#!/bin/bash
# swap - swap two files by renaming via a temp file
#
# Usage:
#   swap <file1> <file2>
#
# Options:
#   -h, --help     Show help message

_swap() (
    local SCRIPT_NAME; SCRIPT_NAME="$(basename "${BASH_SOURCE[0]}")"
    case "${BASH_SOURCE[0]}" in /dev/*|/proc/*) SCRIPT_NAME="" ;; esac
    case "$SCRIPT_NAME" in ""|bash|sh|zsh|dash) SCRIPT_NAME="swap" ;; esac

    _show_help() {
        local s; [ -t 1 ] && s=$'\033[4m'
        local r; [ -t 1 ] && r=$'\033[24m'
        cat <<EOF
NAME
  $SCRIPT_NAME - swap two files by renaming
SYNOPSIS
  $SCRIPT_NAME ${s}file1${r} ${s}file2${r}
  $SCRIPT_NAME -h
DESCRIPTION
  Atomically swaps two files using mv and a temp file at
  /tmp/temp-swap-${s}basename${r} (falls back to
  /tmp/temp-swap-${s}basename${r}-\$\$-\$RANDOM on collision).
  Directories are not supported. On partial failure,
  attempts to restore the original state (see EXIT STATUS).
EXIT STATUS
  0   Success
  2   Not enough arguments (usage error)
  4   First file does not exist
  5   First file is a directory
  6   Second file does not exist
  7   Second file is a directory
  8   Failed to move first file to temp (no restore needed: files untouched)
  9   Failed to move second file to first (restore attempted: temp -> first)
  10  Failed to move temp to second (no restore attempted: second already moved)
EOF
    }

    _error() { echo "[ERR][$SCRIPT_NAME] $*" >&2; }

    case "$1" in
        -h|--help)
            _show_help
            return 0
            ;;
    esac

    if [ $# -lt 2 ]; then
        _error "Must provide two files to swap (received $#). Run \`$SCRIPT_NAME -h\` for usage"
        return 2
    fi
    local file1="$1"
    local file2="$2"

    # Validate first file
    if [ ! -e "$file1" ]; then
        _error "File '$file1' does not exist"
        return 4
    elif [ -d "$file1" ]; then
        _error "'$file1' is a directory"
        return 5
    fi

    # Validate second file
    if [ ! -e "$file2" ]; then
        _error "File '$file2' does not exist"
        return 6
    elif [ -d "$file2" ]; then
        _error "'$file2' is a directory"
        return 7
    fi

    # Create a unique temporary filename for swapping
    local basename; basename="$(basename "$file1")"
    local temp_file="/tmp/temp-swap-$basename"
    while [ -e "$temp_file" ]; do
        temp_file="/tmp/temp-swap-$basename-$$-$RANDOM"
    done

    # Perform the swap using `mv`. Return distinct codes for each failure point
    if ! mv "$file1" "$temp_file"; then # file1 -> temp
        _error "Failed to move '$file1' to temporary file '$temp_file'"
        return 8
    fi
    if ! mv "$file2" "$file1"; then # file2 -> file1
        _error "Failed to move '$file2' to '$file1'"
        # Move failed - Attempt to restore the original file
        if ! mv "$temp_file" "$file1"; then
            _error "Failed to restore '$temp_file' to '$file1'"
        fi
        return 9
    fi
    if ! mv "$temp_file" "$file2"; then # temp -> file2
        _error "Failed to move temporary file to '$file2'"
        return 10
    fi

    # 0 status code is implied, but we will be explicit
    return 0
)

_swap "$@"
__swap_rc=$?
unset -f _swap
if [ -n "${BASH_SOURCE[0]}" ] && [ "${BASH_SOURCE[0]}" != "$0" ]; then
    eval "unset __swap_rc; return $__swap_rc"
fi
eval "unset __swap_rc; exit $__swap_rc"