From c03412837d9ed5e0ae60c50794107ff64343d41c Mon Sep 17 00:00:00 2001
From: Barrett Ruth <br.barrettruth@gmail.com>
Date: Tue, 24 Feb 2026 15:09:36 -0500
Subject: [PATCH] feat(diff): add buffer-to-store diff algorithm

Problem: need to reconcile buffer edits against the JSON store
on :w, handling creates, deletes, updates, reorders, and
duplicate IDs from yank/paste.

Solution: add diff module that parses buffer lines, matches
against stored tasks by ID, creates new tasks for unknown or
duplicate IDs, marks removed tasks as deleted, and updates
changed fields.
---
 lua/todo/diff.lua | 149 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 149 insertions(+)
 create mode 100644 lua/todo/diff.lua

diff --git a/lua/todo/diff.lua b/lua/todo/diff.lua
new file mode 100644
index 0000000..902c064
--- /dev/null
+++ b/lua/todo/diff.lua
@@ -0,0 +1,149 @@
+local config = require('todo.config')
+local parse = require('todo.parse')
+local store = require('todo.store')
+
+---@class todo.ParsedEntry
+---@field type 'task'|'header'|'blank'
+---@field id? integer
+---@field description? string
+---@field priority? integer
+---@field category? string
+---@field due? string
+---@field lnum integer
+
+---@class todo.diff
+local M = {}
+
+---@return string
+local function timestamp()
+  return os.date('!%Y-%m-%dT%H:%M:%SZ')
+end
+
+---@param lines string[]
+---@return todo.ParsedEntry[]
+function M.parse_buffer(lines)
+  local result = {}
+  local current_category = nil
+
+  for i, line in ipairs(lines) do
+    if line == '' then
+      table.insert(result, { type = 'blank', lnum = i })
+    elseif line:match('^%S') then
+      current_category = line
+      table.insert(result, { type = 'header', category = line, lnum = i })
+    else
+      local id, body = line:match('^/(%d+)/(  .+)$')
+      if not id then
+        body = line:match('^(  .+)$')
+      end
+      if body then
+        local stripped = body:match('^  (.+)$') or body
+        local priority = 0
+        if stripped:match('^! ') then
+          priority = 1
+          stripped = stripped:sub(3)
+        end
+        local description, metadata = parse.body(stripped)
+        if description and description ~= '' then
+          table.insert(result, {
+            type = 'task',
+            id = id and tonumber(id) or nil,
+            description = description,
+            priority = priority,
+            category = metadata.cat or current_category or config.get().default_category,
+            due = metadata.due,
+            lnum = i,
+          })
+        end
+      end
+    end
+  end
+
+  return result
+end
+
+---@param lines string[]
+function M.apply(lines)
+  local parsed = M.parse_buffer(lines)
+  local now = timestamp()
+  local data = store.data()
+
+  local old_by_id = {}
+  for _, task in ipairs(data.tasks) do
+    if task.status ~= 'deleted' then
+      old_by_id[task.id] = task
+    end
+  end
+
+  local seen_ids = {}
+  local order_counter = 0
+
+  for _, entry in ipairs(parsed) do
+    if entry.type ~= 'task' then
+      goto continue
+    end
+
+    order_counter = order_counter + 1
+
+    if entry.id and old_by_id[entry.id] then
+      if seen_ids[entry.id] then
+        store.add({
+          description = entry.description,
+          category = entry.category,
+          priority = entry.priority,
+          due = entry.due,
+          order = order_counter,
+        })
+      else
+        seen_ids[entry.id] = true
+        local task = old_by_id[entry.id]
+        local changed = false
+        if task.description ~= entry.description then
+          task.description = entry.description
+          changed = true
+        end
+        if task.category ~= entry.category then
+          task.category = entry.category
+          changed = true
+        end
+        if task.priority ~= entry.priority then
+          task.priority = entry.priority
+          changed = true
+        end
+        if task.due ~= entry.due then
+          task.due = entry.due
+          changed = true
+        end
+        if task.order ~= order_counter then
+          task.order = order_counter
+          changed = true
+        end
+        if changed then
+          task.modified = now
+        end
+      end
+    else
+      store.add({
+        description = entry.description,
+        category = entry.category,
+        priority = entry.priority,
+        due = entry.due,
+        order = order_counter,
+      })
+    end
+
+    ::continue::
+  end
+
+  for id, task in pairs(old_by_id) do
+    if not seen_ids[id] then
+      task.status = 'deleted'
+      task['end'] = now
+      task.modified = now
+    end
+  end
+
+  store.save()
+end
+
+return M