-- Snippet engine. Owns the always-on machinery (expansion fallback, the completion source, the
-- placeholder-jump keymaps) and turns snippets on per filetype: `add(ft, definitions)` registers a
-- FileType autocmd that exposes the definitions to the completion source. Definitions live in
-- plugin/50-snippet.lua.
--
-- API:
--   add(filetype, definitions) — enable `definitions` (trigger word -> LSP snippet body) for
--                                `filetype` (a string or list of filetypes)

local M = {}

----------------------------------------------------------------------------------------------------
-- Snippet expansion fallback
----------------------------------------------------------------------------------------------------
-- Work around Neovim's snippet grammar rejecting placeholder defaults that mix text with nested
-- tabstops (e.g. lua_ls's `${1:pairs(${2:t})}`): the `any_or_text` rule matches a single node, not
-- a `(any + text)^1` sequence. On parse failure, strip the snippet markup and insert it as plain
-- text instead of throwing. Remove once fixed upstream.
-- The `@as table` cast retypes the module here so overriding `expand` isn't flagged as a duplicate
-- field set.
local snippet = vim.snippet --[[@as table]]
local orig_expand = snippet.expand

-- Reduce LSP snippet syntax to plain text: keep placeholder/variable defaults, the first choice,
-- drop bare tabstops.
local function strip_snippet(s)
	local prev
	repeat
		prev = s
		s = s:gsub("%$(%b{})", function(group)
			local body = group:sub(2, -2)
			local choice = body:match("^%d+|(.*)|$")
			if choice then
				return (choice:gsub(",.*$", "")) -- first choice
			end
			local default = body:match("^[%w_]+:(.*)$")
			if default then
				return default -- ${n:default} / ${VAR:default}
			end
			return "" -- bare ${n} / ${VAR}
		end)
	until s == prev
	s = s:gsub("%$[%w_]+", "") -- bare $n / $VAR
	s = s:gsub("\\([%$}{|,\\])", "%1") -- unescape
	return s
end

-- Insert `text` at the cursor as literal lines, re-indenting continuation lines to the current
-- indent, and leave the cursor at the end of the inserted text.
local function insert_plain(text)
	local win = vim.api.nvim_get_current_win()
	local row, col = unpack(vim.api.nvim_win_get_cursor(win))
	local indent = vim.api.nvim_get_current_line():match("^%s*") or ""
	local lines = vim.split(text, "\n", { plain = true })
	for i = 2, #lines do
		lines[i] = indent .. lines[i]
	end
	vim.api.nvim_buf_set_text(0, row - 1, col, row - 1, col, lines)
	local last = #lines
	local new_row = row - 1 + (last - 1)
	local new_col = last == 1 and col + #lines[1] or #lines[last]
	vim.api.nvim_win_set_cursor(win, { new_row + 1, new_col })
end

-- Expand normally; on a parse failure fall back to inserting the stripped snippet as plain text.
snippet.expand = function(input)
	local ok = pcall(orig_expand, input)
	if not ok then
		insert_plain(strip_snippet(input))
		vim.notify(
			"Snippet parse failed, inserted as plain text: " .. vim.inspect(input),
			vim.log.levels.WARN
		)
	end
end

----------------------------------------------------------------------------------------------------
-- Completion source
----------------------------------------------------------------------------------------------------
-- Exposes `vim.b.snippets` (trigger word -> LSP snippet body) as a 'complete' function source. A
-- filetype opts in through `add()` below, which sets `vim.b.snippets`, points 'completefunc' here,
-- and appends "F" to 'complete'. `dotfiles_snippet_source` must be a global so `v:lua` can reach
-- it; see `:help complete-functions` and `:help v:lua-call`.

-- Render a snippet to preview text: resolve placeholders and their mirrors to the placeholder's
-- default value.
local function render_snippet(s)
	local defaults = {}
	for n, d in s:gmatch("%${(%d+):([^{}]*)}") do
		defaults[n] = d
	end
	local prev
	repeat
		prev = s
		s = s:gsub("%$(%b{})", function(group)
			local body = group:sub(2, -2)
			local choice = body:match("^%d+|(.*)|$")
			if choice then
				return (choice:gsub(",.*$", "")) -- first choice
			end
			local default = body:match("^[%w_]+:(.*)$")
			if default then
				return default -- ${n:default} / ${VAR:default}
			end
			return defaults[body] or "" -- ${n} / ${VAR}
		end)
	until s == prev
	s = s:gsub("%$(%d+)", function(n)
		return defaults[n] or "" -- $n mirror
	end)
	s = s:gsub("%$[%w_]+", "") -- bare $VAR
	s = s:gsub("\\([%$}{|,\\])", "%1") -- unescape
	return s
end

-- 'completefunc' source: report the replaced range on the first pass, then the matching snippet
-- triggers (with rendered preview and the body stashed in user_data for CompleteDone).
function _G.dotfiles_snippet_source(findstart, base)
	local snippets = vim.b.snippets or {}
	local line = vim.api.nvim_get_current_line()
	local col = vim.api.nvim_win_get_cursor(0)[2]
	if findstart == 1 then
		-- Walk back over trigger characters to find the range the match replaces.
		local start = col
		while start > 0 and line:sub(start, start):match("[%w_]") do
			start = start - 1
		end
		return start
	end
	local items = {}
	for trigger, body in pairs(snippets) do
		if vim.startswith(trigger, base) then
			items[#items + 1] = {
				word = trigger,
				kind = "Snippet",
				menu = "[snippet]",
				info = render_snippet(body),
				user_data = { snippet = body },
			}
		end
	end
	return items
end

-- Expand the snippet once one of the items above is accepted (its trigger word is inserted first,
-- so remove it).
vim.api.nvim_create_autocmd("CompleteDone", {
	desc = "Expand accepted snippet completion",
	group = vim.g.dotfiles.augroup,
	callback = function()
		local item = vim.v.completed_item
		local data = item.user_data
		if type(data) ~= "table" or not data.snippet then
			return
		end
		local row, col = unpack(vim.api.nvim_win_get_cursor(0))
		vim.api.nvim_buf_set_text(0, row - 1, col - #item.word, row - 1, col, { "" })
		vim.snippet.expand(data.snippet)
	end,
})

----------------------------------------------------------------------------------------------------
-- Placeholder jumping
----------------------------------------------------------------------------------------------------

-- Jump between snippet placeholders, falling back to native behavior when no snippet is active.
vim.keymap.set({ "i", "s" }, "<C-n>", function()
	return vim.snippet.active({ direction = 1 }) and "<Cmd>lua vim.snippet.jump(1)<CR>" or "<C-n>"
end, { expr = true })
vim.keymap.set({ "i", "s" }, "<C-p>", function()
	return vim.snippet.active({ direction = -1 }) and "<Cmd>lua vim.snippet.jump(-1)<CR>" or "<C-p>"
end, { expr = true })

----------------------------------------------------------------------------------------------------
-- Registration
----------------------------------------------------------------------------------------------------

-- Enable `definitions` for `filetype` (a string or list): on FileType, stash them in
-- `vim.b.snippets`, point 'completefunc' at the source, and add "F" to 'complete' so they surface
-- in the completion menu.
function M.add(filetype, definitions)
	vim.api.nvim_create_autocmd("FileType", {
		desc = "Enable snippets",
		group = vim.g.dotfiles.augroup,
		pattern = filetype,
		callback = function()
			vim.b.snippets = definitions
			vim.bo.completefunc = "v:lua.dotfiles_snippet_source"
			vim.opt_local.complete:append("F")
		end,
	})
end

return M