"""Earned Value Management metrics and forecasting formulas.

All functions take cumulative arrays (PV/EV/AC) aligned on a 1-based period index,
matching `src.synthetic.Project` and the tidy schema in `data/README.md`.

References: PMI EVM, NASA EVM handbook, Lipke earned-schedule method.
"""
from __future__ import annotations

import numpy as np


def to_increments(cum) -> np.ndarray:
    """Cumulative -> per-period increments."""
    return np.diff(np.asarray(cum, float), prepend=0.0)


def from_increments(inc, start: float = 0.0) -> np.ndarray:
    """Per-period increments -> cumulative."""
    return start + np.cumsum(np.asarray(inc, float))


def indices(pv, ev, ac) -> dict:
    """Time series of SV, CV, SPI, CPI."""
    pv = np.asarray(pv, float)
    ev = np.asarray(ev, float)
    ac = np.asarray(ac, float)
    with np.errstate(divide="ignore", invalid="ignore"):
        spi = np.where(pv > 0, ev / pv, np.nan)
        cpi = np.where(ac > 0, ev / ac, np.nan)
    return {"sv": ev - pv, "cv": ev - ac, "spi": spi, "cpi": cpi}


def latest(pv, ev, ac, bac) -> dict:
    """Status at the most recent period."""
    pv = np.asarray(pv, float)
    ev = np.asarray(ev, float)
    ac = np.asarray(ac, float)
    pv_n, ev_n, ac_n = pv[-1], ev[-1], ac[-1]
    return {
        "pv": pv_n,
        "ev": ev_n,
        "ac": ac_n,
        "bac": bac,
        "spi": ev_n / pv_n if pv_n > 0 else np.nan,
        "cpi": ev_n / ac_n if ac_n > 0 else np.nan,
        "pct_complete": ev_n / bac if bac > 0 else np.nan,
    }


# --------------------------------------------------------------------------- #
# Cost forecasting (Estimate at Completion)
# --------------------------------------------------------------------------- #
EAC_METHODS = ("bac_cpi", "ac_plus_remaining", "cpi_spi")
EAC_LABELS = {
    "bac_cpi": "EAC = BAC / CPI",
    "ac_plus_remaining": "EAC = AC + (BAC - EV)",
    "cpi_spi": "EAC = AC + (BAC - EV) / (CPI x SPI)",
}


def forecast_eac(pv, ev, ac, bac, method: str = "cpi_spi") -> float:
    s = latest(pv, ev, ac, bac)
    cpi, spi, ev_n, ac_n = s["cpi"], s["spi"], s["ev"], s["ac"]
    if method == "bac_cpi":
        return bac / cpi if cpi and cpi > 0 else np.nan
    if method == "ac_plus_remaining":
        return ac_n + (bac - ev_n)
    if method == "cpi_spi":
        d = cpi * spi
        return ac_n + (bac - ev_n) / d if d and d > 0 else np.nan
    raise ValueError(f"unknown EAC method: {method!r}")


def all_eacs(pv, ev, ac, bac) -> dict:
    return {m: forecast_eac(pv, ev, ac, bac, m) for m in EAC_METHODS}


def etc(pv, ev, ac, bac, method: str = "cpi_spi") -> float:
    return forecast_eac(pv, ev, ac, bac, method) - np.asarray(ac, float)[-1]


def vac(pv, ev, ac, bac, method: str = "cpi_spi") -> float:
    return bac - forecast_eac(pv, ev, ac, bac, method)


def tcpi(ev, ac, bac, target: float | None = None) -> float:
    """To-Complete Performance Index (efficiency required to hit `target`/BAC)."""
    ev_n = np.asarray(ev, float)[-1]
    ac_n = np.asarray(ac, float)[-1]
    target = bac if target is None else target
    denom = target - ac_n
    return (bac - ev_n) / denom if denom != 0 else np.nan


# --------------------------------------------------------------------------- #
# Schedule forecasting (earned-schedule method)
# --------------------------------------------------------------------------- #
def earned_schedule(pv, ev_now: float) -> float:
    """Earned schedule (in period units): the time at which planned PV first
    equals the current EV, by linear interpolation between period boundaries."""
    pv = np.asarray(pv, float)
    if pv[0] > 0 and ev_now <= pv[0]:
        return ev_now / pv[0]
    if ev_now >= pv[-1]:
        return float(len(pv))
    i = int(np.searchsorted(pv, ev_now))            # pv[i-1] < ev_now <= pv[i]
    lo, hi = pv[i - 1], pv[i]
    frac = (ev_now - lo) / (hi - lo) if hi > lo else 0.0
    return i + frac                                  # pv[i-1] sits at time i


def forecast_finish(pv, ev, planned_duration: int) -> float:
    """Independent estimate of finish period via earned schedule: PD / SPI(t)."""
    ev = np.asarray(ev, float)
    at = len(ev)                                     # actual time elapsed
    es = earned_schedule(pv, ev[-1])
    spi_t = es / at if at > 0 else np.nan
    return planned_duration / spi_t if spi_t and spi_t > 0 else np.nan