Skeleton for the Coding Agent. (#1)

Reviewed-on: #1
Co-authored-by: Drew Galbraith <drew@tiramisu.one>
Co-committed-by: Drew Galbraith <drew@tiramisu.one>

src/core/history.rs

@@ -0,0 +1,89 @@
+use crate::core::types::ConversationMessage;
+
+/// The in-memory conversation history for the current session.
+///
+/// Stores messages as a flat ordered list. Each [`push`][`Self::push`] appends
+/// one message; [`messages`][`Self::messages`] returns a slice over all of them.
+///
+/// This is a flat list for Phase 1. Phases 3+ will introduce a tree structure
+/// (each event carrying a `parent_id`) to support conversation branching and
+/// sub-agent threads. The flat model is upward-compatible: a tree is just a
+/// linear chain of parent IDs when there is no branching.
+pub struct ConversationHistory {
+    messages: Vec<ConversationMessage>,
+}
+
+impl ConversationHistory {
+    /// Create an empty history.
+    pub fn new() -> Self {
+        Self {
+            messages: Vec::new(),
+        }
+    }
+
+    /// Append one message to the end of the history.
+    pub fn push(&mut self, message: ConversationMessage) {
+        self.messages.push(message);
+    }
+
+    /// Return the full ordered message list, oldest-first.
+    ///
+    /// This slice is what gets serialised and sent to the provider on each
+    /// turn -- the provider needs the full prior context to generate a coherent
+    /// continuation.
+    pub fn messages(&self) -> &[ConversationMessage] {
+        &self.messages
+    }
+}
+
+impl Default for ConversationHistory {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::core::types::Role;
+
+    #[test]
+    fn new_history_is_empty() {
+        let history = ConversationHistory::new();
+        assert!(history.messages().is_empty());
+    }
+
+    #[test]
+    fn push_and_read_roundtrip() {
+        let mut history = ConversationHistory::new();
+        history.push(ConversationMessage {
+            role: Role::User,
+            content: "hello".to_string(),
+        });
+        history.push(ConversationMessage {
+            role: Role::Assistant,
+            content: "hi there".to_string(),
+        });
+
+        let msgs = history.messages();
+        assert_eq!(msgs.len(), 2);
+        assert_eq!(msgs[0].role, Role::User);
+        assert_eq!(msgs[0].content, "hello");
+        assert_eq!(msgs[1].role, Role::Assistant);
+        assert_eq!(msgs[1].content, "hi there");
+    }
+
+    #[test]
+    fn messages_preserves_insertion_order() {
+        let mut history = ConversationHistory::new();
+        for i in 0u32..5 {
+            history.push(ConversationMessage {
+                role: Role::User,
+                content: format!("msg {i}"),
+            });
+        }
+        for (i, msg) in history.messages().iter().enumerate() {
+            assert_eq!(msg.content, format!("msg {i}"));
+        }
+    }
+}