Add immutable conversation exchange storage

docs/roadmap/implementation.md

@@ -318,6 +318,8 @@ domain representations simply to fan out callbacks.
 
 ## Stage 1 — Implement Immutable Exchange Storage
 
+**Status:** Complete.
+
 **Scope:** internal history value and ownership only.
 
 1. Add the internal exchange representation and construction/flattening helpers

internal/domain/exchange.go

@@ -0,0 +1,60 @@
+package domain
+
+// Exchange is an immutable logical conversation ending in an assistant message.
+type Exchange struct {
+	parent      *Exchange
+	runID       string
+	parentRunID string
+	messages    []RenderedMessage
+	count       int
+}
+
+// CaptureExchange creates a new exchange without modifying its parent.
+func CaptureExchange(parent *Exchange, runID string, messages []RenderedMessage, output string) *Exchange {
+	turn := make([]RenderedMessage, len(messages)+1)
+	copyRenderedMessages(turn, messages)
+	turn[len(messages)] = RenderedMessage{Role: RoleAssistant, Content: output}
+	exchange := &Exchange{parent: parent, runID: runID, messages: turn, count: len(turn)}
+	if parent != nil {
+		exchange.parentRunID = parent.runID
+		exchange.count += parent.count
+	}
+	return exchange
+}
+
+// RunID returns the identity of the run that produced this exchange.
+func (e *Exchange) RunID() string {
+	if e == nil {
+		return ""
+	}
+	return e.runID
+}
+
+// ParentRunID returns the immediate source run identity, if present.
+func (e *Exchange) ParentRunID() string {
+	if e == nil {
+		return ""
+	}
+	return e.parentRunID
+}
+
+// MessageCount returns the number of messages in the logical history.
+func (e *Exchange) MessageCount() int {
+	if e == nil {
+		return 0
+	}
+	return e.count
+}
+
+// Messages returns a detached copy of the complete logical history.
+func (e *Exchange) Messages() []RenderedMessage {
+	if e == nil {
+		return nil
+	}
+	messages := make([]RenderedMessage, e.count)
+	for current := e; current != nil; current = current.parent {
+		start := current.count - len(current.messages)
+		copyRenderedMessages(messages[start:current.count], current.messages)
+	}
+	return messages
+}

internal/domain/exchange_test.go

@@ -0,0 +1,53 @@
+package domain
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+)
+
+func TestExchangeHistoryOwnershipAndBranches(t *testing.T) {
+	control := &CacheControl{Type: CacheControlEphemeral, TTL: "1h"}
+	source := []RenderedMessage{{Role: RoleSystem, Content: "", CacheControl: control}, {Role: RoleUser, Content: "question"}}
+	initial := CaptureExchange(nil, "first", source, " answer ")
+	first := CaptureExchange(initial, "second", []RenderedMessage{{Role: RoleUser, Content: "follow-up"}}, "reply")
+	second := CaptureExchange(initial, "third", []RenderedMessage{{Role: RoleDeveloper, Content: "other"}}, "")
+	source[0].Content = "changed"
+	control.TTL = "changed"
+	got := initial.Messages()
+	want := []RenderedMessage{{Role: RoleSystem, Content: "", CacheControl: &CacheControl{Type: CacheControlEphemeral, TTL: "1h"}}, {Role: RoleUser, Content: "question"}, {Role: RoleAssistant, Content: " answer "}}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("initial messages = %#v, want %#v", got, want)
+	}
+	got[0].CacheControl.TTL = "mutated"
+	got[1].Content = "mutated"
+	if !reflect.DeepEqual(initial.Messages(), want) {
+		t.Fatal("returned messages changed retained history")
+	}
+	if first.RunID() != "second" || first.ParentRunID() != "first" || first.MessageCount() != 5 {
+		t.Fatal("incorrect branch metadata")
+	}
+	if second.ParentRunID() != "first" || second.Messages()[3].Content != "other" || second.Messages()[4].Content != "" {
+		t.Fatal("incorrect sibling history")
+	}
+	if first.Messages()[3].Content != "follow-up" || !reflect.DeepEqual(initial.Messages(), want) {
+		t.Fatal("branches changed each other or their parent")
+	}
+}
+
+func TestExchangeManyTurns(t *testing.T) {
+	var exchange *Exchange
+	const turns = 2000
+	for i := 0; i < turns; i++ {
+		exchange = CaptureExchange(exchange, fmt.Sprint(i), []RenderedMessage{{Role: RoleUser, Content: fmt.Sprint(i)}}, fmt.Sprint(i))
+	}
+	messages := exchange.Messages()
+	if len(messages) != turns*2 || exchange.MessageCount() != len(messages) {
+		t.Fatalf("unexpected history length: %d", len(messages))
+	}
+	for i := 0; i < turns; i++ {
+		if messages[2*i].Content != fmt.Sprint(i) || messages[2*i+1].Role != RoleAssistant || messages[2*i+1].Content != fmt.Sprint(i) {
+			t.Fatalf("unexpected turn %d: %#v", i, messages[2*i:2*i+2])
+		}
+	}
+}