1 files changed, 143 insertions, 0 deletions
diff --git a/runtime/lua/vim/lsp/_snippet_grammar.lua b/runtime/lua/vim/lsp/_snippet_grammar.lua
new file mode 100644
index 0000000000..86f00fea51
--- /dev/null
+++ b/runtime/lua/vim/lsp/_snippet_grammar.lua
@@ -0,0 +1,143 @@
+--- Grammar for LSP snippets, based on https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#snippet_syntax
+
+local lpeg = vim.lpeg
+local P, S, R, V = lpeg.P, lpeg.S, lpeg.R, lpeg.V
+local C, Cg, Ct = lpeg.C, lpeg.Cg, lpeg.Ct
+
+local M = {}
+
+local alpha = R('az', 'AZ')
+local backslash = P('\\')
+local colon = P(':')
+local dollar = P('$')
+local int = R('09') ^ 1
+local l_brace, r_brace = P('{'), P('}')
+local pipe = P('|')
+local slash = P('/')
+local underscore = P('_')
+local var = Cg((underscore + alpha) * ((underscore + alpha + int) ^ 0), 'name')
+local format_capture = Cg(int / tonumber, 'capture')
+local format_modifier = Cg(P('upcase') + P('downcase') + P('capitalize'), 'modifier')
+local tabstop = Cg(int / tonumber, 'tabstop')
+
+--- Returns a function that unescapes occurrences of "special" characters.
+---
+--- @param special string
+--- @return fun(match: string): string
+local function escape_text(special)
+  return function(match)
+    local escaped = match:gsub('\\(.)', function(c)
+      return special:find(c) and c or '\\' .. c
+    end)
+    return escaped
+  end
+end
+
+-- Text nodes match "any character", but $, \, and } must be escaped.
+local escapable = '$}\\'
+local text = (backslash * S(escapable)) + (P(1) - S(escapable))
+local text_0, text_1 = (text ^ 0) / escape_text(escapable), text ^ 1
+-- Within choice nodes, \ also escapes comma and pipe characters.
+local choice_text = C(((backslash * S(escapable .. ',|')) + (P(1) - S(escapable .. ',|'))) ^ 1)
+  / escape_text(escapable .. ',|')
+local if_text, else_text = Cg(text_0, 'if_text'), Cg(text_0, 'else_text')
+-- Within format nodes, make sure we stop at /
+local format_text = C(((backslash * S(escapable)) + (P(1) - S(escapable .. '/'))) ^ 1)
+  / escape_text(escapable)
+-- Within ternary condition format nodes, make sure we stop at :
+local if_till_colon_text = Cg(
+  C(((backslash * S(escapable)) + (P(1) - S(escapable .. ':'))) ^ 1) / escape_text(escapable),
+  'if_text'
+)
+
+-- Matches the string inside //, allowing escaping of the closing slash.
+local regex = Cg(((backslash * slash) + (P(1) - slash)) ^ 1, 'regex')
+
+-- Regex constructor flags (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/RegExp#parameters).
+local options = Cg(S('dgimsuvy') ^ 0, 'options')
+
+--- @enum vim.snippet.Type
+local Type = {
+  Tabstop = 1,
+  Placeholder = 2,
+  Choice = 3,
+  Variable = 4,
+  Format = 5,
+  Text = 6,
+  Snippet = 7,
+}
+M.NodeType = Type
+
+--- @class vim.snippet.Node<T>: { type: vim.snippet.Type, data: T }
+--- @class vim.snippet.TabstopData: { tabstop: number }
+--- @class vim.snippet.TextData: { text: string }
+--- @class vim.snippet.PlaceholderData: { tabstop: vim.snippet.TabstopData, value: vim.snippet.Node<any> }
+--- @class vim.snippet.ChoiceData: { tabstop: vim.snippet.TabstopData, values: string[] }
+--- @class vim.snippet.VariableData: { name: string, default?: vim.snippet.Node<any>, regex?: string, format?: vim.snippet.Node<vim.snippet.FormatData|vim.snippet.TextData>[], options?: string }
+--- @class vim.snippet.FormatData: { capture: number, modifier?: string, if_text?: string, else_text?: string }
+--- @class vim.snippet.SnippetData: { children: vim.snippet.Node<any>[] }
+
+--- Returns a function that constructs a snippet node of the given type.
+---
+--- @generic T
+--- @param type vim.snippet.Type
+--- @return fun(data: T): vim.snippet.Node<T>
+local function node(type)
+  return function(data)
+    return { type = type, data = data }
+  end
+end
+
+-- stylua: ignore
+local G = P({
+  'snippet';
+  snippet = Ct(Cg(
+    Ct((
+      V('any') +
+      (Ct(Cg(text_1 / escape_text(escapable), 'text')) / node(Type.Text))
+    ) ^ 1), 'children'
+  )) / node(Type.Snippet),
+  any_or_text = V('any') + (Ct(Cg(text_0 / escape_text(escapable), 'text')) / node(Type.Text)),
+  any = V('placeholder') + V('tabstop') + V('choice') + V('variable'),
+  tabstop = Ct(dollar * (tabstop + (l_brace * tabstop * r_brace))) / node(Type.Tabstop),
+  placeholder = Ct(dollar * l_brace * tabstop * colon * Cg(V('any_or_text'), 'value') * r_brace) / node(Type.Placeholder),
+  choice = Ct(dollar *
+    l_brace *
+    tabstop *
+    pipe *
+    Cg(Ct(choice_text * (P(',') * choice_text) ^ 0), 'values') *
+    pipe *
+    r_brace) / node(Type.Choice),
+  variable = Ct(dollar * (
+    var + (
+    l_brace * var * (
+      r_brace +
+      (colon * Cg(V('any_or_text'), 'default') * r_brace) +
+      (slash * regex * slash * Cg(Ct((V('format') + (C(format_text) / node(Type.Text))) ^ 1), 'format') * slash * options * r_brace)
+    ))
+  )) / node(Type.Variable),
+  format = Ct(dollar * (
+    format_capture + (
+    l_brace * format_capture * (
+      r_brace +
+      (colon * (
+        (slash * format_modifier * r_brace) +
+        (P('+') * if_text * r_brace) +
+        (P('?') * if_till_colon_text * colon * else_text * r_brace) +
+        (P('-') * else_text * r_brace) +
+        (else_text * r_brace)
+      ))
+    ))
+  )) / node(Type.Format),
+})
+
+--- Parses the given input into a snippet tree.
+--- @param input string
+--- @return vim.snippet.Node<vim.snippet.SnippetData>
+function M.parse(input)
+  local snippet = G:match(input)
+  assert(snippet, 'snippet parsing failed')
+  return snippet --- @type vim.snippet.Node<vim.snippet.SnippetData>
+end
+
+return M