2 files changed, 286 insertions, 0 deletions
diff --git a/cmd/generate/mailstuff/mbox.go b/cmd/generate/mailstuff/mbox.go
new file mode 100644
index 0000000..8700c24
--- /dev/null
+++ b/cmd/generate/mailstuff/mbox.go
@@ -0,0 +1,38 @@
+package mailstuff
+
+import (
+	"bytes"
+	"io"
+	"net/mail"
+)
+
+func ReadMBox(r io.Reader) ([]*mail.Message, error) {
+	rest, err := io.ReadAll(r)
+	if err != nil {
+		return nil, err
+	}
+
+	const terminator = "\nFrom "
+
+	var parts [][]byte
+	for {
+		pos := bytes.Index(rest, []byte(terminator))
+		if pos < 0 {
+			parts = append(parts, rest)
+			break
+		}
+		parts = append(parts, rest[:pos+1])
+		rest = rest[pos+1:]
+	}
+
+	ret := make([]*mail.Message, len(parts))
+	for i := range len(parts) {
+		msg, err := mail.ReadMessage(bytes.NewReader(parts[i]))
+		if err != nil {
+			return nil, err
+		}
+		ret[i] = msg
+	}
+
+	return ret, nil
+}
diff --git a/cmd/generate/mailstuff/thread.go b/cmd/generate/mailstuff/thread.go
new file mode 100644
index 0000000..c6fa181
--- /dev/null
+++ b/cmd/generate/mailstuff/thread.go
@@ -0,0 +1,248 @@
+package mailstuff
+
+import (
+	"regexp"
+	"strings"
+)
+
+type set[T comparable] map[T]struct{}
+
+func (s set[T]) Insert(val T) {
+	s[val] = struct{}{}
+}
+
+func (s set[T]) Has(val T) bool {
+	_, ok := s[val]
+	return ok
+}
+
+func (s set[T]) PickOne() T {
+	for v := range s {
+		return v
+	}
+	var zero T
+	return zero
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+// https://www.jwz.org/doc/threading.html
+
+// Definitions /////////////////////////////////////////////////////////////////
+
+type jwzContainer struct {
+	Message  *jwzMessage
+	Parent   *jwzContainer
+	Children set[*jwzContainer]
+}
+
+type jwzMessage struct {
+	Subject    string
+	ID         jwzID
+	References []jwzID
+}
+
+type jwzID string
+
+func (ancestor *jwzContainer) IsAncestorOf(descendent *jwzContainer) bool {
+	if ancestor == descendent {
+		return true
+	}
+	for child := range ancestor.Children {
+		if child.IsAncestorOf(descendent) {
+			return true
+		}
+	}
+	return false
+}
+
+// The Algorithm ///////////////////////////////////////////////////////////////
+
+var jwzSubjectRE = regexp.MustCompile(`^(?:\s*[Rr][Ee](?:\[[0-9]+\])?:)*`)
+
+func jwzThreadMessages(msgs map[jwzID]*jwzMessage) set[*jwzContainer] {
+	idTable := make(map[jwzID]*jwzContainer, len(msgs))
+
+	// 1. For each message
+	for _, msg := range msgs {
+		// A.
+		msgContainer := idTable[msg.ID]
+		if msgContainer != nil && msgContainer.Message == nil {
+			msgContainer.Message = msg
+		} else {
+			msgContainer = &jwzContainer{
+				Message:  msg,
+				Children: make(set[*jwzContainer]),
+			}
+			idTable[msg.ID] = msgContainer
+		}
+		// B.
+		for _, refID := range msg.References {
+			refContainer := idTable[refID]
+			if refContainer == nil {
+				refContainer = &jwzContainer{
+					Children: make(set[*jwzContainer]),
+				}
+				idTable[refID] = refContainer
+			}
+		}
+		for i := 0; i+1 < len(msg.References); i++ {
+			parent := idTable[msg.References[i]]
+			child := idTable[msg.References[i+1]]
+			if !parent.IsAncestorOf(child) && !child.IsAncestorOf(parent) {
+				parent.Children.Insert(child)
+				child.Parent = parent
+			}
+		}
+		// C.
+		if len(msg.References) == 0 {
+			if msgContainer.Parent != nil {
+				delete(msgContainer.Parent.Children, msgContainer)
+			}
+			msgContainer.Parent = nil
+		} else {
+			msgContainer.Parent = idTable[msg.References[len(msg.References)-1]]
+			msgContainer.Parent.Children.Insert(msgContainer)
+		}
+	}
+
+	// 2. Find the root set
+	roots := make(set[*jwzContainer])
+	for _, container := range idTable {
+		if container.Parent == nil {
+			roots.Insert(container)
+		}
+	}
+
+	// 3. Discard id_table
+	idTable = nil
+
+	// 4. Prune empty containers
+	pseudoRoot := &jwzContainer{
+		Children: roots,
+	}
+	for root := range roots {
+		root.Parent = pseudoRoot
+	}
+	var recurse func(*jwzContainer)
+	recurse = func(container *jwzContainer) {
+		// Recurse.  This is a touch complicated because
+		// `recurse(child)` might insert into
+		// `container.Children`, and those insertions might
+		// not be emitted by the range loop
+		for visited := make(set[*jwzContainer]); ; {
+			beforeSize := len(visited)
+			for child := range container.Children {
+				if visited.Has(child) {
+					continue
+				}
+				recurse(child)
+				visited.Insert(child)
+			}
+			if len(visited) == beforeSize {
+				break
+			}
+		}
+		// Main.
+		if container.Message == nil {
+			if len(container.Children) == 0 { // A.
+				delete(container.Parent.Children, container)
+			} else { // B.
+				if len(container.Children) == 1 || container.Parent != pseudoRoot {
+					for child := range container.Children {
+						container.Parent.Children.Insert(child)
+						child.Parent = container.Parent
+					}
+					delete(container.Parent.Children, container)
+				}
+			}
+		}
+	}
+	for root := range roots {
+		recurse(root)
+	}
+	for root := range roots {
+		root.Parent = nil
+	}
+	pseudoRoot = nil
+
+	// 5. Group root set by subject
+	// A.
+	subjectTable := make(map[string]*jwzContainer)
+	// B.
+	for root := range roots {
+		var subject string
+		if root.Message != nil {
+			subject = root.Message.Subject
+		} else {
+			subject = root.Children.PickOne().Message.Subject
+		}
+		prefix := jwzSubjectRE.FindString(subject)
+		subject = strings.TrimSpace(subject[len(prefix):])
+		if subject == "" {
+			continue
+		}
+		if other := subjectTable[subject]; other == nil {
+			subjectTable[subject] = root
+		} else if other.Message == nil {
+			subjectTable[subject] = root
+		} else if jwzSubjectRE.MatchString(other.Message.Subject) && prefix == "" {
+			subjectTable[subject] = root
+		}
+	}
+	// C.
+	for root := range roots {
+		var subject string
+		if root.Message != nil {
+			subject = root.Message.Subject
+		} else {
+			subject = root.Children.PickOne().Message.Subject
+		}
+		prefix := jwzSubjectRE.FindString(subject)
+		subject = strings.TrimSpace(subject[len(prefix):])
+
+		other := subjectTable[subject]
+		if other == nil || other == root {
+			continue
+		}
+
+		switch {
+		case root.Message == nil && other.Message == nil:
+			for child := range root.Children {
+				other.Children.Insert(child)
+				child.Parent = other
+			}
+			delete(roots, root)
+		case (root.Message == nil) != (other.Message == nil):
+			var empty, nonEmpty *jwzContainer
+			if root.Message == nil {
+				empty = root
+				nonEmpty = other
+			} else {
+				empty = other
+				nonEmpty = root
+			}
+			empty.Children.Insert(nonEmpty)
+			nonEmpty.Parent = empty
+		case other.Message != nil && !jwzSubjectRE.MatchString(other.Message.Subject) && prefix != "":
+			other.Children.Insert(root)
+			root.Parent = other
+		// skip the reverse of the above case--it happened implicitly
+		default:
+			newRoot := &jwzContainer{
+				Children: make(set[*jwzContainer], 2),
+			}
+			newRoot.Children.Insert(root)
+			root.Parent = newRoot
+			newRoot.Children.Insert(other)
+			other.Parent = newRoot
+			subjectTable[subject] = newRoot
+			roots.Insert(newRoot)
+			delete(roots, root)
+			delete(roots, other)
+		}
+	}
+
+	// 6. Now you're done threading
+	return roots
+}