"""Coaching method: cost = Σ(lesson_type_price × hours) over a lesson plan.

Base lesson-type prices reproduce real quotes (GA4/GTM 중급 → ₩1,570,000).
Optional matrix mode prices by subject level (base × level_multiple).
Student-count discount is opt-in (real 1:1 quotes apply none).
"""


def _discount(rate, students, apply):
    if not apply:
        return 0.0
    for mx, r in rate["coaching"]["student_discount"]["bands"]:
        if students <= mx:
            return float(r)
    return 0.0  # 30+ → 별도 협의


def _unit_price(rate, lesson, mode):
    c = rate["coaching"]
    base = c["lesson_type_prices"][lesson["type"]]
    if mode == "matrix":
        lvl = c.get("subject_levels", {}).get(lesson.get("subject"))
        if lvl:
            base = base * c["level_multiple"].get(lvl, 1.0)
    return base


def build(scope, rate, catalog):
    c = rate["coaching"]
    mode = scope.get("pricing_mode") or c.get("pricing_mode", "base")
    if scope.get("lessons"):
        lessons = scope["lessons"]
        title = scope.get("course") or "맞춤 코칭"
    else:
        cname = scope.get("course")
        courses = catalog.get("courses", {})
        if cname not in courses:
            raise SystemExit(f"course '{cname}' not in catalog {list(courses)}")
        lessons = courses[cname]["lessons"]
        title = courses[cname].get("title", cname)

    items, subtotal = [], 0.0
    for L in lessons:
        up = _unit_price(rate, L, mode)
        amt = up * L["hours"]
        items.append({"subject": L.get("subject", ""), "title": L.get("title", ""),
                      "type": L["type"], "hours": L["hours"], "unit_price": up, "amount": amt})
        subtotal += amt

    students = scope.get("students", 1)
    apply = scope.get("apply_student_discount", c["student_discount"]["apply_default"])
    disc = _discount(rate, students, apply)
    disc_amt = subtotal * disc
    total = subtotal - disc_amt
    return {
        "kind": "coaching", "service": catalog["service"], "label": title,
        "pricing_mode": mode, "students": students,
        "discount_rate": disc, "discount_amount": disc_amt,
        "lessons": items, "subtotal_sum": subtotal, "proposal": int(round(total)),
        "stub": bool(catalog.get("_stub", False)),
    }