# Evaluate every R constraint, including patch-level and upper bounds.
r_requirement_satisfied <- function(depends, r_version) {
  if (is.na(depends) || !nzchar(trimws(depends))) {
    return(TRUE)
  }
  entries <- trimws(strsplit(depends, ",", fixed = TRUE)[[1L]])
  entries <- entries[grepl("^R(\\s|\\(|$)", entries)]
  all(vapply(
    entries,
    function(entry) {
      if (entry == "R") {
        return(TRUE)
      }
      match <- regmatches(
        entry,
        regexec(
          "^R\\s*\\(\\s*(>=|<=|==|>|<)\\s*([0-9]+(?:[.-][0-9]+)*)\\s*\\)$",
          entry,
          perl = TRUE
        )
      )[[1L]]
      if (length(match) != 3L) {
        stop("Cannot parse R dependency: ", entry)
      }
      comparison <- utils::compareVersion(r_version, match[3L])
      switch(
        match[2L],
        ">=" = comparison >= 0L,
        "<=" = comparison <= 0L,
        "==" = comparison == 0L,
        ">" = comparison > 0L,
        "<" = comparison < 0L
      )
    },
    logical(1L)
  ))
}

# Use unfiltered current CRAN metadata: available.packages() would hide exactly
# the packages that cannot run under the primary interpreter.
# Propagate R incompatibility through required dependencies, not Suggests.
newer_r_candidates <- function(db, primary_r, target_r) {
  db <- as.data.frame(db, stringsAsFactors = FALSE)
  # CRAN also lists R-specific Recommended copies (e.g. Matrix for R-devel).
  # Prefer the ordinary contrib record, or a future-R copy can incorrectly
  # block every package depending on Matrix on all released interpreters.
  if ("Path" %in% names(db)) {
    db <- db[order(!is.na(db$Path) & nzchar(db$Path)), , drop = FALSE]
  }
  db <- db[!duplicated(db$Package), , drop = FALSE]
  deps <- tools::package_dependencies(
    packages = db$Package,
    db = as.matrix(db),
    which = c("Depends", "Imports", "LinkingTo"),
    recursive = FALSE
  )
  supported <- function(version) {
    ok <- vapply(db$Depends, r_requirement_satisfied, logical(1L), version)
    repeat {
      blocked <- db$Package[!ok]
      next_ok <- ok &
        !vapply(deps, function(x) any(x %in% blocked), logical(1L))
      if (identical(ok, next_ok)) {
        return(ok)
      }
      ok <- next_ok
    }
  }
  selected <- !supported(primary_r) & supported(target_r)
  if ("OS_type" %in% names(db)) {
    selected <- selected & (is.na(db$OS_type) | db$OS_type != "windows")
  }
  result <- db[selected, c("Package", "Version"), drop = FALSE]
  result[order(result$Package), , drop = FALSE]
}