--- Frontmatter as an edit surface (tech-spec §8.3). The daemon renders an
--- editable YAML block atop a node's body on read (`node.get {frontmatter}`)
--- and strips it on write; this module is the **smart-client** half: parse the
--- buffer's frontmatter and translate each changed field into the right
--- structured RPC (title→rename, attention→set_attention, dates→set_schedule,
--- project→set_project, tags→tag.add/remove). The body itself rides `node.update`.
local rpc = require("heph.rpc")

local M = {}

local function trim(s)
  return (s:gsub("^%s+", ""):gsub("%s+$", ""))
end

--- Unquote a YAML scalar (`"…"` with `\"`/`\\` escapes), else return it trimmed.
local function unquote(s)
  s = trim(s)
  local inner = s:match('^"(.*)"$')
  if inner then
    return (inner:gsub('\\"', '"'):gsub("\\\\", "\\"))
  end
  return s
end

--- Parse a YAML flow sequence `[a, b]` (or `[]`) into a list of scalars.
local function parse_flow_seq(s)
  local inner = s:match("^%[(.*)%]$")
  if not inner then
    return {}
  end
  local out = {}
  for item in (inner .. ","):gmatch("(.-),") do
    item = trim(item)
    if item ~= "" then
      out[#out + 1] = unquote(item)
    end
  end
  return out
end

--- Parse a leading `---` frontmatter block. Returns `(fm, body)` where `fm` is a
--- table of string fields plus `tags` (a list), or `(nil, text)` when there is
--- no conforming block (so a frontmatter-less buffer never looks like metadata).
function M.parse(text)
  if text:sub(1, 4) ~= "---\n" then
    return nil, text
  end
  local lines = vim.split(text, "\n", { plain = true })
  local close
  for i = 2, #lines do
    if lines[i] == "---" then
      close = i
      break
    end
  end
  if not close then
    return nil, text
  end
  local fm = { tags = {} }
  for j = 2, close - 1 do
    local key, val = lines[j]:match("^([%w_%-]+):%s*(.*)$")
    if key == "tags" then
      fm.tags = parse_flow_seq(val)
    elseif key then
      fm[key] = unquote(val)
    end
  end
  local body = table.concat({ unpack(lines, close + 1) }, "\n")
  return fm, body
end

--- A `YYYY-MM-DD` string → epoch ms at **local midnight** (matching the
--- daemon's `datespec`). Returns `nil, err` on a malformed date.
local function date_to_ms(s)
  local y, m, d = tostring(s):match("^(%d%d%d%d)%-(%d%d)%-(%d%d)$")
  if not y then
    return nil, "expected a YYYY-MM-DD date, got " .. tostring(s)
  end
  return os.time({ year = tonumber(y), month = tonumber(m), day = tonumber(d), hour = 0, min = 0, sec = 0 }) * 1000
end

--- Whitespace-prefixed inline `#hashtags` in `body`, de-duplicated in order.
--- A markdown heading (`# Title`, `## foo`) has a space after the `#`, so it
--- never matches. (Scanned across the whole body, code fences included — a
--- pragmatic v1 simplification.)
function M.hashtags(body)
  local seen, out = {}, {}
  for tag in (" " .. (body or "")):gmatch("%s#([%w_%-]+)") do
    if not seen[tag] then
      seen[tag] = true
      out[#out + 1] = tag
    end
  end
  return out
end

--- Union of two scalar lists, order-stable, de-duplicated.
local function union(a, b)
  local seen, out = {}, {}
  for _, list in ipairs({ a or {}, b or {} }) do
    for _, x in ipairs(list) do
      if not seen[x] then
        seen[x] = true
        out[#out + 1] = x
      end
    end
  end
  return out
end

--- Set difference of two scalar lists → (added, removed).
local function set_diff(old, new)
  local oldset, newset = {}, {}
  for _, x in ipairs(old or {}) do
    oldset[x] = true
  end
  for _, x in ipairs(new or {}) do
    newset[x] = true
  end
  local added, removed = {}, {}
  for x in pairs(newset) do
    if not oldset[x] then
      added[#added + 1] = x
    end
  end
  for x in pairs(oldset) do
    if not newset[x] then
      removed[#removed + 1] = x
    end
  end
  return added, removed
end

--- Apply the difference between `canonical` (rendered on read) and the buffer's
--- `fm` by issuing the matching RPCs. `node_id` is the opened node (title/tags
--- target); task scalars route to the owning task (`fm.task`). Inline
--- `#hashtags` in `body` are unioned into the desired tag set (so they manage
--- tags too); `fm.tags` is updated to that union so the caller caches it as the
--- new canonical. Raises on any RPC error (a mistyped `state`, an unknown
--- `project`).
function M.apply(node_id, canonical, fm, body)
  canonical = canonical or {}

  -- Tags on the opened node = the frontmatter list ∪ inline #hashtags.
  fm.tags = union(fm.tags, M.hashtags(body))
  local added, removed = set_diff(canonical.tags, fm.tags)
  for _, t in ipairs(added) do
    rpc.call("tag.add", { node_id = node_id, tag = t })
  end
  for _, t in ipairs(removed) do
    rpc.call("tag.remove", { node_id = node_id, tag = t })
  end

  -- Task scalars route to the owning task, if this node is or backs one.
  local task = fm.task or canonical.task
  if task then
    if fm.attention and fm.attention ~= canonical.attention then
      rpc.call("task.set_attention", { id = task, attention = fm.attention })
    end

    -- Schedule (double-option): a present value sets, a removed line clears.
    local patch, changed = { id = task }, false
    local function sched(field)
      if fm[field] == canonical[field] then
        return
      end
      changed = true
      if fm[field] == nil or fm[field] == "" then
        patch[field] = vim.NIL -- cleared
      elseif field == "recurrence" then
        patch[field] = fm[field] -- a raw RRULE
      else
        local ms, err = date_to_ms(fm[field])
        if not ms then
          error("heph: " .. err)
        end
        patch[field] = ms
      end
    end
    sched("do_date")
    sched("late_on")
    sched("recurrence")
    if changed then
      rpc.call("task.set_schedule", patch)
    end

    if fm.state and fm.state ~= canonical.state then
      rpc.call("task.set_state", { id = task, state = fm.state }) -- a typo → rpc error
    end

    if fm.project ~= canonical.project then
      local pid = vim.NIL -- removed / blank → unfile
      if fm.project and fm.project ~= "" then
        local node = rpc.call("node.resolve", { title = fm.project })
        if not node then
          error("heph: no node named '" .. fm.project .. "' to file under")
        end
        pid = node.id
      end
      rpc.call("task.set_project", { id = task, project_id = pid })
    end
  end
end

return M