#' Require usage of `fixed=TRUE` in regular expressions where appropriate
#'
#' Invoking a regular expression engine is overkill for cases when the search
#'   pattern only involves static patterns.
#'
#' NB: for `stringr` functions, that means wrapping the pattern in `stringr::fixed()`.
#'
#' NB: this linter is likely not able to distinguish every possible case when
#'   a fixed regular expression is preferable, rather it seeks to identify
#'   likely cases. It should _never_ report false positives, however; please
#'   report false positives as an error.
#'
#' @param allow_unescaped Logical, default `FALSE`. If `TRUE`, only patterns that
#'   require regex escapes (e.g. `"\\$"` or `"[$]"`) will be linted. See examples.
#' @examples
#' # will produce lints
#' code_lines <- 'gsub("\\\\.", "", x)'
#' writeLines(code_lines)
#' lint(
#'   text = code_lines,
#'   linters = fixed_regex_linter()
#' )
#'
#' lint(
#'   text = 'grepl("a[*]b", x)',
#'   linters = fixed_regex_linter()
#' )
#'
#' lint(
#'   text = 'grepl("a[*]b", x)',
#'   linters = fixed_regex_linter(allow_unescaped = TRUE)
#' )
#'
#' code_lines <- 'stringr::str_subset(x, "\\\\$")'
#' writeLines(code_lines)
#' lint(
#'   text = code_lines,
#'   linters = fixed_regex_linter()
#' )
#'
#' lint(
#'   text = 'grepl("Munich", address)',
#'   linters = fixed_regex_linter()
#' )
#'
#' # okay
#' code_lines <- 'gsub("\\\\.", "", x, fixed = TRUE)'
#' writeLines(code_lines)
#' lint(
#'   text = code_lines,
#'   linters = fixed_regex_linter()
#' )
#'
#' lint(
#'   text = 'grepl("a*b", x, fixed = TRUE)',
#'   linters = fixed_regex_linter()
#' )
#'
#' lint(
#'   text = 'stringr::str_subset(x, stringr::fixed("$"))',
#'   linters = fixed_regex_linter()
#' )
#'
#' lint(
#'   text = 'grepl("Munich", address, fixed = TRUE)',
#'   linters = fixed_regex_linter()
#' )
#'
#' lint(
#'   text = 'grepl("Munich", address)',
#'   linters = fixed_regex_linter(allow_unescaped = TRUE)
#' )
#'
#' @evalRd rd_tags("fixed_regex_linter")
#' @seealso [linters] for a complete list of linters available in lintr.
#' @export
fixed_regex_linter <- function(allow_unescaped = FALSE) {
  # regular expression pattern is the first argument
  pos_1_regex_funs <- xp_text_in_table(c(
    "grep", "gsub", "sub", "regexec", "grepl", "regexpr", "gregexpr"
  ))

  # regular expression pattern is the second argument
  pos_2_regex_funs <- xp_text_in_table(c(
    # base functions.
    "strsplit",
    # data.table functions.
    "tstrsplit",
    # stringr functions.
    #   even though the user action is different
    #   (setting fixed=TRUE vs. wrapping stringr::fixed()),
    #   detection of the lint is the same
    "str_count", "str_detect", "str_ends", "str_extract", "str_extract_all",
    "str_locate", "str_locate_all", "str_match", "str_match_all",
    "str_remove", "str_remove_all", "str_replace", "str_replace_all",
    "str_split", "str_starts", "str_subset",
    "str_view", "str_view_all", "str_which"
  ))

  pipes <- setdiff(magrittr_pipes, c("%$%", "%T>%"))
  in_pipe_cond <- glue("
    parent::expr/preceding-sibling::SPECIAL[{ xp_text_in_table(pipes) }]
    | parent::expr/preceding-sibling::PIPE
  ")

  # NB: strsplit doesn't have an ignore.case argument
  # NB: we intentionally exclude cases like gsub(x, c("a" = "b")), where "b" is fixed
  xpath <- glue("
  //SYMBOL_FUNCTION_CALL[ {pos_1_regex_funs} ]
    /parent::expr[
      not(following-sibling::SYMBOL_SUB[
        (text() = 'fixed' or text() = 'ignore.case')
        and following-sibling::expr[1][NUM_CONST[text() = 'TRUE'] or SYMBOL[text() = 'T']]
      ])
    ]
    /following-sibling::expr[
      (
        position() = 1
        and STR_CONST
        and not(EQ_SUB)
        and not({ in_pipe_cond })
      ) or (
        STR_CONST
        and preceding-sibling::*[2][self::SYMBOL_SUB/text() = 'pattern']
      )
    ]
  |
  //SYMBOL_FUNCTION_CALL[ {pos_2_regex_funs} ]
    /parent::expr[
      not(following-sibling::SYMBOL_SUB[
        text() = 'fixed'
        and following-sibling::expr[1][NUM_CONST[text() = 'TRUE'] or SYMBOL[text() = 'T']]
      ])
    ]
    /following-sibling::expr[
      position() = 2 - count({ in_pipe_cond })
      and STR_CONST
      and not(EQ_SUB)
    ]
  ")

  Linter(function(source_expression) {
    if (!is_lint_level(source_expression, "expression")) {
      return(list())
    }

    xml <- source_expression$xml_parsed_content

    patterns <- xml_find_all(xml, xpath)
    pattern_strings <- get_r_string(patterns)
    is_static <- is_not_regex(pattern_strings, allow_unescaped)

    fixed_equivalent <- encodeString(get_fixed_string(pattern_strings[is_static]), quote = '"', justify = "none")
    call_name <- xml_find_chr(patterns[is_static], "string(preceding-sibling::expr[last()]/SYMBOL_FUNCTION_CALL)")

    is_stringr <- startsWith(call_name, "str_")
    replacement <- ifelse(
      is_stringr,
      sprintf("stringr::fixed(%s)", fixed_equivalent),
      fixed_equivalent
    )
    msg <- paste(
      "This regular expression is static, i.e., its matches can be expressed as a fixed substring expression, which",
      "is faster to compute. Here, you can use",
      replacement, ifelse(is_stringr, "as the pattern.", "with fixed = TRUE.")
    )

    xml_nodes_to_lints(
      patterns[is_static],
      source_expression = source_expression,
      lint_message = msg,
      type = "warning"
    )
  })
}
