EBML: Implement string parsing

Signed-off-by: Serial <69764315+Serial-ATA@users.noreply.github.com>

Author: Serial-ATA

src/ebml/element_reader.rs

@@ -1,6 +1,6 @@
 use crate::ebml::vint::VInt;
 use crate::error::Result;
-use crate::macros::decode_err;
+use crate::macros::{decode_err, try_vec};
 
 use std::io::Read;
 
@@ -342,8 +342,21 @@ where
 		})
 	}
 
-	pub(crate) fn read_string(&mut self) -> Result<String> {
-		todo!()
+	pub(crate) fn read_string(&mut self, element_length: u64) -> Result<String> {
+		// https://www.rfc-editor.org/rfc/rfc8794.html#section-7.4
+		// A String Element MUST declare a length in octets from zero to VINTMAX
+		let mut content = try_vec![0; element_length as usize];
+		self.reader.read_exact(&mut content)?;
+
+		// https://www.rfc-editor.org/rfc/rfc8794.html#section-13
+		// Null Octets, which are octets with all bits set to zero,
+		// MAY follow the value of a String Element or UTF-8 Element to serve as a terminator.
+		if let Some(i) = content.iter().rposition(|x| *x != 0) {
+			let new_len = i + 1;
+			content.truncate(new_len);
+		}
+
+		String::from_utf8(content).map_err(Into::into)
 	}
 
 	pub(crate) fn read_utf8(&mut self) -> Result<String> {

src/ebml/read.rs

@@ -119,7 +119,9 @@ where
 			ElementIdent::EBMLReadVersion => {
 				properties.header.read_version = element_reader.read_unsigned_int(size)?
 			},
-			ElementIdent::DocType => properties.header.doc_type = element_reader.read_string()?,
+			ElementIdent::DocType => {
+				properties.header.doc_type = element_reader.read_string(size)?
+			},
 			ElementIdent::DocTypeVersion => {
 				properties.header.doc_type_version = element_reader.read_unsigned_int(size)?
 			},