ui: allow thread arrow customization

The thread prefix appearance is inspired by mutt and has been regarded
by some as too wide and not very aesthetically pleasing. Nevertheless,
it has some technical limitations, like not being able to show if a
thread is folded.

Allow for full customisation of the thread prefix by introducing 14 new
config options.

Dirlist is not affected.

Changelog-added: Thread arrow prefixes are now fully configurable.
Co-authored-by: Robin Jarry <robin@jarry.cc>
Signed-off-by: inwit <inwit@sindominio.net>
Tested-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

5 files changed, 311 insertions, 20 deletions

diff --git a/app/dirtree.go b/app/dirtree.go
index 559a717e..ce9bd27e 100644
--- a/app/dirtree.go
+++ b/app/dirtree.go
@@ -347,7 +347,9 @@ func (dt *DirectoryTree) countVisible(list []*types.Thread) (n int) {
 
 func (dt *DirectoryTree) displayText(node *types.Thread) string {
 	elems := strings.Split(dt.treeDirs[getAnyUid(node)], dt.DirectoryList.worker.PathSeparator())
-	return fmt.Sprintf("%s%s%s", threadPrefix(node, false, false), getFlag(node), elems[countLevels(node)])
+	return fmt.Sprintf("%s%s%s",
+		threadPrefix(node, false, false),
+		getFlag(node), elems[countLevels(node)])
 }
 
 func (dt *DirectoryTree) getDirectory(node *types.Thread) string {
diff --git a/app/msglist.go b/app/msglist.go
index 7948e6a1..63089536 100644
--- a/app/msglist.go
+++ b/app/msglist.go
@@ -417,36 +417,89 @@ func unreadInThread(thread *types.Thread, store *lib.MessageStore) (ctr int) {
 	return
 }
 
-func threadPrefix(t *types.Thread, reverse bool, point bool) string {
-	var arrow string
-	var tip string
-	if point {
-		tip = ">"
-	}
-	arrowPrefixSibling := "├─" + tip
-	arrowPrefixReverse := "┌─" + tip
-	arrowPrefixEnd := "└─" + tip
-	arrowStem := "│"
-	arrowIndent := strings.Repeat(" ", runewidth.StringWidth(arrowPrefixSibling)-1)
+func threadPrefix(t *types.Thread, reverse bool, msglist bool) string {
+	uiConfig := SelectedAccountUiConfig()
+	var tip, prefix, firstChild, lastSibling, orphan string
+	if msglist {
+		tip = uiConfig.ThreadPrefixTip
+	} else {
+		threadPrefixSibling := "├─"
+		threadPrefixReverse := "┌─"
+		threadPrefixEnd := "└─"
+		threadStem := "│"
+		threadIndent := strings.Repeat(" ", runewidth.StringWidth(threadPrefixSibling)-1)
+
+		switch {
+		case t.Parent != nil && t.NextSibling != nil:
+			prefix += threadPrefixSibling
+		case t.Parent != nil && reverse:
+			prefix += threadPrefixReverse
+		case t.Parent != nil:
+			prefix += threadPrefixEnd
+		}
+
+		for n := t.Parent; n != nil && n.Parent != nil; n = n.Parent {
+			if n.NextSibling != nil {
+				prefix = threadStem + threadIndent + prefix
+			} else {
+				prefix = " " + threadIndent + prefix
+			}
+		}
+
+		return prefix
+	}
+
+	if reverse {
+		firstChild = uiConfig.ThreadPrefixFirstChildReverse
+		lastSibling = uiConfig.ThreadPrefixLastSiblingReverse
+		orphan = uiConfig.ThreadPrefixOrphanReverse
+	} else {
+		firstChild = uiConfig.ThreadPrefixFirstChild
+		lastSibling = uiConfig.ThreadPrefixLastSibling
+		orphan = uiConfig.ThreadPrefixOrphan
+	}
+
+	var hiddenOffspring bool = t.FirstChild != nil && t.FirstChild.Hidden > 0
+	var parentAndSiblings bool = t.Parent != nil && t.NextSibling != nil
 
 	switch {
-	case t.Parent != nil && t.NextSibling != nil:
-		arrow += arrowPrefixSibling
-	case t.Parent != nil && reverse:
-		arrow += arrowPrefixReverse
+	case parentAndSiblings && hiddenOffspring:
+		prefix = uiConfig.ThreadPrefixHasSiblings +
+			uiConfig.ThreadPrefixFolded
+	case parentAndSiblings && t.FirstChild != nil:
+		prefix = uiConfig.ThreadPrefixHasSiblings +
+			firstChild + tip
+	case parentAndSiblings:
+		prefix = uiConfig.ThreadPrefixHasSiblings +
+			uiConfig.ThreadPrefixLimb +
+			uiConfig.ThreadPrefixUnfolded + tip
+	case t.Parent != nil && hiddenOffspring:
+		prefix = lastSibling + uiConfig.ThreadPrefixFolded
+	case t.Parent != nil && t.FirstChild != nil:
+		prefix = lastSibling + firstChild + tip
+	case t.Parent != nil && t.FirstChild == nil:
+		prefix = lastSibling + uiConfig.ThreadPrefixLimb + tip
 	case t.Parent != nil:
-		arrow += arrowPrefixEnd
+		prefix = lastSibling + uiConfig.ThreadPrefixUnfolded +
+			uiConfig.ThreadPrefixTip
+	case t.Parent == nil && hiddenOffspring:
+		prefix = uiConfig.ThreadPrefixFolded
+	case t.Parent == nil && t.FirstChild != nil:
+		prefix = orphan
+	case t.Parent == nil && t.FirstChild == nil:
+		prefix = uiConfig.ThreadPrefixLone
 	}
 
 	for n := t.Parent; n != nil && n.Parent != nil; n = n.Parent {
 		if n.NextSibling != nil {
-			arrow = arrowStem + arrowIndent + arrow
+			prefix = uiConfig.ThreadPrefixStem +
+				uiConfig.ThreadPrefixIndent + prefix
 		} else {
-			arrow = " " + arrowIndent + arrow
+			prefix = " " + uiConfig.ThreadPrefixIndent + prefix
 		}
 	}
 
-	return arrow
+	return prefix
 }
 
 func sameParent(left, right *types.Thread) bool {
diff --git a/config/aerc.conf b/config/aerc.conf
index 30c3c7bb..ac2a6847 100644
--- a/config/aerc.conf
+++ b/config/aerc.conf
@@ -335,6 +335,104 @@
 # Default: 50ms
 #client-threads-delay=50ms
 
+#
+# Thread prefix customization:
+
+#
+# Customize the thread prefix appearance by selecting the arrow head.
+#
+# Default: ">"
+#thread-prefix-tip = ">"
+
+#
+# Customize the thread prefix appearance by selecting the arrow indentation.
+#
+# Default: " "
+#thread-prefix-indent = " "
+
+#
+# Customize the thread prefix appearance by selecting the vertical extension of
+# the arrow.
+#
+# Default: "│"
+#thread-prefix-stem = "│"
+
+#
+# Customize the thread prefix appearance by selecting the horizontal extension
+# of the arrow.
+#
+# Default: ""
+#thread-prefix-limb = ""
+
+#
+# Customize the thread prefix appearance by selecting the folded thread
+# indicator.
+#
+# Default: ""
+#thread-prefix-folded = ""
+
+#
+# Customize the thread prefix appearance by selecting the unfolded thread
+# indicator.
+#
+# Default: ""
+#thread-prefix-unfolded = ""
+
+#
+# Customize the thread prefix appearance by selecting the first child connector.
+#
+# Default: ""
+#thread-prefix-first-child = ""
+
+#
+# Customize the thread prefix appearance by selecting the connector used if
+# the message has siblings.
+#
+# Default: "├─"
+#thread-prefix-has-siblings = "├─"
+
+#
+# Customize the thread prefix appearance by selecting the connector used if the
+# message has no parents and no children.
+#
+# Default: ""
+#thread-prefix-lone = ""
+
+#
+# Customize the thread prefix appearance by selecting the connector used if the
+# message has no parents and has children.
+#
+# Default: ""
+#thread-prefix-orphan = ""
+
+#
+# Customize the thread prefix appearance by selecting the connector for the last
+# sibling.
+#
+# Default: "└─"
+#thread-prefix-last-sibling = "└─"
+
+#
+# Customize the reversed thread prefix appearance by selecting the connector for
+# the last sibling.
+#
+# Default: "┌─"
+#thread-prefix-last-sibling-reverse = "┌─"
+
+#
+# Customize the reversed thread prefix appearance by selecting the first child
+# connector.
+#
+# Default: ""
+#thread-prefix-first-child-reverse = ""
+
+#
+# Customize the reversed thread prefix appearance by selecting the connector
+# used if the message has no parents and has children.
+#
+# Default: ""
+#thread-prefix-orphan-reverse = ""
+
 [statusline]
 #
 # Describes the format for the status line. This is a comma separated list of
diff --git a/config/ui.go b/config/ui.go
index 83b45230..9a97d4ef 100644
--- a/config/ui.go
+++ b/config/ui.go
@@ -78,6 +78,21 @@ type UIConfig struct {
 	ReverseThreadOrder bool `ini:"reverse-thread-order"`
 	SortThreadSiblings bool `ini:"sort-thread-siblings"`
 
+	ThreadPrefixTip                string `ini:"thread-prefix-tip" default:">"`
+	ThreadPrefixIndent             string `ini:"thread-prefix-indent" default:" "`
+	ThreadPrefixStem               string `ini:"thread-prefix-stem" default:"│"`
+	ThreadPrefixLimb               string `ini:"thread-prefix-limb" default:""`
+	ThreadPrefixFolded             string `ini:"thread-prefix-folded" default:""`
+	ThreadPrefixUnfolded           string `ini:"thread-prefix-unfolded" default:""`
+	ThreadPrefixFirstChild         string `ini:"thread-prefix-first-child" default:""`
+	ThreadPrefixHasSiblings        string `ini:"thread-prefix-has-siblings" default:"├─"`
+	ThreadPrefixLone               string `ini:"thread-prefix-lone" default:""`
+	ThreadPrefixOrphan             string `ini:"thread-prefix-orphan" default:""`
+	ThreadPrefixLastSibling        string `ini:"thread-prefix-last-sibling" default:"└─"`
+	ThreadPrefixLastSiblingReverse string `ini:"thread-prefix-last-sibling-reverse" default:"┌─"`
+	ThreadPrefixFirstChildReverse  string `ini:"thread-prefix-first-child-reverse" default:""`
+	ThreadPrefixOrphanReverse      string `ini:"thread-prefix-orphan-reverse" default:""`
+
 	// Tab Templates
 	TabTitleAccount  *template.Template `ini:"tab-title-account" default:"{{.Account}}"`
 	TabTitleComposer *template.Template `ini:"tab-title-composer" default:"{{.Subject}}"`
diff --git a/doc/aerc-config.5.scd b/doc/aerc-config.5.scd
index 1aa9da5f..9ba7a941 100644
--- a/doc/aerc-config.5.scd
+++ b/doc/aerc-config.5.scd
@@ -469,6 +469,129 @@ These options are configured in the *[ui]* section of _aerc.conf_.
 
 	Default: _false_
 
+## THREAD PREFIX CUSTOMIZATION
+
+You can fully customize the thread arrows appearance, which is defined by the
+following configurable prefix parts:
+
+*thread-prefix-tip* = _<string>_
+	Define the arrow head.
+
+	Default: _">"_
+
+*thread-prefix-indent* = _<string>_
+	Define the arrow indentation.
+
+	Default: _" "_
+
+*thread-prefix-stem* = _<string>_
+	Define the vertical extension of the arrow.
+
+	Default: _"│"_
+
+*thread-prefix-limb* = _<string>_
+	Define the horizontal extension of the arrow.
+
+	Default: _""_
+
+*thread-prefix-folded* = _<string>_
+	Define the folded thread indicator.
+
+	Default: _""_
+
+*thread-prefix-unfolded* = _<string>_
+	Define the unfolded thread indicator.
+
+	Default: _""_
+
+*thread-prefix-first-child* = _<string>_
+	Define the first child connector.
+
+	Default: _""_
+
+*thread-prefix-has-siblings* = _<string>_
+	Define the connector used if the message has siblings.
+
+	Default: _├─_
+
+*thread-prefix-lone* = _<string>_
+	Define the connector used if the message has no parents and no children.
+
+	Default: _""_
+
+*thread-prefix-orphan* = _<string>_
+	Define the connector used if the message has no parents and has children.
+
+	Default: _""_
+
+*thread-prefix-last-sibling* = _<string>_
+	Define the connector for the last sibling.
+
+	Default: _└─_
+
+*thread-prefix-last-sibling-reverse* = _<string>_
+	Define the connector for the last sibling in reversed threads.
+
+	Default: _┌─_
+
+*thread-prefix-first-child-reverse* = _<string>_
+
+	Define the arrow appearance by selecting the first child connector in
+	reversed threads.
+
+	Default: _""_
+
+*thread-prefix-orphan-reverse* = _<string>_
+	Customize the reversed threads arrow appearance by selecting the
+	connector used if the message has no parents and has children.
+
+	Default: _""_
+
+
+Default settings (mutt-style):
+
+	```
+	[PATCH aerc v5] ui: allow thread arrow customisation
+	├─>[aerc/patches] build success
+	├─>Re: [PATCH aerc v5] ui: allow thread arrow customisation
+	├─
+	└─>
+	  ├─>
+	  │ ├─>
+	  │ └─>
+	  │   └─>
+	  └─>
+	```
+
+More compact, rounded threads that are also fold-aware:
+
+	```
+	┌[PATCH aerc v5] ui: allow thread arrow customisation
+	├─[aerc/patches] build success
+	├─Re: [PATCH aerc v5] ui: allow thread arrow customisation
+	├+
+	╰┬
+	 ├┬
+	 │├─
+	 │╰┬
+	 │ ╰─
+	 ╰─
+	```
+
+```
+thread-prefix-tip = ""
+thread-prefix-indent = ""
+thread-prefix-stem = "│"
+thread-prefix-limb = "─"
+thread-prefix-folded = "+"
+thread-prefix-unfolded = ""
+thread-prefix-first-child = "┬"
+thread-prefix-has-siblings = "├"
+thread-prefix-orphan = "┌"
+thread-prefix-lone = " "
+thread-prefix-last-sibling = "╰"
+```
+
 ## CONTEXTUAL UI CONFIGURATION
 
 The UI configuration can be specialized for accounts, specific mail