.gitignore

@@ -1,2 +1 @@
 /target
-Cargo.lock

src/bit_buffer.rs

@@ -1,87 +0,0 @@
-#[derive(Debug)]
-pub struct BitWriter {
-    data: Vec<u8>,
-    bits: u8,
-    nextbit: usize,
-}
-impl BitWriter {
-    pub fn new() -> Self {
-        return BitWriter {
-            data: vec![],
-            bits: 0,
-            nextbit: 7,
-        };
-    }
-    pub fn write(&mut self, bit: bool) {
-        if bit {
-            self.bits = 1 << self.nextbit | self.bits;
-        }
-        if self.nextbit == 0 {
-            self.data.push(self.bits);
-            self.bits = 0;
-            self.nextbit = 7;
-        } else {
-            self.nextbit -= 1;
-        }
-    }
-    pub fn flush(mut self) -> Vec<u8> {
-        if self.bits != 0 {
-            self.data.push(self.bits);
-        }
-        return self.data;
-    }
-}
-
-trait Poppable {
-    fn pop(&mut self) -> Option<u8>;
-}
-impl Poppable for &[u8] {
-    fn pop(&mut self) -> Option<u8> {
-        if self.len() == 0 {
-            return None;
-        }
-        let out = self[0];
-        *self = &self[1..];
-        return Some(out);
-    }
-}
-
-pub struct BitReader<'a> {
-    data: &'a [u8],
-    // bits: u8,
-    nextbit: usize,
-}
-
-impl<'a> BitReader<'a> {
-    pub fn new(data: &'a [u8]) -> Self {
-        BitReader { data, nextbit: 7 }
-    }
-}
-
-impl<'a> From<&'a [u8]> for BitReader<'a> {
-    fn from(value: &'a [u8]) -> Self {
-        BitReader::new(value)
-    }
-}
-
-impl BitReader<'_> {
-    pub fn pop(&mut self) -> Option<bool> {
-        if self.data.len() == 0 {
-            return None;
-        }
-        let bit = (self.data[0] >> self.nextbit) & 1;
-        if self.nextbit == 0 {
-            self.data.pop();
-            self.nextbit = 8;
-        }
-        self.nextbit -= 1;
-        return Some(bit > 0);
-    }
-}
-
-impl Iterator for BitReader<'_> {
-    type Item = bool;
-    fn next(&mut self) -> Option<Self::Item> {
-        self.pop()
-    }
-}

src/main.rs

@@ -1,97 +1,53 @@
 // https://marknelson.us/posts/2014/10/19/data-compression-with-arithmetic-coding.html
 use std::collections::HashMap;
 
-mod bit_buffer;
-use bit_buffer::{BitReader, BitWriter};
-
 type Model = HashMap<u8, (f64, f64)>;
 
-fn get_symbol(model: &Model, low: f64, high: f64) -> Option<u8> {
+fn get_symbol(model: &Model, d: f64) -> Option<u8> {
+    // Brute force
     for (symbol, (start, end)) in model {
-        if low >= *start && high < *end {
+        if d >= *start && d < *end {
             return Some(*symbol);
         }
     }
     return None;
 }
 
-fn encode(input: &[u8], model: &Model) -> Vec<u8> {
-    const HALF: u64 = 1 << (u64::BITS - 1);
-    const LOW_CONVERGE: u64 = 0b10 << (u64::BITS - 2);
-    const HIGH_CONVERGE: u64 = 0b01 << (u64::BITS - 2);
-
-    let mut output = BitWriter::new();
-
-    let mut high = u64::MAX;
-    let mut low = u64::MIN;
-    let mut pending_bits = 0;
-
-    for symbol in input {
-        let range = high - low;
+fn encode(data: &[u8], model: &Model) -> f64 {
+    let mut high: f64 = 1.0;
+    let mut low: f64 = 0.0;
+    for symbol in data {
         let p = model.get(symbol).expect("Invalid/Unsupported data");
-        high = low + (range as f64 * p.1) as u64;
-        low = low + (range as f64 * p.0) as u64;
-        loop {
-            if high < HALF {
-                output.write(false);
-                print!("0");
-                while pending_bits > 0 {
-                    output.write(true);
-                    print!("1");
-                    pending_bits -= 1;
-                }
-            } else if low >= HALF {
-                output.write(true);
-                print!("1");
-                while pending_bits > 0 {
-                    output.write(true);
-                    print!("0");
-                    pending_bits -= 1;
-                }
-            } else if low >= LOW_CONVERGE && high < HIGH_CONVERGE {
-                println!("BET");
-                pending_bits += 1;
-                low <<= 1;
-                low &= HALF - 1;
-                high <<= 1;
-                high &= HALF + 1;
-                continue;
-            } else {
-                break;
-            }
-            low <<= 1;
-            high <<= 1;
-            high |= 1;
-        }
+        let range = high - low;
+        high = low + range * p.1;
+        low = low + range * p.0;
     }
-    println!("");
-    return output.flush();
+    return low + (high - low) / 2.0;
 }
 
-fn decode(input: &[u8], model: &Model) -> Vec<u8> {
-    let mut high = 1.0;
-    let mut low = 0.0;
-    let mut output = vec![];
-    for bit in BitReader::new(input) {
-        let diff = high - low;
-        if bit {
-            //print!("1");
-            low = low + (diff / 2.0);
-        } else {
-            high = high - (diff / 2.0);
-            //print!("0");
-        }
-        if let Some(symbol) = get_symbol(model, low, high) {
-            //println!("\nGot sym: {} from [{}, {})", symbol as char, low, high);
-            output.push(symbol);
-            let (slow, shigh) = model.get(&symbol).unwrap();
-            let symdiff = *shigh - *slow;
-            high = (high - *slow) / symdiff;
-            low = (low - *slow) / symdiff;
+fn decode(message: f64, model: &Model) {
+    let mut high: f64 = 1.0;
+    let mut low: f64 = 0.0;
+    loop {
+        let range = high - low;
+        let d = (message - low) / range;
+        let c = match get_symbol(&model, d) {
+            Some(c) => c,
+            None => {
+                println!("");
+                eprintln!("Decode error: d={d}");
+                return;
+            }
+        };
+        if c == b'-' {
+            println!("");
+            return;
         }
+        print!("{}", c as char);
+        let p = model.get(&c).expect("Decode error");
+        high = low + range * p.1;
+        low = low + range * p.0;
     }
-
-    return output;
 }
 
 fn make_model(probabilities: &[(u8, f64)]) -> Model {
@@ -136,17 +92,8 @@ const ENGLISH: &[(u8, f64)] = &[
     (b'-', 0.02),
 ];
 fn main() {
-    let data = b"hello world-";
-    println!("MODEL:");
     let model: Model = make_model(ENGLISH);
-    println!("");
-
-    let _enc = encode(data, &model);
-    let _dec = decode(&_enc, &model);
-
-    println!("{}", String::from_utf8(_dec).unwrap());
-    println!(
-        "Compression Ratio: {}",
-        data.len() as f64 / _enc.len() as f64
-    );
+    let message = encode(b"hello world-", &model);
+    println!("{message}");
+    decode(message, &model);
 }