diff --git a/Cargo.lock b/Cargo.lock
index 3851729..891be45 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -744,6 +744,7 @@ dependencies = [
 name = "qoi2"
 version = "0.1.0"
 dependencies = [
+ "anyhow",
  "bytemuck",
  "imageproc",
  "itertools 0.13.0",
diff --git a/Cargo.toml b/Cargo.toml
index 9775555..7077817 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -4,6 +4,7 @@ version = "0.1.0"
 edition = "2021"
 
 [dependencies]
+anyhow = "1.0.86"
 bytemuck = "1.16.1"
 imageproc = "0.25.0"
 itertools = "0.13.0"
diff --git a/src/encoder.rs b/src/encoder.rs
index b5d42af..045a4aa 100644
--- a/src/encoder.rs
+++ b/src/encoder.rs
@@ -1,11 +1,12 @@
 use std::mem::discriminant;
+
 use itertools::Itertools;
 use crate::common::*;
 
+// @TODO replace Options with custom Errors
 
 impl Ops {
-    // @TODO don't alloc here, get a mut reference and extend here
-    fn format (self) -> Vec<u8> {
+    fn write_encoded (self, buf: &mut Vec<u8>) {
         // cast_diff
         let cd = |diff: i8| cast_with_bias(diff, 2);
         // cast_luma_green
@@ -14,13 +15,13 @@ impl Ops {
         let clrb = |diff: i8| cast_with_bias(diff, 8);
 
         match self {
-            Ops::Nop => vec![],
-            Ops::Rgb([r, g, b]) => vec![254, r, g, b],
-            Ops::Rgba([r, g, b, a]) => vec![255, r, g, b, a],
-            Ops::Index(idx) => vec![idx as u8],
-            Ops::Run(len) => vec![(3 << 6) | (len - 1)],
-            Ops::Diff([dr, dg, db]) => vec![(1 << 6) | ((cd(dr) << 4) | (cd(dg) << 2) | cd(db)) ],
-            Ops::Luma([dr, dg, db]) => vec![(2 << 6) | clg(dg), (clrb(dr) << 4) | clrb(db)],
+            Ops::Nop => (),
+            Ops::Rgb([r, g, b]) => buf.extend_from_slice(&[254, r, g, b]),
+            Ops::Rgba([r, g, b, a]) => buf.extend_from_slice(&[255, r, g, b, a]),
+            Ops::Index(idx) => buf.push(idx as u8),
+            Ops::Run(len) => buf.push((3 << 6) | (len - 1)),
+            Ops::Diff([dr, dg, db]) => buf.push((1 << 6) | ((cd(dr) << 4) | (cd(dg) << 2) | cd(db))),
+            Ops::Luma([dr, dg, db]) => buf.extend_from_slice(&[(2 << 6) | clg(dg), (clrb(dr) << 4) | clrb(db)]),
         }
     }
 }
@@ -57,8 +58,9 @@ fn encode_body (header: Header, data: &[u8]) -> Option<Vec<u8>> {
                 Ops::Run(run) if run < 62 => cur = Ops::Run(run + 1),
                 // if run is full, commit it to out vec, and start a new one
                 Ops::Run(run) if run >= 62 => {
+                    cur = Ops::Run(62); // clamp for safety;
+                    cur.write_encoded(&mut out);
                     cur = Ops::Run(1);
-                    out.extend_from_slice(&Ops::Run(62).format());
                 },
                 // otherwise (default case, when encountering a match for the first time) start a run
                 _ => cur = Ops::Run(1),
@@ -66,19 +68,19 @@ fn encode_body (header: Header, data: &[u8]) -> Option<Vec<u8>> {
         } else {
             // if current Op is Run (meaning we need to and it since there's no match)
             if discriminant(&cur) == discriminant(&Ops::Run(0)) {
-                out.extend_from_slice(&cur.format());
+                cur.write_encoded(&mut out);
             }
 
             let hash = hash(rgba);
             // INDEX -> DIFF -> LUMA -> RGB(A)
             if known[hash] == rgba {
                 cur = Ops::Index(hash);
-            } else if let Some(ops) = diff_ops([r, g, b, a], [last[0], last[1], last[2], last[3]]) {
+            } else if let Some(ops) = diff_ops([r, g, b, a], last) {
                 cur = ops;
             } else {
                 cur = header.channels.ops_color(&rgba[..header.channels.num()])?;
             }
-            out.extend_from_slice(&cur.format());
+            cur.write_encoded(&mut out);
             last = rgba;
             known[hash] = rgba;
         }
@@ -86,7 +88,7 @@ fn encode_body (header: Header, data: &[u8]) -> Option<Vec<u8>> {
 
     // if an image buffer ends with a run, flush it as well
     if discriminant(&cur) == discriminant(&Ops::Run(0)) {
-        out.extend_from_slice(&cur.format());
+        cur.write_encoded(&mut out);
     }
 
     Some(out)