318 lines
9.9 KiB
Markdown
318 lines
9.9 KiB
Markdown
# 11.设置条件
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到目前为止，所有对象的属性都是数据：文本、数字。但属性也可以是代码。
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在上一章中，我们通过关闭洞穴入口（进入洞穴的通道）来限制玩家的行动自由。这已经让游戏变得更有挑战性了，但这并没有带来多少谜题，除非我们为玩家提供一个微小的可能性来打开通道。真正的挑战应该是让玩家找到通道打开的条件。
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让我们举一个简单的例子。为了越过守卫进入山洞，玩家必须杀死或贿赂守卫（或两者兼而有之，这很有价值）。换句话说：
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- 当警卫死亡时（HP=0），入口开放
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- 当警卫拿着银币(贿赂警卫)时，入口开放
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- 两者都不是，入口关闭
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打开一个封闭的通道（在这里是进入洞穴）涉及到改变一些属性值：
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- 目的地从 NULL(空地点)变为洞穴
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- <strong>textGo</strong>从 "警卫阻止你...... "改为 "你走进山洞"
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- 在一些特殊情况下，描述和细节不需要改变。但对于一个门洞或栅栏，其中之一（或两者）通常会包含一些从 "开放 "到 "关闭 "的文字。
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有许多方法来实现这一目标。在这里，我们将讨论一种简单、可维护和通用的方法。
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首先，我们定义了两个独立的通道：一个代表开放通道，另一个代表封闭通道。除了上面列出的那些，这两条通道在每一个属性上都是相同的。(在你下面看到的生成的地图中，注意有两个箭头通向洞穴；一个是实线，一个是虚线)。
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接下来，我们引入一个名为条件的新属性，它决定了某个对象是否存在。这两个通道将被赋予互斥的条件，因此在任何时候都只能有一个存在。
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每个条件将被实现为一个布尔函数：<strong>TRUE</strong>意味着该对象存在，<strong>FALSE</strong>意味着它不存在。
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```c
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bool intoCaveIsOpen(void)
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{
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return guard->health == 0 || silver->location == guard;
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}
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bool intoCaveIsClosed(void)
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{
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return guard->health > 0 && silver->location != guard;
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}
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```
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思考题：你能仿照上面例子自己写一些条件函数吗？
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新的属性条件是一个指向这样一个函数的指针。
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```c
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bool (*condition)(void);
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```
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接下来，我们可以立即开始为 object.txt 中的新属性分配函数。
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# object.txt
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```
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- intoCave
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condition intoCaveIsOpen
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description "a cave entrance to the east"
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tags "east", "entrance"
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location field
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destination cave
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detail "The entrance is just a narrow opening in a small outcrop.\n"
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textGo "You walk into the cave.\n"
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- intoCaveBlocked
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condition intoCaveIsClosed
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description "a cave entrance to the east"
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tags "east", "entrance"
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location field
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prospect cave
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detail "The entrance is just a narrow opening in a small outcrop.\n"
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textGo "The guard stops you from walking into the cave.\n"
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```
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思考题：尝试自己实现上面的伪代码
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这两个 "条件 "函数是如此具体，每一个条件函数都只用这一次。现在，我们可以在我们需要的地方定义这些函数。许多编程语言都支持匿名函数，像这样：
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```
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- intoCave
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condition { return guard->health == 0 || silver->location == guard; }
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...
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- intoCaveBlocked
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condition { return guard->health > 0 && silver->location != guard; }
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...
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```
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所以现在我们可以把额外的段落和条件添加到 object.txt 中，就像前面解释的那样。
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# object.txt
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```
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#include <stdbool.h>
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#include <stdio.h>
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#include "object.h"
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typedef struct object {
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bool (*condition)(void);
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const char *description;
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const char **tags;
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struct object *location;
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struct object *destination;
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struct object *prospect;
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const char *details;
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const char *contents;
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const char *textGo;
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int weight;
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int capacity;
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int health;
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} OBJECT;
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extern OBJECT objs[];
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- field
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description "an open field"
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tags "field"
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details "The field is a nice and quiet place under a clear blue sky."
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capacity 9999
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- cave
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description "a little cave"
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tags "cave"
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details "The cave is just a cold, damp, rocky chamber."
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capacity 9999
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- silver
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description "a silver coin"
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tags "silver", "coin", "silver coin"
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location field
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details "The coin has an eagle on the obverse."
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weight 1
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- gold
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description "a gold coin"
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tags "gold", "coin", "gold coin"
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location cave
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details "The shiny coin seems to be a rare and priceless artefact."
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weight 1
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- guard
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description "a burly guard"
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tags "guard", "burly guard"
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location field
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details "The guard is a really big fellow."
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contents "He has"
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health 100
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capacity 20
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- player
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description "yourself"
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tags "yourself"
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location field
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details "You would need a mirror to look at yourself."
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contents "You have"
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health 100
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capacity 20
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- intoCave
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condition { return guard->health == 0 || silver->location == guard; }
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description "a cave entrance to the east"
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tags "east", "entrance"
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location field
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destination cave
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details "The entrance is just a narrow opening in a small outcrop."
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textGo "You walk into the cave."
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- intoCaveBlocked
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condition { return guard->health > 0 && silver->location != guard; }
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description "a cave entrance to the east"
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tags "east", "entrance"
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location field
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prospect cave
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details "The entrance is just a narrow opening in a small outcrop."
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textGo "The guard stops you from walking into the cave."
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- exitCave
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description "an exit to the west"
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tags "west", "exit"
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location cave
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destination field
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details "Sunlight pours in through an opening in the cave's wall."
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textGo "You walk out of the cave."
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- wallField
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description "dense forest all around"
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tags "west", "north", "south", "forest"
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location field
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details "The field is surrounded by trees and undergrowth."
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textGo "Dense forest is blocking the way."
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- wallCave
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description "solid rock all around"
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tags "east", "north", "south", "rock"
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location cave
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details "Carved in stone is a secret password 'abccb'."
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textGo "Solid rock is blocking the way."
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```
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思考题：尝试自己实现这些功能，并看看与你之前设计的有何不同
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为了使这些条件发挥作用，我们需要调整函数 isHolding 和 getDistance。
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# misc.c
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```c
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#include <stdbool.h>
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#include <stdio.h>
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#include "object.h"
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#include "misc.h"
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bool isHolding(OBJECT *container, OBJECT *obj)
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{
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return validObject(obj) && obj->location == container;
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}
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OBJECT *getPassage(OBJECT *from, OBJECT *to)
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{
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if (from != NULL && to != NULL)
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{
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OBJECT *obj;
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for (obj = objs; obj < endOfObjs; obj++)
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{
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if (isHolding(from, obj) && obj->prospect == to)
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{
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return obj;
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}
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}
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}
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return NULL;
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}
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DISTANCE getDistance(OBJECT *from, OBJECT *to)
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{
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return to == NULL ? distUnknownObject :
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!validObject(to) ? distNotHere :
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to == from ? distSelf :
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isHolding(from, to) ? distHeld :
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isHolding(to, from) ? distLocation :
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isHolding(from->location, to) ? distHere :
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isHolding(from, to->location) ? distHeldContained :
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isHolding(from->location, to->location) ? distHereContained :
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getPassage(from->location, to) != NULL ? distOverthere :
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distNotHere;
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}
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OBJECT *actorHere(void)
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{
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OBJECT *obj;
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for (obj = objs; obj < endOfObjs; obj++)
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{
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if (isHolding(player->location, obj) && obj != player &&
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obj->health > 0)
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{
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return obj;
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}
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}
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return NULL;
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}
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int listObjectsAtLocation(OBJECT *location)
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{
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int count = 0;
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OBJECT *obj;
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for (obj = objs; obj < endOfObjs; obj++)
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{
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if (obj != player && isHolding(location, obj))
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{
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if (count++ == 0)
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{
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printf("%s:\n", location->contents);
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}
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printf("%s\n", obj->description);
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}
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}
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return count;
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}
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```
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思考题：想想我们调整了什么
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注意：
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1. 警卫不可能会死，所以可以说我们的条件函数中的<strong>HP</strong>是很无用的。当然，这很容易通过添加一个 kill 命令来解决，见第 20 章。
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2. 这两个条件函数是互补的；它们有资格成为重复的代码。为了消除这一点，我们可能决定让一个函数调用另一个函数（用'！'操作符来否定结果）。一个匿名函数没有（稳定的）名字，但我们可以用它的对象来指代它。我们可以用 intoCaveBlocked 的条件函数代替。
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3. 为了简单起见，条件函数没有参数。实际上，传递一个参数 OBJECT *obj 可能更好；这使得编写更多的通用条件函数成为可能，可以在多个对象中重复使用。
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4. 在理论上，任何对象都可以成为 "条件"。在下一章，你可以看到一个类似的技术被应用于此。
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思考题：想一想上面第二点要怎么用 C 来实现？
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输出样例
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Welcome to Little Cave Adventure.
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You are in an open field.
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You see:
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a silver coin
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a burly guard
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a cave entrance to the east
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dense forest all around
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--> go entrance
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The guard stops you from walking into the cave.
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--> get coin
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You pick up a silver coin.
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--> give coin
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You give a silver coin to a burly guard.
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--> go entrance
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You walk into the cave.
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You are in a little cave.
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You see:
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a gold coin
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an exit to the west
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solid rock all around
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--> quit
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Bye!
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