add sample size option

index.js

@@ -73,13 +73,45 @@ module.exports.detect = function(buffer) {
   return match ? match.name : null;
 };
 
-module.exports.detectFile = function(filepath, fn) {
-  fs.readFile(filepath, function(err, res) {
-    if (err) return fn(err, null);
-    fn(null, self.detect(res));
+module.exports.detectFile = function(filepath, opts, cb) {
+  if (typeof opts === 'function') {
+    cb = opts;
+    opts = undefined;
+  }
+
+  var fd;
+
+  var handler = function(err, buffer) {
+    if (fd) {
+      fs.closeSync(fd);
+    }
+
+    if (err) return cb(err, null);
+    cb(null, self.detect(buffer));
+  };
+
+  if (opts && opts.sampleSize) {
+    fd = fs.openSync(filepath, 'r'),
+      sample = new Buffer(opts.sampleSize);
+
+    fs.read(fd, sample, 0, opts.sampleSize, null, function(err) {
+      handler(err, sample);
     });
+    return;
+  }
+
+  fs.readFile(filepath, handler);
 };
 
-module.exports.detectFileSync = function(filepath) {
+module.exports.detectFileSync = function(filepath, opts) {
+  if (opts && opts.sampleSize) {
+    var fd = fs.openSync(filepath, 'r'),
+      sample = new Buffer(opts.sampleSize);
+
+    fs.readSync(fd, sample, 0, opts.sampleSize);
+    fs.closeSync(fd);
+    return self.detect(sample);
+  }
+
   return self.detect(fs.readFileSync(filepath));
 };

test/chardet.js

@@ -20,11 +20,23 @@ describe('chardet', function() {
         done();
       });
     });
+
+    it('should detect encoding with smaller sample size', function(done) {
+      chardet.detectFile(path, { sampleSize: 32 }, function(err, res) {
+        assert.equal(err, null);
+        assert.equal(res, 'UTF-8');
+        done();
+      });
+    });
   });
 
   describe('#detectFileSync', function() {
     it('should detect encoding', function() {
       assert.equal(chardet.detectFileSync(path), 'UTF-8');
     });
+
+    it('should detect encoding with smaller sample size', function() {
+      assert.equal(chardet.detectFileSync(path, { sampleSize: 32 }), 'UTF-8');
+    });
   });
 });