Add encode_sections() helper

Author: maurolacy

packages/std/src/sections.rs

@@ -1,3 +1,6 @@
+use crate::conversion::to_u32;
+use crate::errors::StdResult;
+
 /// A sections decoder for the special case of two elements
 #[allow(dead_code)] // used in Wasm and tests only
 pub fn decode_sections2(data: Vec<u8>) -> (Vec<u8>, Vec<u8>) {
@@ -6,6 +9,33 @@ pub fn decode_sections2(data: Vec<u8>) -> (Vec<u8>, Vec<u8>) {
     (first, second)
 }
 
+/// Encodes multiple sections of data into one vector.
+///
+/// Each section is suffixed by a section length encoded as big endian uint32.
+/// Using suffixes instead of prefixes allows reading sections in reverse order,
+/// such that the first element does not need to be re-allocated if the contract's
+/// data structure supports truncation (such as a Rust vector).
+///
+/// The resulting data looks like this:
+///
+/// ```ignore
+/// section1 || section1_len || section2 || section2_len || section3 || section3_len || …
+/// ```
+#[allow(dead_code)] // used in Wasm and tests only
+pub fn encode_sections(sections: &[Vec<u8>]) -> StdResult<Vec<u8>> {
+    let mut out_len: usize = sections.iter().map(|section| section.len()).sum();
+    out_len += 4 * sections.len();
+    let mut out_data = Vec::with_capacity(out_len);
+    for section in sections {
+        let section_len = to_u32(section.len())?.to_be_bytes();
+        out_data.extend(section);
+        out_data.extend_from_slice(&section_len);
+    }
+    debug_assert_eq!(out_data.len(), out_len);
+    debug_assert_eq!(out_data.capacity(), out_len);
+    Ok(out_data)
+}
+
 /// Splits data into the last section ("tail") and the rest.
 /// The tail's length information is cut off, such that it is ready to use.
 /// The rest is basically unparsed and contails the lengths of the remaining sections.
@@ -76,4 +106,40 @@ mod tests {
         assert_ne!(second.capacity(), original_capacity);
         assert_ne!(second.as_ptr(), original_ptr);
     }
+
+    #[test]
+    fn encode_sections_works_for_empty_sections() {
+        let enc = encode_sections(&[]).unwrap();
+        assert_eq!(enc, b"" as &[u8]);
+        let enc = encode_sections(&[vec![]]).unwrap();
+        assert_eq!(enc, b"\0\0\0\0" as &[u8]);
+        let enc = encode_sections(&[vec![], vec![]]).unwrap();
+        assert_eq!(enc, b"\0\0\0\0\0\0\0\0" as &[u8]);
+        let enc = encode_sections(&[vec![], vec![], vec![]]).unwrap();
+        assert_eq!(enc, b"\0\0\0\0\0\0\0\0\0\0\0\0" as &[u8]);
+    }
+
+    #[test]
+    fn encode_sections_works_for_one_element() {
+        let enc = encode_sections(&[]).unwrap();
+        assert_eq!(enc, b"" as &[u8]);
+        let enc = encode_sections(&[vec![0xAA]]).unwrap();
+        assert_eq!(enc, b"\xAA\0\0\0\x01" as &[u8]);
+        let enc = encode_sections(&[vec![0xAA, 0xBB]]).unwrap();
+        assert_eq!(enc, b"\xAA\xBB\0\0\0\x02" as &[u8]);
+        let enc = encode_sections(&[vec![0x9D; 277]]).unwrap();
+        assert_eq!(enc, b"\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\x9D\0\0\x01\x15" as &[u8]);
+    }
+
+    #[test]
+    fn encode_sections_works_for_multiple_elements() {
+        let enc = encode_sections(&[vec![0xAA]]).unwrap();
+        assert_eq!(enc, b"\xAA\0\0\0\x01" as &[u8]);
+        let enc = encode_sections(&[vec![0xAA], vec![0xDE, 0xDE]]).unwrap();
+        assert_eq!(enc, b"\xAA\0\0\0\x01\xDE\xDE\0\0\0\x02" as &[u8]);
+        let enc = encode_sections(&[vec![0xAA], vec![0xDE, 0xDE], vec![]]).unwrap();
+        assert_eq!(enc, b"\xAA\0\0\0\x01\xDE\xDE\0\0\0\x02\0\0\0\0" as &[u8]);
+        let enc = encode_sections(&[vec![0xAA], vec![0xDE, 0xDE], vec![], vec![0xFF; 19]]).unwrap();
+        assert_eq!(enc, b"\xAA\0\0\0\x01\xDE\xDE\0\0\0\x02\0\0\0\0\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\0\0\0\x13" as &[u8]);
+    }
 }