"""SAP 10.2 Table 13 — high-rate fraction for electric DHW heating.

Sourced verbatim from `domain/sap10_calculator/docs/specs/sap-10-2-full-
specification-2025-03-14.pdf`, page 197 (Table 13). RdSAP10 §10.5 (PDF
p.54) routes electric immersion water heating here via Table 12a's
"Immersion water heater" row, whose Water-heating column reads "Fraction
from Table 13".

The table gives the fraction of DHW electricity consumed at the HIGH
rate for a cylinder with a single or dual immersion heater on an
off-peak tariff; the remainder is at the low rate. Note 2 of the table
supplies exact equations equivalent to the tabulated (rounded) grid —
this module evaluates those equations, so no floor-area interpolation
is needed:

    7-hour tariff (>= 7 hours/day at the low rate)
      Dual:   [(6.8 - 0.024 V) N + 14 - 0.07 V] / 100
      Single: [(14530 - 762 N) / V - 80 + 10 N] / 100

    10-hour tariff (>= 10 hours/day at the low rate)
      Dual:   [(6.8 - 0.036 V) N + 14 - 0.105 V] / 100
      Single: [(14530 - 762 N) / (1.5 V) - 80 + 10 N] / 100

where V is the cylinder volume (litres) and N is the assumed occupancy
(Appendix J Table 1b). Per Note 2 the result is clamped to [0, 1].

Table 12a (PDF p.191) — whose title reads "High-rate fractions for
systems using 7-hour and 10-hour tariffs" — routes the "Immersion water
heater" row to Table 13 for the tariff "7-hour or 10-hour" ONLY. An
18-hour or 24-hour tariff is outside Table 12a/13's scope: it provides
at least 18 hours/day at the low rate, more than enough to heat any
immersion cylinder off-peak, so the high-rate fraction is 0 (all DHW
billed at the low rate). The Elmhurst dr87 worksheet for solid fuel 5
(cert 001431: 18-hour meter, 110 L dual immersion, WHC 903) confirms
this — HW (245) high-rate = 0.0 kWh, (246) low-rate = 100%. (An earlier
reading mapped 18-/24-hour to the 10-hour column via Note 1's "at least
10 hours"; the worksheet refutes it — Table 12a's title bounds the table
to the two named tariffs.)

Heat pumps providing water heating only are treated as dual immersion
(Note 1) — out of scope of this helper (callers route those via Table
12a).
"""

from __future__ import annotations

from domain.sap10_calculator.tables.table_12a import Tariff


def electric_dhw_high_rate_fraction(
    *,
    cylinder_volume_l: float,
    occupancy_n: float,
    single_immersion: bool,
    tariff: Tariff,
) -> float:
    """SAP 10.2 Table 13 (PDF p.197) high-rate fraction for an electric
    immersion DHW cylinder on an off-peak tariff.

    `single_immersion` selects the single- vs dual-immersion equation
    (RdSAP10 §10.5 p.54: an immersion is assumed dual on a dual meter).
    The 7-hour tariff uses the 7-hour equations; the 10-hour tariff uses
    the 10-hour equations. The 18-hour and 24-hour tariffs are outside
    Table 12a/13's "7-hour and 10-hour" scope (PDF p.191 title) — they
    provide >= 18 hours/day at the low rate, so the high-rate fraction is
    0. STANDARD has no off-peak split and is rejected — callers must
    early-return before this fires.
    """
    if tariff is Tariff.STANDARD:
        raise ValueError("Table 13 high-rate fraction is undefined for STANDARD")
    if tariff in (Tariff.EIGHTEEN_HOUR, Tariff.TWENTY_FOUR_HOUR):
        # Outside Table 12a's 7-hour/10-hour scope — >= 18 h/day low rate
        # heats the cylinder entirely off-peak (high-rate fraction 0).
        return 0.0
    v = cylinder_volume_l
    n = occupancy_n
    if tariff is Tariff.SEVEN_HOUR:
        if single_immersion:
            fraction = ((14530 - 762 * n) / v - 80 + 10 * n) / 100
        else:
            fraction = ((6.8 - 0.024 * v) * n + 14 - 0.07 * v) / 100
    else:
        # 10-hour tariff (18-/24-hour handled above as out-of-scope).
        if single_immersion:
            fraction = ((14530 - 762 * n) / (1.5 * v) - 80 + 10 * n) / 100
        else:
            fraction = ((6.8 - 0.036 * v) * n + 14 - 0.105 * v) / 100
    return max(0.0, min(1.0, fraction))