"""Session summarization: compact a session's raw exchanges into a stored gist.

This is the compaction half of the tiered memory. Raw exchanges stay for detail
recall; the summary is what surfaces when an *older* session is recalled later —
"a month ago is a general idea," per the design.
"""
from __future__ import annotations

from lyra import config, llm, logbus, memory
from lyra.llm import Backend

# Re-summarize a session once it has accumulated this many new raw exchanges
# beyond what its current summary covers.
SUMMARIZE_AFTER = 20

_PROMPT = """You are compacting a conversation into a long-term memory record \
(not replying to anyone). Write a concise gist of the session below: what was \
discussed, key decisions or outcomes, concrete specifics worth keeping (names, \
places, numbers, hands), and the user's apparent mood/state. Third person, \
referring to the user as "Brian". 4-8 sentences. No preamble."""


def _transcript(exchanges: list[memory.Exchange]) -> str:
    return "\n".join(f"{ex.role}: {ex.content}" for ex in exchanges)


def summarize_session(session_id: str, backend: Backend | None = None) -> str | None:
    """(Re)generate and store the gist for a session. Returns the summary text.

    Returns None if the session has no exchanges. The summarizer defaults to the
    local backend so routine compaction stays free.
    """
    exchanges = memory.history(session_id)
    if not exchanges:
        return None

    backend = backend or config.load().summary_backend
    messages = [
        {"role": "system", "content": _PROMPT},
        {"role": "user", "content": _transcript(exchanges)},
    ]
    gist = llm.complete(messages, backend=backend)

    last_id = exchanges[-1].id
    memory.store_summary(session_id, gist, last_id)
    logbus.log(
        "info", "summarized session", session=session_id,
        exchanges=len(exchanges), backend=backend,
    )
    return gist


def maybe_summarize(session_id: str, backend: Backend | None = None) -> None:
    """Summarize the session if enough new turns have accumulated since last time."""
    if memory.unsummarized_count(session_id) >= SUMMARIZE_AFTER:
        summarize_session(session_id, backend=backend)