"""Topological Scheduling Engine for Construction Projects.

Features:
- Cycle Detection (DAG validation)
- Forward Pass (Early Start / Early Finish propagation based on FS, SS, FF, SF + lag)
- Backward Pass (Late Start / Late Finish & Float calculation)
- Critical Path Method (CPM) identification
- Summary task rollup (parent tasks envelop child start/end and progress)
"""
from collections import defaultdict, deque
from datetime import date, timedelta
import uuid

from app.utils.date_service import date_service


class CycleError(Exception):
    """Raised when a dependency would introduce a cycle in the project graph."""
    pass


def check_for_cycle(
    existing_edges: list[tuple[uuid.UUID, uuid.UUID]],
    new_from: uuid.UUID,
    new_to: uuid.UUID,
) -> bool:
    """Return True if adding (new_from -> new_to) creates a cycle in the directed graph."""
    if new_from == new_to:
        return True

    adj: dict[uuid.UUID, list[uuid.UUID]] = defaultdict(list)
    for u, v in existing_edges:
        adj[u].append(v)
    adj[new_from].append(new_to)

    visited: set[uuid.UUID] = set()
    rec_stack: set[uuid.UUID] = set()

    def dfs(node: uuid.UUID) -> bool:
        visited.add(node)
        rec_stack.add(node)
        for neighbor in adj.get(node, []):
            if neighbor not in visited:
                if dfs(neighbor):
                    return True
            elif neighbor in rec_stack:
                return True
        rec_stack.remove(node)
        return False

    all_nodes = set(adj.keys())
    for node in all_nodes:
        if node not in visited:
            if dfs(node):
                return True
    return False


def calculate_schedule(
    tasks: list,
    relations: list,
    project_start: date | None = None,
) -> dict:
    """Perform CPM forward and backward pass on tasks according to relations.

    Returns dict with:
    - tasks_schedule: {task_id: {"planned_start": date, "planned_end": date, "duration": int, "is_critical": bool, "float_days": int}}
    - critical_path: list of task_ids
    """
    if not tasks:
        return {"tasks_schedule": {}, "critical_path": []}

    task_map = {t.id: t for t in tasks}
    in_degree: dict[uuid.UUID, int] = {t.id: 0 for t in tasks}
    adj: dict[uuid.UUID, list] = defaultdict(list)
    reverse_adj: dict[uuid.UUID, list] = defaultdict(list)

    for rel in relations:
        if rel.from_task_id in task_map and rel.to_task_id in task_map:
            adj[rel.from_task_id].append(rel)
            reverse_adj[rel.to_task_id].append(rel)
            in_degree[rel.to_task_id] += 1

    # 1. Topological Sort via Kahn's algorithm
    queue = deque([tid for tid, deg in in_degree.items() if deg == 0])
    topo_order: list[uuid.UUID] = []

    while queue:
        u = queue.popleft()
        topo_order.append(u)
        for rel in adj[u]:
            v = rel.to_task_id
            in_degree[v] -= 1
            if in_degree[v] == 0:
                queue.append(v)

    if len(topo_order) < len(tasks):
        # Fallback if a cycle somehow exists: process in default order
        topo_order = [t.id for t in tasks]

    # Baseline anchor date
    base_date = project_start or date_service.today()

    # 2. Forward Pass (Calculate Early Start ES and Early Finish EF)
    es: dict[uuid.UUID, date] = {}
    ef: dict[uuid.UUID, date] = {}
    durations: dict[uuid.UUID, int] = {}

    for tid in topo_order:
        t = task_map[tid]
        # مدت = فاصلهٔ روزها؛ صفر یعنی شروع و پایان همان روز
        duration = max(t.duration or 0, 0)
        durations[tid] = duration

        # Default start from current task planned_start or project base
        initial_start = t.planned_start or base_date

        incoming = reverse_adj.get(tid, [])
        if not incoming:
            es[tid] = initial_start
        else:
            cand_starts = [initial_start]
            for rel in incoming:
                p_id = rel.from_task_id
                p_ef = ef.get(p_id, initial_start)
                p_es = es.get(p_id, initial_start)
                lag = rel.lag_days or 0
                rtype = (rel.relation_type or "FS").upper()

                if rtype == "FS":
                    # Successor starts after Predecessor finishes + lag
                    cand_starts.append(p_ef + timedelta(days=lag))
                elif rtype == "SS":
                    # Successor starts after Predecessor starts + lag
                    cand_starts.append(p_es + timedelta(days=lag))
                elif rtype == "FF":
                    # Successor finishes after Predecessor finishes + lag -> adjust start
                    needed_ef = p_ef + timedelta(days=lag)
                    cand_starts.append(needed_ef - timedelta(days=duration))
                elif rtype == "SF":
                    # Successor finishes after Predecessor starts + lag
                    needed_ef = p_es + timedelta(days=lag)
                    cand_starts.append(needed_ef - timedelta(days=duration))

            es[tid] = max(cand_starts)

        ef[tid] = es[tid] + timedelta(days=duration)

    # 3. Backward Pass (Calculate Late Start LS, Late Finish LF, Float)
    project_max_ef = max(ef.values()) if ef else base_date
    lf: dict[uuid.UUID, date] = {}
    ls: dict[uuid.UUID, date] = {}

    for tid in reversed(topo_order):
        duration = durations[tid]
        outgoing = adj.get(tid, [])

        if not outgoing:
            lf[tid] = project_max_ef
        else:
            cand_finishes = []
            for rel in outgoing:
                s_id = rel.to_task_id
                s_ls = ls.get(s_id, project_max_ef)
                s_lf = lf.get(s_id, project_max_ef)
                lag = rel.lag_days or 0
                rtype = (rel.relation_type or "FS").upper()

                if rtype == "FS":
                    cand_finishes.append(s_ls - timedelta(days=lag))
                elif rtype == "SS":
                    needed_s_start = s_ls - timedelta(days=lag)
                    cand_finishes.append(needed_s_start + timedelta(days=duration))
                elif rtype == "FF":
                    cand_finishes.append(s_lf - timedelta(days=lag))
                elif rtype == "SF":
                    needed_s_finish = s_lf - timedelta(days=lag)
                    cand_finishes.append(needed_s_finish + timedelta(days=duration))

            lf[tid] = min(cand_finishes) if cand_finishes else project_max_ef

        ls[tid] = lf[tid] - timedelta(days=duration)

    # 4. Critical Path Identification (Total Float == 0)
    critical_path: list[uuid.UUID] = []
    tasks_schedule: dict[uuid.UUID, dict] = {}

    for tid in topo_order:
        t_es = es[tid]
        t_ef = ef[tid]
        t_ls = ls.get(tid, t_es)
        t_lf = lf.get(tid, t_ef)
        float_days = (t_ls - t_es).days
        is_critical = float_days <= 0

        if is_critical:
            critical_path.append(tid)

        tasks_schedule[tid] = {
            "planned_start": t_es,
            "planned_end": t_ef,
            "duration": durations[tid],
            "float_days": max(float_days, 0),
            "is_critical": is_critical,
        }

    return {
        "tasks_schedule": tasks_schedule,
        "critical_path": critical_path,
        "project_finish": project_max_ef,
    }


def rollup_parent_tasks(tasks: list) -> dict[uuid.UUID, dict]:
    """Roll up dates and progress from children to parent tasks."""
    parent_map: dict[uuid.UUID, list] = defaultdict(list)
    task_map = {t.id: t for t in tasks}

    for t in tasks:
        if t.parent_id and t.parent_id in task_map:
            parent_map[t.parent_id].append(t)

    rollups: dict[uuid.UUID, dict] = {}
    for pid, children in parent_map.items():
        child_starts = [c.planned_start for c in children if c.planned_start]
        child_ends = [c.planned_end for c in children if c.planned_end]
        child_progs = [c.progress or 0 for c in children]

        p_start = min(child_starts) if child_starts else None
        p_end = max(child_ends) if child_ends else None
        p_dur = None
        if p_start and p_end:
            p_dur = date_service.calculate_duration(p_start, p_end)

        rollups[pid] = {
            "planned_start": p_start,
            "planned_end": p_end,
            "duration": p_dur,
            "progress": round(sum(child_progs) / len(child_progs)) if child_progs else 0,
        }

    return rollups


async def sync_project_lifecycle_dates(db, project_id: uuid.UUID):
    """Synchronize Project planned_start, planned_end, actual_start, actual_end, and progress from its tasks.

    Business rules:
    - planned_start: planned_start of the first task (earliest planned_start)
    - planned_end: planned_end of the last task (latest planned_end)
    - actual_start: actual_start of the first task (or earliest actual_start across tasks)
    - actual_end: actual_end of the last task
    """
    from sqlalchemy import select
    from app.models import Project, Task

    project = await db.get(Project, project_id)
    if not project:
        return None

    tasks_result = await db.execute(select(Task).where(Task.project_id == project_id))
    tasks = tasks_result.scalars().all()

    if not tasks:
        project.planned_start = None
        project.planned_end = None
        project.actual_start = None
        project.actual_end = None
        project.progress = 0.0
        return project

    valid_starts = [t for t in tasks if t.planned_start is not None]
    valid_ends = [t for t in tasks if t.planned_end is not None]

    if valid_starts:
        first_task = min(valid_starts, key=lambda t: (t.planned_start, t.created_at))
        project.planned_start = first_task.planned_start
        all_actual_starts = [t.actual_start for t in tasks if t.actual_start is not None]
        project.actual_start = first_task.actual_start or (min(all_actual_starts) if all_actual_starts else None)
    else:
        project.planned_start = None
        all_actual_starts = [t.actual_start for t in tasks if t.actual_start is not None]
        project.actual_start = min(all_actual_starts) if all_actual_starts else None

    if valid_ends:
        last_task = max(valid_ends, key=lambda t: (t.planned_end, t.created_at))
        project.planned_end = last_task.planned_end
        project.actual_end = last_task.actual_end
    else:
        project.planned_end = None
        project.actual_end = None

    progs = [t.progress or 0 for t in tasks]
    if progs:
        project.progress = round(sum(progs) / len(progs), 1)

    return project