# References Header Threading Implementation Plan

> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.

**Goal:** Add `References` header fallback so threading resolves correctly when `In-Reply-To` is absent.

**Architecture:** Extract a `resolveThreadID` helper from `processMessage` that accepts a DB lookup function and parsed headers. This makes the threading logic testable without needing IMAP client infrastructure. The helper tries `In-Reply-To` first, then iterates `References` last-to-first on miss.

**Tech Stack:** Go, `github.com/emersion/go-message/mail`, `gorm.io/gorm`.

---

### Task 1: Write failing tests for resolveThreadID

**Files:**
- Create: `internal/mail/resolve_thread_test.go`
- Modify: `internal/mail/imap.go` (add `resolveThreadID` stub)

- [ ] **Step 1: Write failing tests**

Create `internal/mail/resolve_thread_test.go`:

```go
package mail

import (
	"errors"
	"testing"

	"thuanle.me/claw-email-bridge/internal/database"
)

func TestResolveThreadID_NoHeaders_ReturnsMessageID(t *testing.T) {
	got := resolveThreadID("msg-1", "", "", nilLookup)
	if got != "msg-1" {
		t.Fatalf("expected msg-1, got %s", got)
	}
}

func TestResolveThreadID_InReplyToMatches_ReturnsParentThreadID(t *testing.T) {
	lookup := func(messageID string) (*database.Task, error) {
		if messageID == "<parent>" {
			return &database.Task{ThreadID: "thread-abc"}, nil
		}
		return nil, nil
	}
	got := resolveThreadID("msg-2", "<parent>", "", lookup)
	if got != "thread-abc" {
		t.Fatalf("expected thread-abc, got %s", got)
	}
}

func TestResolveThreadID_ReferencesOnly_LastIDMatches(t *testing.T) {
	lookup := func(messageID string) (*database.Task, error) {
		if messageID == "<C>" {
			return &database.Task{ThreadID: "thread-c"}, nil
		}
		return nil, nil
	}
	got := resolveThreadID("msg-3", "", "<A> <B> <C>", lookup)
	if got != "thread-c" {
		t.Fatalf("expected thread-c, got %s", got)
	}
}

func TestResolveThreadID_ReferencesOnly_MiddleIDMatches(t *testing.T) {
	lookup := func(messageID string) (*database.Task, error) {
		if messageID == "<B>" {
			return &database.Task{ThreadID: "thread-b"}, nil
		}
		return nil, nil
	}
	got := resolveThreadID("msg-4", "", "<A> <B> <C>", lookup)
	if got != "thread-b" {
		t.Fatalf("expected thread-b, got %s", got)
	}
}

func TestResolveThreadID_ReferencesOnly_NoMatch_ReturnsMessageID(t *testing.T) {
	got := resolveThreadID("msg-5", "", "<X> <Y>", nilLookup)
	if got != "msg-5" {
		t.Fatalf("expected msg-5, got %s", got)
	}
}

func TestResolveThreadID_InReplyToMiss_ReferencesMatches(t *testing.T) {
	lookup := func(messageID string) (*database.Task, error) {
		if messageID == "<ref>" {
			return &database.Task{ThreadID: "thread-ref"}, nil
		}
		return nil, nil
	}
	got := resolveThreadID("msg-6", "<unknown>", "<ref>", lookup)
	if got != "thread-ref" {
		t.Fatalf("expected thread-ref, got %s", got)
	}
}

func TestResolveThreadID_LookupError_ReturnsMessageID(t *testing.T) {
	lookup := func(messageID string) (*database.Task, error) {
		return nil, errors.New("db error")
	}
	got := resolveThreadID("msg-7", "<parent>", "", lookup)
	if got != "msg-7" {
		t.Fatalf("expected msg-7, got %s", got)
	}
}

var nilLookup = func(string) (*database.Task, error) { return nil, nil }
```

- [ ] **Step 2: Run tests to verify they fail**

Run: `go test ./internal/mail -run ResolveThreadID -v`
Expected: FAIL — `resolveThreadID` not defined.

- [ ] **Step 3: Commit test file**

```bash
git add internal/mail/resolve_thread_test.go
git commit -m "test(mail): add resolveThreadID tests (RED)"
```

### Task 2: Implement resolveThreadID

**Files:**
- Modify: `internal/mail/imap.go`

- [ ] **Step 1: Add resolveThreadID function**

Add to `internal/mail/imap.go`, before the `processMessage` function:

```go
// resolveThreadID determines the thread_id for a message.
// It tries In-Reply-To first, then falls back to References (last-to-first).
// Returns the message's own ID if no parent is found.
func resolveThreadID(messageID, inReplyTo, references string, lookup func(string) (*database.Task, error)) string {
	if inReplyTo != "" {
		if parent, err := lookup(inReplyTo); err == nil && parent != nil {
			return parent.ThreadID
		}
	}

	for _, refID := range parseReferences(references) {
		if parent, err := lookup(refID); err == nil && parent != nil {
			return parent.ThreadID
		}
	}

	return messageID
}

// parseReferences splits a References header value into individual message IDs,
// returned in reverse order (most recent first).
func parseReferences(header string) []string {
	if header == "" {
		return nil
	}
	raw := strings.Fields(header)
	for i := range raw {
		raw[i] = strings.TrimSpace(raw[i])
	}
	// Reverse: last (most recent) first.
	for i, j := 0, len(raw)-1; i < j; i, j = i+1, j-1 {
		raw[i], raw[j] = raw[j], raw[i]
	}
	return raw
}
```

- [ ] **Step 2: Run tests to verify they pass**

Run: `go test ./internal/mail -run ResolveThreadID -v`
Expected: PASS.

- [ ] **Step 3: Commit**

```bash
git add internal/mail/imap.go
git commit -m "feat(mail): implement resolveThreadID with References fallback"
```

### Task 3: Wire resolveThreadID into processMessage

**Files:**
- Modify: `internal/mail/imap.go`

- [ ] **Step 1: Replace threading block in processMessage**

Replace the block at `imap.go:333-369` (the threading section) with:

```go
		// 4. Threading: determine thread_id.
		threadID := messageID
		inReplyTo := ""
		references := ""

		// Parse body to get In-Reply-To / References headers and plain text.
		bodyPlain := ""
		rawBody := buf.FindBodySection(bodySection)
		if rawBody != nil {
			mr, err := gomessage.CreateReader(io.NopCloser(strings.NewReader(string(rawBody))))
			if err == nil {
				if irt, err := mr.Header.Text("In-Reply-To"); err == nil && irt != "" {
					inReplyTo = strings.TrimSpace(irt)
				}
				if refs, err := mr.Header.Text("References"); err == nil && refs != "" {
					references = refs
				}

				for {
					p, err := mr.NextPart()
					if err != nil {
						break
					}
					if _, ok := p.Header.(*gomessage.InlineHeader); ok {
						b, _ := io.ReadAll(p.Body)
						if bodyPlain == "" {
							bodyPlain = string(b)
						}
					}
				}
			}
		}

		threadID = resolveThreadID(messageID, inReplyTo, references, func(mid string) (*database.Task, error) {
			return database.FindByMessageID(w.db, mid)
		})
```

- [ ] **Step 2: Run full test suite**

Run: `go test ./internal/mail -v`
Expected: PASS (all ResolveThreadID + existing dial tests).

- [ ] **Step 3: Run full project tests**

Run: `go test ./...`
Expected: PASS.

- [ ] **Step 4: Commit**

```bash
git add internal/mail/imap.go
git commit -m "feat(mail): wire resolveThreadID into processMessage"
```

### Task 4: Final verification

- [ ] **Step 1: Run full validation**

Run: `go build ./... && go vet ./... && go test ./... -v`
Expected: all PASS.

- [ ] **Step 2: Push and create PR**

```bash
git push -u origin <branch>
```

PR body must include `Fixes #4`.