11006：位域样式的可移植编码

题目

把位域结构表示的文本样式编码为布局固定的 32 位无符号整数，并完成反向解码。交换格式不得依赖位域的对象表示。

解析

位域适合表达字段含义，但字段排列顺序、填充方式和结构体大小都可能随实现变化。因此，编码时应逐个读取字段，再用掩码和移位构造数值；解码时也应从数值中逐段提取字段。

两个函数都先完成全部检查和计算，最后才写入输出对象。这样，保留位、字号或对齐方式非法时，调用方原有结果不会被部分覆盖。

解析

#include <stdbool.h>
#include <stdint.h>

enum {
    TEXT_ALIGN_LEFT = 0,
    TEXT_ALIGN_CENTER = 1,
    TEXT_ALIGN_RIGHT = 2
};

typedef struct {
    unsigned int font_id : 8;
    unsigned int size : 7;
    unsigned int alignment : 2;
    unsigned int bold : 1;
    unsigned int italic : 1;
    unsigned int underline : 1;
} TextStyle;

bool text_style_encode(
    const TextStyle *style,
    uint32_t *out_code
);

bool text_style_decode(
    uint32_t code,
    TextStyle *out_style
);

enum {
    FONT_ID_SHIFT = 0,
    SIZE_SHIFT = 8,
    ALIGNMENT_SHIFT = 15,
    BOLD_SHIFT = 17,
    ITALIC_SHIFT = 18,
    UNDERLINE_SHIFT = 19
};

static const uint32_t FONT_ID_MASK = UINT32_C(0xff);
static const uint32_t SIZE_MASK = UINT32_C(0x7f);
static const uint32_t ALIGNMENT_MASK = UINT32_C(0x3);
static const uint32_t USED_MASK = UINT32_C(0x000fffff);

static bool valid_alignment(unsigned int alignment) {
    return alignment == TEXT_ALIGN_LEFT
        || alignment == TEXT_ALIGN_CENTER
        || alignment == TEXT_ALIGN_RIGHT;
}

bool text_style_encode(
    const TextStyle *style,
    uint32_t *out_code
) {
    if (style == NULL || out_code == NULL) {
        return false;
    }
    if (style->size == 0
        || !valid_alignment(style->alignment)) {
        return false;
    }

    uint32_t code = 0;
    code |= (uint32_t)style->font_id << FONT_ID_SHIFT;
    code |= (uint32_t)style->size << SIZE_SHIFT;
    code |= (uint32_t)style->alignment << ALIGNMENT_SHIFT;
    code |= (uint32_t)style->bold << BOLD_SHIFT;
    code |= (uint32_t)style->italic << ITALIC_SHIFT;
    code |= (uint32_t)style->underline << UNDERLINE_SHIFT;

    *out_code = code;
    return true;
}

bool text_style_decode(
    uint32_t code,
    TextStyle *out_style
) {
    if (out_style == NULL || (code & ~USED_MASK) != 0) {
        return false;
    }

    unsigned int size =
        (unsigned int)((code >> SIZE_SHIFT) & SIZE_MASK);
    unsigned int alignment =
        (unsigned int)(
            (code >> ALIGNMENT_SHIFT) & ALIGNMENT_MASK
        );

    if (size == 0 || !valid_alignment(alignment)) {
        return false;
    }

    TextStyle style = {0};
    style.font_id =
        (unsigned int)(
            (code >> FONT_ID_SHIFT) & FONT_ID_MASK
        );
    style.size = size;
    style.alignment = alignment;
    style.bold = (unsigned int)((code >> BOLD_SHIFT) & 1u);
    style.italic =
        (unsigned int)((code >> ITALIC_SHIFT) & 1u);
    style.underline =
        (unsigned int)((code >> UNDERLINE_SHIFT) & 1u);

    *out_style = style;
    return true;
}

sizeof(TextStyle) 相等也只能说明某个实现恰好分配了相同数量的字节，不能证明字段位序、填充位或字节序符合交换格式。显式编码才给出了跨实现稳定的数值布局。