#!/usr/bin/env bash
set -euo pipefail

usage() {
    echo 'Usage: zstdedit <file.zst>'
}

die() {
    echo "zstdedit: $*" >&2
    exit 1
}

choose_editor() {
    if [[ -n "${VISUAL:-}" ]]; then
        echo "$VISUAL"
        return 0
    fi

    if [[ -n "${EDITOR:-}" ]]; then
        echo "$EDITOR"
        return 0
    fi

    local candidate
    for candidate in micro nano vim vi; do
        if command -v "$candidate" >/dev/null 2>&1; then
            echo "$candidate"
            return 0
        fi
    done

    return 1
}

run_editor() {
    local file="$1"
    local editor
    editor=$(choose_editor) || die 'No editor found. Set VISUAL or EDITOR.'

    local -a editor_cmd
    read -r -a editor_cmd <<<"$editor"
    "${editor_cmd[@]}" "$file"
}

if [[ $# -ne 1 ]]; then
    usage >&2
    exit 1
fi

command -v zstd >/dev/null 2>&1 || die 'zstd is not installed.'

target="$1"
target_dir=$(dirname -- "$target")
target_base=$(basename -- "$target")

[[ -d "$target_dir" ]] || die "Parent directory does not exist: $target_dir"

tmpdir=$(mktemp -d "${TMPDIR:-/tmp}/zstdedit.XXXXXX")
compressed_tmp=''
cleanup() {
    if [[ -n "$compressed_tmp" ]]; then
        rm -f -- "$compressed_tmp"
    fi
    rm -rf -- "$tmpdir"
}
trap cleanup EXIT

before_file="$tmpdir/before"
edit_file="$tmpdir/${target_base%.zst}"

if [[ -e "$target" ]]; then
    [[ -f "$target" ]] || die "Not a regular file: $target"
    zstd -q -dc -- "$target" >"$before_file" || die "Failed to decompress: $target"
else
    : >"$before_file"
fi

cp -- "$before_file" "$edit_file"
run_editor "$edit_file"

if cmp -s -- "$before_file" "$edit_file"; then
    echo 'No changes.'
    exit 0
fi

compressed_tmp=$(mktemp -- "$target_dir/.${target_base}.zstdedit.XXXXXX")
zstd -q -f -o "$compressed_tmp" -- "$edit_file" || die 'Failed to recompress edited file.'

if [[ -e "$target" ]]; then
    chmod --reference="$target" "$compressed_tmp" 2>/dev/null || true
fi

mv -f -- "$compressed_tmp" "$target"
compressed_tmp=''
echo "Updated $target"