Array deserialization: optimize with for (not collect)

Author: optout21

lightning-invoice/src/de.rs

@@ -50,13 +50,24 @@ impl<const N: usize> FromBase32 for [u8; N] {
 	type Err = Bolt11ParseError;
 
 	fn from_base32(data: &[Fe32]) -> Result<Self, Self::Err> {
-		data.iter().copied().fes_to_bytes().collect::<Vec<_>>().try_into().map_err(|_| {
-			Bolt11ParseError::InvalidSliceLength(
+		let mut res_arr = [0; N];
+		// Do in a for loop to place in the array directly, not using `collect`
+		let mut idx = 0;
+		for elem in data.iter().copied().fes_to_bytes() {
+			if idx >= N {
+				break; // too many inputs, stop early, OK
+			}
+			res_arr[idx] = elem;
+			idx += 1;
+		}
+		if idx != N {
+			return Err(Bolt11ParseError::InvalidSliceLength(
 				data.len(),
 				(N * 8 + 4) / 5,
 				"<[u8; N]>",
-			)
-		})
+			));
+		}
+		Ok(res_arr)
 	}
 }
 

lightning-invoice/src/test_ser_de.rs

@@ -59,15 +59,34 @@ fn array_u8() {
 
 #[test]
 fn array_u8_error_invalid_length() {
+	// correct len -- 5 fe32 -> 3 bytes
 	assert_eq!(
 		<[u8; 3]>::from_base32(
-			&"qqqs".to_string().chars().map(|c| Fe32::from_char(c).unwrap()).collect::<Vec<_>>()[..]
+			&"pqqql".to_string().chars().map(|c| Fe32::from_char(c).unwrap()).collect::<Vec<_>>()[..]
+		)
+		.unwrap(),
+		[8, 0, 15]
+	);
+
+	// input too short
+	assert_eq!(
+		<[u8; 3]>::from_base32(
+			&"pqql".to_string().chars().map(|c| Fe32::from_char(c).unwrap()).collect::<Vec<_>>()[..]
 		)
 		.err()
 		.unwrap()
 		.to_string(),
 		"Slice had length 4 instead of 5 for element <[u8; N]>"
 	);
+
+	// input too long
+	assert_eq!(
+		<[u8; 3]>::from_base32(
+			&"pqqqql".to_string().chars().map(|c| Fe32::from_char(c).unwrap()).collect::<Vec<_>>()[..]
+		)
+		.unwrap(),
+		[8, 0, 0]
+	);
 }
 
 #[test]