import torch
import torch.nn as nn
import torch.nn.functional as F


class ConvBNAct(nn.Module):
    def __init__(self, in_channels, out_channels, kernel_size=3, stride=1, groups=1, act=True):
        super().__init__()
        padding = kernel_size // 2
        self.block = nn.Sequential(
            nn.Conv2d(in_channels, out_channels, kernel_size, stride, padding, groups=groups, bias=False),
            nn.BatchNorm2d(out_channels),
            nn.SiLU(inplace=True) if act else nn.Identity(),
        )

    def forward(self, x):
        return self.block(x)


class SimpleFPN(nn.Module):
    """Top-down FPN for features ordered high-resolution to low-resolution."""

    def __init__(self, in_channels, out_channels=128):
        super().__init__()
        self.lateral = nn.ModuleList([ConvBNAct(c, out_channels, kernel_size=1) for c in in_channels])
        self.output = nn.ModuleList([ConvBNAct(out_channels, out_channels, kernel_size=3) for _ in in_channels])

    def forward(self, features):
        laterals = [layer(feature) for layer, feature in zip(self.lateral, features)]
        for idx in range(len(laterals) - 1, 0, -1):
            up = F.interpolate(laterals[idx], size=laterals[idx - 1].shape[-2:], mode="nearest")
            laterals[idx - 1] = laterals[idx - 1] + up
        return [layer(feature) for layer, feature in zip(self.output, laterals)]


class BottleneckLite(nn.Module):
    def __init__(self, channels, shortcut=True):
        super().__init__()
        hidden = max(8, channels // 2)
        self.shortcut = shortcut
        self.cv1 = ConvBNAct(channels, hidden, kernel_size=1)
        self.cv2 = ConvBNAct(hidden, channels, kernel_size=3)

    def forward(self, x):
        y = self.cv2(self.cv1(x))
        return x + y if self.shortcut else y


class C2fLite(nn.Module):
    """Small YOLO-style concat block for neck fusion."""

    def __init__(self, in_channels, out_channels, num_blocks=2):
        super().__init__()
        hidden = max(8, out_channels // 2)
        self.reduce = ConvBNAct(in_channels, hidden * 2, kernel_size=1)
        self.blocks = nn.ModuleList([BottleneckLite(hidden, shortcut=True) for _ in range(num_blocks)])
        self.fuse = ConvBNAct(hidden * (2 + num_blocks), out_channels, kernel_size=1)

    def forward(self, x):
        y = list(self.reduce(x).chunk(2, dim=1))
        for block in self.blocks:
            y.append(block(y[-1]))
        return self.fuse(torch.cat(y, dim=1))


class YOLOPANNeck(nn.Module):
    """PAN-FPN style neck: upsample/concat top-down, then downsample/concat bottom-up."""

    def __init__(self, in_channels, out_channels=128, num_blocks=2):
        super().__init__()
        c3, c4, c5 = in_channels
        self.p3_lateral = ConvBNAct(c3, out_channels, kernel_size=1)
        self.p4_lateral = ConvBNAct(c4, out_channels, kernel_size=1)
        self.p5_lateral = ConvBNAct(c5, out_channels, kernel_size=1)

        self.top_p4 = C2fLite(out_channels * 2, out_channels, num_blocks=num_blocks)
        self.top_p3 = C2fLite(out_channels * 2, out_channels, num_blocks=num_blocks)
        self.down_p3 = ConvBNAct(out_channels, out_channels, kernel_size=3, stride=2)
        self.bottom_p4 = C2fLite(out_channels * 2, out_channels, num_blocks=num_blocks)
        self.down_p4 = ConvBNAct(out_channels, out_channels, kernel_size=3, stride=2)
        self.bottom_p5 = C2fLite(out_channels * 2, out_channels, num_blocks=num_blocks)

    def forward(self, features):
        p3, p4, p5 = features
        p3 = self.p3_lateral(p3)
        p4 = self.p4_lateral(p4)
        p5 = self.p5_lateral(p5)

        p4_td = self.top_p4(torch.cat([F.interpolate(p5, size=p4.shape[-2:], mode="nearest"), p4], dim=1))
        p3_out = self.top_p3(torch.cat([F.interpolate(p4_td, size=p3.shape[-2:], mode="nearest"), p3], dim=1))
        p4_out = self.bottom_p4(torch.cat([self.down_p3(p3_out), p4_td], dim=1))
        p5_out = self.bottom_p5(torch.cat([self.down_p4(p4_out), p5], dim=1))
        return [p3_out, p4_out, p5_out]


def make_locations(feature_shapes, strides, device):
    locations = []
    for (height, width), stride in zip(feature_shapes, strides):
        shifts_x = (torch.arange(width, device=device, dtype=torch.float32) + 0.5) * stride
        shifts_y = (torch.arange(height, device=device, dtype=torch.float32) + 0.5) * stride
        grid_y, grid_x = torch.meshgrid(shifts_y, shifts_x, indexing="ij")
        locations.append(torch.stack((grid_x.reshape(-1), grid_y.reshape(-1)), dim=1))
    return locations