"""Staged sync: source dir -> diff vs target -> apply with deletion gate."""
from __future__ import annotations

import shutil
from dataclasses import dataclass, field
from pathlib import Path


_DIFF_PREVIEW_CAP = 5


class UnconfirmedDeletionsError(RuntimeError):
    pass


@dataclass
class Diff:
    additions: list[str] = field(default_factory=list)
    modifications: list[str] = field(default_factory=list)
    deletions: list[str] = field(default_factory=list)


def _walk_relative(d: Path) -> set[str]:
    return {p.relative_to(d).as_posix() for p in d.rglob("*") if p.is_file()}


def stage_and_diff(src: Path, tgt: Path) -> Diff:
    """Compare src and tgt trees and return the diff."""
    src_files = _walk_relative(src)
    tgt_files = _walk_relative(tgt)

    additions = sorted(src_files - tgt_files)
    deletions = sorted(tgt_files - src_files)
    modifications = sorted(
        rel for rel in (src_files & tgt_files)
        if (src / rel).read_bytes() != (tgt / rel).read_bytes()
    )
    return Diff(additions=additions, modifications=modifications, deletions=deletions)


def apply(diff: Diff, src: Path, tgt: Path, *, allow_deletes: bool) -> None:
    """Apply additions/modifications. Apply deletions only if allow_deletes."""
    if diff.deletions and not allow_deletes:
        raise UnconfirmedDeletionsError(
            f"refusing to apply {len(diff.deletions)} deletions without allow_deletes=True; "
            f"first few: {diff.deletions[:_DIFF_PREVIEW_CAP]}"
        )

    for rel in diff.additions + diff.modifications:
        src_path = src / rel
        tgt_path = tgt / rel
        tgt_path.parent.mkdir(parents=True, exist_ok=True)
        shutil.copy2(src_path, tgt_path)

    if allow_deletes:
        for rel in diff.deletions:
            (tgt / rel).unlink()
            parent = (tgt / rel).parent
            while parent != tgt and parent.exists():
                try:
                    if any(parent.iterdir()):
                        break
                    parent.rmdir()
                except OSError:
                    break
                parent = parent.parent


def _format_section(label_count: str, sigil: str, items: list[str]) -> list[str]:
    if not items:
        return []
    lines = [f"  {label_count}"]
    for r in items[:_DIFF_PREVIEW_CAP]:
        lines.append(f"    {sigil} {r}")
    if len(items) > _DIFF_PREVIEW_CAP:
        lines.append(f"    {sigil} ... and {len(items) - _DIFF_PREVIEW_CAP} more")
    return lines


def format_diff_summary(diff: Diff) -> str:
    """Human-readable summary for confirmation prompts."""
    lines: list[str] = []
    lines.extend(_format_section(f"+{len(diff.additions)} additions", "+", diff.additions))
    lines.extend(_format_section(f"~{len(diff.modifications)} modifications", "~", diff.modifications))
    lines.extend(_format_section(f"-{len(diff.deletions)} deletions", "-", diff.deletions))
    if not lines:
        return "  (no changes)"
    return "\n".join(lines)