src/diffusion_lm/chat_server.py

"""OpenAI-compatible chat endpoint over the reasoning engine.

Serves ``/v1/chat/completions`` and ``/v1/models`` so any OpenAI-API chat client can talk to a
hybrid checkpoint with its own sampler — the model cannot run under llama.cpp-family runtimes,
whose autoregressive decoding never matches the block-denoising objective.

Clients resend the full message history on every request and carry no thinking notes, while the
training layout replaces messages outside the visible window with the ledger of notes taken on
them. The server therefore caches each turn's notes keyed by a hash of the exact history that
produced it: a client that resends history verbatim reconstructs the same ledger the playground
would hold. On a cache miss (server restart, edited history) the older turns simply contribute
nothing, which is the playground's restart behaviour as well.
"""

from __future__ import annotations

import argparse
import hashlib
import json
import threading
import time
import uuid
from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
from pathlib import Path

from diffusion_lm.claims import (
    KEEP_MESSAGES,
    SYSTEM,
    chat_prefix,
    ledger_line,
    ledger_notes,
    merge_notes,
)
from diffusion_lm.reasoning_playground import ReasoningEngine
from diffusion_lm.train import resolve_device

MAX_ANSWER_TOKENS = 384
NOTE_CACHE_LIMIT = 4096


def _turn_key(system: str, messages: list[dict[str, str]]) -> str:
    payload = json.dumps(
        [system] + [[m['role'], m['content']] for m in messages],
        ensure_ascii=False, sort_keys=False,
    )
    return hashlib.sha256(payload.encode('utf-8')).hexdigest()


class ChatService:
    """One engine plus the note cache; generation is serialized on the GPU."""

    def __init__(self, engine: ReasoningEngine, model_id: str, keep_messages: int) -> None:
        self.engine = engine
        self.model_id = model_id
        self.keep_messages = keep_messages
        self.notes: dict[str, str] = {}
        self.lock = threading.Lock()

    def _attach_notes(self, system: str, messages: list[dict[str, str]]) -> list[dict[str, str]]:
        attached = []
        for index, message in enumerate(messages):
            entry = {'role': message['role'], 'content': message['content']}
            if message['role'] == 'assistant':
                note = self.notes.get(_turn_key(system, messages[: index + 1]))
                if note:
                    entry['note'] = note
            attached.append(entry)
        return attached

    def build_prefix(self, system: str, messages: list[dict[str, str]]) -> str:
        attached = self._attach_notes(system, messages)
        older = merge_notes(ledger_notes(attached, self.keep_messages))
        window = attached[max(0, len(attached) - self.keep_messages):]
        return chat_prefix(
            [{'role': m['role'], 'content': m['content']} for m in window],
            system=system, extra=ledger_line(older),
        )

    def remember(self, system: str, messages: list[dict[str, str]],
                 answer: str, note: str) -> None:
        if not note:
            return
        if len(self.notes) >= NOTE_CACHE_LIMIT:
            self.notes.clear()
        turn = messages + [{'role': 'assistant', 'content': answer}]
        self.notes[_turn_key(system, turn)] = note

    def generate(self, system: str, messages: list[dict[str, str]], *, temperature: float,
                 top_p: float, max_tokens: int, seed: int):
        """Yield ``(answer_so_far, note, done)``; the note arrives with the final snapshot."""

        prefix = self.build_prefix(system, messages)
        blocks: list[tuple[int, str]] = []
        answer = ''
        with self.lock:
            for _, answer, _ in self.engine.stream_chat(
                prefix,
                temperature=temperature,
                # 16 measured optimal on qwen06b-genchat-sft (2026-08-12 five-point sweep):
                # best numeric fidelity, clean doubling, half the latency of 32. Below 8 both
                # fidelity and fluency degrade while latency barely moves.
                steps_per_block=16,
                max_answer_tokens=min(MAX_ANSWER_TOKENS, max_tokens),
                top_p=top_p,
                seed=seed,
                blocks_out=blocks,
            ):
                yield answer, '', False
        note = '; '.join(text for _, text in blocks if text)
        self.remember(system, messages, answer.strip(), note)
        yield answer, note, True


def _split_messages(raw: list[dict]) -> tuple[str, list[dict[str, str]]]:
    system = SYSTEM
    messages = []
    for message in raw:
        role, content = message.get('role'), str(message.get('content') or '')
        if role == 'system':
            system = content or system
        elif role in ('user', 'assistant'):
            messages.append({'role': role, 'content': content})
    return system, messages


class Handler(BaseHTTPRequestHandler):
    service: ChatService

    def log_message(self, format: str, *args) -> None:  # noqa: A002
        pass

    def _json(self, status: int, body: dict) -> None:
        data = json.dumps(body, ensure_ascii=False).encode('utf-8')
        self.send_response(status)
        self.send_header('Content-Type', 'application/json')
        self.send_header('Content-Length', str(len(data)))
        self.send_header('Access-Control-Allow-Origin', '*')
        self.end_headers()
        self.wfile.write(data)

    def do_OPTIONS(self) -> None:  # noqa: N802
        self.send_response(204)
        self.send_header('Access-Control-Allow-Origin', '*')
        self.send_header('Access-Control-Allow-Methods', 'GET, POST, OPTIONS')
        self.send_header('Access-Control-Allow-Headers', 'Content-Type, Authorization')
        self.end_headers()

    def do_GET(self) -> None:  # noqa: N802
        if self.path.rstrip('/') in ('/v1/models', '/models'):
            self._json(200, {'object': 'list', 'data': [
                {'id': self.service.model_id, 'object': 'model', 'owned_by': 'mini-mdlm'},
            ]})
        else:
            self._json(404, {'error': 'not found'})

    def do_POST(self) -> None:  # noqa: N802
        if self.path.rstrip('/') not in ('/v1/chat/completions', '/chat/completions'):
            self._json(404, {'error': 'not found'})
            return
        try:
            length = int(self.headers.get('Content-Length', 0))
            request = json.loads(self.rfile.read(length))
            system, messages = _split_messages(request.get('messages') or [])
            if not messages or messages[-1]['role'] != 'user':
                raise ValueError('last message must be from the user')
        except (ValueError, json.JSONDecodeError) as error:
            self._json(400, {'error': {'message': str(error), 'type': 'invalid_request_error'}})
            return

        temperature = float(request.get('temperature') or 0.8)
        top_p = float(request.get('top_p') or 0.95)
        max_tokens = int(request.get('max_tokens') or MAX_ANSWER_TOKENS)
        seed = int(request.get('seed') or 0)
        stream = bool(request.get('stream'))
        completion_id = f'chatcmpl-{uuid.uuid4().hex[:24]}'
        created = int(time.time())

        snapshots = self.service.generate(
            system, messages, temperature=temperature, top_p=top_p,
            max_tokens=max_tokens, seed=seed,
        )
        if not stream:
            answer = note = ''
            for answer, note, _ in snapshots:
                pass
            message = {'role': 'assistant', 'content': answer.strip()}
            if note:
                message['reasoning_content'] = note
            self._json(200, {
                'id': completion_id, 'object': 'chat.completion', 'created': created,
                'model': self.service.model_id,
                'choices': [{'index': 0, 'message': message, 'finish_reason': 'stop'}],
            })
            return

        self.send_response(200)
        self.send_header('Content-Type', 'text/event-stream')
        self.send_header('Cache-Control', 'no-cache')
        self.send_header('Access-Control-Allow-Origin', '*')
        self.end_headers()

        def chunk(delta: dict, finish: str | None = None) -> bytes:
            body = {
                'id': completion_id, 'object': 'chat.completion.chunk', 'created': created,
                'model': self.service.model_id,
                'choices': [{'index': 0, 'delta': delta, 'finish_reason': finish}],
            }
            return f'data: {json.dumps(body, ensure_ascii=False)}\n\n'.encode('utf-8')

        try:
            self.wfile.write(chunk({'role': 'assistant'}))
            sent = ''
            for answer, _, done in snapshots:
                if len(answer) > len(sent):
                    self.wfile.write(chunk({'content': answer[len(sent):]}))
                    self.wfile.flush()
                    sent = answer
            self.wfile.write(chunk({}, finish='stop'))
            self.wfile.write(b'data: [DONE]\n\n')
        except (BrokenPipeError, ConnectionResetError):
            pass


def main() -> None:
    parser = argparse.ArgumentParser(description=__doc__)
    parser.add_argument('--checkpoint', type=Path,
                        default=Path('outputs/qwen06b-genchat-sft/inference-latest.pt'))
    parser.add_argument('--tokenizer', type=Path,
                        default=Path('artifacts/tokenizer-qwen3-adaptive.json'))
    parser.add_argument('--model-id', default=None,
                        help='name reported to clients; defaults to the checkpoint directory')
    parser.add_argument('--keep-messages', type=int, default=KEEP_MESSAGES)
    parser.add_argument('--host', default='127.0.0.1')
    parser.add_argument('--port', type=int, default=7998)
    parser.add_argument('--device', default='auto')
    args = parser.parse_args()

    device = resolve_device(args.device)
    engine = ReasoningEngine(args.checkpoint, args.tokenizer, device)
    if not engine.chat_ready:
        raise SystemExit(f'{args.checkpoint} is not an adaptive hybrid over a ChatML tokenizer')
    Handler.service = ChatService(
        engine, args.model_id or args.checkpoint.parent.name, args.keep_messages,
    )
    server = ThreadingHTTPServer((args.host, args.port), Handler)
    print(f'serving {Handler.service.model_id} on http://{args.host}:{args.port}/v1')
    server.serve_forever()


if __name__ == '__main__':
    main()