pr.R

#' Helpers for GitHub pull requests
#'
#' @description
#' The `pr_*` family of functions is designed to make working with GitHub pull
#' requests (PRs) as painless as possible for both contributors and package
#' maintainers.
#'
#' To use the `pr_*` functions, your project must be a Git repo and have one of
#' these GitHub remote configurations:
#' * "ours": You can push to the GitHub remote configured as `origin` and it's
#'   not a fork.
#' * "fork": You can push to the GitHub remote configured as `origin`, it's a
#'   fork, and its parent is configured as `upstream`. `origin` points to your
#'   **personal** copy and `upstream` points to the **source repo**.
#'
#' "Ours" and "fork" are two of several GitHub remote configurations examined in
#' [Common remote setups](https://happygitwithr.com/common-remote-setups.html)
#' in Happy Git and GitHub for the useR.
#'
#' The [Pull Request
#' Helpers](https://usethis.r-lib.org/articles/articles/pr-functions.html)
#' article walks through the process of making a pull request with the `pr_*`
#' functions.
#'

#' The `pr_*` functions also use your Git/GitHub credentials to carry out
#' various remote operations; see below for more about auth. The `pr_*`
#' functions also proactively check for agreement re: the default branch in your
#' local repo and the source repo. See [git_default_branch()] for more.
#'

#' @template double-auth
#'

#' @section For contributors:

#' To contribute to a package, first use `create_from_github("OWNER/REPO")`.
#' This forks the source repository and checks out a local copy.

#'
#' Next use `pr_init()` to create a branch for your PR. It is best practice to
#' never make commits to the default branch branch of a fork (usually named
#' `main` or `master`), because you do not own it. A pull request should always
#' come from a feature branch. It will be much easier to pull upstream changes
#' from the fork parent if you only allow yourself to work in feature branches.
#' It is also much easier for a maintainer to explore and extend your PR if you
#' create a feature branch.
#'
#' Work locally, in your branch, making changes to files, and committing your
#' work. Once you're ready to create the PR, run `pr_push()` to push your local
#' branch to GitHub, and open a webpage that lets you initiate the PR (or draft
#' PR).
#'
#' To learn more about the process of making a pull request, read the [Pull
#' Request
#' Helpers](https://usethis.r-lib.org/articles/articles/pr-functions.html)
#' vignette.
#'
#' If you are lucky, your PR will be perfect, and the maintainer will accept it.
#' You can then run `pr_finish()` to delete your PR branch. In most cases,
#' however, the maintainer will ask you to make some changes. Make the changes,
#' then run `pr_push()` to update your PR.
#'
#' It's also possible that the maintainer will contribute some code to your PR:
#' to get those changes back onto your computer, run `pr_pull()`. It can also
#' happen that other changes have occurred in the package since you first
#' created your PR. You might need to merge the default branch (usually named
#' `main` or `master`) into your PR branch. Do that by running
#' `pr_merge_main()`: this makes sure that your PR is compatible with the
#' primary repo's main line of development. Both `pr_pull()` and
#' `pr_merge_main()` can result in merge conflicts, so be prepared to resolve
#' before continuing.
#'
#' @section For maintainers:
#' To download a PR locally so that you can experiment with it, run
#' `pr_fetch()` and select the PR or, if you already know its number, call
#' `pr_fetch(<pr_number>)`. If you make changes, run `pr_push()` to push them
#' back to GitHub. After you have merged the PR, run `pr_finish()` to delete the
#' local branch and remove the remote associated with the contributor's fork.
#'
#' @section Overview of all the functions:

#' * `pr_init()`: As a contributor, start work on a new PR by ensuring that
#' your local repo is up-to-date, then creating and checking out a new branch.
#' Nothing is pushed to or created on GitHub until you call `pr_push()`.

#' * `pr_fetch()`: As a maintainer, review or contribute changes to an existing
#' PR by creating a local branch that tracks the remote PR. `pr_fetch()` does as
#' little work as possible, so you can also use it to resume work on an PR that
#' already has a local branch (where it will also ensure your local branch is
#' up-to-date). If called with no arguments, up to 9 open PRs are offered for
#' interactive selection.

#' * `pr_resume()`: Resume work on a PR by switching to an existing local branch
#' and pulling any changes from its upstream tracking branch, if it has one. If
#' called with no arguments, up to 9 local branches are offered for interactive
#' selection, with a preference for branches connected to PRs and for branches
#' with recent activity.

#' * `pr_push()`: The first time it's called, a PR branch is pushed to GitHub
#' and you're taken to a webpage where a new PR (or draft PR) can be created.
#' This also sets up the local branch to track its remote counterpart.
#' Subsequent calls to `pr_push()` make sure the local branch has all the remote
#' changes and, if so, pushes local changes, thereby updating the PR.

#' * `pr_pull()`: Pulls changes from the local branch's remote tracking branch.
#' If a maintainer has extended your PR, this is how you bring those changes
#' back into your local work.

#' * `pr_merge_main()`: Pulls changes from the default branch of the source repo
#' into the current local branch. This can be used when the local branch is the
#' default branch or when it's a PR branch.

#' * `pr_pause()`: Makes sure you're up-to-date with any remote changes in the
#' PR. Then switches back to the default branch and pulls from the source repo.
#' Use `pr_resume()` with name of branch or use `pr_fetch()` to resume using PR
#' number.

#' * `pr_view()`: Visits the PR associated with the current branch in the
#' browser (default) or the specific PR identified by `number`.
#' (FYI [browse_github_pulls()] is a handy way to visit the list of all PRs for
#' the current project.)

#' * `pr_forget()`: Does local clean up when the current branch is an actual or
#' notional PR that you want to abandon. Maybe you initiated it yourself, via
#' `pr_init()`, or you used `pr_fetch()` to explore a PR from GitHub. Only does
#' *local* operations: does not update or delete any remote branches, nor does
#' it close any PRs. Alerts the user to any uncommitted or unpushed work that is
#' at risk of being lost. If user chooses to proceed, switches back to the
#' default branch, pulls changes from source repo, and deletes local PR branch.
#' Any associated Git remote is deleted, if the "forgotten" PR was the only
#' branch using it.

#' * `pr_finish()`: Does post-PR clean up, but does NOT actually merge or close
#' a PR (maintainer should do this in the browser). If `number` is not given,
#' infers the PR from the upstream tracking branch of the current branch. If
#' `number` is given, it does not matter whether the PR exists locally. If PR
#' exists locally, alerts the user to uncommitted or unpushed changes, then
#' switches back to the default branch, pulls changes from source repo, and
#' deletes local PR branch. If the PR came from an external fork, any associated
#' Git remote is deleted, provided it's not in use by any other local branches.
#' If the PR has been merged and user has permission, deletes the remote branch
#' (this is the only remote operation that `pr_finish()` potentially does).
#'
#' @name pull-requests
NULL

#' @export
#' @rdname pull-requests
#' @param branch Name of a new or existing local branch. If creating a new
#'   branch, note this should usually consist of lower case letters, numbers,
#'   and `-`.
pr_init <- function(branch) {
  check_string(branch)
  repo <- git_repo()

  if (gert::git_branch_exists(branch, local = TRUE, repo = repo)) {
    code <- glue('pr_resume("{branch}")')
    ui_bullets(c(
      "i" = "Branch {.val {branch}} already exists, calling {.code {code}}."
    ))
    return(pr_resume(branch))
  }

  # don't absolutely require PAT success, because we could be offline
  # or in another salvageable situation, e.g. need to configure PAT
  cfg <- github_remote_config(github_get = NA)
  check_for_bad_config(cfg)
  tr <- target_repo(cfg, ask = FALSE)

  maybe_good_configs <- c("maybe_ours_or_theirs", "maybe_fork")
  if (cfg$type %in% maybe_good_configs) {
    ui_bullets(c(
      "x" = 'Unable to confirm the GitHub remote configuration is
             "pull request ready".',
      "i" = "You probably need to configure a personal access token for
             {.val {tr$host}}.",
      "i" = "See {.run usethis::gh_token_help()} for help with that.",
      "i" = "(Or maybe we're just offline?)"
    ))
    if (ui_github_remote_config_wat(cfg)) {
      ui_bullets(c("x" = "Cancelling."))
      return(invisible())
    }
  }

  default_branch <- git_default_branch()
  challenge_non_default_branch(
    "Are you sure you want to create a PR branch based on a non-default branch?",
    default_branch = default_branch
  )

  online <- is_online(tr$host)
  if (online) {
    # this is not pr_pull_source_override() because:
    # a) we may NOT be on default branch (although we probably are)
    # b) we didn't just switch to the branch we're on, therefore we have to
    #    consider that the pull may be affected by uncommitted changes or a
    #    merge
    current_branch <- git_branch()
    if (current_branch == default_branch) {
      # override for mis-configured forks, that have default branch tracking
      # the fork (origin) instead of the source (upstream)
      remref <- glue("{tr$remote}/{default_branch}")
    } else {
      remref <- git_branch_tracking(current_branch)
    }
    if (!is.na(remref)) {
      comparison <- git_branch_compare(current_branch, remref)
      if (comparison$remote_only > 0) {
        challenge_uncommitted_changes()
      }
      ui_bullets(c("v" = "Pulling changes from {.val {remref}}."))
      git_pull(remref = remref, verbose = FALSE)
    }
  } else {
    ui_bullets(c(
      "!" = "Unable to pull changes for current branch, since we are offline."
    ))
  }

  ui_bullets(c("v" = "Creating and switching to local branch {.val {branch}}."))
  gert::git_branch_create(branch, repo = repo)
  config_key <- glue("branch.{branch}.created-by")
  gert::git_config_set(config_key, value = "usethis::pr_init", repo = repo)

  ui_bullets(c("_" = "Use {.run usethis::pr_push()} to create a PR."))
  invisible()
}

#' @export
#' @rdname pull-requests
pr_resume <- function(branch = NULL) {
  repo <- git_repo()

  if (is.null(branch)) {
    ui_bullets(c(
      "i" = "No branch specified ... looking up local branches and associated PRs."
    ))
    default_branch <- git_default_branch()
    branch <- choose_branch(exclude = default_branch)
    if (is.null(branch)) {
      ui_bullets(c("x" = "Repo doesn't seem to have any non-default branches."))
      return(invisible())
    }
    if (length(branch) == 0) {
      ui_bullets(c("x" = "No branch selected, exiting."))
      return(invisible())
    }
  }
  check_string(branch)

  if (!gert::git_branch_exists(branch, local = TRUE, repo = repo)) {
    code <- glue('usethis::pr_init("{branch}")')
    ui_abort(c(
      "x" = "No branch named {.val {branch}} exists.",
      "_" = "Call {.run {code}} to create a new PR branch."
    ))
  }

  challenge_uncommitted_changes()

  ui_bullets(c("v" = "Switching to branch {.val {branch}}."))
  gert::git_branch_checkout(branch, repo = repo)
  git_pull()

  ui_bullets(c("_" = "Use {.run usethis::pr_push()} to create or update PR."))
  invisible()
}

#' @export
#' @rdname pull-requests
#' @param number Number of PR.
#' @param target Which repo to target? This is only a question in the case of a
#'   fork. In a fork, there is some slim chance that you want to consider pull
#'   requests against your fork (the primary repo, i.e. `origin`) instead of
#'   those against the source repo (i.e. `upstream`, which is the default).
#'
#' @examples
#' \dontrun{
#' pr_fetch(123)
#' }
pr_fetch <- function(number = NULL, target = c("source", "primary")) {
  repo <- git_repo()
  tr <- target_repo(github_get = NA, role = target, ask = FALSE)
  challenge_uncommitted_changes()

  if (is.null(number)) {
    ui_bullets(c("i" = "No PR specified ... looking up open PRs."))
    pr <- choose_pr(tr = tr)
    if (is.null(pr)) {
      ui_bullets(c("x" = "No open PRs found for {.val {tr$repo_spec}}."))
      return(invisible())
    }
    if (min(lengths(pr)) == 0) {
      ui_bullets(c("x" = "No PR selected, exiting."))
      return(invisible())
    }
  } else {
    pr <- pr_get(number = number, tr = tr)
  }

  if (is.na(pr$pr_repo_owner)) {
    ui_abort("
      The repo or branch where {.href [PR #{pr$pr_number}]({pr$pr_html_url})} originates seems to have been
      deleted.")
  }

  pr_user <- glue("@{pr$pr_user}")
  ui_bullets(c(
    "v" = "Checking out PR {.val {pr$pr_string}} ({.field {pr_user}}):
           {.val {pr$pr_title}}."
  ))

  if (pr$pr_from_fork && isFALSE(pr$maintainer_can_modify)) {
    ui_bullets(c(
      "!" = "Note that user does NOT allow maintainer to modify this PR at this
             time, although this can be changed."
    ))
  }

  remote <- github_remote_list(pr$pr_remote)
  if (nrow(remote) == 0) {
    url <- switch(tr$protocol, https = pr$pr_https_url, ssh = pr$pr_ssh_url)
    ui_bullets(c("v" = "Adding remote {.val {pr$pr_remote}} as {.val {url}}."))
    gert::git_remote_add(url = url, name = pr$pr_remote, repo = repo)
    config_key <- glue("remote.{pr$pr_remote}.created-by")
    gert::git_config_set(config_key, "usethis::pr_fetch", repo = repo)
  }
  pr_remref <- glue_data(pr, "{pr_remote}/{pr_ref}")
  gert::git_fetch(
    remote = pr$pr_remote,
    refspec = pr$pr_ref,
    repo = repo,
    verbose = FALSE
  )

  if (is.na(pr$pr_local_branch)) {
    pr$pr_local_branch <-
      if (pr$pr_from_fork) sub(":", "-", pr$pr_label) else pr$pr_ref
  }

  # Create local branch, if necessary, and switch to it ----
  if (!gert::git_branch_exists(pr$pr_local_branch, local = TRUE, repo = repo)) {
    ui_bullets(c(
      "v" = "Creating and switching to local branch {.val {pr$pr_local_branch}}.",
      "v" = "Setting {.val {pr_remref}} as remote tracking branch."
    ))
    gert::git_branch_create(pr$pr_local_branch, ref = pr_remref, repo = repo)
    config_key <- glue("branch.{pr$pr_local_branch}.created-by")
    gert::git_config_set(config_key, "usethis::pr_fetch", repo = repo)
    config_url <- glue("branch.{pr$pr_local_branch}.pr-url")
    gert::git_config_set(config_url, pr$pr_html_url, repo = repo)
    return(invisible())
  }

  # Local branch pre-existed; make sure tracking branch is set, switch, & pull
  ui_bullets(c("v" = "Switching to branch {.val {pr$pr_local_branch}}."))
  gert::git_branch_checkout(pr$pr_local_branch, repo = repo)
  config_url <- glue("branch.{pr$pr_local_branch}.pr-url")
  gert::git_config_set(config_url, pr$pr_html_url, repo = repo)

  pr_branch_ours_tracking <- git_branch_tracking(pr$pr_local_branch)
  if (is.na(pr_branch_ours_tracking) ||
      pr_branch_ours_tracking != pr_remref) {
    ui_bullets(c("v" = "Setting {.val {pr_remref}} as remote tracking branch."))
    gert::git_branch_set_upstream(pr_remref, repo = repo)
  }
  git_pull(verbose = FALSE)
}

#' @export
#' @rdname pull-requests
pr_push <- function() {
  repo <- git_repo()
  cfg <- github_remote_config(github_get = TRUE)
  check_for_config(cfg, ok_configs = c("ours", "fork"))
  default_branch <- git_default_branch()
  check_pr_branch(default_branch)
  challenge_uncommitted_changes()

  branch <- git_branch()
  remref <- git_branch_tracking(branch)
  if (is.na(remref)) {
    # this is the first push
    if (cfg$type == "fork" && cfg$upstream$can_push && is_interactive()) {
      choices <- c(
        origin   = ui_pre_glue("
          <<cfg$origin$repo_spec>> = {.val origin} (external PR)"),
        upstream = ui_pre_glue("
          <<cfg$upstream$repo_spec>> = {.val upstream} (internal PR)")
      )
      choices_formatted <- map_chr(choices, cli::format_inline)
      title <- glue("Which repo do you want to push to?")
      choice <- utils::menu(choices_formatted, graphics = FALSE, title = title)
      remote <- names(choices)[[choice]]
    } else {
      remote <- "origin"
    }

    git_push_first(branch, remote)
  } else {
    check_branch_pulled(use = "pr_pull()")
    git_push(branch, remref)
  }

  # Prompt to create PR if does not exist yet
  tr <- target_repo(cfg, ask = FALSE)
  pr <- pr_find(branch, tr = tr)
  if (is.null(pr)) {
    pr_create()
  } else {
    ui_bullets(c(
      "_" = "View PR at {.url {pr$pr_html_url}} or call {.run usethis::pr_view()}."
    ))
  }

  invisible()
}

#' @export
#' @rdname pull-requests
pr_pull <- function() {
  cfg <- github_remote_config(github_get = TRUE)
  check_for_config(cfg)
  default_branch <- git_default_branch()
  check_pr_branch(default_branch)
  challenge_uncommitted_changes()

  git_pull()

  # note associated PR in git config, if applicable
  tr <- target_repo(cfg, ask = FALSE)
  pr_find(tr = tr)

  invisible(TRUE)
}

#' @export
#' @rdname pull-requests
pr_merge_main <- function() {
  tr <- target_repo(github_get = TRUE, ask = FALSE)
  challenge_uncommitted_changes()
  remref <- glue("{tr$remote}/{tr$default_branch}")
  ui_bullets(c("v" = "Pulling changes from {.val {remref}}."))
  git_pull(remref, verbose = FALSE)
}

#' @export
#' @rdname pull-requests
pr_view <- function(number = NULL, target = c("source", "primary")) {
  tr <- target_repo(github_get = NA, role = target, ask = FALSE)
  url <- NULL
  if (is.null(number)) {
    branch <- git_branch()
    default_branch <- git_default_branch()
    if (branch != default_branch) {
      url <- pr_url(branch = branch, tr = tr)
      if (is.null(url)) {
        ui_bullets(c(
          "i" = "Current branch ({.val {branch}}) does not appear to be
                 connected to a PR."
        ))
      } else {
        number <- sub("^.+pull/", "", url)
        ui_bullets(c(
          "i" = "Current branch ({.val {branch}}) is connected to PR #{number}."
        ))
      }
    }
  } else {
    pr <- pr_get(number = number, tr = tr)
    url <- pr$pr_html_url
  }
  if (is.null(url)) {
    ui_bullets(c("i" = "No PR specified ... looking up open PRs."))
    pr <- choose_pr(tr = tr)
    if (is.null(pr)) {
      ui_bullets(c("x" = "No open PRs found for {.val {tr$repo_spec}}."))
      return(invisible())
    }
    if (min(lengths(pr)) == 0) {
      ui_bullets(c("x" = "No PR selected, exiting."))
      return(invisible())
    }
    url <- pr$pr_html_url
  }
  view_url(url)
}

#' @export
#' @rdname pull-requests
pr_pause <- function() {
  # intentionally naive selection of target repo
  tr <- target_repo(github_get = FALSE, ask = FALSE)

  ui_bullets(c("v" = "Switching back to the default branch."))
  default_branch <- git_default_branch()
  if (git_branch() == default_branch) {
    ui_bullets(c(
      "!" = "Already on this repo's default branch ({.val {default_branch}}),
             nothing to do."
    ))
    return(invisible())
  }
  challenge_uncommitted_changes()
  # TODO: what happens here if offline?
  check_branch_pulled(use = "pr_pull()")

  ui_bullets(c(
    "v" = "Switching back to default branch ({.val {default_branch}})."
  ))
  gert::git_branch_checkout(default_branch, repo = git_repo())
  pr_pull_source_override(tr = tr, default_branch = default_branch)
}

#' @export
#' @rdname pull-requests
pr_finish <- function(number = NULL, target = c("source", "primary")) {
  pr_clean(number = number, target = target, mode = "finish")
}

#' @export
#' @rdname pull-requests
pr_forget <- function() pr_clean(mode = "forget")

# unexported helpers ----

# Removes local evidence of PRs that you're done with or wish you'd never
# started or fetched
# Only possible remote action is to delete the remote branch for a merged PR
pr_clean <- function(number = NULL,
                     target = c("source", "primary"),
                     mode = c("finish", "forget")) {
  mode <- match.arg(mode)
  repo <- git_repo()
  tr <- target_repo(github_get = NA, role = target, ask = FALSE)
  default_branch <- git_default_branch()

  if (is.null(number)) {
    check_pr_branch(default_branch)
    pr <- pr_find(git_branch(), tr = tr, state = "all")
    # if the remote branch has already been deleted (probably post-merge), we
    # can't always reverse engineer what the corresponding local branch was, but
    # we already know it -- it's how we found the PR in the first place!
    if (!is.null(pr)) {
      pr$pr_local_branch <- pr$pr_local_branch %|% git_branch()
    }
  } else {
    pr <- pr_get(number = number, tr = tr)
  }

  pr_local_branch <- if (is.null(pr)) git_branch() else pr$pr_local_branch

  if (!is.na(pr_local_branch)) {
    if (pr_local_branch == git_branch()) {
      challenge_uncommitted_changes()
    }
    tracking_branch <- git_branch_tracking(pr_local_branch)
    if (is.na(tracking_branch)) {
      if (ui_nah(c(
        "!" = "Local branch {.val {pr_local_branch}} has no associated remote
               branch.",
        "i" = "If we delete {.val {pr_local_branch}}, any work that exists only
               on this branch may be hard for you to recover.",
        " " = "Proceed anyway?"
      ))) {
        ui_bullets(c("x" = "Cancelling."))
        return(invisible())
      }
    } else {
      cmp <- git_branch_compare(
        branch = pr_local_branch,
        remref = tracking_branch
      )
      if (cmp$local_only > 0 && ui_nah(c(
        "!" = "Local branch {.val {pr_local_branch}} has 1 or more commits that
               have not been pushed to {.val {tracking_branch}}.",
        "i" = "If we delete {.val {pr_local_branch}}, this work may be hard for
               you to recover.",
        " " = "Proceed anyway?"
      ))) {
        ui_bullets(c("x" = "Cancelling."))
        return(invisible())
      }
    }
  }

  if (git_branch() != default_branch) {
    ui_bullets(c(
      "v" = "Switching back to default branch ({.val {default_branch}})."
    ))
    gert::git_branch_checkout(default_branch, force = TRUE, repo = repo)
    pr_pull_source_override(tr = tr, default_branch = default_branch)
  }

  if (!is.na(pr_local_branch)) {
    ui_bullets(c(
      "v" = "Deleting local {.val {pr_local_branch}} branch."
    ))
    gert::git_branch_delete(pr_local_branch, repo = repo)
  }

  if (is.null(pr)) {
    return(invisible())
  }

  if (mode == "finish") {
    pr_branch_delete(pr)
  }

  # delete remote, if we (usethis) added it AND no remaining tracking branches
  created_by <- git_cfg_get(glue("remote.{pr$pr_remote}.created-by"))
  if (is.null(created_by) || !grepl("^usethis::", created_by)) {
    return(invisible())
  }

  branches <- gert::git_branch_list(local = TRUE, repo = repo)
  branches <- branches[!is.na(branches$upstream), ]
  if (sum(grepl(glue("^refs/remotes/{pr$pr_remote}"), branches$upstream)) == 0) {
    ui_bullets(c("v" = "Removing remote {.val {pr$pr_remote}}."))
    gert::git_remote_remove(remote = pr$pr_remote, repo = repo)
  }
  invisible()
}

# Make sure to pull from upstream/DEFAULT (as opposed to origin/DEFAULT) if
# we're in DEFAULT branch of a fork. I wish everyone set up DEFAULT to track the
# DEFAULT branch in the source repo, but this protects us against sub-optimal
# setup.
pr_pull_source_override <- function(tr = NULL, default_branch = NULL) {
  # naive selection of target repo; calling function should analyse the config
  tr <- tr %||% target_repo(github_get = FALSE, ask = FALSE)

  # TODO: why does this not use a check_*() function, i.e. shared helper?
  # I guess to issue a specific error message?
  current_branch <- git_branch()
  default_branch <- default_branch %||% git_default_branch()
  if (current_branch != default_branch) {
    ui_abort("
      Internal error: {.fun pr_pull_source_override} should only be used when on
      default branch.")
  }

  # guard against mis-configured forks, that have default branch tracking
  # the fork (origin) instead of the source (upstream)
  # TODO: should I just change the upstream tracking branch, i.e. fix it?
  remref <- glue("{tr$remote}/{default_branch}")
  if (is_online(tr$host)) {
    ui_bullets(c("v" = "Pulling changes from {.val {remref}}."))
    git_pull(remref = remref, verbose = FALSE)
  } else {
    ui_bullets(c(
      "!" = "Can't reach {.val {tr$host}}, therefore unable to pull changes from
             {.val {remref}}."
    ))
  }
}

pr_create <- function() {
  branch <- git_branch()
  tracking_branch <- git_branch_tracking(branch)
  remote <- remref_remote(tracking_branch)
  remote_dat <- github_remotes(remote, github_get = FALSE)
  ui_bullets(c("_" = "Create PR at link given below."))
  view_url(glue_data(remote_dat, "{host_url}/{repo_spec}/compare/{branch}"))
}

# retrieves 1 PR, if:
# * we can establish a tracking relationship between `branch` and a PR branch
# * we can get the user to choose 1
pr_find <- function(branch = git_branch(),
                    tr = NULL,
                    state = c("open", "closed", "all")) {
  # Have we done this before? Check if we've cached pr-url in git config.
  config_url <- glue("branch.{branch}.pr-url")
  url <- git_cfg_get(config_url, where = "local")
  if (!is.null(url)) {
    return(pr_get(number = sub("^.+pull/", "", url), tr = tr))
  }

  tracking_branch <- git_branch_tracking(branch)
  if (is.na(tracking_branch)) {
    return(NULL)
  }

  state <- match.arg(state)
  remote <- remref_remote(tracking_branch)
  remote_dat <- github_remotes(remote)

  pr_head <- glue("{remote_dat$repo_owner}:{remref_branch(tracking_branch)}")
  pr_dat <- pr_list(tr = tr, state = state, head = pr_head)
  if (nrow(pr_dat) == 0) {
    return(NULL)
  }
  if (nrow(pr_dat) > 1) {
    spec <- sub(":", "/", pr_head)
    ui_bullets(c("!" = "Multiple PRs are associated with {.val {spec}}."))
    pr_dat <- choose_pr(pr_dat = pr_dat)
    if (min(lengths(pr_dat)) == 0) {
      ui_abort("
        One of these PRs must be specified explicitly or interactively: \\
        {.or {paste0('#', pr_dat$pr_number)}}.")
    }
  }

  gert::git_config_set(config_url, pr_dat$pr_html_url, repo = git_repo())
  as.list(pr_dat)
}

pr_url <- function(branch = git_branch(),
                   tr = NULL,
                   state = c("open", "closed", "all")) {
  state <- match.arg(state)
  pr <- pr_find(branch, tr = tr, state = state)
  if (is.null(pr)) {
    NULL
  } else {
    pr$pr_html_url
  }
}

pr_data_tidy <- function(pr) {
  out <- list(
    pr_number     = pluck_int(pr, "number"),
    pr_title      = pluck_chr(pr, "title"),
    pr_state      = pluck_chr(pr, "state"),
    pr_user       = pluck_chr(pr, "user", "login"),
    pr_created_at = pluck_chr(pr, "created_at"),
    pr_updated_at = pluck_chr(pr, "updated_at"),
    pr_merged_at  = pluck_chr(pr, "merged_at"),
    pr_label      = pluck_chr(pr, "head", "label"),
    # the 'repo' element of 'head' is NULL when fork has been deleted
    pr_repo_owner = pluck_chr(pr, "head", "repo", "owner", "login"),
    pr_ref        = pluck_chr(pr, "head", "ref"),
    pr_repo_spec  = pluck_chr(pr, "head", "repo", "full_name"),
    pr_from_fork  = pluck_lgl(pr, "head", "repo", "fork"),
    # 'maintainer_can_modify' is only present when we GET one specific PR
    pr_maintainer_can_modify = pluck_lgl(pr, "maintainer_can_modify"),
    pr_https_url  = pluck_chr(pr, "head", "repo", "clone_url"),
    pr_ssh_url    = pluck_chr(pr, "head", "repo", "ssh_url"),
    pr_html_url   = pluck_chr(pr, "html_url"),
    pr_string     = glue("
      {pluck_chr(pr, 'base', 'repo', 'full_name')}/#{pluck_int(pr, 'number')}")
  )

  grl <- github_remote_list(these = NULL)
  m <- match(out$pr_repo_spec, grl$repo_spec)
  out$pr_remote <- if (is.na(m)) out$pr_repo_owner else grl$remote[m]

  pr_remref <- glue("{out$pr_remote}/{out$pr_ref}")
  gbl <- gert::git_branch_list(local = TRUE, repo = git_repo())
  gbl <- gbl[!is.na(gbl$upstream), c("name", "upstream")]
  gbl$upstream <- sub("^refs/remotes/", "", gbl$upstream)
  m <- match(pr_remref, gbl$upstream)
  out$pr_local_branch <- if (is.na(m)) NA_character_ else gbl$name[m]

  # If the fork has been deleted, these are all NA
  # - Because pr$head$repo is NULL:
  #   pr_repo_owner, pr_repo_spec, pr_from_fork, pr_https_url, pr_ssh_url
  # - Because derived from those above:
  #   pr_remote, pr_remref pr_local_branch
  # I suppose one could already have a local branch, if you fetched the PR
  # before the fork got deleted.
  # But an initial pr_fetch() won't work if the fork has been deleted.
  # I'm willing to accept that the pr_*() functions don't necessarily address
  # the "deleted fork" scenario. It's relatively rare.
  # example: https://github.com/r-lib/httr/pull/634

  out
}

pr_list <- function(tr = NULL,
                    github_get = NA,
                    state = c("open", "closed", "all"),
                    head = NULL) {
  tr <- tr %||% target_repo(github_get = github_get, ask = FALSE)
  state <- match.arg(state)
  gh <- gh_tr(tr)
  safely_gh <- purrr::safely(gh, otherwise = NULL)
  out <- safely_gh(
    "GET /repos/{owner}/{repo}/pulls",
    state = state, head = head, .limit = Inf
  )
  if (is.null(out$error)) {
    prs <- out$result
  } else {
    ui_bullets(c("x" = "Unable to retrieve PRs for {.value {tr$repo_spec}}."))
    prs <- NULL
  }
  no_prs <- length(prs) == 0
  if (no_prs) {
    prs <- list(list())
  }
  out <- map(prs, pr_data_tidy)
  out <- map(out, ~ as.data.frame(.x, stringsAsFactors = FALSE))
  out <- do.call(rbind, out)
  if (no_prs) {
    out[0, ]
  } else {
    pr_is_open <- out$pr_state == "open"
    rbind(out[pr_is_open, ], out[!pr_is_open, ])
  }
}

# retrieves specific PR by number
pr_get <- function(number, tr = NULL, github_get = NA) {
  tr <- tr %||% target_repo(github_get = github_get, ask = FALSE)
  gh <- gh_tr(tr)
  raw <- gh("GET /repos/{owner}/{repo}/pulls/{number}", number = number)
  pr_data_tidy(raw)
}

branches_with_no_upstream_or_github_upstream <- function(tr = NULL) {
  repo <- git_repo()
  gb_dat <- gert::git_branch_list(local = TRUE, repo = repo)
  gb_dat <- gb_dat[, c("name", "upstream", "updated")]
  gb_dat$remref     <- sub("^refs/remotes/", "", gb_dat$upstream)
  gb_dat$upstream   <- NULL
  gb_dat$remote     <- remref_remote(gb_dat$remref)
  gb_dat$ref        <- remref_branch(gb_dat$remref)
  gb_dat$cfg_pr_url <- map_chr(
    glue("branch.{gb_dat$name}.pr-url"),
    ~ git_cfg_get(.x, where = "local") %||% NA_character_
  )

  ghr <- github_remote_list(these = NULL)[["remote"]]
  gb_dat <- gb_dat[is.na(gb_dat$remref) | (gb_dat$remote %in% ghr), ]

  pr_dat <- pr_list(tr = tr)
  dat <- merge(
    x    = gb_dat, y    = pr_dat,
    by.x = "name", by.y = "pr_local_branch",
    all.x = TRUE
  )
  dat <- dat[order(dat$pr_number, dat$pr_updated_at, dat$updated, decreasing = TRUE), ]

  missing_cfg <- is.na(dat$cfg_pr_url) & !is.na(dat$pr_html_url)
  purrr::walk2(
    glue("branch.{dat$name[missing_cfg]}.pr-url"),
    dat$pr_html_url[missing_cfg],
    ~ gert::git_config_set(.x, .y, repo = repo)
  )

  dat
}

choose_branch <- function(exclude = character()) {
  if (!is_interactive()) {
    return(character())
  }
  dat <- branches_with_no_upstream_or_github_upstream()
  dat <- dat[!dat$name %in% exclude, ]
  if (nrow(dat) == 0) {
    return()
  }
  prompt <- "Which branch do you want to checkout? (0 to exit)"
  n_show_max <- 9
  n <- nrow(dat)
  n_shown <- compute_n_show(n, n_show_nominal = n_show_max)
  n_not_shown <- n - n_shown
  if (n_not_shown > 0) {
    branches_not_shown <- utils::tail(dat$name, -n_shown)
    dat <- dat[seq_len(n_shown), ]
    fine_print <- cli::format_inline(
      "{n_not_shown} branch{?/es} not listed: {.val {branches_not_shown}}"
    )
    prompt <- glue("{prompt}\n{fine_print}")
  }
  dat$pretty_user <- glue_data(dat, "@{pr_user}")
  dat$pretty_name <- format(dat$name, justify = "right")
  dat_pretty <- purrr::pmap_chr(
    dat[c("pretty_name", "pr_number", "pretty_user", "pr_title")],
    function(pretty_name, pr_number, pretty_user, pr_title) {
      if (is.na(pr_number)) {
        pretty_name
      } else {
        template <- ui_pre_glue(
          "{pretty_name} {cli::symbol$arrow_right} #{pr_number} ({.field <<pretty_user>>}): {pr_title}"
        )
        cli::format_inline(template)
      }
    }
  )
  choice <- utils::menu(title = prompt, choices = cli::ansi_strtrim(dat_pretty))
  dat$name[choice]
}

choose_pr <- function(tr = NULL, pr_dat = NULL) {
  if (!is_interactive()) {
    return(list(pr_number = list()))
  }
  if (is.null(pr_dat)) {
    tr <- tr %||% target_repo()
    pr_dat <- pr_list(tr)
  }
  if (nrow(pr_dat) == 0) {
    return()
  }

  # wording needs to make sense for several PR-choosing tasks, e.g. fetch, view,
  # finish, forget
  prompt <- "Which PR are you interested in? (0 to exit)"
  n_show_max <- 9
  n <- nrow(pr_dat)
  n_shown <- compute_n_show(n, n_show_nominal = n_show_max)
  n_not_shown <- n - n_shown
  if (n_not_shown > 0) {
    pr_dat <- pr_dat[seq_len(n_shown), ]
    info1 <- cli::format_inline("Not shown: {n_not_shown} more PR{?s}.")
    info2 <- cli::format_inline(
      "Call {.run usethis::browse_github_pulls()} to browse all PRs."
    )
    prompt <- glue("{prompt}\n{info1}\n{info2}")
  }

  some_closed <- any(pr_dat$pr_state == "closed")
  pr_pretty <- purrr::pmap_chr(
    pr_dat[c("pr_number", "pr_html_url", "pr_user", "pr_state", "pr_title")],
    function(pr_number, pr_html_url, pr_user, pr_state, pr_title) {
      href_number <- ui_pre_glue("{.href [PR #<<pr_number>>](<<pr_html_url>>)}")
      at_user <- glue("@{pr_user}")
      if (some_closed) {
        template <- ui_pre_glue(
          "<<href_number>> ({.field <<at_user>>}, {pr_state}): {.val <<pr_title>>}"
        )
        cli::format_inline(template)
      } else {
        template <- ui_pre_glue(
          "<<href_number>> ({.field <<at_user>>}): {.val <<pr_title>>}"
        )
        cli::format_inline(template)
      }
    }
  )
  choice <- utils::menu(
    title = prompt,
    choices = cli::ansi_strtrim(pr_pretty)
  )
  as.list(pr_dat[choice, ])
}

# deletes the remote branch associated with a PR
# returns invisible TRUE/FALSE re: whether a deletion actually occurred
# reasons this returns FALSE
# * don't have push permission on remote where PR branch lives
# * PR has not been merged
# * remote branch has already been deleted
pr_branch_delete <- function(pr) {
  remote <- pr$pr_remote
  remote_dat <- github_remotes(remote)
  if (!isTRUE(remote_dat$can_push)) {
    return(invisible(FALSE))
  }

  gh <- gh_tr(remote_dat)
  pr_ref <- tryCatch(
    gh(
      "GET /repos/{owner}/{repo}/git/ref/{ref}",
      ref = glue("heads/{pr$pr_ref}")
    ),
    http_error_404 = function(cnd) NULL
  )

  pr_remref <- glue_data(pr, "{pr_remote}/{pr_ref}")

  if (is.null(pr_ref)) {
    ui_bullets(c(
      "i" = "PR {.val {pr$pr_string}} originated from branch {.val {pr_remref}},
             which no longer exists."
    ))
    return(invisible(FALSE))
  }

  if (is.na(pr$pr_merged_at)) {
    ui_bullets(c(
      "i" = "PR {.val {pr$pr_string}} is unmerged, we will not delete the
             remote branch {.val {pr_remref}}."
    ))
    return(invisible(FALSE))
  }

  ui_bullets(c(
    "v" = "PR {.val {pr$pr_string}} has been merged, deleting remote branch
           {.val {pr_remref}}."
  ))
  # TODO: tryCatch here?
  gh(
    "DELETE /repos/{owner}/{repo}/git/refs/{ref}",
    ref = glue("heads/{pr$pr_ref}")
  )
  invisible(TRUE)
}

check_pr_branch <- function(default_branch = git_default_branch()) {
  # the glue-ing happens inside check_current_branch(), where `gb` gives the
  # current git branch
  check_current_branch(
    is_not = default_branch,
    message = c(
      "i" = "The {.code pr_*()} functions facilitate pull requests.",
      "i" = "The current branch ({.val {gb}}) is this repo's default branch, but
             pull requests should NOT come from the default branch.",
      "i" = "Do you need to call {.fun usethis::pr_init} (new PR)?
             Or {.fun usethis::pr_resume} or
             {.fun usethis::pr_fetch} (existing PR)?"
    )
  )
}