feat(picker): flexible filename format (#2294)

## Description

This PR builds on top of #743 to add flexible filename formatting in the
picker.

### Changes:
- **Flexible path truncation**: Added support for different truncation
strategies (`left`, `center`, `right`) instead of just a fixed number
- `left`: truncates the beginning of the path (e.g.,
`…/path/to/file.lua`)
- `center`: truncates the middle of the path (default behavior, e.g.,
`~/pro…/file.lua`)
  - `right`: truncates the end of the path (e.g., `~/projects/long…`)
- **Dynamic width calculation**: The filename formatter now adapts to
available window width using a resolve function
- **Enhanced truncate utility**: Updated `M.truncate()` to support
left-side truncation

### Implementation:
The filename formatter now uses a `resolve` function that receives
context including the picker, item, current offset, and maximum
available width. This allows the formatter to make intelligent decisions
about how to display the path based on actual available space.

## Related Issue(s)
- Based on #743
- Addresses dynamic path formatting needs

## Technical Details
The implementation introduces:
1. `snacks.picker.format.ctx` - context passed to resolve functions
2. `snacks.picker.format.resolve` - callback type for dynamic formatting
3. `Snacks.picker.highlight.resolve()` - resolves flex text elements in
highlight arrays

---------

Co-authored-by: qw457812 <37494864+qw457812@users.noreply.github.com>

lua/snacks/picker/config/defaults.lua

@@ -1,7 +1,14 @@
 local M = {}
 
+---@class snacks.picker.format.ctx
+---@field picker snacks.Picker
+---@field item snacks.picker.Item
+---@field offset number
+---@field max_width number
+
+---@alias snacks.picker.format.resolve fun(ctx:snacks.picker.format.ctx):snacks.picker.Highlight[]
 ---@alias snacks.picker.Extmark vim.api.keyset.set_extmark|{col:number, row?:number, field?:string}
----@alias snacks.picker.Text {[1]:string, [2]:string?, virtual?:boolean, field?:string}
+---@alias snacks.picker.Text {[1]:string, [2]:string?, virtual?:boolean, field?:string, resolve?:snacks.picker.format.resolve}
 ---@alias snacks.picker.Highlight snacks.picker.Text|snacks.picker.Extmark
 ---@alias snacks.picker.format fun(item:snacks.picker.Item, picker:snacks.Picker):snacks.picker.Highlight[]
 ---@alias snacks.picker.preview fun(ctx: snacks.picker.preview.ctx):boolean?
@@ -146,7 +153,11 @@ local defaults = {
     },
     file = {
       filename_first = false, -- display filename before the file path
-      truncate = 40, -- truncate the file path to (roughly) this length
+      --- * left: truncate the beginning of the path
+      --- * center: truncate the middle of the path
+      --- * right: truncate the end of the path
+      ---@type "left"|"center"|"right"
+      truncate = "center",
       filename_only = false, -- only show the filename
       icon_width = 2, -- width of the icon (in characters)
       git_status_hl = true, -- use the git status highlight group for the filename

lua/snacks/picker/core/list.lua

@@ -475,6 +475,16 @@ function M:format(item)
 
   -- Add the formatted item
   local line = self.picker.format(item, self.picker)
+
+  ---@type snacks.picker.format.ctx
+  local ctx = {
+    picker = self.picker,
+    item = item,
+    offset = vim.api.nvim_strwidth(text),
+    max_width = vim.api.nvim_win_get_width(self.win.win) - 5,
+  }
+  line = Snacks.picker.highlight.resolve(line, ctx)
+
   while #line > 0 and type(line[#line][1]) == "string" and line[#line][1]:find("^%s*$") do
     table.remove(line)
   end

lua/snacks/picker/format.lua

@@ -41,13 +41,26 @@ end
 
 ---@param item snacks.picker.Item
 function M.filename(item, picker)
+  ---@type snacks.picker.Text[]
+  return {
+    {
+      "",
+      resolve = function(ctx)
+        return M._filename(ctx)
+      end,
+    },
+  }
+end
+
+---@type snacks.picker.format.resolve
+function M._filename(ctx)
+  local picker, item = ctx.picker, ctx.item
   ---@type snacks.picker.Highlight[]
   local ret = {}
   if not item.file then
     return ret
   end
   local path = Snacks.picker.util.path(item) or item.file
-  path = Snacks.picker.util.truncpath(path, picker.opts.formatters.file.truncate or 40, { cwd = picker:cwd() })
   local name, cat = path, "file"
   if item.buf and vim.api.nvim_buf_is_loaded(item.buf) then
     name = vim.bo[item.buf].filetype
@@ -67,6 +80,9 @@ function M.filename(item, picker)
     ret[#ret + 1] = { icon, hl, virtual = true }
   end
 
+  local truncate = picker.opts.formatters.file.truncate
+  path = Snacks.picker.util.truncpath(path, ctx.max_width, { cwd = picker:cwd(), kind = truncate })
+
   local base_hl = item.dir and "SnacksPickerDirectory" or "SnacksPickerFile"
   local function is(prop)
     local it = item

lua/snacks/picker/util/highlight.lua

@@ -202,6 +202,42 @@ function M.winhl(prefix, links)
   return table.concat(ret, ",")
 end
 
+--- Resolves the first flex text in the line.
+---@param line snacks.picker.Highlight[]
+---@param ctx snacks.picker.format.ctx
+function M.resolve(line, ctx)
+  local ret = {} ---@type snacks.picker.Highlight[]
+  local offset = 0
+  local width = 0
+  local resolve ---@type number?
+
+  for t, text in ipairs(line) do
+    local w = M.offset({ text }, { char_idx = true })
+    if not resolve and type(text) == "table" and text.resolve then
+      ---@cast text snacks.picker.Text
+      resolve = t
+    elseif resolve then
+      width = width + w
+    else
+      width = width + w
+      offset = offset + w
+    end
+  end
+
+  if resolve then
+    ctx.offset = offset
+    ctx.max_width = ctx.max_width - width
+    vim.list_extend(ret, line, 1, resolve - 1)
+    offset = M.offset(ret)
+    vim.list_extend(ret, line[resolve].resolve(ctx))
+    local diff = M.offset(ret) - offset
+    vim.list_extend(ret, line, resolve + 1)
+    M.fix_offset(ret, diff, resolve + 1)
+  end
+
+  return ret
+end
+
 ---@param line snacks.picker.Highlight[]
 ---@param opts? {offset?:number}
 function M.to_text(line, opts)
@@ -248,13 +284,16 @@ function M.to_text(line, opts)
 end
 
 ---@param hl snacks.picker.Highlight[]
-function M.fix_offset(hl, offset)
-  for _, t in ipairs(hl) do
-    if t.col then
-      t.col = t.col + offset
-    end
-    if t.end_col then
-      t.end_col = t.end_col + offset
+---@param start_idx? number
+function M.fix_offset(hl, offset, start_idx)
+  for i, t in ipairs(hl) do
+    if start_idx == nil or i >= start_idx then
+      if t.col then
+        t.col = t.col + offset
+      end
+      if t.end_col then
+        t.end_col = t.end_col + offset
+      end
     end
   end
 end

lua/snacks/picker/util/init.lua

@@ -15,9 +15,10 @@ function M.path(item)
 end
 
 ---@param path string
----@param len? number
----@param opts? {cwd?: string}
+---@param len number
+---@param opts? {cwd?: string, kind?: "left" | "center" | "right"}
 function M.truncpath(path, len, opts)
+  opts = opts or {}
   local cwd = svim.fs.normalize(opts and opts.cwd or vim.fn.getcwd(), { _fast = true, expand_env = false })
   local home = svim.fs.normalize("~")
   path = svim.fs.normalize(path, { _fast = true, expand_env = false })
@@ -35,6 +36,12 @@ function M.truncpath(path, len, opts)
   end
   path = path:gsub("/$", "")
 
+  if opts.kind == "left" then
+    return M.truncate(path, len, true)
+  elseif opts.kind == "right" then
+    return M.truncate(path, len, false)
+  end
+
   if vim.api.nvim_strwidth(path) <= len then
     return path
   end
@@ -135,9 +142,12 @@ end
 
 ---@param text string
 ---@param width number
-function M.truncate(text, width)
-  if vim.api.nvim_strwidth(text) > width then
-    return vim.fn.strcharpart(text, 0, width - 1) .. "…"
+---@param left? boolean
+function M.truncate(text, width, left)
+  local tw = vim.api.nvim_strwidth(text)
+  if tw > width then
+    return left and "…" .. vim.fn.strcharpart(text, tw - width + 1, width - 1)
+      or vim.fn.strcharpart(text, 0, width - 1) .. "…"
   end
   return text
 end