"""Local prefix diagnostics; retain hashes only, never prompt/image payloads.

These compare adapted messages, not the gateway's tokenized wire prefix.
Only provider-reported cached_tokens is a measured cache hit.
"""

import hashlib
import json

from langchain_core.messages import BaseMessage


def _digest(value) -> str:
    return hashlib.sha256(
        json.dumps(value, ensure_ascii=False, sort_keys=True, default=str).encode("utf-8")
    ).hexdigest()


class PromptPrefixTracker:
    def __init__(self):
        self._previous: dict[str, tuple[str, tuple[str, ...]]] = {}

    def observe(self, identity: str, messages, options: dict) -> dict:
        identity = _digest(identity)
        if not isinstance(messages, (list, tuple)) or not all(
            isinstance(message, BaseMessage) for message in messages
        ):
            return {"comparison": "unavailable"}
        fingerprints = tuple(
            _digest(
                {
                    "role": message.type.removesuffix("MessageChunk").lower(),
                    "content": message.content,
                    "name": message.name,
                    "tool_calls": getattr(message, "tool_calls", None),
                    "tool_call_id": getattr(message, "tool_call_id", None),
                }
            )
            for message in messages
        )
        options_hash = _digest(options)
        previous = self._previous.get(identity)
        # Task-scoped and bounded even if a caller tries many endpoint roles.
        if identity not in self._previous and len(self._previous) >= 32:
            self._previous.pop(next(iter(self._previous)))
        self._previous[identity] = (options_hash, fingerprints)
        common = 0
        if previous:
            for old, new in zip(previous[1], fingerprints):
                if old != new:
                    break
                common += 1
        schema_changed = previous is not None and previous[0] != options_hash
        return {
            "comparison": "local_message_fingerprints",
            "message_count": len(fingerprints),
            "previous_message_count": len(previous[1]) if previous else None,
            "shared_prefix_messages": common if previous else None,
            "first_changed_message": (
                common + 1 if previous and common < len(previous[1]) else None
            ),
            "append_only": bool(previous and common == len(previous[1]) and not schema_changed),
            "tools_or_format_changed": schema_changed,
            "cold_start": previous is None,
        }