diff --git a/src/core/directory.rs b/src/core/directory.rs
index a018e1bc7..088717b84 100644
--- a/src/core/directory.rs
+++ b/src/core/directory.rs
@@ -30,6 +30,11 @@ impl Deref for ReadOnlySource {
 }
 
 impl ReadOnlySource {
+
+    pub fn len(&self,) -> usize {
+        self.as_slice().len()
+    }
+
     pub fn as_slice(&self,) -> &[u8] {
         match *self {
             ReadOnlySource::Mmap(ref mmap_read_only) => unsafe { mmap_read_only.as_slice() },
diff --git a/src/core/fastfield.rs b/src/core/fastfield.rs
index c612bcd3c..df023257a 100644
--- a/src/core/fastfield.rs
+++ b/src/core/fastfield.rs
@@ -1,9 +1,44 @@
 use std::io::Write;
 use std::io;
+use std::io::Cursor;
+use std::io::Seek;
+use std::io::SeekFrom;
 use core::serialize::BinarySerializable;
+use core::directory::ReadOnlySource;
+use core::schema::DocId;
+
 
 struct IntFastFieldWriter {
-    vals: Vec<u64>,
+    vals: Vec<u32>,
+}
+
+
+pub fn compute_num_bits(amplitude: u32) -> u8 {
+    if amplitude == 0 {
+        0
+    }
+    else {
+        1 + compute_num_bits(amplitude / 2)
+    }
+}
+
+// only works for big-endian
+fn serialize_packed_ints<I: Iterator<Item=u32>>(vals_it: I, num_bits: u8, write: &mut Write) -> io::Result<()> {
+    let mut mini_buffer_written = 0;
+    let mut mini_buffer = 0u64;
+    for val in vals_it {
+        if mini_buffer_written + num_bits > 64 {
+            try!(mini_buffer.serialize(write));
+            mini_buffer = 0;
+            mini_buffer_written = 0;
+        }
+        mini_buffer |= (val as u64) << mini_buffer_written;
+        mini_buffer_written += num_bits;
+    }
+    if mini_buffer_written > 0 {
+        try!(mini_buffer.serialize(write));
+    }
+    Ok(())
 }
 
 impl IntFastFieldWriter {
@@ -14,49 +49,189 @@ impl IntFastFieldWriter {
         }
     }
 
-    pub fn add(&mut self, val: u64) {
+    pub fn add(&mut self, val: u32) {
         self.vals.push(val);
     }
 
-    pub fn compute_num_bits(&self, amplitude: u64) -> u8 {
-        if amplitude == 0 {
-            0
-        }
-        else {
-            1 + self.compute_num_bits(amplitude / 2)
-        }
-    }
-
     pub fn close(&self, write: &mut Write) -> io::Result<()> {
-        try!((self.vals.len() as u32).serialize(write));
         if self.vals.is_empty() {
             return Ok(())
         }
         let min = self.vals.iter().min().unwrap();
         let max = self.vals.iter().max().unwrap();
-        let amplitude: u64 = max - min;
-        let num_bits = self.compute_num_bits(amplitude);
-        for val in self.vals.iter() {
-            try!(val.serialize(write));
-        }
-        Ok(())
+        try!(min.serialize(write));
+        let amplitude: u32 = max - min;
+        let num_bits: u8 = compute_num_bits(amplitude);
+        try!(num_bits.serialize(write));
+        let vals_it = self.vals.iter().map(|i| i-min);
+        serialize_packed_ints(vals_it, num_bits, write)
     }
 }
 
+pub struct IntFastFieldReader {
+    data: ReadOnlySource,
+    min_val: u32,
+    num_bits: u32,
+    mask: u32,
+    num_in_pack: u32,
+}
+
+impl IntFastFieldReader {
+    pub fn open(data: &ReadOnlySource) -> io::Result<IntFastFieldReader> {
+        let mut cursor: Cursor<&[u8]> = Cursor::new(&*data);
+        let min_val = try!(u32::deserialize(&mut cursor));
+        let num_bits = try!(u8::deserialize(&mut cursor));
+        let mask = (1 << num_bits) - 1;
+        let num_in_pack = 64u32 / (num_bits as u32);
+        Ok(IntFastFieldReader {
+            min_val: min_val,
+            num_bits: num_bits as u32,
+            data: data.slice(5, data.len()),
+            mask: mask,
+            num_in_pack: num_in_pack,
+        })
+    }
+
+    pub fn get(&self, doc: DocId) -> u32 {
+        let mut cursor = Cursor::new(&*self.data);
+        let long_addr = doc / self.num_in_pack;
+        let ord_within_long = doc - long_addr * self.num_in_pack;
+        let bit_shift = (self.num_bits as u32) * ord_within_long;
+        cursor.seek(SeekFrom::Start((long_addr as u64)   * 8u64)).unwrap();
+        let val_unshifted_unmasked = u64::deserialize(&mut cursor).unwrap();
+        let val_shifted = (val_unshifted_unmasked >> bit_shift) as u32;
+        return self.min_val + (val_shifted & self.mask);
+    }
+}
 
 #[cfg(test)]
 mod tests {
 
+    use super::compute_num_bits;
     use super::IntFastFieldWriter;
+    use super::IntFastFieldReader;
+    use core::directory::ReadOnlySource;
+    use test::Bencher;
+    use test;
+    use rand::Rng;
+    use rand::SeedableRng;
+    use rand::XorShiftRng;
 
     #[test]
-    fn test_intfastfieldwriter() {
-        let mut write: Vec<u8> = Vec::new();
-        let mut int_fast_field_writer = IntFastFieldWriter::new();
-        int_fast_field_writer.add(4u64);
-        int_fast_field_writer.add(14u64);
-        int_fast_field_writer.add(2u64);
-        int_fast_field_writer.close(&mut write).unwrap();
-        assert_eq!(write.len(), 8 * 3 + 4);
+    fn test_compute_num_bits() {
+        assert_eq!(compute_num_bits(1), 1u8);
+        assert_eq!(compute_num_bits(0), 0u8);
+        assert_eq!(compute_num_bits(2), 2u8);
+        assert_eq!(compute_num_bits(3), 2u8);
+        assert_eq!(compute_num_bits(4), 3u8);
+        assert_eq!(compute_num_bits(255), 8u8);
+        assert_eq!(compute_num_bits(256), 9u8);
+    }
+
+    #[test]
+    fn test_intfastfield_small() {
+        let mut buffer: Vec<u8> = Vec::new();
+        {
+            let mut int_fast_field_writer = IntFastFieldWriter::new();
+            int_fast_field_writer.add(4u32);
+            int_fast_field_writer.add(14u32);
+            int_fast_field_writer.add(2u32);
+            int_fast_field_writer.close(&mut buffer).unwrap();
+            assert_eq!(buffer.len(), 4 + 1 + 8 as usize);
+        }
+        {
+            let source = ReadOnlySource::Anonymous(buffer);
+            let fast_field_reader = IntFastFieldReader::open(&source).unwrap();
+            assert_eq!(fast_field_reader.get(0), 4u32);
+            assert_eq!(fast_field_reader.get(1), 14u32);
+            assert_eq!(fast_field_reader.get(2), 2u32);
+        }
+    }
+
+
+    #[test]
+    fn test_intfastfield_large() {
+        let mut buffer: Vec<u8> = Vec::new();
+        {
+            let mut int_fast_field_writer = IntFastFieldWriter::new();
+            int_fast_field_writer.add(4u32);
+            int_fast_field_writer.add(14_082_001u32);
+            int_fast_field_writer.add(3_052u32);
+            int_fast_field_writer.close(&mut buffer).unwrap();
+            assert_eq!(buffer.len(), 21 as usize);
+        }
+        {
+            let source = ReadOnlySource::Anonymous(buffer);
+            let fast_field_reader = IntFastFieldReader::open(&source).unwrap();
+            assert_eq!(fast_field_reader.get(0), 4u32);
+            assert_eq!(fast_field_reader.get(1), 14_082_001u32);
+            assert_eq!(fast_field_reader.get(2), 3_052u32);
+        }
+    }
+
+
+    fn generate_permutation() -> Vec<u32> {
+        let seed: &[u32; 4] = &[1, 2, 3, 4];
+        let mut rng = XorShiftRng::from_seed(*seed);
+        let mut permutation: Vec<u32> = (0u32..1_000_000u32).collect();
+        rng.shuffle(&mut permutation);
+        permutation
+    }
+
+    #[test]
+    fn test_intfastfield_permutation() {
+        let mut buffer: Vec<u8> = Vec::new();
+        let permutation = generate_permutation();
+        {
+            let mut int_fast_field_writer = IntFastFieldWriter::new();
+            for x in permutation.iter() {
+                int_fast_field_writer.add(*x);
+            }
+            int_fast_field_writer.close(&mut buffer).unwrap();
+        }
+        let source = ReadOnlySource::Anonymous(buffer);
+        let int_fast_field_reader = IntFastFieldReader::open(&source).unwrap();
+
+        let n = test::black_box(100);
+        let mut a = 0u32;
+        for _ in 0..n {
+            assert_eq!(int_fast_field_reader.get(a as u32), permutation[a as usize]);
+            a = int_fast_field_reader.get(a as u32);
+        }
+    }
+
+
+    #[bench]
+    fn bench_intfastfield_veclookup(b: &mut Bencher) {
+        let permutation = generate_permutation();
+        b.iter(|| {
+            let n = test::black_box(100);
+            let mut a = 0u32;
+            for _ in 0..n {
+                a = permutation[a as usize];
+            }
+        });
+    }
+
+    #[bench]
+    fn bench_intfastfield_fflookup(b: &mut Bencher) {
+        let mut buffer: Vec<u8> = Vec::new();
+        {
+            let permutation = generate_permutation();
+            let mut int_fast_field_writer = IntFastFieldWriter::new();
+            for x in permutation.iter() {
+                int_fast_field_writer.add(*x);
+            }
+            int_fast_field_writer.close(&mut buffer).unwrap();
+        }
+        let source = ReadOnlySource::Anonymous(buffer);
+        let int_fast_field_reader = IntFastFieldReader::open(&source).unwrap();
+        b.iter(|| {
+            let n = test::black_box(100);
+            let mut a = 0u32;
+            for _ in 0..n {
+                a = int_fast_field_reader.get(a as u32);
+            }
+        });
     }
 }