#!/usr/bin/env bash

set -Eeuo pipefail

script_name="git wt"

usage() {
    cat <<EOF
Usage:
  $script_name <branch>

Creates a worktree for <branch> under <gj.root>/worktree/<owner>/<repo>/<branch>
and prints its path. Creates the branch if it does not exist.
If <branch> is already checked out in a worktree, prints that path. If it is
checked out in the main repository, that checkout is switched to the default
branch (or detached) first, so the branch can move to its own worktree.

Config:
  gj.root   Directory where repositories are stored.

Example:
  cd ($script_name pb/my-feature)
EOF
}

# Prints the path of the worktree that has <branch> checked out, if any.
worktree_for_branch() {
    git worktree list --porcelain | awk -v ref="branch refs/heads/$1" '
        /^worktree / { path = substr($0, 10) }
        $0 == ref { print path; exit }'
}

# Frees <branch> from the worktree at <path> so it can be checked out elsewhere.
release_branch() {
    local branch=$1 path=$2 default
    [[ -z "$(git -C "$path" status --porcelain)" ]] ||
        die "$branch is checked out in $path with uncommitted changes; commit or stash first"
    default=$(git -C "$path" default-branch 2>/dev/null || true)
    if [[ -n "$default" && "$default" != "$branch" ]] && git -C "$path" switch --quiet "$default" 2>/dev/null; then
        printf 'Switched %s to %s\n' "$path" "$default" >&2
    else
        git -C "$path" switch --quiet --detach
        printf 'Detached HEAD in %s\n' "$path" >&2
    fi
}

die() {
    printf '%s\n' "$*" >&2
    exit 1
}

main() {
    [[ $# -eq 1 ]] || {
        usage
        exit 2
    }
    case "$1" in
    -h | --help)
        usage
        exit 0
        ;;
    esac

    local branch=$1 root toplevel common main dest existing

    root=$(git config --global --get gj.root || true)
    [[ -n "$root" ]] || die "gj.root is not set in git config"
    root="${root/#\~/$HOME}"

    toplevel=$(git rev-parse --show-toplevel 2>/dev/null) || die "Not inside a git repository"
    [[ "$toplevel" == "$root"/* ]] || die "Repository is not under gj.root: $toplevel"

    # Use the main repository path even when run from inside a linked worktree.
    common=$(git rev-parse --path-format=absolute --git-common-dir)
    main=${common%/.git}
    if [[ "$toplevel" == "$root"/worktree/* ]]; then
        toplevel=$main
    fi

    dest="$root/worktree/${toplevel#"$root"/}/${branch//\//-}"

    existing=$(worktree_for_branch "$branch")
    if [[ -n "$existing" && "$existing" != "$main" ]]; then
        printf '%s\n' "$existing"
        return
    fi
    [[ -z "$existing" ]] || release_branch "$branch" "$existing"

    if [[ ! -d "$dest" ]]; then
        git fetch --prune --quiet
        if git show-ref --verify --quiet "refs/heads/$branch" || git show-ref --verify --quiet "refs/remotes/origin/$branch"; then
            git worktree add --quiet "$dest" "$branch"
        else
            git worktree add --quiet -b "$branch" "$dest"
        fi
    fi

    printf '%s\n' "$dest"
}

main "$@"