diff --git a/package-lock.json b/package-lock.json
index 02ea185..ad11e5d 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -581,6 +581,11 @@
       "integrity": "sha1-PYpcZog6FqMMqGQ+hR8Zuqd5eRc=",
       "dev": true
     },
+    "fastfile": {
+      "version": "0.0.1",
+      "resolved": "https://registry.npmjs.org/fastfile/-/fastfile-0.0.1.tgz",
+      "integrity": "sha512-Fk8PWafGWGEUw7oPq/dJen92ASxknCEy4ZC8n4VEvSwCp/jcReyEmVoWsRIWTf+IvAp2MzvFi54vOPeK2LQZtQ=="
+    },
     "ffiasm": {
       "version": "0.0.2",
       "resolved": "https://registry.npmjs.org/ffiasm/-/ffiasm-0.0.2.tgz",
@@ -1123,10 +1128,11 @@
       "dev": true
     },
     "r1csfile": {
-      "version": "0.0.4",
-      "resolved": "https://registry.npmjs.org/r1csfile/-/r1csfile-0.0.4.tgz",
-      "integrity": "sha512-1Y/zzzEjQVTR/gPlduRaKi2K+yU+UxqtsS+obDLEEb4WAzwCkKGybRfp037CUW5OApeleS1WdGmtKv9K9FPhsA==",
+      "version": "0.0.5",
+      "resolved": "https://registry.npmjs.org/r1csfile/-/r1csfile-0.0.5.tgz",
+      "integrity": "sha512-B+BdKPb/WUTp4N/3X4d1Spgx9Ojx5tFVejGZRJxpTtzq34mC8Vi/czWfiPj85V8kud31lCfYcZ16z7+czvM0Sw==",
       "requires": {
+        "fastfile": "0.0.1",
         "ffjavascript": "0.1.0"
       }
     },
diff --git a/package.json b/package.json
index 8750724..db405a6 100644
--- a/package.json
+++ b/package.json
@@ -31,11 +31,12 @@
   "dependencies": {
     "chai": "^4.2.0",
     "circom_runtime": "0.0.5",
+    "fastfile": "0.0.1",
     "ffiasm": "0.0.2",
     "ffjavascript": "0.1.0",
     "ffwasm": "0.0.7",
     "fnv-plus": "^1.3.1",
-    "r1csfile": "0.0.4",
+    "r1csfile": "0.0.5",
     "tmp-promise": "^2.0.2",
     "wasmbuilder": "0.0.10"
   },
diff --git a/src/r1csfile.js b/src/r1csfile.js
index f046205..ba722a9 100644
--- a/src/r1csfile.js
+++ b/src/r1csfile.js
@@ -1,16 +1,15 @@
 
-const fs = require("fs");
+const fastFile = require("fastfile");
 const assert = require("assert");
-const Scalar = require("ffjavascript").Scalar;
 
 module.exports.buildR1cs = buildR1cs;
 
 async function buildR1cs(ctx, fileName) {
 
-    const fd = await fs.promises.open(fileName, "w");
+    const fd = await fastFile.createOverride(fileName);
 
 
-    await fd.write("r1cs"); // Magic "r1cs"
+    await fd.write(Buffer.from("r1cs"), 0); // Magic "r1cs"
 
     let p = 4;
     await writeU32(1); // Version
@@ -53,7 +52,6 @@ async function buildR1cs(ctx, fileName) {
     for (let i=0; i<ctx.constraints.length; i++) {
         if ((ctx.verbose)&&(i%10000 == 0)) {
             if (ctx.verbose) console.log("writing constraint: ", i);
-            await fd.datasync();
         }
         await writeConstraint(ctx.constraints[i]);
     }
@@ -89,19 +87,22 @@ async function buildR1cs(ctx, fileName) {
     await writeU32(constraintsSize, pConstraintsSize);
     await writeU32(mapSize, pMapSize);
 
-    await fd.sync();
     await fd.close();
 
     async function writeU32(v, pos) {
+        let o = (typeof pos == "undefined") ? p : pos;
+
         const b = Buffer.allocUnsafe(4);
         b.writeInt32LE(v);
 
-        await fd.write(b, 0, 4, pos);
+        await fd.write(b, o);
 
         if (typeof(pos) == "undefined") p += 4;
     }
 
     async function writeU64(v, pos) {
+        let o = (typeof pos == "undefined") ? p : pos;
+
         const b = Buffer.allocUnsafe(8);
 
         const LSB = v & 0xFFFFFFFF;
@@ -109,7 +110,7 @@ async function buildR1cs(ctx, fileName) {
         b.writeInt32LE(LSB, 0);
         b.writeInt32LE(MSB, 4);
 
-        await fd.write(b, 0, 8, pos);
+        await fd.write(b, o);
 
         if (typeof(pos) == "undefined") p += 8;
     }
@@ -134,16 +135,15 @@ async function buildR1cs(ctx, fileName) {
     }
 
     async function writeBigInt(n, pos) {
-        const b = Buffer.allocUnsafe(n8);
 
-        const dwords = Scalar.toArray(n, 0x100000000);
+        let o = (typeof pos == "undefined") ? p : pos;
 
-        for (let i=0; i<dwords.length; i++) {
-            b.writeUInt32LE(dwords[dwords.length-1-i], i*4, 4 );
-        }
-        b.fill(0, dwords.length*4);
+        const s = n.toString(16);
+        const b = Buffer.from(s.padStart(n8*2, "0"), "hex");
+        const buff = Buffer.allocUnsafe(b.length);
+        for (let i=0; i<b.length; i++) buff[i] = b[b.length-1-i];
 
-        await fd.write(b, 0, fs, pos);
+        await fd.write(buff, o);
 
         if (typeof(pos) == "undefined") p += n8;
     }