"""Minimal coverage for scripts/cache_aware_proxy pick_instance + cache LRU (S1)."""

from __future__ import annotations

import importlib.util
import sys
import types
from pathlib import Path

import pytest

PROXY_PATH = Path(__file__).resolve().parent.parent / "scripts" / "cache_aware_proxy.py"


def _install_stub_modules() -> None:
    """Provide minimal stand-ins for fastapi/uvicorn/httpx so the proxy
    module imports cleanly without the full server deps."""
    if "uvicorn" not in sys.modules:
        sys.modules["uvicorn"] = types.ModuleType("uvicorn")

    if "fastapi" not in sys.modules:
        fastapi_mod = types.ModuleType("fastapi")

        class _FastAPI:
            def __init__(self, *a, **kw):
                self.state = types.SimpleNamespace()

            def post(self, *a, **kw):
                def deco(fn): return fn
                return deco

            def get(self, *a, **kw):
                def deco(fn): return fn
                return deco

        class _HTTPException(Exception):
            def __init__(self, status_code=500, detail=""):
                self.status_code = status_code
                self.detail = detail

        class _Request:  # not actually instantiated by the routing tests
            pass

        fastapi_mod.FastAPI = _FastAPI
        fastapi_mod.HTTPException = _HTTPException
        fastapi_mod.Request = _Request
        sys.modules["fastapi"] = fastapi_mod

        responses_mod = types.ModuleType("fastapi.responses")

        class _StreamingResponse:
            def __init__(self, *a, **kw): pass

        responses_mod.StreamingResponse = _StreamingResponse
        sys.modules["fastapi.responses"] = responses_mod

    if "httpx" not in sys.modules:
        httpx_mod = types.ModuleType("httpx")

        class _AsyncClient:
            def __init__(self, *a, **kw): pass
            async def aclose(self): pass

        class _Limits:
            def __init__(self, *a, **kw): pass

        httpx_mod.AsyncClient = _AsyncClient
        httpx_mod.Limits = _Limits
        sys.modules["httpx"] = httpx_mod


@pytest.fixture(scope="module")
def proxy():
    _install_stub_modules()
    spec = importlib.util.spec_from_file_location("cache_aware_proxy", PROXY_PATH)
    if spec is None or spec.loader is None:
        pytest.skip(f"cannot load proxy module at {PROXY_PATH}")
    mod = importlib.util.module_from_spec(spec)
    sys.modules["cache_aware_proxy"] = mod
    try:
        spec.loader.exec_module(mod)
    except ModuleNotFoundError as exc:
        pytest.skip(f"proxy dependency missing: {exc}")
    return mod


def _make_inst(proxy, url: str, ongoing_tokens: int = 0,
               active_p_offloads: int = 0):
    inst = proxy.InstanceState(url)
    inst.ongoing_tokens = ongoing_tokens
    inst.active_p_offloads = active_p_offloads
    return inst


def test_record_prefix_evicts_oldest_block(proxy):
    """LRU bound on cached_blocks must evict the oldest entry once full."""
    inst = proxy.InstanceState("http://x")
    saved = proxy.SETTINGS.cache_capacity_blocks
    proxy.SETTINGS.cache_capacity_blocks = 2
    try:
        block_size = proxy.BLOCK_SIZE
        # Three distinct one-block prefixes; first must be evicted.
        prefix_a = [1] * block_size
        prefix_b = [2] * block_size
        prefix_c = [3] * block_size
        inst.record_prefix(prefix_a)
        inst.record_prefix(prefix_b)
        inst.record_prefix(prefix_c)
        assert len(inst.cached_blocks) == 2
        # A should have been evicted.
        assert inst.estimate_cache_hit(prefix_a) == 0
        assert inst.estimate_cache_hit(prefix_b) == block_size
        assert inst.estimate_cache_hit(prefix_c) == block_size
    finally:
        proxy.SETTINGS.cache_capacity_blocks = saved


def test_estimate_cache_hit_touches_lru(proxy):
    """A cache hit must move the block to the MRU position."""
    inst = proxy.InstanceState("http://x")
    saved = proxy.SETTINGS.cache_capacity_blocks
    proxy.SETTINGS.cache_capacity_blocks = 2
    try:
        block_size = proxy.BLOCK_SIZE
        a = [1] * block_size
        b = [2] * block_size
        c = [3] * block_size
        inst.record_prefix(a)
        inst.record_prefix(b)
        # Touch A so it becomes MRU; B is now LRU.
        assert inst.estimate_cache_hit(a) == block_size
        # Insert C: B should be evicted, A should remain.
        inst.record_prefix(c)
        assert inst.estimate_cache_hit(a) == block_size
        assert inst.estimate_cache_hit(b) == 0
    finally:
        proxy.SETTINGS.cache_capacity_blocks = saved


def test_pick_instance_session_affinity_sticks(proxy):
    insts = [_make_inst(proxy, "http://a"), _make_inst(proxy, "http://b")]
    affinity = {"sess1": 1}
    chosen, idx = proxy.pick_instance(insts, None, "sess1", 1000, affinity)
    assert idx == 1 and chosen is insts[1]


def test_pick_instance_session_affinity_breaks_on_overload(proxy):
    """When the pinned instance is heavily overloaded, fallback to load-aware pick."""
    insts = [
        _make_inst(proxy, "http://a", ongoing_tokens=100),
        _make_inst(proxy, "http://b", ongoing_tokens=1_000_000),
        _make_inst(proxy, "http://c", ongoing_tokens=100),
    ]
    affinity = {"sess1": 1}
    chosen, idx = proxy.pick_instance(insts, None, "sess1", 1000, affinity)
    # avg ~333k; B at 1M is ~3x avg, well above OVERLOAD_FACTOR=2.0 -> fallback.
    assert idx != 1
    assert chosen is not insts[1]


def test_pick_instance_p_offload_penalty_steers_away(proxy):
    """Instances actively running offloaded HEAVY prefills get penalized."""
    insts = [
        _make_inst(proxy, "http://a", ongoing_tokens=0, active_p_offloads=2),
        _make_inst(proxy, "http://b", ongoing_tokens=1000),
    ]
    chosen, idx = proxy.pick_instance(insts, None, None, 5000, {})
    # B's 1000-token load is much smaller than A's 2 * HEAVY_THRESHOLD penalty.
    assert idx == 1 and chosen is insts[1]


def test_pick_instance_lmetric_picks_lowest_score(proxy):
    insts = [_make_inst(proxy, "http://a"), _make_inst(proxy, "http://b")]
    insts[0].pending_prefill_tokens = 0
    insts[0].num_requests = 0
    insts[1].pending_prefill_tokens = 5000
    insts[1].num_requests = 4
    chosen, idx = proxy.pick_instance_lmetric(insts, None, None, 1000, {})
    # Empty instance has score = 1000 * 0 = 0; busy one has (5000+1000)*4.
    assert idx == 0 and chosen is insts[0]


def test_pick_instance_lmetric_ignores_session_affinity(proxy):
    """Review #3: pure --policy lmetric must remain affinity-free."""
    insts = [_make_inst(proxy, "http://a"), _make_inst(proxy, "http://b")]
    # Make inst[1] look much busier than inst[0]; LMetric must still pick 0
    # even though affinity points at 1.
    insts[0].pending_prefill_tokens = 0
    insts[0].num_requests = 0
    insts[1].pending_prefill_tokens = 5000
    insts[1].num_requests = 4
    affinity = {"sess1": 1}
    chosen, idx = proxy.pick_instance_lmetric(insts, None, "sess1", 1000, affinity)
    assert idx == 0
    # Picker must not mutate the affinity dict either.
    assert affinity == {"sess1": 1}


def _record_n_blocks(proxy, inst, n: int) -> list[int]:
    """Record n distinct one-block prefixes on inst; return token_ids covering them."""
    block_size = proxy.BLOCK_SIZE
    tokens: list[int] = []
    for b in range(n):
        tokens.extend([1000 + b] * block_size)
    inst.record_prefix(tokens)
    return tokens


def test_hybrid_high_cache_session_sticks_to_affinity(proxy):
    """Hybrid: affinity instance with cache_ratio > 0.5 and no overload → stick."""
    insts = [_make_inst(proxy, "http://a"), _make_inst(proxy, "http://b")]
    tokens = _record_n_blocks(proxy, insts[1], 2)  # 2 blocks cached on inst[1]
    affinity = {"sess1": 1}
    chosen, idx, decision = proxy.pick_instance_unified_hybrid(
        insts, tokens, "sess1", len(tokens), affinity)
    assert idx == 1 and chosen is insts[1]
    assert decision["decision"] == "affinity"
    assert decision["affinity_idx"] == 1
    assert decision["chosen_idx"] == 1
    assert decision["affinity_cache_ratio"] > 0.5
    assert decision["tie_break_used"] is False


def test_hybrid_high_cache_breaks_on_overload(proxy):
    """Hybrid: affinity num_requests > avg * overload_factor → fall back to LMetric,
    and with realistic new-token tail the LMetric fallback steers off the hot instance."""
    insts = [
        _make_inst(proxy, "http://a"),
        _make_inst(proxy, "http://b"),
        _make_inst(proxy, "http://c"),
    ]
    cached = _record_n_blocks(proxy, insts[1], 2)
    # Append one more uncached block so LMetric sees a real prefill cost on the
    # cached instance too (BS multiplier becomes visible). Without this, the
    # cached instance scores 0 * BS = 0 regardless of how loaded it is.
    tokens = cached + [999_999] * proxy.BLOCK_SIZE
    insts[1].num_requests = 300  # avg = 100; 300 > 100 * 2.0 ✓ breaks the gate
    affinity = {"sess1": 1}
    chosen, idx, decision = proxy.pick_instance_unified_hybrid(
        insts, tokens, "sess1", len(tokens), affinity)
    assert decision["decision"] == "lmetric_fallback"
    assert decision["affinity_idx"] == 1
    assert idx != 1, "affinity instance is overloaded; fallback should steer away"


def test_hybrid_low_cache_falls_back(proxy):
    """Hybrid: cache_ratio <= 0.5 on affinity → fall back to LMetric."""
    insts = [_make_inst(proxy, "http://a"), _make_inst(proxy, "http://b")]
    tokens = [1] * (proxy.BLOCK_SIZE * 2)  # 1024 tokens, nothing cached anywhere
    affinity = {"sess1": 1}
    chosen, idx, decision = proxy.pick_instance_unified_hybrid(
        insts, tokens, "sess1", len(tokens), affinity)
    assert decision["decision"] == "lmetric_fallback"
    assert decision["affinity_cache_ratio"] == 0.0


def test_hybrid_new_session_tie_break_does_not_always_pick_index_0(proxy):
    """Review #4: when all instances tie (e.g. BS=0), tie-break must rotate."""
    insts = [_make_inst(proxy, "http://a") for _ in range(3)]
    seen = set()
    for _ in range(12):
        # No session_id, all empty → score = 0 for everyone → ties → rotate.
        chosen, idx, decision = proxy.pick_instance_unified_hybrid(
            insts, None, None, 100, {})
        seen.add(idx)
        assert decision["decision"] == "lmetric_fallback"
        assert decision["tie_break_used"] is True
    assert seen == {0, 1, 2}, f"tie-breaker did not rotate; only saw {seen}"


def test_hybrid_decision_fields_populated(proxy):
    """Review #7: decision dict must carry the breakdown fields."""
    insts = [_make_inst(proxy, "http://a"), _make_inst(proxy, "http://b")]
    _, _, decision = proxy.pick_instance_unified_hybrid(
        insts, None, None, 100, {})
    expected_keys = {
        "decision", "affinity_idx", "chosen_idx",
        "affinity_cache_hit", "affinity_cache_ratio", "affinity_num_requests",
        "avg_num_requests", "fallback_score", "tie_break_used",
    }
    assert expected_keys.issubset(decision.keys())


def test_settings_has_runtime_knobs(proxy):
    """D5/B4/M3: Settings dataclass exposes the previously-hardcoded knobs."""
    s = proxy.SETTINGS
    for field in (
        "heavy_threshold",
        "overload_factor",
        "max_offload_inflight",
        "cache_gate_ratio",
        "prefill_throughput",
        "rdma_overhead_s",
        "cache_capacity_blocks",
    ):
        assert hasattr(s, field), f"SETTINGS missing {field}"
    # Runtime mutability matters for tests + __main__ override.
    saved = s.cache_gate_ratio
    s.cache_gate_ratio = 0.55
    assert proxy.SETTINGS.cache_gate_ratio == 0.55
    s.cache_gate_ratio = saved


def test_pick_instance_load_only_picks_min_num_requests(proxy):
    insts = [_make_inst(proxy, "http://a"), _make_inst(proxy, "http://b"),
             _make_inst(proxy, "http://c")]
    insts[0].num_requests = 5
    insts[1].num_requests = 2
    insts[2].num_requests = 8
    chosen, idx = proxy.pick_instance_load_only(insts, None, "sess1", 1000, {})
    assert idx == 1 and chosen is insts[1]


def test_pick_instance_load_only_ignores_cache_hits(proxy):
    insts = [_make_inst(proxy, "http://a"), _make_inst(proxy, "http://b")]
    block_size = proxy.BLOCK_SIZE
    prefix = [123] * (block_size * 4)
    insts[0].record_prefix(prefix)
    insts[0].num_requests = 10
    insts[1].num_requests = 0
    chosen, idx = proxy.pick_instance_load_only(insts, prefix, None,
                                                  len(prefix), {})
    assert idx == 1, "load_only must ignore cache hit on inst[0]"


def test_pick_instance_sticky_first_turn_picks_min_load(proxy):
    insts = [_make_inst(proxy, "http://a"), _make_inst(proxy, "http://b")]
    insts[0].num_requests = 10
    insts[1].num_requests = 2
    affinity = {}
    chosen, idx = proxy.pick_instance_sticky(insts, None, "sess1", 1000, affinity)
    assert idx == 1
    assert affinity == {"sess1": 1}


def test_pick_instance_sticky_subsequent_never_breaks(proxy):
    """Once assigned, sticky must never re-route even under massive overload."""
    insts = [_make_inst(proxy, "http://a"), _make_inst(proxy, "http://b")]
    affinity = {"sess1": 0}
    insts[0].num_requests = 1_000_000
    insts[1].num_requests = 0
    chosen, idx = proxy.pick_instance_sticky(insts, None, "sess1", 1000, affinity)
    assert idx == 0, "sticky must stay even when pinned instance is saturated"


def test_p_offload_penalty_uses_settings_heavy_threshold(proxy):
    """M2: tweaking SETTINGS.heavy_threshold changes the P-offload penalty."""
    inst = proxy.InstanceState("http://x")
    inst.active_p_offloads = 3
    saved = proxy.SETTINGS.heavy_threshold
    try:
        proxy.SETTINGS.heavy_threshold = 10000
        assert proxy._p_offload_penalty(inst) == 30000
        proxy.SETTINGS.heavy_threshold = 50000
        assert proxy._p_offload_penalty(inst) == 150000
    finally:
        proxy.SETTINGS.heavy_threshold = saved