C语言编程实现矩形窗、汉宁窗、汉明窗、布莱克曼窗。定义窗的类型为变量，用switch...case 语句实现不同窗的选择。

在C语言中，窗口函数（Window Functions）通常用于信号处理和数字滤波，它们会对输入数据施加特定的衰减或加权，以减少频谱泄露或改善滤波特性。以下是四种常见的窗函数及其实现：

1. **矩形窗（Rectangular Window）**:

typedef enum {RECTANGULAR = 0} WindowType;

void rectangular_window(float* data, int length) {
    for (int i = 0; i < length; ++i) {
        data[i] = 1.0f;
    }
}

2. **汉宁窗（Hanning Window）**:

typedef enum {HANNING = 1} WindowType;

void hanning_window(float* data, int length) {
    float alpha = 0.5f;
    for (int i = 0; i < length; ++i) {
        data[i] = alpha * (1 - cos(2 * M_PI * i / (length - 1)));
    }
}

3. **汉明窗（Hamming Window）**:

typedef enum {HAMMING = 2} WindowType;

void hamming_window(float* data, int length) {
    float alpha = 0.54 + 0.46 * cos(2 * M_PI * i / (length - 1));
    for (int i = 0; i < length; ++i) {
        data[i] = alpha;
    }
}

4. **布莱克曼窗（Blackman Window）**:

typedef enum {BLACKMAN = 3} WindowType;

void blackman_window(float* data, int length) {
    float a = 0.42f, b = 0.50f, c = 0.08f;
    for (int i = 0; i < length; ++i) {
        float w = a - 1))) + c * cos(4 * M_PI * i / (length - 1));
        data[i] = w;
    }
}

在上述代码中，`WindowType`是一个枚举类型，用于指示当前选择的窗口类型。你可以通过`switch`语句动态地切换到不同的窗口类型：

WindowType window_type = RECTANGULAR; // 初始化窗口类型

switch (window_type) {
    case RECTANGULAR:
        rectangular_window(data, length);
        break;
    case HANNING:
        hanning_window(data, length);
        break;
    case HAMMING:
        hamming_window(data, length);
        break;
    case BLACKMAN:
        blackman_window(data, length);
        break;
    default:
        printf("Invalid window type!\n");
}