feat: spec complaint program encoding

Co-authored-by: rvcas <x@rvcas.dev>

crates/flat/src/encode.rs

@@ -28,6 +28,14 @@ impl Encode for isize {
     }
 }
 
+impl Encode for usize {
+    fn encode(&self, e: &mut Encoder) -> Result<(), String> {
+        e.word(*self);
+
+        Ok(())
+    }
+}
+
 impl Encode for char {
     fn encode(&self, e: &mut Encoder) -> Result<(), String> {
         e.char(*self)?;

crates/flat/src/encoder.rs

@@ -1,4 +1,4 @@
-use crate::encode::Encode;
+use crate::{encode::Encode, zigzag};
 
 pub struct Encoder {
     pub buffer: Vec<u8>,
@@ -66,12 +66,13 @@ impl Encoder {
     }
 
     pub fn integer(&mut self, i: isize) -> Result<&mut Self, String> {
+        let i = zigzag::to_usize(i);
         self.word(i);
         Ok(self)
     }
 
     pub fn char(&mut self, c: char) -> Result<&mut Self, String> {
-        self.word(c as isize);
+        self.word(c as usize);
         Ok(self)
     }
 
@@ -132,7 +133,7 @@ impl Encoder {
         self.write_blk(arr, src_ptr);
     }
 
-    fn word(&mut self, c: isize) {
+    pub fn word(&mut self, c: usize) {
         loop {
             let mut w = (c & 127) as u8;
             let c = c >> 7;

crates/flat/src/lib.rs

@@ -1,6 +1,7 @@
 mod encode;
 mod encoder;
 mod filler;
+mod zigzag;
 
 pub mod en {
     pub use super::encode::*;

crates/flat/src/zigzag.rs

@@ -0,0 +1,27 @@
+pub fn to_usize(x: isize) -> usize {
+    let double_x = x << 1;
+
+    if x >= 0 {
+        double_x as usize
+    } else {
+        (-double_x - 1) as usize
+    }
+}
+
+pub fn to_isize(u: usize) -> isize {
+    let s = u as isize;
+
+    (s >> 1) ^ -(s & 1)
+}
+
+#[cfg(test)]
+mod test {
+    #[test]
+    fn convert() {
+        let n = -12;
+        let unsigned = super::to_usize(n);
+        let signed = super::to_isize(unsigned);
+
+        assert_eq!(n, signed)
+    }
+}