// Copyright 2021 Google LLC
// 
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// 
// https://www.apache.org/licenses/LICENSE-2.0
// 
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

identifier = @{ (ASCII_ALPHANUMERIC | "@" | "/" | ".")+ } 
symbol = {
  identifier
  | literal_string
}
literal_string = { "\"" ~ (!"\"" ~ ANY)+ ~ "\"" }
whitespace = _{ " " }

parents_op = { ":" }
ancestors_op = { ",," }
prefix_op = _{ parents_op | ancestors_op }

children_op = { ":" }
descendants_op = { ",," }
postfix_op = _{ children_op | descendants_op }

union_op = { "|" }
intersection_op = { "&" }
difference_op = { "-" }
infix_op = _{ union_op | intersection_op | difference_op }

function_name = @{ (ASCII_ALPHANUMERIC | "_")+ }
function_arguments = {
  (whitespace* ~ expression ~ whitespace* ~ ",")* ~ whitespace* ~ expression ~ whitespace*
  | whitespace*
}

primary = {
  function_name ~ "(" ~ function_arguments ~ ")"
  | "(" ~ expression ~ ")"
  | symbol
}

prefix_expression = { prefix_op* ~ primary }

postfix_expression = { prefix_expression ~ postfix_op* }

infix_expression = {
  whitespace* ~ postfix_expression ~ whitespace* ~ (infix_op ~ whitespace* ~ postfix_expression ~ whitespace*)*
}

expression = {
  whitespace* ~ infix_expression ~ whitespace*
}