#!/usr/bin/env bash
# usage: smart-cp SRC DST
set -euo pipefail

src=$1
dst=$2

find "$src" -type f -print0 | while IFS= read -r -d '' f; do
    rel=${f#"$src"/}
    target=$dst/$rel
    mkdir -p "$(dirname "$target")"

    if [[ ! -e $target ]]; then
        cp -p "$f" "$target"
        continue
    fi

    src_size=$(stat -c%s "$f")
    dst_size=$(stat -c%s "$target")

    if (( src_size == dst_size )); then
        continue  # same size — leave it alone
    fi

    printf '%s differs (src=%d, dst=%d) — [o]verwrite/[s]kip/[d]iff/[q]uit? ' \
        "$rel" "$src_size" "$dst_size" >&2
    read -r choice </dev/tty
    case $choice in
        o|O) cp -p "$f" "$target" ;;
        s|S) ;;
        d|D) diff "$target" "$f" || true; exec "$0" "$src" "$dst" ;;  # crude: restart
        q|Q) exit 0 ;;
        *)   echo "skipping" >&2 ;;
    esac
done