"""Shared plumbing for the four extraction branches.

Prompts live in `prompts/*.txt`, never in code, for two reasons: a prompt change
is not a code change, and **the fixed prefix must stay byte-identical across
calls** or prompt caching silently stops engaging at roughly 10x the input cost.
"""

from __future__ import annotations

from functools import lru_cache
from pathlib import Path

from ..models import Chunk
from ..settings import CACHE_MIN_TOKENS

PROMPT_DIR = Path(__file__).parent / "prompts"


def est_tokens(text: str) -> int:
    """Cheap estimate, for dry-run budgeting only. Real counts come from the
    API's usage object — never report a cached price from an estimate."""
    return max(1, int(len(text) / 3.6))


@lru_cache(maxsize=8)
def load_prompt(branch: str) -> str:
    return (PROMPT_DIR / f"{branch}.txt").read_text(encoding="utf-8")


def prefix_tokens(branch: str) -> int:
    return est_tokens(load_prompt(branch))


def cacheable(branch: str) -> bool:
    """Whether the fixed prefix is long enough to cache at all.

    Reported, never assumed: caching does not engage below the floor, so a
    shorter prefix caches nothing. Only the API's `cached_tokens` proves a hit.
    """
    return prefix_tokens(branch) >= CACHE_MIN_TOKENS


def evidence_block(chunks: list[Chunk], scores: list[float] | None = None) -> str:
    """Evidence labelled with chunk_id, section and page so the model can cite
    provenance and we can trace which evidence produced which field.

    **The heading is included, and must stay included.** Two reasons:

    1. Indonesian standards name the term in the heading and open the body with
       the definition — "2.1.3 Physical of Availability (PA)" / "Adalah
       ketersediaan fisik…" — so the body often never repeats the term. Without
       the heading the model is asked to define a term the evidence never names.
    2. It keeps one invariant true: **what the model reads is exactly what the
       span check searches.** `validate.evidence_text` composes heading + text;
       if this block showed only the text, the model could never quote a
       section title, and any field that did quote one would be rejected as
       unlocatable.
    """
    parts = []
    for i, chunk in enumerate(chunks):
        score = f" score={scores[i]:.1f}" if scores and i < len(scores) else ""
        head = f"{chunk.heading}\n" if chunk.heading else ""
        parts.append(
            f"[chunk_id={chunk.chunk_id} section={chunk.section_no or '-'} "
            f"page={chunk.page_start}{score}]\n{head}{chunk.text}"
        )
    return "EVIDENCE\n" + "\n\n---\n\n".join(parts)