First commit

.gitignore

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+/target

Cargo.toml

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+[package]
+name = "sac"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

src/main.rs

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+// https://marknelson.us/posts/2014/10/19/data-compression-with-arithmetic-coding.html
+use std::collections::HashMap;
+
+type Model = HashMap<u8, (f64, f64)>;
+
+fn get_symbol(model: &Model, d: f64) -> Option<u8> {
+    // Brute force
+    for (symbol, (start, end)) in model {
+        if d >= *start && d < *end {
+            return Some(*symbol);
+        }
+    }
+    return None;
+}
+
+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");
+        let range = high - low;
+        high = low + range * p.1;
+        low = low + range * p.0;
+    }
+    return low + (high - low) / 2.0;
+}
+
+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;
+    }
+}
+
+fn make_model(probabilities: &[(u8, f64)]) -> Model {
+    let mut model = HashMap::new();
+    let mut end: f64 = 0.0;
+    for (symbol, probability) in probabilities {
+        let start: f64 = end;
+        end = start + probability;
+        model.insert(*symbol, (start, end));
+        println!("{}: [{}, {})", *symbol as char, start, end);
+    }
+    return model;
+}
+const ENGLISH: &[(u8, f64)] = &[
+    (b'a', 0.08),
+    (b'b', 0.01),
+    (b'c', 0.02),
+    (b'd', 0.04),
+    (b'e', 0.12),
+    (b'f', 0.02),
+    (b'g', 0.02),
+    (b'h', 0.06),
+    (b'i', 0.07),
+    (b'j', 0.01),
+    (b'k', 0.01),
+    (b'l', 0.04),
+    (b'm', 0.02),
+    (b'n', 0.06),
+    (b'o', 0.07),
+    (b'p', 0.01),
+    (b'q', 0.01),
+    (b'r', 0.06),
+    (b's', 0.06),
+    (b't', 0.09),
+    (b'u', 0.02),
+    (b'v', 0.01),
+    (b'w', 0.02),
+    (b'x', 0.01),
+    (b'y', 0.02),
+    (b'z', 0.01),
+    (b' ', 0.01),
+    (b'-', 0.02),
+];
+fn main() {
+    let model: Model = make_model(ENGLISH);
+    let message = encode(b"hello world-", &model);
+    println!("{message}");
+    decode(message, &model);
+}