"""GPU detection, memory calculation, and auto batch size."""

import math


def detect_device() -> tuple[str, str]:
    """Detect available device. Returns (device_string, human_name)."""
    try:
        import torch

        if torch.cuda.is_available():
            name = torch.cuda.get_device_name(0)
            return "cuda", name
        if hasattr(torch.backends, "mps") and torch.backends.mps.is_available():
            return "mps", "Apple Silicon (MPS)"
    except ImportError:
        pass

    return "cpu", "CPU (no GPU detected)"


def get_gpu_info() -> dict:
    """Get GPU memory info."""
    try:
        import torch

        if torch.cuda.is_available():
            total = torch.cuda.get_device_properties(0).total_memory
            total_gb = total / (1024**3)
            return {
                "memory_total": f"{total_gb:.1f} GB",
                "memory_total_bytes": total,
                "gpu_count": torch.cuda.device_count(),
            }
        if hasattr(torch.backends, "mps") and torch.backends.mps.is_available():
            # MPS doesn't expose memory easily, estimate from system
            return {
                "memory_total": "shared (Apple Silicon)",
                "memory_total_bytes": 0,
                "gpu_count": 1,
            }
    except ImportError:
        pass

    return {
        "memory_total": "N/A (CPU mode)",
        "memory_total_bytes": 0,
        "gpu_count": 0,
    }


def estimate_batch_size(
    model_params_b: float,
    seq_length: int,
    gpu_memory_bytes: int,
    quantization: str = "4bit",
    lora_r: int = 64,
) -> int:
    """Estimate max batch size that fits in GPU memory.

    Conservative estimate — better to start smaller and gradient accumulate.
    """
    if gpu_memory_bytes == 0:
        return 1  # CPU fallback

    gpu_gb = gpu_memory_bytes / (1024**3)

    # Rough memory per param based on quantization
    bytes_per_param = {"4bit": 0.5, "8bit": 1.0, "none": 2.0}  # FP16
    bpp = bytes_per_param.get(quantization, 2.0)

    # Model memory (static)
    model_mem_gb = model_params_b * bpp

    # LoRA trainable params (usually ~1-3% of total)
    lora_ratio = min(lora_r * 2 / 4096, 0.05)  # rough estimate
    trainable_mem_gb = model_params_b * 2 * lora_ratio  # FP16 for trainable

    # Optimizer states (Adam: 2x params)
    optimizer_mem_gb = trainable_mem_gb * 2

    # Available for activations
    overhead_gb = 1.5  # CUDA overhead, fragmentation
    available_gb = gpu_gb - model_mem_gb - trainable_mem_gb - optimizer_mem_gb - overhead_gb

    if available_gb <= 0:
        return 1

    # Rough activation memory per sample per token
    # ~2 bytes per hidden dim per layer per token for a transformer
    activation_per_sample_gb = (seq_length * model_params_b * 0.001)  # very rough
    activation_per_sample_gb = max(activation_per_sample_gb, 0.5)  # minimum 0.5 GB

    batch_size = max(1, int(available_gb / activation_per_sample_gb))
    # Clamp to power of 2 (common practice)
    batch_size = 2 ** int(math.log2(batch_size)) if batch_size > 1 else 1

    return min(batch_size, 32)  # cap at 32


def model_size_from_name(model_name: str) -> float:
    """Guess model size in billions from model name."""
    name_lower = model_name.lower()

    size_markers = [
        ("70b", 70), ("65b", 65), ("34b", 34), ("33b", 33),
        ("13b", 13), ("8b", 8), ("7b", 7), ("3b", 3),
        ("1.5b", 1.5), ("1b", 1), ("0.5b", 0.5),
    ]

    for marker, size in size_markers:
        if marker in name_lower:
            return size

    return 7.0  # default guess


def get_compute_dtype():
    """Return the best compute dtype for the current device.

    Uses bfloat16 on CUDA GPUs that support it, float16 otherwise.
    On CPU, uses float32 to avoid dtype mismatch errors.
    """
    import torch

    if torch.cuda.is_available():
        if torch.cuda.is_bf16_supported():
            return torch.bfloat16
        return torch.float16
    return torch.float32