import re
import shutil
import subprocess
import time
from pathlib import Path

from .models import Toolchain


class ToolchainError(RuntimeError):
    pass


def _decode(value):
    return value if isinstance(value, str) else (value or b"").decode("utf-8", errors="replace")


def discover_toolchain(root: Path, *, which=shutil.which, run=subprocess.run, deadline=None, clock=time.monotonic) -> Toolchain:
    root = root.resolve()
    executable = which("dotnet")
    if executable is None or not Path(executable).is_file():
        raise ToolchainError("dotnet not found; install .NET SDK 8 and check PATH and global.json")
    executable = Path(executable).resolve()
    argv = [str(executable), "--version"]
    context = f"argv={argv!r}; cwd={str(root)!r}; check .NET SDK 8 and global.json"
    timeout = 1800 if deadline is None else min(1800, deadline - clock())
    if timeout <= 0:
        raise ToolchainError(f"SDK probe timed out before it could start; {context}")
    try:
        result = run(argv, cwd=root, capture_output=True, check=False, shell=False, timeout=timeout)
    except subprocess.TimeoutExpired as exc:
        raise ToolchainError(f"SDK probe timed out; {context}") from exc
    except OSError as exc:
        raise ToolchainError(f"SDK probe could not start: {exc}; {context}") from exc
    if result.returncode != 0:
        diagnostic = (_decode(result.stdout) + "\n" + _decode(result.stderr)).strip()[-8000:]
        raise ToolchainError(f"SDK probe failed with exit code {result.returncode}; {context}; output tail: {diagnostic}")
    version = _decode(result.stdout).strip()
    if not re.fullmatch(r"8\.0\.\d+", version):
        raise ToolchainError(f"expected stable .NET SDK 8, got {version!r}; {context}")
    return Toolchain(executable, version)