"""
app/utils/storage.py
---------------------
Storage abstraction for uploaded photo files (R2 object-storage migration).

One interface, backend selected by config ``STORAGE_BACKEND``:
  - ``local`` (default): files under ``app/static/uploads``, served via
    ``url_for('static', ...)``. **Byte-for-byte identical** to the behavior
    before this abstraction existed — Phase 1 is a no-op.
  - ``s3`` (added in Phase 2): Cloudflare R2 / any S3-compatible store.

The stored **key** is always the relative path ``uploads/<subfolder>/<file>`` —
the exact string persisted in the DB (`Issue.photo_path`, `result_photos[]`,
`mobile_photo_paths[]`, `Inspection.form_data` image values,
`InspectionResult.photo_path`). It never changes between backends, so there is
**no schema migration**.

Public module-level API (delegates to the active backend):

    save(file_obj, subfolder) -> key    # write an uploaded file, return its key
    media_url(key) -> str               # browser/API URL for a key ('' if falsy)
    read(key) -> bytes                  # raw bytes (e.g. PDF export image embed)
    exists(key) -> bool
    delete(key) -> None                 # best-effort file removal
    abs_local_path(key) -> str          # physical path under static/ (local backend)

NOTE: extension / magic-byte validation stays in the CALLER (e.g. `_save_photo`,
`api/photos.upload_photo`) — this module only moves bytes and builds URLs.
"""

import os
import uuid
import logging

from flask import current_app, url_for

logger = logging.getLogger(__name__)


def _ext_of(filename):
    """Lower-case extension of a filename, defaulting to 'jpg' when absent."""
    name = filename or ''
    return name.rsplit('.', 1)[-1].lower() if '.' in name else 'jpg'


# ── Local filesystem backend (current behavior) ───────────────────────────────

class LocalBackend:
    """Files under ``app/static/uploads``, served by Flask/Nginx static route."""

    name = 'local'

    def _uploads_root(self):
        # config UPLOAD_FOLDER == <root>/app/static/uploads
        return current_app.config['UPLOAD_FOLDER']

    def _static_folder(self):
        # <root>/app/static — key 'uploads/...' resolves under here
        return os.path.join(current_app.root_path, 'static')

    def save(self, file_obj, subfolder):
        """Write ``file_obj`` under ``subfolder``; return the ``uploads/...`` key.

        Replicates the previous inline logic in `_save_photo` / `upload_photo`
        exactly: random uuid filename, `os.makedirs(exist_ok=True)`, same key.
        """
        ext      = _ext_of(file_obj.filename)
        filename = f'{uuid.uuid4().hex}.{ext}'
        dest_dir = os.path.join(self._uploads_root(), subfolder)
        os.makedirs(dest_dir, exist_ok=True)
        file_obj.save(os.path.join(dest_dir, filename))
        return f'uploads/{subfolder}/{filename}'

    def abs_local_path(self, key):
        return os.path.normpath(os.path.join(self._static_folder(), key))

    def media_url(self, key):
        if not key:
            return ''
        return url_for('static', filename=key)

    def read(self, key):
        with open(self.abs_local_path(key), 'rb') as fh:
            return fh.read()

    def exists(self, key):
        return bool(key) and os.path.isfile(self.abs_local_path(key))

    def delete(self, key):
        if not key:
            return
        try:
            path = self.abs_local_path(key)
            if os.path.isfile(path):
                os.remove(path)
        except OSError:
            pass


# S3Backend (Cloudflare R2) is added in Phase 2 and registered below.

_BACKENDS = {
    'local': LocalBackend,
}


def get_backend():
    """Return the active storage backend (one cached instance per app)."""
    name = (current_app.config.get('STORAGE_BACKEND') or 'local').lower()
    cls  = _BACKENDS.get(name)
    if cls is None:
        logger.warning('Unknown STORAGE_BACKEND=%r — falling back to local', name)
        cls = LocalBackend
    cache_key = f'_storage_backend_{cls.name}'
    inst = current_app.extensions.get(cache_key)
    if inst is None:
        inst = cls()
        current_app.extensions[cache_key] = inst
    return inst


# ── Module-level convenience delegates ────────────────────────────────────────

def save(file_obj, subfolder):
    return get_backend().save(file_obj, subfolder)


def media_url(key):
    return get_backend().media_url(key)


def read(key):
    return get_backend().read(key)


def exists(key):
    return get_backend().exists(key)


def delete(key):
    return get_backend().delete(key)


def abs_local_path(key):
    return get_backend().abs_local_path(key)