import os
import io
import json
from typing import Optional
from contextlib import asynccontextmanager
from pathlib import Path

import pandas as pd
from fastapi import FastAPI, HTTPException
from fastapi.middleware.cors import CORSMiddleware
from fastapi.responses import HTMLResponse
from pydantic import BaseModel
from huggingface_hub import HfApi, hf_hub_download

# ── Config ────────────────────────────────────────────────────────────────────
HF_TOKEN   = os.environ.get("HF_TOKEN")
HF_REPO_ID = os.environ.get("HF_REPO_ID")

VACANCY_FILE     = "hostel_vacancy.xlsx"
CASTE_FILE       = "caste.xlsx"
MERIT_FILE       = "Merit Data.xlsx"
MASTERS_FILE     = "AI Masters Data 13052026 (1).xlsx"
ALLOCATIONS_FILE = "hostel_allocations.json"

# ── In-memory state ───────────────────────────────────────────────────────────
vacancy_df:       pd.DataFrame   = pd.DataFrame()
merit_df:         pd.DataFrame   = pd.DataFrame()
caste_map:        dict[int, str] = {}   # CasteCategoryID → name
caste_detail_map: dict[int, dict]= {}   # caste_id → {name, category_id, category_name}
hostel_map:       dict[int, dict]= {}   # HostelID → {name, type, district, division}
district_map:     dict[int, str] = {}   # Districtcode → Districtname
course_map        = {1: "Undergraduate", 2: "Postgraduate"}
gender_map        = {1: "Male", 2: "Female"}
hostel_type_map   = {1: "Boys", 2: "Girls", 0: "Other"}   # 1=Boys, 2=Girls per spec
allocations:      list[dict]     = []

# ── HF helpers ────────────────────────────────────────────────────────────────
hf_api = HfApi()
_repo_file_cache: dict[str, str] = {}   # lowercase → actual filename


def _build_file_cache():
    global _repo_file_cache
    try:
        files = list(hf_api.list_repo_files(repo_id=HF_REPO_ID, repo_type="dataset", token=HF_TOKEN))
        _repo_file_cache = {f.lower(): f for f in files}
        print(f"HF repo files: {list(_repo_file_cache.values())}")
    except Exception as e:
        print(f"Warning: could not list repo files: {e}")


def hf_download(filename: str) -> bytes | None:
    # Try exact name first
    try:
        path = hf_hub_download(
            repo_id=HF_REPO_ID, filename=filename,
            repo_type="dataset", token=HF_TOKEN,
        )
        with open(path, "rb") as f:
            return f.read()
    except Exception:
        pass

    # Build cache and try case-insensitive match
    if not _repo_file_cache:
        _build_file_cache()

    actual = _repo_file_cache.get(filename.lower())
    if actual and actual != filename:
        print(f"Case-insensitive match: '{filename}' -> '{actual}'")
        try:
            path = hf_hub_download(
                repo_id=HF_REPO_ID, filename=actual,
                repo_type="dataset", token=HF_TOKEN,
            )
            with open(path, "rb") as f:
                return f.read()
        except Exception as e:
            print(f"Failed to download '{actual}': {e}")

    print(f"File not found: '{filename}'. Available: {list(_repo_file_cache.values())}")
    return None


def hf_upload_bytes(content: bytes, filename: str):
    hf_api.upload_file(
        path_or_fileobj=io.BytesIO(content),
        path_in_repo=filename,
        repo_id=HF_REPO_ID,
        repo_type="dataset",
        token=HF_TOKEN,
    )


def clean_cols(df: pd.DataFrame) -> pd.DataFrame:
    df.columns = [c.lstrip("# ").strip() for c in df.columns]
    return df


# ── Loaders ───────────────────────────────────────────────────────────────────
def load_vacancy() -> pd.DataFrame:
    raw = hf_download(VACANCY_FILE)
    if raw is None:
        raise RuntimeError(f"{VACANCY_FILE} not found in HF repo.")
    df = clean_cols(pd.read_excel(io.BytesIO(raw)))
    return df


def load_merit() -> pd.DataFrame:
    raw = hf_download(MERIT_FILE)
    if raw is None:
        raise RuntimeError(f"{MERIT_FILE} not found in HF repo.")
    df = clean_cols(pd.read_excel(io.BytesIO(raw)))
    return df


def load_masters() -> dict:
    raw = hf_download(MASTERS_FILE)
    if raw is None:
        raise RuntimeError(f"{MASTERS_FILE} not found in HF repo.")
    sheets = pd.read_excel(io.BytesIO(raw), sheet_name=None)
    return {k: clean_cols(v) for k, v in sheets.items()}


def load_caste_file() -> dict[int, str]:
    raw = hf_download(CASTE_FILE)
    if raw is None:
        return {}
    df = clean_cols(pd.read_excel(io.BytesIO(raw)))
    return dict(zip(df["CasteCategoryID"].astype(int), df["CasteCategoryName"].str.strip()))


def load_allocations_from_hf() -> list[dict]:
    raw = hf_download(ALLOCATIONS_FILE)
    return json.loads(raw.decode()) if raw else []


def save_allocations_to_hf(data: list[dict]):
    hf_upload_bytes(json.dumps(data, indent=2).encode(), ALLOCATIONS_FILE)


# ── Lifespan ──────────────────────────────────────────────────────────────────
@asynccontextmanager
async def lifespan(app: FastAPI):
    global vacancy_df, merit_df, caste_map, caste_detail_map
    global hostel_map, district_map, allocations

    _build_file_cache()   # list repo files once for case-insensitive matching
    vacancy_df  = load_vacancy()
    merit_df    = load_merit()
    masters     = load_masters()
    caste_map   = load_caste_file()

    # District map
    dist_df = masters.get("District", pd.DataFrame())
    if not dist_df.empty:
        district_map = dict(zip(
            dist_df["Districtcode"].astype(int),
            dist_df["Districtname"].str.strip()
        ))

    # Category map (from masters sheet, supplement caste.xlsx)
    cat_df = masters.get("Category", pd.DataFrame())
    if not cat_df.empty:
        for _, row in cat_df.iterrows():
            cid = int(row["CasteCategoryID"])
            if cid not in caste_map:
                caste_map[cid] = str(row["CasteCategoryName"]).strip()

    # Caste detail map: caste_id → {name, category_id, category_name}
    caste_df = masters.get("Caste", pd.DataFrame())
    if not caste_df.empty:
        for _, row in caste_df.iterrows():
            cid  = int(row["caste_id"])
            catid = int(row["caste_category_id"])
            caste_detail_map[cid] = {
                "caste_id":     cid,
                "caste_name":   str(row["caste_name"]).strip(),
                "category_id":  catid,
                "category_name": caste_map.get(catid, f"Category {catid}"),
            }

    # Build reverse district name→code map for hostel enrichment
    name_to_dist_code: dict[str, int] = {}
    for code, name in district_map.items():
        name_to_dist_code[name.lower().strip()] = code
    # aliases
    name_to_dist_code["ahmednagar"] = name_to_dist_code.get("ahilyanagar", 0)

    # Hostel map
    hostel_df = masters.get("Hostels", pd.DataFrame())
    if not hostel_df.empty:
        for _, row in hostel_df.iterrows():
            raw_id = str(row["HostelID"]).strip()
            if not raw_id.isdigit():
                continue   # skip bad rows (address in ID column etc.)
            hid        = int(raw_id)
            h_type_int = int(row["HostelType"]) if pd.notna(row.get("HostelType")) else 0
            dist_name  = str(row.get("District_in_English", "")).strip()
            dist_code  = name_to_dist_code.get(dist_name.lower().strip(), 0)
            hostel_map[hid] = {
                "hostel_id":    hid,
                "name":         str(row.get("HostelName", row.get("HostelName_Old", ""))).strip(),
                "type":         hostel_type_map.get(h_type_int, "Other"),
                "type_code":    h_type_int,   # 1=Boys, 2=Girls
                "district":     dist_name,
                "district_code":dist_code,
                "division":     str(row.get("Division_in_English", "")).strip(),
            }

    allocations = load_allocations_from_hf()
    print(f"✓ {len(vacancy_df)} vacancy rows | {len(merit_df)} merit students | "
          f"{len(hostel_map)} hostels | {len(caste_map)} categories | "
          f"{len(caste_detail_map)} castes | {len(allocations)} allocations")
    yield


app = FastAPI(title="Hostel Allocation API", lifespan=lifespan)
app.add_middleware(CORSMiddleware, allow_origins=["*"], allow_methods=["*"], allow_headers=["*"])

BASE_DIR = Path(__file__).parent

@app.get("/", response_class=HTMLResponse)
def serve_ui():
    p = BASE_DIR / "index.html"
    return HTMLResponse(p.read_text() if p.exists() else "<h2>index.html not found</h2>")


# ── Schemas ───────────────────────────────────────────────────────────────────
class StudentRequest(BaseModel):
    application_no:            int
    hostel_id:                 int
    course_type_id:            int
    caste_category_vacancy_id: int

class AllocateResponse(BaseModel):
    application_no:            int
    student_name:              str
    hostel_id:                 int
    hostel_name:               str
    course_type_id:            int
    course_type_name:          str
    caste_category_vacancy_id: int
    caste_category_name:       str
    marks_percentage:          float
    merit_rank:                int
    priority:                  int
    allocation_percentage:     float
    status:                    str
    message:                   str


# ── Core helpers ──────────────────────────────────────────────────────────────
def get_vacancy_row(hostel_id: int, course_type_id: int, caste_id: int) -> pd.Series | None:
    mask = (
        (vacancy_df["HostelId"]                 == hostel_id)
        & (vacancy_df["CourseTypeId"]           == course_type_id)
        & (vacancy_df["CasteCategoryVacancyId"] == caste_id)
    )
    rows = vacancy_df[mask]
    return rows.iloc[0] if not rows.empty else None


def get_merit_student(application_no: int) -> pd.Series | None:
    rows = merit_df[merit_df["applicationno"] == application_no]
    return rows.iloc[0] if not rows.empty else None


def get_slot_allocations(hostel_id: int, course_type_id: int, caste_id: int) -> list[dict]:
    return [
        a for a in allocations
        if a["hostel_id"]                  == hostel_id
        and a["course_type_id"]            == course_type_id
        and a["caste_category_vacancy_id"] == caste_id
        and a["status"] in ("allocated", "waitlisted")
    ]


def recalculate_priorities(slot_list: list[dict]) -> list[dict]:
    slot_list.sort(key=lambda x: (x["marks_percentage"], -x["merit_rank"]), reverse=True)
    n = len(slot_list)
    for rank, s in enumerate(slot_list, start=1):
        s["priority"]              = rank
        s["allocation_percentage"] = round((n - rank + 1) / n * 100, 2)
    return slot_list


def update_global_allocations(updated_slot: list[dict]):
    if not updated_slot:
        return
    h, c, k = updated_slot[0]["hostel_id"], updated_slot[0]["course_type_id"], updated_slot[0]["caste_category_vacancy_id"]
    global allocations
    allocations = [
        a for a in allocations
        if not (a["hostel_id"] == h and a["course_type_id"] == c and a["caste_category_vacancy_id"] == k)
    ]
    allocations.extend(updated_slot)


def enrich_record(record: dict) -> dict:
    r = dict(record)
    hid = r.get("hostel_id", 0)
    h = hostel_map.get(hid, {})
    r["hostel_name"]        = h.get("name", f"Hostel {hid}")
    r["course_type_name"]   = course_map.get(r.get("course_type_id", 0), "Unknown")
    r["caste_category_name"]= caste_map.get(r.get("caste_category_vacancy_id", 0), "Unknown")
    return r


# ── Routes ────────────────────────────────────────────────────────────────────
@app.get("/health")
def health():
    return {
        "status": "ok",
        "vacancy_rows":    len(vacancy_df),
        "merit_students":  len(merit_df),
        "hostels":         len(hostel_map),
        "caste_categories":len(caste_map),
        "allocations":     len(allocations),
    }


@app.get("/debug/files")
def debug_files():
    """List all files found in the HF dataset repo. Use this to diagnose filename issues."""
    _build_file_cache()
    return {
        "repo_id": HF_REPO_ID,
        "files": list(_repo_file_cache.values()),
        "expected": [VACANCY_FILE, CASTE_FILE, MERIT_FILE, MASTERS_FILE, ALLOCATIONS_FILE],
        "missing": [
            f for f in [VACANCY_FILE, CASTE_FILE, MERIT_FILE, MASTERS_FILE]
            if f.lower() not in _repo_file_cache
        ]
    }


# ── Masters ───────────────────────────────────────────────────────────────────
@app.get("/masters/castes")
def list_castes():
    return [{"CasteCategoryID": k, "CasteCategoryName": v} for k, v in sorted(caste_map.items())]


@app.get("/masters/hostels")
def list_hostels(gender: Optional[int] = None, district_code: Optional[int] = None):
    """gender: 1=Male->Boys hostels, 2=Female->Girls hostels. district_code = student DistrictID."""
    result = list(hostel_map.values())
    if gender in (1, 2):
        result = [h for h in result if h.get("type_code") == gender]
    if district_code:
        result = [h for h in result if h.get("district_code") == district_code]
    return sorted(result, key=lambda x: x["hostel_id"])


@app.get("/masters/districts")
def list_districts():
    return [{"code": k, "name": v} for k, v in sorted(district_map.items())]


@app.get("/masters/course-types")
def list_course_types():
    return [{"id": k, "name": v} for k, v in course_map.items()]


# ── Student lookup ────────────────────────────────────────────────────────────
@app.get("/student/{application_no}")
def get_student(application_no: int):
    s = get_merit_student(application_no)
    if s is None:
        raise HTTPException(404, f"Application no {application_no} not found in merit data.")

    caste_id   = int(s["Caste"]) if pd.notna(s["Caste"]) else None
    caste_info = caste_detail_map.get(caste_id, {}) if caste_id else {}
    cat_id     = caste_info.get("category_id")
    dist_id    = int(s["DistrictID"]) if pd.notna(s["DistrictID"]) else None
    hostel_id  = int(s["HostelID"]) if pd.notna(s["HostelID"]) else None

    gender_int = int(s["gender"]) if pd.notna(s["gender"]) else 0
    # gender 1=Male→Boys hostels(type 1), gender 2=Female→Girls hostels(type 2)
    expected_hostel_type = gender_int  # 1 or 2 maps directly

    return {
        "application_no":          int(s["applicationno"]),
        "user_id":                 int(s["UserId"]) if pd.notna(s["UserId"]) else None,
        "gender":                  gender_map.get(gender_int, "Unknown"),
        "gender_int":              gender_int,
        "expected_hostel_type":    expected_hostel_type,
        "course_type_id":          int(s["CourseType"]),
        "course_type_name":        course_map.get(int(s["CourseType"]), "Unknown"),
        "caste_id":                caste_id,
        "caste_name":              caste_info.get("caste_name", "Unknown"),
        "caste_category_id":       cat_id,
        "caste_category_name":     caste_map.get(cat_id, "Unknown") if cat_id else "Unknown",
        "current_percentage":      float(s["CurrentPercentage"]) if pd.notna(s["CurrentPercentage"]) else None,
        "past_percentage":         float(s["PastPercentage"]) if pd.notna(s["PastPercentage"]) else None,
        "merit_rank":              int(s["MeritRank"]) if pd.notna(s["MeritRank"]) else None,
        "general_merit_rank":      int(s["GeneralMeritRank"]) if pd.notna(s["GeneralMeritRank"]) else None,
        "district_id":             dist_id,
        "district_name":           district_map.get(dist_id, "Unknown") if dist_id else "Unknown",
        "district_code":           dist_id,
        "is_orphan":               bool(s["IsOrphan"]),
        "is_disabled":             bool(s["IsDisable"]) if pd.notna(s["IsDisable"]) else False,
        "is_already_allotted":     bool(s["isalloted"]) if pd.notna(s["isalloted"]) else False,
        "existing_hostel_id":      hostel_id,
        "existing_hostel_name":    hostel_map.get(hostel_id, {}).get("name", "") if hostel_id else None,
        "scrutiny_approved":       bool(s["IsScrutinyApproved"]) if pd.notna(s["IsScrutinyApproved"]) else False,
    }


# ── Vacancy ───────────────────────────────────────────────────────────────────
@app.get("/vacancy")
def list_vacancy(hostel_id: Optional[int] = None, caste_id: Optional[int] = None,
                 gender: Optional[int] = None, district_id: Optional[int] = None):
    rows = vacancy_df.to_dict(orient="records")
    result = []
    for r in rows:
        hid  = int(r["HostelId"])
        cid  = int(r["CasteCategoryVacancyId"])
        dist = int(r["DistrictId"])
        h    = hostel_map.get(hid, {})
        if hostel_id  and hid != hostel_id:   continue
        if caste_id   and cid != caste_id:    continue
        if district_id and dist != district_id: continue
        if gender in (1, 2) and h.get("type_code", 0) != gender: continue
        result.append({
            **r,
            "HostelName":        h.get("name", f"Hostel {hid}"),
            "HostelType":        h.get("type", ""),
            "HostelTypeInt":     h.get("type_code", 0),
            "DistrictName":      district_map.get(dist, str(dist)),
            "CasteCategoryName": caste_map.get(cid, f"Category {cid}"),
        })
    return result


@app.get("/vacancy/{hostel_id}/{course_type_id}/{caste_id}")
def get_vacancy(hostel_id: int, course_type_id: int, caste_id: int):
    row = get_vacancy_row(hostel_id, course_type_id, caste_id)
    if row is None:
        raise HTTPException(404, "Vacancy record not found.")
    h = hostel_map.get(hostel_id, {})
    return {
        **row.to_dict(),
        "HostelName":        h.get("name", f"Hostel {hostel_id}"),
        "CasteCategoryName": caste_map.get(caste_id, f"Category {caste_id}"),
    }


# ── Allocation chances preview ────────────────────────────────────────────────
@app.get("/allocation-chances/{hostel_id}/{course_type_id}/{caste_id}")
def allocation_chances(hostel_id: int, course_type_id: int, caste_id: int,
                       marks: float, merit_rank: Optional[int] = None,
                       gender: Optional[int] = None, district_id: Optional[int] = None):
    h_info   = hostel_map.get(hostel_id, {})
    h_name   = h_info.get("name", f"Hostel {hostel_id}")
    h_type   = h_info.get("type_code", 0)

    # Gender check
    if gender in (1, 2) and h_type in (1, 2) and h_type != gender:
        gender_label = gender_map.get(gender, "Unknown")
        return {
            "hostel_name": h_name, "caste_category": caste_map.get(caste_id, ""),
            "chance_percentage": 0,
            "reason": f"Gender mismatch: {gender_label} student cannot apply to a {h_info.get('type','')} hostel.",
            "remaining_seats": 0, "total_seats": 0, "applicants_in_slot": 0,
            "warning": "gender_mismatch"
        }

    # District check
    if district_id:
        vac_rows = vacancy_df[vacancy_df["HostelId"] == hostel_id]
        if not vac_rows.empty:
            hostel_dist = int(vac_rows.iloc[0]["DistrictId"])
            if district_id != hostel_dist:
                return {
                    "hostel_name": h_name, "caste_category": caste_map.get(caste_id, ""),
                    "chance_percentage": 0,
                    "reason": f"District mismatch: your district ({district_map.get(district_id,district_id)}) "
                              f"vs hostel district ({district_map.get(hostel_dist,hostel_dist)}).",
                    "remaining_seats": 0, "total_seats": 0, "applicants_in_slot": 0,
                    "warning": "district_mismatch"
                }

    row = get_vacancy_row(hostel_id, course_type_id, caste_id)
    if row is None:
        raise HTTPException(404, "Vacancy record not found.")

    total     = int(row["TotalSeat"])
    remaining = int(row["RemainingSeat"])
    cat_name  = caste_map.get(caste_id, f"Category {caste_id}")

    if total == 0:
        return {"chance_percentage": 0, "reason": "No seats in this category.",
                "remaining_seats": 0, "total_seats": 0}

    slot_list = get_slot_allocations(hostel_id, course_type_id, caste_id)
    allocated = [a for a in slot_list if a["status"] == "allocated"]

    if len(allocated) < remaining:
        chance = 100.0
        reason = f"{remaining - len(allocated)} seat(s) still available under '{cat_name}' at {h_name}."
    else:
        weakest_marks = min(a["marks_percentage"] for a in allocated) if allocated else 0
        if marks > weakest_marks:
            chance = round(marks / 100 * 100, 2)
            reason = f"You would displace the weakest applicant ({weakest_marks}% marks) under '{cat_name}'."
        else:
            above  = sum(1 for a in allocated if a["marks_percentage"] >= marks)
            chance = round(max(0, (remaining - above) / total * 100), 2)
            reason = f"All seats full under '{cat_name}'. Waitlisted; rank depends on your merit."

    return {
        "hostel_name":       h_name,
        "caste_category":    cat_name,
        "chance_percentage": chance,
        "reason":            reason,
        "remaining_seats":   remaining,
        "total_seats":       total,
        "applicants_in_slot": len(slot_list),
    }


# ── Allocate ──────────────────────────────────────────────────────────────────
@app.post("/allocate", response_model=AllocateResponse)
def allocate_student(req: StudentRequest):
    global allocations

    # Lookup student from merit data
    s = get_merit_student(req.application_no)
    if s is None:
        raise HTTPException(404, f"Application no {req.application_no} not found in merit data.")

    marks      = float(s["CurrentPercentage"]) if pd.notna(s["CurrentPercentage"]) else 0.0
    merit_rank = int(s["MeritRank"])           if pd.notna(s["MeritRank"])           else 9999
    caste_id   = int(s["Caste"])               if pd.notna(s["Caste"])               else None
    caste_info = caste_detail_map.get(caste_id, {}) if caste_id else {}

    # Validate caste category matches
    if req.caste_category_vacancy_id not in caste_map:
        raise HTTPException(400, f"Invalid caste_category_vacancy_id {req.caste_category_vacancy_id}.")

    # Gender validation: Male→Boys(1), Female→Girls(2)
    gender_int    = int(s["gender"]) if pd.notna(s["gender"]) else 0
    h_info        = hostel_map.get(req.hostel_id, {})
    h_name        = h_info.get("name", f"Hostel {req.hostel_id}")
    h_type_int    = h_info.get("type_code", 0)
    gender_label  = gender_map.get(gender_int, "Unknown")
    hostel_type_label = h_info.get("type", "Unknown")

    if h_type_int in (1, 2) and h_type_int != gender_int:
        raise HTTPException(400,
            f"Gender mismatch: {gender_label} student cannot apply to a {hostel_type_label} hostel ({h_name}).")

    # District: student district must match hostel district (via vacancy DistrictId)
    student_dist_id = int(s["DistrictID"]) if pd.notna(s["DistrictID"]) else None
    vac_rows = vacancy_df[vacancy_df["HostelId"] == req.hostel_id]
    if student_dist_id and not vac_rows.empty:
        hostel_dist_id = int(vac_rows.iloc[0]["DistrictId"])
        if student_dist_id != hostel_dist_id:
            student_dist_name = district_map.get(student_dist_id, str(student_dist_id))
            hostel_dist_name  = district_map.get(hostel_dist_id, str(hostel_dist_id))
            raise HTTPException(400,
                f"District mismatch: Student belongs to '{student_dist_name}' district "
                f"but hostel '{h_name}' is in '{hostel_dist_name}' district.")

    row = get_vacancy_row(req.hostel_id, req.course_type_id, req.caste_category_vacancy_id)
    if row is None:
        raise HTTPException(404, "No vacancy record for given hostel/course/caste combination.")

    total_seats     = int(row["TotalSeat"])
    remaining_seats = int(row["RemainingSeat"])
    cat_name        = caste_map.get(req.caste_category_vacancy_id, "Unknown")

    if total_seats == 0:
        raise HTTPException(400, f"Caste '{cat_name}' has 0 total seats in {h_name}.")

    if any(a["application_no"] == req.application_no for a in allocations):
        raise HTTPException(400, f"Application {req.application_no} already has an allocation entry.")

    slot_list       = get_slot_allocations(req.hostel_id, req.course_type_id, req.caste_category_vacancy_id)
    allocated_count = sum(1 for a in slot_list if a["status"] == "allocated")

    new_record = {
        "application_no":            req.application_no,
        "student_name":              f"Applicant {req.application_no}",
        "hostel_id":                 req.hostel_id,
        "hostel_name":               h_name,
        "hostel_type_code":          h_info.get("type_code", 0),
        "hostel_type":               h_info.get("type", "Unknown"),
        "hostel_district":           h_info.get("district", ""),
        "hostel_district_code":      h_info.get("district_code", 0),
        "course_type_id":            req.course_type_id,
        "course_type_name":          course_map.get(req.course_type_id, "Unknown"),
        "caste_category_vacancy_id": req.caste_category_vacancy_id,
        "caste_category_name":       cat_name,
        "marks_percentage":          marks,
        "merit_rank":                merit_rank,
        "priority":                  0,
        "allocation_percentage":     0.0,
        "status":                    "",
        "message":                   "",
    }

    if allocated_count < remaining_seats:
        new_record["status"] = "allocated"
        slot_list.append(new_record)
        slot_list = recalculate_priorities(slot_list)
        new_record["message"] = (
            f"Room allocated at {h_name} under '{cat_name}'. "
            f"Ranked #{new_record['priority']} of {len(slot_list)} applicant(s). "
            f"Marks: {marks}%, Merit Rank: {merit_rank}."
        )
    else:
        allocated_students = [a for a in slot_list if a["status"] == "allocated"]
        weakest = min(allocated_students, key=lambda x: (x["marks_percentage"], -x["merit_rank"])) \
                  if allocated_students else None

        if weakest and (marks > weakest["marks_percentage"] or
                        (marks == weakest["marks_percentage"] and merit_rank < weakest["merit_rank"])):
            weakest["status"]  = "waitlisted"
            weakest["message"] = (
                f"Displaced by application {req.application_no} "
                f"(marks: {marks}%, rank: {merit_rank}) under '{cat_name}'."
            )
            new_record["status"] = "allocated"
            slot_list.append(new_record)
            slot_list = recalculate_priorities(slot_list)
            new_record["message"] = (
                f"Room allocated at {h_name} under '{cat_name}' by displacing "
                f"application {weakest['application_no']} "
                f"({weakest['marks_percentage']}% marks). Ranked #{new_record['priority']}."
            )
        else:
            new_record["status"] = "waitlisted"
            slot_list.append(new_record)
            slot_list = recalculate_priorities(slot_list)
            new_record["message"] = (
                f"No seat available at {h_name} under '{cat_name}'. "
                f"Waitlisted at #{new_record['priority']} of {len(slot_list)}."
            )

    update_global_allocations(slot_list)
    save_allocations_to_hf(allocations)
    return AllocateResponse(**new_record)


# ── Allocations list ──────────────────────────────────────────────────────────
@app.get("/allocations")
def list_allocations(
    hostel_id:      Optional[int] = None,
    course_type_id: Optional[int] = None,
    caste_id:       Optional[int] = None,
    status:         Optional[str] = None,
):
    result = allocations
    if hostel_id:      result = [a for a in result if a["hostel_id"]                  == hostel_id]
    if course_type_id: result = [a for a in result if a["course_type_id"]             == course_type_id]
    if caste_id:       result = [a for a in result if a["caste_category_vacancy_id"]  == caste_id]
    if status:         result = [a for a in result if a["status"]                     == status]
    return result


@app.delete("/allocations/{application_no}")
def remove_allocation(application_no: int):
    global allocations
    record = next((a for a in allocations if a["application_no"] == application_no), None)
    if record is None:
        raise HTTPException(404, f"Application {application_no} not found.")

    h, c, k = record["hostel_id"], record["course_type_id"], record["caste_category_vacancy_id"]
    allocations = [a for a in allocations if a["application_no"] != application_no]

    slot_list = get_slot_allocations(h, c, k)
    if record["status"] == "allocated":
        waitlisted = [a for a in slot_list if a["status"] == "waitlisted"]
        if waitlisted:
            top = max(waitlisted, key=lambda x: (x["marks_percentage"], -x["merit_rank"]))
            top["status"]  = "allocated"
            top["message"] = (
                f"Promoted from waitlist under '{caste_map.get(k, k)}' due to a vacancy."
            )

    slot_list = recalculate_priorities(slot_list)
    update_global_allocations(slot_list)
    save_allocations_to_hf(allocations)
    return {"message": f"Application {application_no} removed and slot recalculated."}