diff --git a/src/freq.rs b/src/freq.rs
new file mode 100644
index 0000000..269dae4
--- /dev/null
+++ b/src/freq.rs
@@ -0,0 +1,77 @@
+//! Sort bytes by frequency, then encode like so:
+//! - Most popular: `0`
+//! - Second most popular: `10`
+//! - Third most popular: `110`
+//! - Least popular: `(1 * 255) 0`
+//!
+//! For example, `aaabbc` would become `0001010110`
+
+use std::{array, fmt};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct Bits {
+    pub one_count: u8,
+}
+
+impl fmt::Display for Bits {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        for _ in 0..self.one_count {
+            write!(f, "1")?;
+        }
+        write!(f, "0")
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct Encoder<'src> {
+    src: &'src [u8],
+    byte_mapping: [u8; 256],
+}
+
+impl<'src> Encoder<'src> {
+    pub fn new(src: &'src [u8]) -> Self {
+        let mut byte_counts = [0u32; 256];
+
+        for &byte in src {
+            byte_counts[byte as usize] += 1;
+        }
+
+        #[allow(
+            clippy::cast_possible_truncation,
+            reason = "`i` is in the range `0..256`"
+        )]
+        let mut ranking: [u8; 256] = array::from_fn(|i| i as u8);
+        ranking.sort_by_key(|&byte| -i64::from(byte_counts[byte as usize]));
+
+        #[allow(
+            clippy::cast_possible_truncation,
+            reason = "`i` is in the range `0..256`"
+        )]
+        let byte_mapping =
+            array::from_fn(|i| ranking.iter().position(|b| *b == (i as u8)).unwrap() as u8);
+
+        Self { src, byte_mapping }
+    }
+
+    pub fn print_mapping(&self) {
+        for (one_count, byte) in self.byte_mapping.iter().zip(0u8..) {
+            println!(
+                "0x{byte:02x}: {:3} bit{s} ({:?})",
+                one_count + 1,
+                (byte as char),
+                s = if *one_count == 0 { "s" } else { "" },
+            );
+        }
+    }
+}
+
+impl Iterator for Encoder<'_> {
+    type Item = Bits;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let one_count = self.byte_mapping[*self.src.first()? as usize];
+        self.src = &self.src[1..];
+
+        Some(Bits { one_count })
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index 669c29b..509ccd5 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,15 +1,34 @@
-use std::io::{self, Read};
+use std::{
+    env,
+    io::{self, Read},
+};
 
+mod freq;
 mod rle;
 
 fn main() -> Result<(), io::Error> {
+    let debug = env::args().any(|arg| arg == "--debug" || arg == "-d");
+
     let mut buf = Vec::new();
     let len_src = io::stdin().read_to_end(&mut buf)?;
 
-    println!("Original size: {len_src}");
+    println!("Original: {len_src}");
 
     let len_rle = rle::Encoder::new(&buf).count() * 2;
-    println!("Rle'd size:    {len_rle}");
+    println!("   Rle'd: {len_rle}");
+
+    let encoder_freq = freq::Encoder::new(&buf);
+    if debug {
+        encoder_freq.print_mapping();
+    }
+
+    let len_freq_table = 256;
+    let len_freq_bits = encoder_freq
+        .map(|freq::Bits { one_count }| one_count as usize + 1)
+        .sum::<usize>()
+        .div_ceil(8);
+    let len_freq = len_freq_table + len_freq_bits;
+    println!("  Freq'd: {len_freq}");
 
     Ok(())
 }