feat(filter): oil-like editable filter line (#43)

* feat(filter): oil-like editable filter line with predicate dispatch

Problem: no way to narrow the pending buffer to a subset of tasks
without manual scrolling; filtered-out tasks would be silently deleted
on :w because diff.apply() marks unseen IDs as deleted.

Solution: add a FILTER: line rendered at the top of the buffer when a
filter is active. The line is editable — :w re-parses it and updates
the hidden set. diff.apply() gains a hidden_ids param that prevents
filtered-out tasks from being marked deleted. Predicates: cat:X,
overdue, today, priority (space-separated AND). :Pending filter sets
it programmatically; :Pending filter clear removes it.

* ci: format

lua/pending/buffer.lua

@@ -16,6 +16,10 @@ local current_view = nil
 local _meta = {}
 ---@type table<integer, table<string, boolean>>
 local _fold_state = {}
+---@type string[]
+local _filter_predicates = {}
+---@type table<integer, true>
+local _hidden_ids = {}
 
 ---@return pending.LineMeta[]
 function M.meta()
@@ -37,6 +41,24 @@ function M.current_view_name()
   return current_view
 end
 
+---@return string[]
+function M.filter_predicates()
+  return _filter_predicates
+end
+
+---@return table<integer, true>
+function M.hidden_ids()
+  return _hidden_ids
+end
+
+---@param predicates string[]
+---@param hidden table<integer, true>
+---@return nil
+function M.set_filter(predicates, hidden)
+  _filter_predicates = predicates
+  _hidden_ids = hidden
+end
+
 ---@return nil
 function M.clear_winid()
   task_winid = nil
@@ -124,7 +146,13 @@ local function apply_extmarks(bufnr, line_meta)
   vim.api.nvim_buf_clear_namespace(bufnr, task_ns, 0, -1)
   for i, m in ipairs(line_meta) do
     local row = i - 1
-    if m.type == 'task' then
+    if m.type == 'filter' then
+      local line = vim.api.nvim_buf_get_lines(bufnr, row, row + 1, false)[1] or ''
+      vim.api.nvim_buf_set_extmark(bufnr, task_ns, row, 0, {
+        end_col = #line,
+        hl_group = 'PendingFilter',
+      })
+    elseif m.type == 'task' then
       local due_hl = m.overdue and 'PendingOverdue' or 'PendingDue'
       local virt_parts = {}
       if m.show_category and m.category then
@@ -170,6 +198,7 @@ local function setup_highlights()
   vim.api.nvim_set_hl(0, 'PendingDone', { link = 'Comment', default = true })
   vim.api.nvim_set_hl(0, 'PendingPriority', { link = 'DiagnosticWarn', default = true })
   vim.api.nvim_set_hl(0, 'PendingRecur', { link = 'DiagnosticInfo', default = true })
+  vim.api.nvim_set_hl(0, 'PendingFilter', { link = 'DiagnosticWarn', default = true })
 end
 
 local function snapshot_folds(bufnr)
@@ -225,7 +254,13 @@ function M.render(bufnr)
   current_view = current_view or config.get().default_view
   local view_label = current_view == 'priority' and 'queue' or current_view
   vim.api.nvim_buf_set_name(bufnr, 'pending://' .. view_label)
-  local tasks = store.active_tasks()
+  local all_tasks = store.active_tasks()
+  local tasks = {}
+  for _, task in ipairs(all_tasks) do
+    if not _hidden_ids[task.id] then
+      table.insert(tasks, task)
+    end
+  end
 
   local lines, line_meta
   if current_view == 'priority' then
@@ -234,6 +269,11 @@ function M.render(bufnr)
     lines, line_meta = views.category_view(tasks)
   end
 
+  if #_filter_predicates > 0 then
+    table.insert(lines, 1, 'FILTER: ' .. table.concat(_filter_predicates, ' '))
+    table.insert(line_meta, 1, { type = 'filter' })
+  end
+
   _meta = line_meta
 
   snapshot_folds(bufnr)

lua/pending/diff.lua

@@ -27,8 +27,13 @@ end
 function M.parse_buffer(lines)
   local result = {}
   local current_category = nil
+  local start = 1
+  if lines[1] and lines[1]:match('^FILTER:') then
+    start = 2
+  end
 
-  for i, line in ipairs(lines) do
+  for i = start, #lines do
+    local line = lines[i]
     local id, body = line:match('^/(%d+)/(- %[.%] .*)$')
     if not id then
       body = line:match('^(- %[.%] .*)$')
@@ -65,8 +70,9 @@ function M.parse_buffer(lines)
 end
 
 ---@param lines string[]
+---@param hidden_ids? table<integer, true>
 ---@return nil
-function M.apply(lines)
+function M.apply(lines, hidden_ids)
   local parsed = M.parse_buffer(lines)
   local now = timestamp()
   local data = store.data()
@@ -160,7 +166,7 @@ function M.apply(lines)
   end
 
   for id, task in pairs(old_by_id) do
-    if not seen_ids[id] then
+    if not seen_ids[id] and not (hidden_ids and hidden_ids[id]) then
       task.status = 'deleted'
       task['end'] = now
       task.modified = now

lua/pending/init.lua

@@ -94,6 +94,47 @@ function M.has_due()
   return c.overdue > 0 or c.today > 0
 end
 
+---@param tasks pending.Task[]
+---@param predicates string[]
+---@return table<integer, true>
+local function compute_hidden_ids(tasks, predicates)
+  if #predicates == 0 then
+    return {}
+  end
+  local hidden = {}
+  for _, task in ipairs(tasks) do
+    local visible = true
+    for _, pred in ipairs(predicates) do
+      local cat_val = pred:match('^cat:(.+)$')
+      if cat_val then
+        if task.category ~= cat_val then
+          visible = false
+          break
+        end
+      elseif pred == 'overdue' then
+        if not (task.status == 'pending' and task.due and parse.is_overdue(task.due)) then
+          visible = false
+          break
+        end
+      elseif pred == 'today' then
+        if not (task.status == 'pending' and task.due and parse.is_today(task.due)) then
+          visible = false
+          break
+        end
+      elseif pred == 'priority' then
+        if not (task.priority and task.priority > 0) then
+          visible = false
+          break
+        end
+      end
+    end
+    if not visible then
+      hidden[task.id] = true
+    end
+  end
+  return hidden
+end
+
 ---@return integer bufnr
 function M.open()
   local bufnr = buffer.open()
@@ -102,6 +143,30 @@ function M.open()
   return bufnr
 end
 
+---@param pred_str string
+---@return nil
+function M.filter(pred_str)
+  if pred_str == 'clear' or pred_str == '' then
+    buffer.set_filter({}, {})
+    local bufnr = buffer.bufnr()
+    if bufnr then
+      buffer.render(bufnr)
+    end
+    return
+  end
+  local predicates = {}
+  for word in pred_str:gmatch('%S+') do
+    table.insert(predicates, word)
+  end
+  local tasks = store.active_tasks()
+  local hidden = compute_hidden_ids(tasks, predicates)
+  buffer.set_filter(predicates, hidden)
+  local bufnr = buffer.bufnr()
+  if bufnr then
+    buffer.render(bufnr)
+  end
+end
+
 ---@param bufnr integer
 ---@return nil
 function M._setup_autocmds(bufnr)
@@ -246,13 +311,27 @@ end
 ---@return nil
 function M._on_write(bufnr)
   local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)
+  local predicates = buffer.filter_predicates()
+  if lines[1] and lines[1]:match('^FILTER:') then
+    local pred_str = lines[1]:match('^FILTER:%s*(.*)$') or ''
+    predicates = {}
+    for word in pred_str:gmatch('%S+') do
+      table.insert(predicates, word)
+    end
+    lines = vim.list_slice(lines, 2)
+  elseif #buffer.filter_predicates() > 0 then
+    predicates = {}
+  end
+  local tasks = store.active_tasks()
+  local hidden = compute_hidden_ids(tasks, predicates)
+  buffer.set_filter(predicates, hidden)
   local snapshot = store.snapshot()
   local stack = store.undo_stack()
   table.insert(stack, snapshot)
   if #stack > UNDO_MAX then
     table.remove(stack, 1)
   end
-  diff.apply(lines)
+  diff.apply(lines, hidden)
   M._recompute_counts()
   buffer.render(bufnr)
 end
@@ -718,6 +797,8 @@ function M.command(args)
     M.archive(d)
   elseif cmd == 'due' then
     M.due()
+  elseif cmd == 'filter' then
+    M.filter(rest)
   elseif cmd == 'undo' then
     M.undo_write()
   else

lua/pending/views.lua

@@ -2,7 +2,7 @@ local config = require('pending.config')
 local parse = require('pending.parse')
 
 ---@class pending.LineMeta
----@field type 'task'|'header'|'blank'
+---@field type 'task'|'header'|'blank'|'filter'
 ---@field id? integer
 ---@field due? string
 ---@field raw_due? string