gandalf/cutsheet.py

"""Cutsheets: where every cable is meant to go, compared with what Gandalf sees.

Pure functions with no I/O, like devices.py. app.py loads rows from the
database and the monitor's cached link stats and passes them in.

Model
-----
* An **item** is something Gandalf can't discover: a patch panel, a wall jack,
  a shelf, or a rack position for a device. Items of a passive kind (panels and
  jacks) pass a signal straight through, from a port's front to the same port's
  rear.
* A **cable** joins two endpoints. An endpoint is a device or item name, a port
  (a switch's port number, a host's interface name, or a panel port), and for
  passive items a side. The far end may be left blank for a cable that ends
  nowhere yet, such as an unterminated fibre.
* A **path** is the chain of cables joined through passive items, e.g. switch
  port 13 → panel port 5 (front) … panel port 5 (rear) → bedroom jack.
"""
import csv
import io
import re
from typing import Dict, List, Optional, Tuple

ITEM_KINDS = ('patch_panel', 'wall_jack', 'device', 'shelf', 'other')
# Which side of a 4-post rack an item is mounted from. A front and a rear item can share
# a unit; a full-depth one (a server) takes the unit on both sides.
MOUNTS = ('front', 'rear', 'full')
RACK_UNITS_MAX = 60
PASSIVE_KINDS = ('patch_panel', 'wall_jack')
SIDES = ('front', 'rear')
STATUSES = ('planned', 'installed', 'dark', 'retired')
CABLE_TYPES = ('Cat5e', 'Cat6', 'Cat6a', 'DAC', 'AOC', 'OM3', 'OM4', 'OS2', 'Power', 'Other')

LABEL_RE = re.compile(r'^[A-Za-z0-9][A-Za-z0-9._-]{0,31}$')
NAME_MAX = 100
PORT_MAX = 64
TEXT_MAX = 255
NOTES_MAX = 2000
RACK_NAME_MAX = 100

# Comparison outcomes, and whether each counts as a problem
VERDICTS = {
    'match':          'Matches what Gandalf sees',
    'wrong_neighbor': 'A different device is connected',
    'no_link':        'Planned device is not linked',
    'missing_port':   'That port is not reported by the device',
    'unverified':     "Gandalf can't confirm the far end",
    'idle':           'Ends at a jack or panel with nothing linked',
    'not_checked':    'No switch port to check against',
    'planned':        'Planned, not installed yet',
    'dark':           'Unused (dark)',
    'retired':        'Retired',
    'conflict':       'Another cable uses the same port',
}
PROBLEM_VERDICTS = ('wrong_neighbor', 'no_link', 'missing_port', 'conflict')


class ValidationError(ValueError):
    """A request the cutsheet can't accept, with a message for the user."""


# ── Validation ──────────────────────────────────────────────────────────────

def _text(data: dict, key: str, limit: int = TEXT_MAX, required: bool = False, label: str = '') -> str:
    value = data.get(key)
    value = '' if value is None else str(value).strip()
    if required and not value:
        raise ValidationError(f'{label or key} is required')
    if len(value) > limit:
        raise ValidationError(f'{label or key} must be {limit} characters or fewer')
    return value


def _int(data: dict, key: str, low: int, high: int, label: str) -> Optional[int]:
    value = data.get(key)
    if value in (None, ''):
        return None
    try:
        number = int(value)
    except (TypeError, ValueError):
        raise ValidationError(f'{label} must be a whole number')
    if not low <= number <= high:
        raise ValidationError(f'{label} must be between {low} and {high}')
    return number


def validate_item(data: dict, items: Optional[Dict[str, dict]] = None) -> dict:
    """An item from a form. `items` (by name) is needed to check a shelf placement."""
    kind = _text(data, 'kind', 20) or 'other'
    if kind not in ITEM_KINDS:
        raise ValidationError(f'kind must be one of {", ".join(ITEM_KINDS)}')
    item = {
        'name':        _text(data, 'name', NAME_MAX, required=True, label='Name'),
        'kind':        kind,
        'port_count':  _int(data, 'port_count', 1, 512, 'Port count'),
        'location':    _text(data, 'location'),
        'rack_u':      _int(data, 'rack_u', 1, 60, 'Rack unit'),
        'rack_height': _int(data, 'rack_height', 1, 20, 'Height'),
        'rack':        _text(data, 'rack', RACK_NAME_MAX),
        'device':      _text(data, 'device'),
        'mount':       _text(data, 'mount', 10) or 'front',
        'shelf':       _text(data, 'shelf', NAME_MAX),
        'notes':       _text(data, 'notes', NOTES_MAX),
    }
    if item['mount'] not in MOUNTS:
        raise ValidationError(f'mount must be one of {", ".join(MOUNTS)}')
    if item['shelf']:
        shelf = (items or {}).get(item['shelf'])
        if not shelf or shelf['kind'] != 'shelf':
            raise ValidationError(f'{item["shelf"]} is not a shelf on the cutsheet')
        if kind == 'shelf' or shelf['name'] == item['name']:
            raise ValidationError('A shelf cannot sit on a shelf')
        if item['rack_u'] or item['rack_height']:
            raise ValidationError('An item on a shelf takes its rack position from the shelf; clear the rack unit')
        item['rack'] = ''  # the shelf's rack and side
        item['mount'] = 'front'
    if item['rack_height'] and not item['rack_u']:
        raise ValidationError('Set a rack unit to go with the height')
    if item['rack_u'] and not item['rack_height']:
        item['rack_height'] = 1
    if kind == 'patch_panel' and not item['port_count']:
        raise ValidationError('A patch panel needs a port count')
    return item


def _endpoint(data: dict, prefix: str, items: Dict[str, dict], required: bool) -> Tuple[str, str, str]:
    device = _text(data, f'{prefix}_device', NAME_MAX, required=required,
                   label='From device' if prefix == 'a' else 'To device')
    port = _text(data, f'{prefix}_port', PORT_MAX)
    side = _text(data, f'{prefix}_side', 10)
    if not device:
        if port or side:
            raise ValidationError('Choose a device before a port')
        return '', '', ''
    item = items.get(device)
    passive = bool(item and item['kind'] in PASSIVE_KINDS)
    if passive:
        if side not in SIDES:
            raise ValidationError(f'Choose the front or rear of {device}')
        if not port:
            raise ValidationError(f'Choose a port on {device}')
        count = item.get('port_count')
        if count and (not port.isdigit() or not 1 <= int(port) <= count):
            raise ValidationError(f'{device} has ports 1 to {count}')
    else:
        side = ''
    return device, port, side


def validate_cable(data: dict, items: Dict[str, dict]) -> dict:
    label = _text(data, 'label', 32)
    if label and not LABEL_RE.match(label):
        raise ValidationError('Label may use letters, digits, dot, dash and underscore (up to 32)')
    status = _text(data, 'status', 20) or 'installed'
    if status not in STATUSES:
        raise ValidationError(f'status must be one of {", ".join(STATUSES)}')
    cable_type = _text(data, 'cable_type', 32)
    if cable_type and cable_type not in CABLE_TYPES:
        raise ValidationError(f'Type must be one of {", ".join(CABLE_TYPES)}')
    length = data.get('length_m')
    if length in (None, ''):
        length = None
    else:
        try:
            length = round(float(length), 2)
        except (TypeError, ValueError):
            raise ValidationError('Length must be a number of metres')
        if not 0 < length <= 999:
            raise ValidationError('Length must be between 0 and 999 metres')
    a = _endpoint(data, 'a', items, required=True)
    b = _endpoint(data, 'b', items, required=False)
    if a == b:
        raise ValidationError('Both ends are the same port')
    return {
        'label': label or None,
        'a_device': a[0], 'a_port': a[1], 'a_side': a[2],
        'b_device': b[0], 'b_port': b[1], 'b_side': b[2],
        'cable_type': cable_type, 'length_m': length,
        'color': _text(data, 'color', 32), 'status': status,
        'notes': _text(data, 'notes', NOTES_MAX),
        'label_applied': bool(data.get('label_applied')),
    }


def next_label(cables: List[dict], prefix: str = 'C') -> str:
    """The next free cable ID: C01, C02, ... (wider once past 99)."""
    used = set()
    for c in cables:
        m = re.match(rf'^{re.escape(prefix)}(\d+)$', c.get('label') or '', re.IGNORECASE)
        if m:
            used.add(int(m.group(1)))
    n = 1
    while n in used:
        n += 1
    return f'{prefix}{n:02d}'


# ── Paths ───────────────────────────────────────────────────────────────────

def _node(cable: dict, end: str) -> Optional[tuple]:
    device = cable.get(f'{end}_device') or ''
    if not device:
        return None
    return (device, cable.get(f'{end}_port') or '', cable.get(f'{end}_side') or '')


def _other_side(node: tuple, items: Dict[str, dict]) -> Optional[tuple]:
    """Through a passive item, the same port's other side; otherwise the path ends."""
    item = items.get(node[0])
    if not item or item['kind'] not in PASSIVE_KINDS or node[2] not in SIDES:
        return None
    return (node[0], node[1], 'rear' if node[2] == 'front' else 'front')


def _live(cable: dict) -> bool:
    return cable.get('status') != 'retired'


def node_index(cables: List[dict]) -> Dict[tuple, List[dict]]:
    """Map each endpoint to the live cables plugged into it."""
    index: Dict[tuple, List[dict]] = {}
    for c in cables:
        if not _live(c):
            continue
        for end in ('a', 'b'):
            node = _node(c, end)
            if node:
                index.setdefault(node, []).append(c)
    return index


def trace(cable: dict, cables: List[dict], items: Dict[str, dict],
          index: Optional[Dict[tuple, List[dict]]] = None) -> dict:
    """Follow a cable through passive items in both directions.

    Returns {'hops': [node, cable_id, node, ...], 'cable_ids': [...]} with the
    hops ordered from one end of the run to the other. A node is
    (device, port, side), or None for an unterminated end.
    """
    index = index if index is not None else node_index(cables)

    def walk(node: Optional[tuple], came_from: dict, seen: set) -> list:
        hops = []
        while node is not None:
            through = _other_side(node, items)
            if through is None:
                break
            nxt = next((c for c in index.get(through, []) if c['id'] != came_from['id'] and c['id'] not in seen),
                       None)
            hops.append(through)
            if nxt is None:
                break
            seen.add(nxt['id'])
            hops.append(nxt['id'])
            far = 'b' if _node(nxt, 'a') == through else 'a'
            node = _node(nxt, far)
            hops.append(node)
            came_from = nxt
        return hops

    seen = {cable['id']}
    toward_a = walk(_node(cable, 'a'), cable, seen)
    toward_b = walk(_node(cable, 'b'), cable, seen)
    hops = list(reversed(toward_a)) + [_node(cable, 'a'), cable['id'], _node(cable, 'b')] + toward_b
    return {'hops': hops, 'cable_ids': [h for h in hops if isinstance(h, int)]}


# ── Plan vs. actual ─────────────────────────────────────────────────────────

def _switch_port(node: Optional[tuple], switches: dict) -> Optional[dict]:
    """The discovered port behind an endpoint, if it is a switch or router port."""
    if not node or node[0] not in switches or not node[1].isdigit():
        return None
    for port in (switches[node[0]].get('ports') or {}).values():
        if port.get('port_idx') == int(node[1]):
            return port
    return {}  # the device is known but doesn't report this port


def _same(a: str, b: str) -> bool:
    return bool(a) and bool(b) and a.strip().lower() == b.strip().lower()


def device_of(name: str, items: Dict[str, dict]) -> str:
    """The live device an item name stands for: its linked device, or the name itself."""
    return (items.get(name) or {}).get('device') or name


def with_aliases(switches: dict, items: Dict[str, dict]) -> dict:
    """Switches also reachable by the name of an item linked to them (item "UDM-Pro" -> "Dream Machine Pro")."""
    out = dict(switches or {})
    for name, item in (items or {}).items():
        device = item.get('device')
        if device and device in out and name not in out:
            out[name] = out[device]
    return out


def _check_end(node: tuple, other: Optional[tuple], switches: dict, items: Dict[str, dict]) -> Optional[dict]:
    port = _switch_port(node, switches)
    if port is None:
        return None
    if port == {}:
        return {'verdict': 'missing_port', 'detail': f'{node[0]} reports no port {node[1]}'}
    other_passive = other is None or (items.get(other[0], {}).get('kind') in PASSIVE_KINDS)
    expected = None if other_passive else device_of(other[0], items)
    lldp = port.get('lldp') or {}
    seen = lldp.get('system_name') or ''
    sfp_peer = (port.get('sfp') or {}).get('cable_peer')

    if sfp_peer and other is not None and not other_passive:
        if _same(sfp_peer.get('switch', ''), device_of(other[0], items)) and str(sfp_peer.get('port_idx')) == other[1]:
            return {'verdict': 'match', 'detail': 'Both ends report the same DAC serial'}
    if not port.get('up'):
        if expected:
            return {'verdict': 'no_link', 'detail': f'{node[0]} port {node[1]} is down'}
        return {'verdict': 'idle', 'detail': f'{node[0]} port {node[1]} is down'}
    if expected:
        if not seen:
            return {'verdict': 'unverified', 'detail': f'Link up on port {node[1]}, but no neighbor is identified'}
        if _same(seen, expected):
            return {'verdict': 'match', 'detail': f'{node[0]} port {node[1]} sees {seen}'}
        return {'verdict': 'wrong_neighbor', 'detail': f'{node[0]} port {node[1]} sees {seen}, not {expected}'}
    detail = f'Link up on port {node[1]}' + (f', sees {seen}' if seen else '')
    return {'verdict': 'unverified', 'detail': detail}


_VERDICT_RANK = ['wrong_neighbor', 'no_link', 'missing_port', 'unverified', 'idle', 'match']


def compare_path(path: dict, cables_by_id: Dict[int, dict], switches: dict, items: Dict[str, dict]) -> dict:
    """Check a traced run against discovery. Returns {'verdict', 'detail'}."""
    run = [cables_by_id[i] for i in path['cable_ids']]
    statuses = {c['status'] for c in run}
    for status in ('retired', 'dark', 'planned'):
        if status in statuses:
            return {'verdict': status, 'detail': VERDICTS[status]}
    ends = (path['hops'][0], path['hops'][-1])
    checks = [c for c in (_check_end(ends[0], ends[1], switches, items),
                          _check_end(ends[1], ends[0], switches, items)) if c]
    if not checks:
        return {'verdict': 'not_checked', 'detail': VERDICTS['not_checked']}
    # A switch-to-switch run is checked from both ends: report the worst
    return min(checks, key=lambda c: _VERDICT_RANK.index(c['verdict']))


def evaluate(cables: List[dict], items: Dict[str, dict], switches: dict) -> Dict[int, dict]:
    """Trace and compare every cable. Returns {cable_id: {'path', 'verdict', 'detail'}}."""
    switches = with_aliases(switches, items)
    index = node_index(cables)
    by_id = {c['id']: c for c in cables}
    result: Dict[int, dict] = {}
    for c in cables:
        if not _live(c):
            result[c['id']] = {'path': {'hops': [_node(c, 'a'), c['id'], _node(c, 'b')], 'cable_ids': [c['id']]},
                               'verdict': 'retired', 'detail': VERDICTS['retired']}
            continue
        conflict = next((n for n in (_node(c, 'a'), _node(c, 'b'))
                         if n and len([x for x in index.get(n, []) if x['status'] in ('installed', 'dark')]) > 1
                         and c['status'] in ('installed', 'dark')), None)
        path = trace(c, cables, items, index)
        if conflict:
            others = [x.get('label') or f'#{x["id"]}' for x in index[conflict] if x['id'] != c['id']]
            result[c['id']] = {'path': path, 'verdict': 'conflict',
                               'detail': f'{describe_node(conflict)} is also used by {", ".join(others)}'}
            continue
        result[c['id']] = dict(compare_path(path, by_id, switches, items), path=path)
    return result


def describe_node(node: Optional[tuple]) -> str:
    if node is None:
        return 'unterminated'
    device, port, side = node
    text = device
    if port:
        text += f' port {port}' if port.isdigit() else f' {port}'
    if side:
        text += f' ({side})'
    return text


# ── Undocumented links ──────────────────────────────────────────────────────

def undocumented(cables: List[dict], switches: dict, items: Optional[Dict[str, dict]] = None) -> List[dict]:
    """Linked switch ports that no live cable mentions, as suggestions to add."""
    documented = set()
    for c in cables:
        if not _live(c):
            continue
        for end in ('a', 'b'):
            node = _node(c, end)
            if node:
                documented.add((device_of(node[0], items or {}).lower(), node[1]))
    suggestions = []
    for sw_name, sw in sorted((switches or {}).items()):
        for port_name, port in (sw.get('ports') or {}).items():
            idx = port.get('port_idx')
            if not port.get('up') or idx is None or (sw_name.lower(), str(idx)) in documented:
                continue
            lldp = port.get('lldp') or {}
            sfp = port.get('sfp') or {}
            peer = sfp.get('cable_peer')
            suggestion = {
                'switch': sw_name, 'port_idx': idx, 'port_name': port_name,
                'neighbor': lldp.get('system_name') or '', 'neighbor_port': lldp.get('port_id') or '',
                'speed_mbps': port.get('speed_mbps') or 0,
                'cable_type': 'DAC' if sfp.get('kind') == 'dac' else '',
                'via': 'lldp' if lldp.get('system_name') else '',
            }
            if lldp.get('source') == 'unifi-clients':
                suggestion['via'] = 'client table'
            if peer:
                suggestion.update(neighbor=peer['switch'], neighbor_port=str(peer['port_idx']), via='DAC serial')
                # Offer each DAC once, from the end that sorts first
                if (peer['switch'].lower(), str(peer['port_idx'])) in documented:
                    continue
                if (peer['switch'], peer['port_idx']) < (sw_name, idx):
                    continue
            suggestions.append(suggestion)
    return suggestions


# ── Labels to make ──────────────────────────────────────────────────────────

def labels_to_make(cables: List[dict], items: Dict[str, dict], port_labels: List[dict]) -> dict:
    """Cable IDs and panel-port labels that haven't been stuck on yet."""
    cords = [
        {'id': c['id'], 'text': c['label'], 'copies': 2 if c.get('b_device') else 1,
         'where': f'{describe_node(_node(c, "a"))} ↔ {describe_node(_node(c, "b"))}'}
        for c in sorted(cables, key=lambda c: _natural(c.get('label') or ''))
        if c.get('label') and not c.get('label_applied') and c['status'] in ('installed', 'dark')
    ]
    unlabeled = [c['id'] for c in cables if not c.get('label') and c['status'] in ('installed', 'dark')]
    ports = [
        {'item': p['item_name'], 'port': p['port'], 'text': p['label']}
        for p in sorted(port_labels, key=lambda p: (p['item_name'].lower(), _natural(p['port'])))
        if p.get('label') and not p.get('applied')
    ]
    return {'cables': cords, 'ports': ports, 'unlabeled_cable_ids': unlabeled}


def _natural(text: str) -> list:
    return [int(t) if t.isdigit() else t.lower() for t in re.split(r'(\d+)', text or '')]


# ── CSV ─────────────────────────────────────────────────────────────────────

CSV_FIELDS = ['label', 'a_device', 'a_port', 'a_side', 'b_device', 'b_port', 'b_side',
              'cable_type', 'length_m', 'color', 'status', 'label_applied', 'notes']


def cables_to_csv(cables: List[dict]) -> str:
    out = io.StringIO()
    writer = csv.DictWriter(out, fieldnames=CSV_FIELDS, extrasaction='ignore', lineterminator='\n')
    writer.writeheader()
    for c in sorted(cables, key=lambda c: _natural(c.get('label') or '')):
        row = {k: c.get(k) for k in CSV_FIELDS}
        row['label_applied'] = 'yes' if c.get('label_applied') else ''
        row['length_m'] = '' if c.get('length_m') is None else f'{float(c["length_m"]):g}'
        # Keep spreadsheet apps from running a cell as a formula
        writer.writerow({k: _csv_safe(v) for k, v in row.items()})
    return out.getvalue()


def _csv_safe(value) -> str:
    text = '' if value is None else str(value)
    return "'" + text if text[:1] in ('=', '+', '-', '@', '\t', '\r') and not _is_number(text) else text


def _is_number(text: str) -> bool:
    try:
        float(text)
        return True
    except ValueError:
        return False


def parse_csv(text: str, items: Dict[str, dict], max_rows: int = 2000) -> Tuple[List[dict], List[str]]:
    """Validate a cable CSV. Returns (cables, errors); nothing is applied on any error."""
    reader = csv.DictReader(io.StringIO(text))
    missing = [f for f in ('a_device',) if f not in (reader.fieldnames or [])]
    if missing:
        return [], [f'Missing column: {", ".join(missing)}']
    rows, errors, labels = [], [], set()
    for line, raw in enumerate(reader, start=2):
        if line - 1 > max_rows:
            errors.append(f'More than {max_rows} rows')
            break
        data = {k: (v or '').strip() for k, v in raw.items() if k}
        for k in list(data):
            if data[k].startswith("'") and data[k][1:2] in ('=', '+', '-', '@'):
                data[k] = data[k][1:]
        if not any(data.values()):
            continue
        data['label_applied'] = data.get('label_applied', '').lower() in ('yes', 'true', '1', 'y')
        try:
            cable = validate_cable(data, items)
        except ValidationError as e:
            errors.append(f'Line {line}: {e}')
            continue
        if cable['label']:
            if cable['label'].lower() in labels:
                errors.append(f'Line {line}: label {cable["label"]} appears twice')
                continue
            labels.add(cable['label'].lower())
        rows.append(cable)
    return rows, errors


# ── Problems, for the monitor ───────────────────────────────────────────────

def path_text(hops: list) -> str:
    return ' → '.join(describe_node(h) for h in hops if not isinstance(h, int))


def runs_for(device: str, cables: List[dict], items: Dict[str, dict], switches: dict,
             port: Optional[str] = None) -> List[dict]:
    """The live cables ending on a device (or one of its ports), each with its full run.

    Returns [{'cable_id', 'label', 'port', 'status', 'verdict', 'detail', 'path'}].
    """
    results = None
    out = []
    for c in sorted(cables, key=lambda c: _natural(c.get('label') or '')):
        if not _live(c):
            continue
        for end in ('a', 'b'):
            if not _same(device_of(c.get(f'{end}_device') or '', items), device):
                continue
            if port is not None and str(c.get(f'{end}_port') or '') != str(port):
                continue
            if results is None:
                results = evaluate(cables, items, switches or {})
            r = results[c['id']]
            out.append({
                'cable_id': c['id'], 'label': c.get('label') or f'#{c["id"]}',
                'port': c.get(f'{end}_port') or '', 'status': c['status'],
                'verdict': r['verdict'], 'detail': r['detail'], 'path': path_text(r['path']['hops']),
            })
    return out


def problems(cables: List[dict], items: Dict[str, dict], switches: dict) -> List[dict]:
    """One entry per installed run whose check found a problem.

    A run through a panel is several cables with the same verdict; it is reported
    once, under the cable on the switch port (the one people look for).
    Returns [{'key', 'cable_id', 'label', 'switch', 'port', 'verdict', 'detail', 'path'}].
    """
    switches = switches or {}
    results = evaluate(cables, items, switches)
    by_id = {c['id']: c for c in cables}
    seen_runs = set()
    found = []
    for cid, r in sorted(results.items()):
        if r['verdict'] not in PROBLEM_VERDICTS or by_id[cid]['status'] != 'installed':
            continue
        run = tuple(sorted(r['path']['cable_ids']))
        if run in seen_runs and r['verdict'] != 'conflict':
            continue
        seen_runs.add(run)
        hops = r['path']['hops']
        ends = [n for n in (hops[0], hops[-1]) if n and n[0] in switches]
        switch_end = ends[0] if ends else _node(by_id[cid], 'a')
        # Name the run after the cable plugged into the switch, when there is one
        lead = next((by_id[i] for i in r['path']['cable_ids']
                     if switch_end in (_node(by_id[i], 'a'), _node(by_id[i], 'b'))), by_id[cid])
        if r['verdict'] == 'conflict':
            lead = by_id[cid]
        found.append({
            'key': f'cable {lead["id"]}',
            'cable_id': lead['id'],
            'label': lead.get('label') or f'#{lead["id"]}',
            'switch': switch_end[0] if switch_end else lead['a_device'],
            'port': switch_end[1] if switch_end else lead['a_port'],
            'verdict': r['verdict'],
            'detail': r['detail'],
            'path': path_text(hops),
        })
    return found


# ── Rack elevation ──────────────────────────────────────────────────────────

def _sides(item: dict) -> Tuple[str, ...]:
    mount = item.get('mount') or 'front'
    return ('front', 'rear') if mount == 'full' else (mount,)


def rack_layout(items: List[dict], units: int, face: str = 'front') -> dict:
    """Place items that have a rack unit, top of the rack first, as seen from one face.

    Front and rear items may share a unit; full-depth items take it on both sides.
    An item that would share a unit and side with one already placed, or runs past the
    top of the rack, is reported (whichever face is asked for) rather than drawn over another.
    Returns {'placed': [item + 'rack_top' + 'through'], 'overlaps': [{'name', 'units', 'with'}],
    'outside': [name]}, where 'through' marks a full-depth item seen from the side it isn't faced to
    (its back, for the rear view).
    """
    taken: Dict[tuple, str] = {}
    placed, overlaps, outside = [], [], []
    racked = [i for i in items if i.get('rack_u')]
    # Top first; where two claim the same top unit, the one recorded first is drawn
    for item in sorted(racked, key=lambda i: (-(i['rack_u'] + (i.get('rack_height') or 1)), i.get('id') or 0)):
        low = item['rack_u']
        high = low + (item.get('rack_height') or 1) - 1
        span = f'U{low}' if high == low else f'U{low}–{high}'
        if high > units:
            outside.append({'name': item['name'], 'units': span})
            continue
        sides = _sides(item)
        cells = [(u, s) for u in range(low, high + 1) for s in sides]
        clash = sorted({taken[c] for c in cells if c in taken})
        if clash:
            overlaps.append({'name': item['name'], 'units': span, 'with': clash})
            continue
        for c in cells:
            taken[c] = item['name']
        if face in sides:
            placed.append(dict(item, rack_top=high, through=item.get('mount') == 'full' and face == 'rear'))
    return {'placed': placed, 'overlaps': overlaps, 'outside': outside}


_MARK_RANK = {'bad': 3, 'doc': 2, 'ok': 1}


def port_marks(cables: List[dict]) -> Dict[tuple, dict]:
    """Per (device, port): the worst check of the live cables plugged in there.

    `cables` are evaluated cables (with 'verdict', 'detail' and 'run', the IDs of
    every cable in its run). Returns {(device, port): {'mark', 'text', 'cable_id', 'run'}},
    where mark is ok (matches), bad (a problem) or doc (documented, not confirmed).
    """
    out: Dict[tuple, dict] = {}
    for c in cables:
        if not _live(c):
            continue
        mark = 'bad' if c['verdict'] in PROBLEM_VERDICTS else ('ok' if c['verdict'] == 'match' else 'doc')
        label = c.get('label') or f'#{c["id"]}'
        for end in ('a', 'b'):
            device, port = c.get(f'{end}_device'), c.get(f'{end}_port')
            if not device or not port:
                continue
            entry = out.setdefault((device, str(port)), {'mark': mark, 'cable_id': c['id'], 'run': [], 'by_detail': {}})
            if _MARK_RANK[mark] > _MARK_RANK[entry['mark']]:
                entry.update(mark=mark, cable_id=c['id'])
            entry['by_detail'].setdefault(c['detail'], []).append(label)
            entry['run'] = sorted(set(entry['run']) | set(c.get('run') or [c['id']]))
    for entry in out.values():
        # A panel port's front and rear cables share one check: say it once
        entry['text'] = '; '.join(f'cable{"s" if len(labels) > 1 else ""} {", ".join(labels)}: {detail}'
                                  for detail, labels in entry.pop('by_detail').items())
    return out


def validate_rack(data: dict) -> dict:
    """A rack from the rack settings form."""
    rack = {
        'name':  _text(data, 'name', RACK_NAME_MAX, required=True, label='Rack name'),
        'units': _int(data, 'units', 1, RACK_UNITS_MAX, 'Height'),
        'notes': _text(data, 'notes', NOTES_MAX),
    }
    if not rack['units']:
        raise ValidationError('Set the rack height in units')
    return rack