"""Filesystem diff engine for ISO images.

Compares two ISO images at the *filesystem* level — which entries exist
in one but not the other, which have changed size or modification time —
without comparing file *contents*.  Like :mod:`iso_handler`, this module
is GUI-agnostic: it depends only on pycdlib (via IsoHandler) and the
standard library, so it can be used from the CLI or a test harness with
no display attached.

The diff is computed in the *default* naming convention of image A (or
an explicit :class:`~iso_scalpel.iso_record.NameType`).  Entries that
exist in both images are matched by path within that convention.
"""

# This file is part of ISO Scalpel.
# Copyright (C) 2025 Jeremy Anderson <info@dcos.net>
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, see <https://www.gnu.org/licenses/>
# or write to the Free Software Foundation, Inc., 51 Franklin Street,
# Fifth Floor, Boston, MA  02110-1301 USA.

from __future__ import annotations

import enum
from dataclasses import dataclass, field

from .iso_handler import IsoHandler
from .iso_record import IsoRecord, NameType


class DiffStatus(enum.Enum):
    """How an entry differs between the two images."""

    SAME = "same"          # present in both, same size
    MODIFIED = "modified"  # present in both, different size or date
    ADDED = "added"        # only in B (the second image)
    REMOVED = "removed"    # only in A (the first image)


@dataclass
class DiffEntry:
    """A single row in the diff output."""

    status: DiffStatus
    path: str                # path relative to root (always "/"-prefixed)
    is_dir: bool
    a: IsoRecord | None   # record from image A (None if ADDED)
    b: IsoRecord | None   # record from image B (None if REMOVED)

    @property
    def status_label(self) -> str:
        return {
            DiffStatus.SAME: "=",
            DiffStatus.MODIFIED: "M",
            DiffStatus.ADDED: "+",
            DiffStatus.REMOVED: "-",
        }[self.status]

    @property
    def a_size(self) -> int:
        return self.a.size if self.a else 0

    @property
    def b_size(self) -> int:
        return self.b.size if self.b else 0

    @property
    def a_date(self) -> str:
        return self.a.date_label if self.a else ""

    @property
    def b_date(self) -> str:
        return self.b.date_label if self.b else ""


@dataclass
class DiffResult:
    """The full result of comparing two images."""

    a_file: str | None       # on-disk filename of A
    b_file: str | None       # on-disk filename of B
    entries: list[DiffEntry] = field(default_factory=list)

    @property
    def same_count(self) -> int:
        return sum(1 for e in self.entries if e.status == DiffStatus.SAME)

    @property
    def modified_count(self) -> int:
        return sum(1 for e in self.entries if e.status == DiffStatus.MODIFIED)

    @property
    def added_count(self) -> int:
        return sum(1 for e in self.entries if e.status == DiffStatus.ADDED)

    @property
    def removed_count(self) -> int:
        return sum(1 for e in self.entries if e.status == DiffStatus.REMOVED)

    @property
    def is_identical(self) -> bool:
        return not self.modified_count and not self.added_count and not self.removed_count


def diff_images(a: IsoHandler, b: IsoHandler,
                name_type: NameType | None = None) -> DiffResult:
    """Compare two open images and return a :class:`DiffResult`.

    ``name_type`` selects which naming convention to compare in.  If
    ``None``, the default convention of image A is used (falling back to
    ISO9660 if A has no UDF/RR/Joliet).
    """
    if not a.is_open or not b.is_open:
        raise ValueError("Both images must be open")

    if name_type is None:
        # Use the richest convention common to both images.
        a_types = set(a.available_name_types())
        b_types = set(b.available_name_types())
        common = a_types & b_types
        for preferred in (NameType.UDF, NameType.ROCK_RIDGE,
                          NameType.JOLIET, NameType.ISO9660):
            if preferred in common:
                name_type = preferred
                break
        if name_type is None:
            name_type = NameType.ISO9660

    tree_a = _walk(a, "/", name_type)
    tree_b = _walk(b, "/", name_type)

    entries: list[DiffEntry] = []
    all_paths = sorted(set(tree_a) | set(tree_b))

    for path in all_paths:
        ra = tree_a.get(path)
        rb = tree_b.get(path)
        if ra and rb:
            if (ra.is_dir and rb.is_dir) or (ra.size == rb.size and ra.modified == rb.modified):
                status = DiffStatus.SAME
            else:
                status = DiffStatus.MODIFIED
        elif rb and not ra:
            status = DiffStatus.ADDED
        else:
            status = DiffStatus.REMOVED
        entries.append(DiffEntry(status=status, path=path,
                                 is_dir=(ra or rb).is_dir, a=ra, b=rb))

    return DiffResult(a_file=a.filename, b_file=b.filename, entries=entries)


def _walk(handler: IsoHandler, root: str,
          name_type: NameType) -> dict[str, IsoRecord]:
    """Recursively walk an image, returning ``{path: IsoRecord}``."""
    out: dict[str, IsoRecord] = {}
    _walk_into(handler, root, name_type, out)
    return out


def _walk_into(handler: IsoHandler, path: str, name_type: NameType,
               out: dict[str, IsoRecord]) -> None:
    try:
        records = handler.list_dir(path, name_type)
    except (OSError, ValueError, KeyError):
        # A directory that cannot be listed is skipped; its descendants are
        # simply absent from the diff rather than aborting the whole walk.
        return
    for rec in records:
        out[rec.path] = rec
        if rec.is_dir:
            _walk_into(handler, rec.path, name_type, out)


def format_diff_text(result: DiffResult) -> str:
    """Render a :class:`DiffResult` as plain text (for the CLI)."""
    lines: list[str] = []
    a_name = result.a_file or "A"
    b_name = result.b_file or "B"
    lines.append(f"--- {a_name}")
    lines.append(f"+++ {b_name}")
    lines.append("")
    if result.is_identical:
        lines.append("Filesystems are identical.")
        return "\n".join(lines)

    lines.append(f"{result.added_count} added, {result.removed_count} removed, "
                 f"{result.modified_count} modified, {result.same_count} unchanged")
    lines.append("")

    # Each row formats one diff entry.  ``SAME`` entries are skipped here and
    # summarised by count at the end, so the formatter is a flat lookup over
    # the three "interesting" statuses.
    def _line_for(e: DiffEntry) -> str | None:
        mark = e.status_label
        tag = "dir " if e.is_dir else ""
        if e.status == DiffStatus.MODIFIED:
            return f" {mark} {e.path}  ({e.a_size} -> {e.b_size} bytes)"
        if e.status == DiffStatus.ADDED:
            return f" {mark} {tag}{e.path}  ({e.b_size} bytes)"
        if e.status == DiffStatus.REMOVED:
            return f" {mark} {tag}{e.path}  ({e.a_size} bytes)"
        return None

    changed = [e for e in result.entries if e.status != DiffStatus.SAME]
    for e in changed:
        line = _line_for(e)
        if line is not None:
            lines.append(line)

    if result.same_count:
        lines.append("")
        lines.append(f"({result.same_count} entries unchanged)")
    return "\n".join(lines)