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Revision: 1.2
Committed: Thu Sep 7 21:42:49 2006 UTC (17 years, 8 months ago) by pippijn
Branch: MAIN
CVS Tags: HEAD
Changes since 1.1: +0 -0 lines
State: FILE REMOVED
Log Message: 
Moved documents to doc/historic

File Contents

# Content
1 Fields that the rest of loader.l doesn't understand are recorded in the
2 object as 'key/value fields'. The first word on the line is the key, and the
3 rest of the line (leading and trailing whitespace is stripped) is the value
4 (which may be an empty string!).
5
6 These allow arbitrary addition of new fields to the object definition
7 without any changes to the loader/saver, and without any increase in object
8 space. Use these to reduce the overloading of various fields for sets of
9 infrequently used objects.
10
11 Storing as key/value pairing makes it so that it is only parsed once during
12 loading - this makes accessing data much faster than if it was just a free
13 block of text that needed parsing each time something might need to get a
14 key. The data itself does remain as text, so if the data is desired in
15 another format, function will need to convert it (eg, atoi) as needed.
16
17 How to use them:
18 =================
19
20 From inside crossfire-server (these are defined in object.c):
21
22 const char *get_ob_key_value(object *op, const char *key)
23
24 Returns the value parameter for the given key, or NULL if the
25 object doesn't have such an extra_field. The string returned is
26 a shared string so should not be modified.
27
28 Note there is no way to tell if the field does not exist or has
29 been set to an empty value. If this differentiation is necessary,
30 a value should be stored away.
31
32 int set_ob_key_value(object *op, const char *key, const char *value, int add_key)
33
34 Sets the value for the given key. If add_key is true, it will add the key
35 if not defined, otherwise, it only updates existing keys. Returns
36 true/false based on success.
37
38 The passed in value is converted to a shared string. Thus, once this function
39 is called, modifying value should not be done.
40
41 Note - usually add_key should be false - there should be little reason
42 to only update existing keys - if you want to store that data away, you should
43 do it regardless of the type.
44
45 Passing in NULL as the value effectively clears the value.
46
47
48 Use in objects/maps:
49 --------------------
50
51 Just add the lines to the object, eg
52 foo bar
53
54 will automatically be handled so that key 'foo' is set to 'bar'
55
56
57 When to use key/value compared to adding new fields:
58 ====================================================
59
60 The following should be considered:
61
62 1) Access to key/value fields is slower than accessing a field directly.
63 Thus, should not be used in time sensitive areas (combat related
64 basically)
65
66 For example, if the only time the key/value would need to be used is by
67 player activating the object, that doesn't happen all that often, so good
68 option.
69
70 If however the key/value would be examined for map updates (say
71 glow_radius) probably not a good option.
72
73 Note also that the performance impact of key/values depend on the
74 number of key/values used for the specific object. If an object has
75 10 key/value pairs, access is much worse than if it just has one.
76
77 However, in all cases, performance is slow because a the key has to
78 looked up in the shared string database and then pointer comparisons
79 done.
80
81 This performance consideration is only something that needs to be if the
82 actual key/value is used in the time critical area, and not the object
83 as a whole. For example, setting a key/value for a monster isn't
84 a problem if that key/value isn't examined during combat (maybe
85 something related to conversation). However, storing attack
86 related value there would not be a good idea.
87
88 2) Use of key values is more memory intensive than just adding fields
89 for objects. On a 32 bit system, eg key/value structure is 12 bytes,
90 compared to just 4 bytes for a pointer to a string.
91
92 One big advantage of the key value is this memory is only used when
93 a key is set. So if you do the math, if more than about 1/3 of the
94 items would have that key/value pair, actually uses less memory to
95 just add a field.
96
97 As of this writing (August 2005) there are 30 values used by less then
98 10 archetypes. Some number of these should perhaps be moved to
99 key/value lists.
100
101 3) Key/value fields shouldn't be used if they are related to an existing
102 field. For example, if there is a 'foo' field, add you want a
103 maxfoo value, it should be done as another field and not as a
104 get_ob_key_value(op, "maxfoo") - trying to main that is difficult.
105
106 Caveats:
107 =========
108
109 - key/values don't guard against real fields being used:
110 set_ob_key_value(ob, "hp", "acidic")
111 will not have the desired effect.
112
113 The load/save code is such that it will save the key/value lists
114 before actual field values. When loading, the last value is
115 used, so in the above example, there may be two lines:
116
117 hp acidic
118 hp 20
119
120 After load, the value of hp will be 20, since that is the last value
121 loaded.
122
123 - Using get_ob_key_value(...) with a key for an actual field in the object
124 will not work. Eg, get_ob_key_value(op, "hp") will not return the
125 hp value of the object - there is no easy way for the functions to
126 to access all the fields.
127
128 - Thanks to its partial dependence on the loader, Crossedit will correctly
129 save and load extra_fields. It can't set or change them, however; there's a
130 whitelist in Attr.c::allowed_variables.
131
132 - Since all lines are valid, catching errors in arches is now difficult.
133 For now, there is a debug statement when we get key/values. When
134 key/values get more use, this will be removed.
135
136 - Key/values are limited to single lines - you can't do something like
137
138 startfoo
139 line1
140 line2
141 endfoo
142
143 and have it work. However, the line length allowed in the loader is
144 quite long.
145
146 - There is no function that walks the key/value list for you. If this
147 is needed, should write your own. However, sucn functionality really
148 shouldn't be needed.
149
150 - key names with no value are an indicator to the loader that there is
151 no value, and that key should be deleted.
152 This is most often used by the saver to denote that the object
153 has cleared the key/value. Eg, the archetype has a key/value
154 pair, but this object doesn't have a key/value that name.
155
156 - Checking two objects against each other can be costly with key/values -
157 its basically a O(n^2) operation because have to check of objects 1
158 keys are in object 2 and vice versa. At a cost of a higher load time,
159 if the fields were sorted, this could be much quicker, as logic is then
160 becomes iterating both lists at the same time, and if any mismatches,
161 know it is different right there. However, this is likely to only
162 become an issue if the number of key/value pairs for any objects go
163 above some certain amount. It's also conceivable since the real
164 high cost here is the comparisons, hashing the keys or values and
165 storing that hash in the object could be a big gain, but once again,
166 depends on how often the values change (as you'd have to recalcuate
167 the entire hash)