64) Mul(e Dec64) Dec64 { return Dec64(C.mul64(C.Dec64(d), C.Dec64(e))) }
+func (d Dec64) Quo(e Dec64) Dec64 { return Dec64(C.quo64(C.Dec64(d), C.Dec64(e))) }
diff --git a/reference/decref/decref.h b/reference/decref/decref.h
new file mode 100644
index 0000000..a603266
--- /dev/null
+++ b/reference/decref/decref.h
@@ -0,0 +1,26 @@
+#pragma once
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef uint64_t Dec64;
+
+Dec64 parse64(const char* str);
+char* string64(Dec64 wrapper);
+
+Dec64 frombid64(uint64_t b);
+uint64_t tobid64(Dec64 a);
+
+int isNaN64(Dec64 a);
+
+Dec64 add64(Dec64 a, Dec64 b);
+Dec64 sub64(Dec64 a, Dec64 b);
+Dec64 mul64(Dec64 a, Dec64 b);
+Dec64 quo64(Dec64 a, Dec64 b);
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/reference/decref/decref_test.go b/reference/decref/decref_test.go
new file mode 100644
index 0000000..b72abc8
--- /dev/null
+++ b/reference/decref/decref_test.go
@@ -0,0 +1,82 @@
+package decref_test
+
+import (
+	"fmt"
+	"math/rand/v2"
+	"strings"
+	"testing"
+
+	"github.com/anz-bank/decimal/d64"
+	"github.com/anz-bank/decimal/reference/decref"
+)
+
+func TestParseString(t *testing.T) {
+	t.Parallel()
+
+	d := decref.Parse64("3.142")
+	if d.String() != "3.142" {
+		t.Errorf("expected 3.142, got %s", d.String())
+	}
+}
+
+func TestAdd(t *testing.T) {
+	t.Parallel()
+
+	rng := rand.New(rand.NewPCG(0, 0))
+	for i := range 1000000 {
+		decrefA := randDec(i, rng)
+		decrefB := randDec(i, rng)
+
+		aStr := formatDec(decrefA)
+		bStr := formatDec(decrefB)
+
+		d64A := d64.MustParse(aStr)
+		d64B := d64.MustParse(bStr)
+
+		// Perform addition
+		decrefResult := decrefA.Add(decrefB)
+		d64Result := d64A.Add(d64B)
+
+		d64Str := d64Result.Text('e', -1)
+		decrefStr := formatDec(decrefResult)
+
+		if d64Str != decrefStr {
+			t.Errorf("Mismatch: d64Result=%s, decrefResult=%s for a=%s, b=%s, %s", d64Str, decrefStr, aStr, bStr, decrefResult.String())
+			t.FailNow()
+		}
+	}
+}
+
+func formatDec(d decref.Dec64) string {
+	decrefStr := d.String()
+	// Trim mantissa trailing zeros.
+	if a, b, cut := strings.Cut(decrefStr, "0e"); cut {
+		decrefStr = strings.TrimRight(a, "0") + "e" + b
+	}
+	// Trim exponent leading zeros and all-zeros.
+	if a, b, cut := strings.Cut(decrefStr, "+0"); cut {
+		decrefStr = a[:len(a)-1]
+		if exp := strings.TrimLeft(b, "0"); exp != "" {
+			decrefStr += "e+" + exp
+		}
+	}
+	// Trim negative-exponent leading zeros.
+	if a, b, cut := strings.Cut(decrefStr, "-0"); cut {
+		decrefStr = a + "-" + strings.TrimLeft(b, "0")
+	}
+	return decrefStr
+}
+
+func randDec(i int, rng *rand.Rand) decref.Dec64 {
+	sign := [2]string{"", "-"}[rng.IntN(2)]
+	const explimit = 385
+	exp := rng.IntN(2*explimit+1) - (explimit - 1)
+	hidigit := rng.IntN(10)
+	lodigits := rng.Int64N(1_000_000_000_000_000)
+	s := fmt.Sprintf("%s%d.%dE%d", sign, hidigit, lodigits, exp)
+	d := decref.Parse64(s)
+	if d.IsNaN() {
+		panic(fmt.Errorf("[%d] unparsable: %s (d = %s)", i, s, d))
+	}
+	return d
+}
diff --git a/reference/decref/go.mod b/reference/decref/go.mod
new file mode 100644
index 0000000..f7005f6
--- /dev/null
+++ b/reference/decref/go.mod
@@ -0,0 +1,5 @@
+module github.com/anz-bank/decimal/reference/decref
+
+go 1.24.0
+
+require github.com/anz-bank/decimal v1.15.0
diff --git a/reference/decref/go.sum b/reference/decref/go.sum
new file mode 100644
index 0000000..f1bf4a8
--- /dev/null
+++ b/reference/decref/go.sum
@@ -0,0 +1,2 @@
+github.com/anz-bank/decimal v1.15.0 h1:wJ9X6XCVpBUqEwapFamKd0eTbCkJPQe9JJSOoShKykU=
+github.com/anz-bank/decimal v1.15.0/go.mod h1:lSAX9MYcSaTIxYaII8tXEJbPom/njRKSAX2VMFVh434=