Add hintfile crate scaffolding

Cargo.toml

@@ -1,6 +1,6 @@
 [workspace]
-members = ["accumulator", "network"]
-default-members = ["accumulator", "network"]
+members = ["accumulator", "hintfile", "network"]
+default-members = ["accumulator", "hintfile", "network"]
 resolver = "3"
 
 [workspace.dependencies]

hintfile/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "hintfile"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]

hintfile/src/lib.rs

@@ -0,0 +1,65 @@
+use std::io::{self, Read, Write};
+
+pub fn write_compact_size<W: Write>(value: u64, writer: &mut W) -> Result<(), io::Error> {
+    match value {
+        0..=0xFC => {
+            writer.write_all(&[value as u8]) // Cast ok because of match.
+        }
+        0xFD..=0xFFFF => {
+            let v = (value as u16).to_le_bytes(); // Cast ok because of match.
+            writer.write_all(&[0xFD, v[0], v[1]])
+        }
+        0x10000..=0xFFFFFFFF => {
+            let v = (value as u32).to_le_bytes(); // Cast ok because of match.
+            writer.write_all(&[0xFE, v[0], v[1], v[2], v[3]])
+        }
+        _ => panic!("unexpected large offset"),
+    }
+}
+
+pub fn read_compact_size<R: Read>(reader: &mut R) -> Result<u64, io::Error> {
+    let mut buf: [u8; 1] = [0; 1];
+    reader.read_exact(&mut buf)?;
+    let prefix = buf[0];
+    match prefix {
+        0xFD => {
+            let mut buf: [u8; 2] = [0; 2];
+            reader.read_exact(&mut buf)?;
+            Ok(u16::from_le_bytes(buf) as u64)
+        }
+        0xFE => {
+            let mut buf: [u8; 4] = [0; 4];
+            reader.read_exact(&mut buf)?;
+            Ok(u32::from_le_bytes(buf) as u64)
+        }
+        0..=0xFC => Ok(prefix as u64),
+        _ => panic!("unexpected large offset"),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{read_compact_size, write_compact_size};
+
+    #[test]
+    fn deser_roundtrip() {
+        let mut buf = Vec::new();
+        let less: u8 = 0xFB;
+        write_compact_size(less as u64, &mut buf).unwrap();
+        let read_cs = read_compact_size(&mut buf.as_slice()).unwrap();
+        let cast_less = read_cs as u8;
+        assert_eq!(less, cast_less);
+        let mut buf = Vec::new();
+        let median: u16 = 0xFFF;
+        write_compact_size(median as u64, &mut buf).unwrap();
+        let read_cs = read_compact_size(&mut buf.as_slice()).unwrap();
+        let cast_median = read_cs as u16;
+        assert_eq!(median, cast_median);
+        let mut buf = Vec::new();
+        let more: u32 = 0xFFFFF;
+        write_compact_size(more as u64, &mut buf).unwrap();
+        let read_cs = read_compact_size(&mut buf.as_slice()).unwrap();
+        let cast_more = read_cs as u32;
+        assert_eq!(more, cast_more);
+    }
+}