transaction/hash.rs: Add FromStr implementation (issue #299)

Co-authored-by: Deirdre Connolly <durumcrustulum@gmail.com>

zebra-chain/src/transaction/hash.rs

@@ -3,7 +3,10 @@ use std::fmt;
 #[cfg(test)]
 use proptest_derive::Arbitrary;
 
-use crate::{serialization::ZcashSerialize, sha256d_writer::Sha256dWriter};
+use crate::{
+    serialization::{SerializationError, ZcashSerialize},
+    sha256d_writer::Sha256dWriter,
+};
 
 use super::Transaction;
 
@@ -33,6 +36,19 @@ impl fmt::Debug for TransactionHash {
     }
 }
 
+impl std::str::FromStr for TransactionHash {
+    type Err = SerializationError;
+    
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let mut bytes = [0; 32];
+        if hex::decode_to_slice(s, &mut bytes[..]).is_err() {
+            Err(SerializationError::Parse("hex decoding error"))
+        } else {
+            Ok(TransactionHash(bytes))
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::io::Write;
@@ -51,7 +67,16 @@ mod tests {
 
         assert_eq!(
             format!("{:?}", hash),
-            "TransactionHash(\"bf46b4b5030752fedac6f884976162bbfb29a9398f104a280b3e34d51b416631\")"
+            r#"TransactionHash("bf46b4b5030752fedac6f884976162bbfb29a9398f104a280b3e34d51b416631")"#
+        );
+    }
+
+    #[test]
+    fn transactionhash_from_str() {
+        let hash:TransactionHash = "bf46b4b5030752fedac6f884976162bbfb29a9398f104a280b3e34d51b416631".parse().unwrap();
+        assert_eq!(
+            format!("{:?}", hash),
+            r#"TransactionHash("bf46b4b5030752fedac6f884976162bbfb29a9398f104a280b3e34d51b416631")"#
         );
     }
 }