track the time offset during parsing

3 files changed, 26 insertions, 11 deletions

diff --git a/src/frame.rs b/src/frame.rs
index 17c6c5a..c617056 100644
--- a/src/frame.rs
+++ b/src/frame.rs
@@ -1,8 +1,9 @@
 /// Represents a single ttyrec frame.
 ///
-/// Ttyrec files are a raw concatenation of frames. Note that the `time` field
-/// in each frame is the time since the start of the entire file, and it is
-/// invalid for the `time` fields in a ttyrec file to be decreasing.
+/// Ttyrec files are a raw concatenation of frames. Note that the timestamps
+/// in the frame are absolute timestamps, and only the difference between them
+/// is relevant. The base timestamp can be anything (common choices are 0 or
+/// the actual time that the frame was generated).
 ///
 /// Frame objects are typically created via the `Creator`, `Parser`, or
 /// `Reader` classes.
diff --git a/src/parser.rs b/src/parser.rs
index 7acbbe9..45b900e 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -25,10 +25,11 @@ impl Header {
 /// call the `add_bytes` method to add them to the internal buffer. At any
 /// point, you can call `next_frame` to then return the next complete frame if
 /// one has been read.
-#[derive(Debug, Clone)]
+#[derive(Debug, Default, Clone)]
 pub struct Parser {
     reading: std::collections::VecDeque<u8>,
     read_state: Option<Header>,
+    offset: Option<std::time::Duration>,
 }
 
 impl Parser {
@@ -91,16 +92,22 @@ impl Parser {
         let time = header.time();
 
         self.read_state = None;
+        if self.offset.is_none() {
+            self.offset = Some(time);
+        }
         Some(crate::frame::Frame { time, data })
     }
-}
 
-impl Default for Parser {
-    fn default() -> Self {
-        Self {
-            reading: std::collections::VecDeque::new(),
-            read_state: None,
-        }
+    /// How much the timestamps in this file should be offset by.
+    ///
+    /// Ttyrec files are allowed to be generated by just inserting the current
+    /// absolute timestamp as the header. This means that during playback, we
+    /// need to take the timestamp of the first frame as the start time, and
+    /// each frame timestamp after that should be offset by that same amount.
+    ///
+    /// Returns `None` if no frames have been read yet.
+    pub fn offset(&self) -> Option<std::time::Duration> {
+        self.offset
     }
 }
 
diff --git a/src/reader.rs b/src/reader.rs
index fd87cd5..ff5f7f9 100644
--- a/src/reader.rs
+++ b/src/reader.rs
@@ -46,4 +46,11 @@ impl<R: tokio::io::AsyncRead> Reader<R> {
             }
         }
     }
+
+    /// How much the timestamps in this file should be offset by.
+    ///
+    /// See `Parser::offset`.
+    pub fn offset(&self) -> Option<std::time::Duration> {
+        self.parser.offset()
+    }
 }