《编写可读代码的艺术》第8章 拆分超长的表达式

    代码中的表达式越长，它就越难理解。本章看看各种操作和拆分代码以使它们更容易理解的方法。

1. 用做解释的变量

if line.split(':')[0].strip() == "root":

-- username 为解释变量
username = line.split(':')[0].strip()
if username == "root":

 2. 总结变量

if (request.user.id == document.owner_id) {
    // user can edit this document...
}
if (request.user.id != document.owner_id) {
    // document is read-only...
}

// user_owns_document 为总结变量，它使得表达更加清楚
final boolean user_owns_document = (request.user.id == document.owner_id);
if (user_owns_document) {
    // user can edit this document...
}
if (!user_owns_document) {
    // document is read-only...
}

3. 使用摩根定理

    1) not (a or b or c) ⇔ (not a) and (not b) and (not c)
    2) not (a and b and c) ⇔ (not a) or (not b) or (not c)

if (!(file_exists && !is_protected)) Error("Sorry, could not read file.");

// 可写为
if (!file_exists || is_protected) Error("Sorry, could not read file.");

4. 不要滥用短路逻辑

assert((!(bucket = FindBucket(key))) || !bucket->IsOccupied());

// 以下方式更易懂
bucket = FindBucket(key);
if (bucket != NULL) assert(!bucket->IsOccupied());

5. 找到更优雅的方式

    OverlapsWith() 反方向是不重叠

bool Range::OverlapsWith(Range other) {
    if (other.end <= begin) return false; // They end before we begin
    if (other.begin >= end) return false; // They begin after we end
    return true; // Only possibility left: they overlap
}

6. 简化表达

void AddStats(const Stats& add_from, Stats* add_to) {
    add_to->set_total_memory(add_from.total_memory() + add_to->total_memory());
    add_to->set_free_memory(add_from.free_memory() + add_to->free_memory());
    add_to->set_swap_memory(add_from.swap_memory() + add_to->swap_memory());
    add_to->set_status_string(add_from.status_string() + add_to->status_string());
    add_to->set_num_processes(add_from.num_processes() + add_to->num_processes());
    //...
}

void AddStats(const Stats& add_from, Stats* add_to) {
    #define ADD_FIELD(field) add_to->set_##field(add_from.field() + add_to->field())
    ADD_FIELD(total_memory);
    ADD_FIELD(free_memory);
    ADD_FIELD(swap_memory);
    ADD_FIELD(status_string);
    ADD_FIELD(num_processes);
    //...
    #undef ADD_FIELD
}