app.py

import numpy as np
import pandas as pd
import streamlit as st


# -----------------------
# Core math
# -----------------------
def normalize_probs(p: np.ndarray) -> np.ndarray:
    p = np.asarray(p, dtype=float)
    p = np.clip(p, 0.0, None)
    s = float(p.sum())
    if s <= 0:
        return np.ones_like(p) / len(p)
    return p / s


def expected_loss(loss: np.ndarray, p: np.ndarray) -> np.ndarray:
    # loss: (A, S), p: (S,)
    return loss @ p


def regret_matrix(loss: np.ndarray) -> np.ndarray:
    # regret[a,s] = loss[a,s] - min_a loss[a,s]
    return loss - loss.min(axis=0, keepdims=True)


def max_regret(regret: np.ndarray) -> np.ndarray:
    return regret.max(axis=1)


def cvar_discrete(losses: np.ndarray, probs: np.ndarray, alpha: float = 0.8) -> float:
    """
    CVaRα for discrete outcomes:
    - Sort by loss ascending
    - Find tail mass beyond alpha (i.e., worst 1-alpha probability)
    - Return probability-weighted average loss over the tail
    """
    alpha = float(alpha)
    alpha = min(max(alpha, 0.0), 1.0)

    order = np.argsort(losses)
    l = np.asarray(losses, dtype=float)[order]
    p = np.asarray(probs, dtype=float)[order]

    p = normalize_probs(p)
    cum = np.cumsum(p)

    # tail = outcomes with cum_prob >= alpha
    tail = cum >= alpha
    if not np.any(tail):
        # alpha==1 with numerical edge cases; take worst outcome
        tail[-1] = True

    tail_p = p[tail].sum()
    if tail_p <= 0:
        return float(l[-1])

    return float((l[tail] * p[tail]).sum() / tail_p)


def cvar_per_action(loss: np.ndarray, p: np.ndarray, alpha: float) -> np.ndarray:
    return np.array([cvar_discrete(loss[i, :], p, alpha=alpha) for i in range(loss.shape[0])], dtype=float)


# -----------------------
# UI
# -----------------------
st.set_page_config(page_title="Decision Kernel Lite", layout="wide")
st.markdown(
    """
    <style>
        section[data-testid="stSidebar"] {
            width: 420px !important;
        }
        section[data-testid="stSidebar"] > div {
            width: 420px !important;
        }
    </style>
    """,
    unsafe_allow_html=True,
)

st.title("Decision Kernel Lite")
st.caption("One output: choose an action under uncertainty. Three lenses: Expected Loss, Regret, CVaR.")

# Defaults
default_actions = ["A1", "A2", "A3"]
default_scenarios = ["Low", "Medium", "High"]
default_probs = [0.3, 0.4, 0.3]
default_loss = np.array([[10, 5, 1], [6, 4, 6], [2, 6, 12]])

st.sidebar.header("Controls")
alpha = st.sidebar.slider("CVaR alpha (tail threshold)", 0.50, 0.99, 0.80, 0.01)
tie_policy = st.sidebar.selectbox("Tie policy", ["First", "Show all"], index=1)

st.sidebar.header("Decision rule")
primary_rule = st.sidebar.radio("Choose action by", ["Expected Loss", "Minimax Regret", "CVaR"], index=0)

# Editable inputs
left, right = st.columns([1.2, 1])

with left:
    st.subheader("1) Define scenarios + probabilities")
    scen_df = pd.DataFrame({"Scenario": default_scenarios, "Probability": default_probs})
    scen_df = st.data_editor(scen_df, num_rows="dynamic", use_container_width=True)

    # clean scenarios/probs
    scen_df = scen_df.dropna(subset=["Scenario"]).copy()
    scen_df["Scenario"] = scen_df["Scenario"].astype(str).str.strip()
    scen_df = scen_df[scen_df["Scenario"] != ""]
    if scen_df.empty:
        st.error("Add at least one scenario.")
        st.stop()

    scenarios = scen_df["Scenario"].tolist()
    probs_raw = scen_df["Probability"].fillna(0.0).astype(float).to_numpy()
    probs = normalize_probs(probs_raw)

    if not np.isclose(probs_raw.sum(), 1.0):
        st.info(f"Probabilities normalized to sum to 1.0 (raw sum was {probs_raw.sum():.3f}).")

with right:
    st.subheader("2) Define actions + losses")
    # loss table editor
    loss_df = pd.DataFrame(default_loss, index=default_actions, columns=default_scenarios)

    # If user changed scenarios count, reindex to match
    # Start from current editor state if available by reconstructing using scenarios
    loss_df = loss_df.reindex(columns=scenarios)
    for c in scenarios:
        if c not in loss_df.columns:
            loss_df[c] = 0.0
    loss_df = loss_df[scenarios]

    loss_df = st.data_editor(
        loss_df.reset_index().rename(columns={"index": "Action"}),
        num_rows="dynamic",
        use_container_width=True,
    )

    loss_df = loss_df.dropna(subset=["Action"]).copy()
    loss_df["Action"] = loss_df["Action"].astype(str).str.strip()
    loss_df = loss_df[loss_df["Action"] != ""]
    if loss_df.empty:
        st.error("Add at least one action.")
        st.stop()

    actions = loss_df["Action"].tolist()
    loss_vals = loss_df.drop(columns=["Action"]).fillna(0.0).astype(float).to_numpy()

# Compute
loss_mat = loss_vals  # shape (A, S)
A, S = loss_mat.shape

exp = expected_loss(loss_mat, probs)
reg = regret_matrix(loss_mat)
mxr = max_regret(reg)
cvar = cvar_per_action(loss_mat, probs, alpha=alpha)

results = pd.DataFrame(
    {
        "Expected Loss": exp,
        "Max Regret": mxr,
        f"CVaR@{alpha:.2f}": cvar,
    },
    index=actions,
)

# -----------------------
# Heuristic recommendation (rule suggestion)
# -----------------------
# Minimal heuristic: if tail risk is materially worse than average, recommend CVaR;
# if probabilities are weak/unknown, recommend Minimax Regret; otherwise Expected Loss.

tail_ratio = float(results[f"CVaR@{alpha:.2f}"].min() / max(results["Expected Loss"].min(), 1e-9))

if tail_ratio >= 1.5:
    rule_reco = "CVaR"
    rule_reason = f"Tail risk dominates average (best CVaR / best Expected Loss = {tail_ratio:.2f})."
else:
    rule_reco = "Expected Loss"
    rule_reason = f"Tail risk is not extreme (ratio = {tail_ratio:.2f}); average-optimal is defensible."

# Let user override the heuristic explicitly (keeps governance clean)
use_rule_reco = st.sidebar.checkbox("Use recommended rule (heuristic)", value=False)
if use_rule_reco:
    primary_rule = rule_reco


# Choose by rule
if primary_rule == "Expected Loss":
    metric = results["Expected Loss"]
    best_val = metric.min()
    best_actions = metric[metric == best_val].index.tolist()
elif primary_rule == "Minimax Regret":
    metric = results["Max Regret"]
    best_val = metric.min()
    best_actions = metric[metric == best_val].index.tolist()
else:
    col = f"CVaR@{alpha:.2f}"
    metric = results[col]
    best_val = metric.min()
    best_actions = metric[metric == best_val].index.tolist()

chosen = best_actions[0] if tie_policy == "First" else ", ".join(best_actions)
st.sidebar.header("Rule guidance (when to use what)")

st.sidebar.markdown(
    """
**Expected Loss (risk-neutral)**
- Use when decisions repeat frequently and you can tolerate variance.
- Use when probabilities are reasonably trusted.
- Optimizes *average* pain.

**Minimax Regret (robust to bad probability estimates)**
- Use when probabilities are unreliable or politically contested.
- Use for one-shot / high-accountability decisions.
- Minimizes “I should have done X” exposure.

**CVaR (tail-risk protection)**
- Use when rare bad outcomes are unacceptable (ruin / safety / bankruptcy).
- Use when downside is asymmetric and must be bounded.
- Optimizes the *average of worst cases* (tail), not the average overall.
"""
)

# Layout output
st.divider()
topL, topR = st.columns([2, 1], vertical_alignment="center")
with topL:
    st.subheader("Decision")
    st.markdown(f"### Choose **{chosen}**")
    st.caption(f"Primary rule: **{primary_rule}**")
with topR:
    st.metric("Scenarios", S)
    st.metric("Actions", A)

st.subheader("Evidence table")
st.dataframe(results.style.format("{:.3f}"), use_container_width=True)

st.subheader("Regret table (per action × scenario)")
reg_df = pd.DataFrame(reg, index=actions, columns=scenarios)
st.dataframe(reg_df.style.format("{:.3f}"), use_container_width=True)

# Decision card
st.subheader("Decision Card")
st.info(f"Recommended rule (heuristic): **{rule_reco}** — {rule_reason}")

prob_str = ", ".join([f"{s}={p:.2f}" for s, p in zip(scenarios, probs)])

exp_best = results["Expected Loss"].idxmin()
mxr_best = results["Max Regret"].idxmin()
cvar_best = results[f"CVaR@{alpha:.2f}"].idxmin()

st.code(
    f"""DECISION KERNEL LITE — DECISION CARD

Decision:
Choose action {chosen}

Context:
- Actions evaluated: {", ".join(actions)}
- Scenarios considered: {", ".join(scenarios)}
- Probabilities: {prob_str}

Results:
- Expected Loss optimal: {exp_best} ({results.loc[exp_best, "Expected Loss"]:.3f})
- Minimax Regret optimal: {mxr_best} ({results.loc[mxr_best, "Max Regret"]:.3f})
- CVaR@{alpha:.2f} optimal: {cvar_best} ({results.loc[cvar_best, f"CVaR@{alpha:.2f}"]:.3f})

Rule guidance:
- Expected Loss: repeated decisions + trusted probabilities
- Minimax Regret: probabilities unreliable + high accountability
- CVaR: tail-risk unacceptable / ruin protection

Recommended rule (heuristic): {rule_reco} — {rule_reason}


Primary rule used: {primary_rule}
""",
    language="text",
)

with st.expander("Raw inputs"):
    st.write("Probabilities (normalized):", probs)
    st.dataframe(pd.DataFrame(loss_mat, index=actions, columns=scenarios), use_container_width=True)