nvim: after/plugin/snipcomp: LuaSnip companion plugin for omni completion

The reddit announcement
https://www.reddit.com/r/neovim/comments/rddugs/snipcomplua_luasnip_companion_plugin_for_omni/

nvim/.config/nvim/after/plugin/snipcomp.lua

@@ -0,0 +1,61 @@
+--[[
+  Companion plugin for the LuaSnip snippet engine. Defines a completion
+  function which can be used for built-in insert mode completion (e.g. omni
+  completion). See 'ins-completion' for details. After completion the snippet
+  is expanded.
+  https://github.com/potamides/dotfiles/blob/master/.config/nvim/plugin/snipcomp.lua
+--]]
+
+-- lazy load LuaSnip, only useful when LuaSnip wasn't already loaded elsewhere
+local luasnip = setmetatable({}, {__index = function(_, key) return require("luasnip")[key] end})
+vim.luasnip = {}
+
+-- https://github.com/hrsh7th/nvim-cmp/issues/180#issuecomment-915405589
+require'luasnip'.config.set_config { history = true }
+require('luasnip.loaders.from_vscode').load {}
+
+local function snippet2completion(snippet)
+  return {
+    word      = snippet.trigger,
+    menu      = snippet.name,
+    info      = vim.trim(table.concat(vim.tbl_flatten({snippet.dscr or "", "", snippet:get_docstring()}), "\n")),
+    dup       = true,
+    user_data = "luasnip"
+  }
+end
+
+local function snippetfilter(line_to_cursor, base)
+  return function(s)
+    return not s.hidden and vim.startswith(s.trigger, base) and s.show_condition(line_to_cursor)
+  end
+end
+
+-- Set 'completefunc' or 'omnifunc' to 'v:lua.vim.luasnip.completefunc' to get
+-- completion.
+function vim.luasnip.completefunc(findstart, base)
+  local line_to_cursor = vim.fn.getline("."):sub(1, vim.fn.col("."))
+  if findstart == 1 then
+    return vim.fn.match(line_to_cursor, '\\k*$')
+  end
+
+  local snippets = vim.list_extend(vim.list_slice(luasnip.snippets.all), luasnip.snippets[vim.bo.filetype] or {})
+  snippets = vim.tbl_filter(snippetfilter(line_to_cursor, base), snippets)
+  snippets = vim.tbl_map(snippet2completion, snippets)
+  table.sort(snippets, function(s1, s2) return s1.word < s2.word end)
+  return snippets
+end
+
+function vim.luasnip.completion_expand(item)
+  if item.user_data == "luasnip" and luasnip.expandable() then
+    luasnip.expand()
+  end
+end
+
+vim.cmd([[
+  augroup luasnip_completion_expand
+    autocmd!
+    autocmd CompleteDone * call v:lua.vim.luasnip.completion_expand(v:completed_item)
+  augroup END
+]])
+
+vim.o.completefunc = 'v:lua.vim.luasnip.completefunc'