"""Boolean scope evaluation for Configuration Policies.

A policy's scopejson is a small typed boolean AST over two endpoint dimensions,
PC type and installed application. This module is the ONLY place that interprets
that AST: it validates the shape (known node types, bounded depth) and evaluates
a validated tree against a resolved endpoint context.

Node grammar (all dict nodes carry a "type"):
    {"type": "and",    "nodes": [<node>, ...]}
    {"type": "or",     "nodes": [<node>, ...]}
    {"type": "not",    "node": <node>}
    {"type": "pctype", "id": <computertypeid:int>}
    {"type": "app",    "id": <appid:int>}
    {"type": "allpcs"} # every PC EXCEPT those whose type is designated a server
    {"type": "true"}   # matches everything, servers included

"allpcs" vs "true": authoring an estate-wide policy almost always means "every
PC an operator sits at", not "every computer we have a row for". Those differ by
the servers, where a reboot or a script push is a materially different event.
"true" is retained because a stored scope of None is equivalent to it and older
policies were written with it, but the builder offers "All PCs" instead.

Server designation is read from the computer type's ICON slug ('server'). That
field is nominally a UI hint, which is a real wart - it is load-bearing here. It
was chosen over matching the type NAME because a site can rename "Server" but a
renamed type keeps its icon, and over a new isserver column because that needs a
migration this did not warrant. If server designation ever becomes a first-class
flag, this is the single place that reads it.

Kept pure and dependency-free (no db, no Flask) so it is trivially unit-tested
and so a future GE-Enforce assembly engine can reuse the exact same evaluator on
the exact same AST when it composes policies.
"""

# Bound the tree so a hand-crafted or imported AST cannot cause pathological
# recursion. Ten levels is far past any policy a human would author.
MAX_SCOPE_DEPTH = 10

# The computer-type icon slug that designates a type as a server. See the module
# docstring for why the icon carries this.
SERVER_ICON = 'server'

_LOGICAL = {'and', 'or', 'not'}
_LEAF = {'pctype', 'app', 'true', 'allpcs'}


class ScopeError(ValueError):
    """Raised when a scope AST is malformed. Carries a human-readable reason."""


def validate_scope(node, _depth=0):
    """Validate a scope AST, raising ScopeError on any problem.

    Returns None on success. Call this at write time (CRUD) so a bad tree never
    reaches the database; evaluate_scope then trusts the stored shape.
    """
    if _depth > MAX_SCOPE_DEPTH:
        raise ScopeError(f'scope nested deeper than {MAX_SCOPE_DEPTH} levels')
    if not isinstance(node, dict):
        raise ScopeError('scope node must be an object')

    ntype = node.get('type')
    if ntype not in _LOGICAL and ntype not in _LEAF:
        raise ScopeError(f'unknown scope node type: {ntype!r}')

    if ntype in ('and', 'or'):
        nodes = node.get('nodes')
        if not isinstance(nodes, list) or not nodes:
            raise ScopeError(f'{ntype} requires a non-empty "nodes" list')
        for child in nodes:
            validate_scope(child, _depth + 1)
    elif ntype == 'not':
        child = node.get('node')
        if child is None:
            raise ScopeError('not requires a "node"')
        validate_scope(child, _depth + 1)
    elif ntype in ('pctype', 'app'):
        ident = node.get('id')
        if not isinstance(ident, int) or isinstance(ident, bool):
            raise ScopeError(f'{ntype} requires an integer "id"')
    # 'true' and 'allpcs' carry no payload.


def evaluate_scope(node, context):
    """Evaluate a validated scope AST against an endpoint context.

    context is expected to expose:
        context['computertypeid'] -> int or None
        context['appids']         -> set/collection of installed appids
        context['isserver']       -> bool, whether the type is server-designated

    Returns True when the endpoint falls in scope. An empty/None scope means the
    policy is unscoped and matches everything (same as {"type": "true"}), which
    keeps a freshly-created policy usable before a scope is drawn.
    """
    if node is None:
        return True

    ntype = node['type']
    if ntype == 'true':
        return True
    if ntype == 'allpcs':
        # Absent designation means "not a server": an endpoint with no type set
        # is an ordinary PC until something says otherwise.
        return not context.get('isserver', False)
    if ntype == 'and':
        return all(evaluate_scope(child, context) for child in node['nodes'])
    if ntype == 'or':
        return any(evaluate_scope(child, context) for child in node['nodes'])
    if ntype == 'not':
        return not evaluate_scope(node['node'], context)
    if ntype == 'pctype':
        return context.get('computertypeid') == node['id']
    if ntype == 'app':
        return node['id'] in (context.get('appids') or ())

    # validate_scope should have caught this; be strict rather than silently
    # matching an endpoint against an unknown node.
    raise ScopeError(f'unknown scope node type at evaluation: {ntype!r}')