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168
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1 The New Dicing System: A Proposal for OpenRPG
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2 ----------------------------------------------
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3
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4 The current dice system for OpenRPG has several limitations. Foremost
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5 among these are the fact that adding a new, non-standard dicing mechanism
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6 requires editing of the basic dice code. There are several secondary
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7 limitations, such as the fact that while the dice system can handle math,
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8 it cannot be used as a calculator -- it will not allow expressions that do
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9 not involve dice.
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10
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11 This proposal is for a new dicing system to replace the current one in
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12 OpenRPG. Since the dicing system is something that users will interact
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13 with frequently, a new system needs to be considered carefully. This
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14 document attempts to describe the new dicing system so that such
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15 consideration can be given to it. It is expected that this document will
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16 grow and change as it is scrutinized.
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17
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18
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19 Design goals for this dicing system:
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20
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21 1. Should be easy for new users to get started with, based on knowing
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22 standard RPG dice notation (NdX) and basic math.
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23
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24 2. Should, as far as practical, maintain compatibility with existing
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25 character sheets, etc., that use the current dice system.
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26
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27 3. Should allow users to create new dice types and new ways of
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28 counting dice. Ideally, this should not require programming, except
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29 in exceptional cases.
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30
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31 4. The dice system should be usable for doing basic math that does
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32 not involve dice.
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33
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34 5. The dice system should be able to handle most current RPG dicing
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35 systems.
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36
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37 Things this dicing system is designed to NOT do:
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38
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39 1. Be a programming language. There are no facilities in it for
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40 user input, output formatting, loops, if-then-else, or similar
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41 things. If these are desired for something involving dice, an
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42 appropriate node and nodehandler can be created.
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43
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44 2. Handle all theoretically possible dicing systems without the
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45 need for programming plugins. First off, this is impossible.
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46 Second, even making an attempt to would require supporting
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47 dicing methods that don't actually turn up in any real game.
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48
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49 3. Handle floating-point math. I don't know of any systems that
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50 use it in their dice schemes right now. If there are some,
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51 we might have to consider adding it.
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52
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53
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54 Syntax Specification
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55
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56 What follows is a BNF specification for the proposed dicing system, with
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57 explanatory text interleaved. At the end of this document is a copy of
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58 the BNF with no explanations, for those who would like to look at it "all
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59 together". Note that BNF describes only syntax, and not semantics; thus,
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60 while anything generated with this grammar should be syntactically correct,
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61 that doesn't mean it will make sense or be allowed.
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62
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63
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64 dice string ::= <expression>
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65 <expression> of <comparison> | <comparison>
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66
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67 This is the top level. The major thing of note here is that comparisons
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68 only occur at this level. This is intentional; the result of a comparison
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69 is a boolean true/false flag rather than a number. Thus, it makes no
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70 sense to allow people to perform further numerical operations on the
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71 result of a comparison. Systems where dice are triggered by the results
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72 of other dice are left for the realm of plugins.
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73
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74
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75 comparison ::= <expression> <relation> <expression>
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76
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77 expression ::= <factor> | <factor> <low-op> <factor>
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78
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79 The separation into "low-op" and "high-op" of the operators is to allow
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80 order of operations to be handled more easily. Syntactically, it's not
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81 really necessary, but it should be helpful in implementation.
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82
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83
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84 factor :: = <term> | <term> <high-op> <term> |
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85 <multi-dice> | <multi-dice> <high-op> <term> |
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86 <term> <high-op> <multi-dice>
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87
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88 Here we start to hit some complication. The intent of the different
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89 entries for multi-dice is that we don't want to allow things like
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90 [3d6 each * 2d6 each]. We are *not* doing vector multiplication!
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91 The "expression" level doesn't have any such limitation on syntax;
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92 things like [3d6 each + 2d6 each] we'll have to either think of a
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93 logical way to handle, or disallow on a semantic level. (Well... I
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94 suppose it could be handled in the BNF, but I think it would get
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95 kind of messy.)
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96
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97
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98 term ::= <dice> | <unit>
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99
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100 unit ::= <number> | ( <expression> )
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101
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102 Dice are not considered a unit. This means that things like [3d6d10] can't
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103 be done without using parentheses. I consider that to be a win for
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104 clarity.
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105
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106
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107 dice ::= <unit>d<unit> | <unit>d<name> | <dice> <flag> | lastroll
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108
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109 The <name> entry here allows for user-created dice (in the syntax, at
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110 least...).
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111
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112
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113 multi-dice ::= ( <dice>, <dice>+ ) | # (1d6,1d8)
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114 <dice> each | # 3d6 each
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115 ( <expression> of <expression> ) | # (3 of 2d6)
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116 lastroll | # lastroll
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117 <multi-dice> <flag> # (3 of 2d6) best 2
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118
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119 "lastroll" by itself can be either dice or multi-dice. I'm thinking that it
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120
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121 should be whatever type the last roll was.
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122
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123
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124 flag ::= reroll <condition> | # repeats
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125 reroll <slice> | # once only
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126 grow <condition> | # reroll and add
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127 shrink <condition> | # reroll and subtract
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128 drop <condition> |
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129 drop <slice> |
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130 take <condition> |
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131 take <slice> |
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132 <slice> | # implied "take"
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133 <name> <condition> | # user-created
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134 <name> # user-created
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135
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136 Technically, we don't need both "drop" and "take" -- one implies the
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137 other. However, having both should make the language easier to use.
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138
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139 "reroll" will work differently depending on whether a condition or a
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140 slice is given. If a condition is given, it will reroll until none of
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141 the dice in the set meet the reroll condition (or until it hits a maximum
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142 allowed number of rerolls). If a slice is given, it will reroll those
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143 dice once. IMHO, this behavior makes the most sense.
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144
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145 The <name> entries here are to allow for user-created flags. Note that
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146 as I've specified things right now, a user-created flag can have a
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147 condition,
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148 but not a slice. That's mostly because I couldn't think of a case where
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149 a slice would be useful... should we add it anyways?
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150
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151
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152 slice ::= highest | lowest | highest <number> | lowest <number>
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153
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154 "highest" and "lowest" without a number are equivalent to doing them with
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155 1 as the number. This is to simplify things like [4d6 drop lowest].
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156
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157
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158 condition ::= <relation> <unit>
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159
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160 This is for conditions on flags. Note that it can take a unit, so you could
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161
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162 use dice in a condition; however, I think the unit should only be evaluated
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163
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164 once, to make things faster. Anyone for repetitive evaluation?
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165
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166
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167 low-op ::= + | - | min | max
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168
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169 "min" takes two values and returns the highest of them, and "max"
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170 returns the lowest of them. This might seem counterintuitive, but
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171 it's meant to be used with dice, like so:
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172
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173 3d6 min 8 - always returns 8 or higher
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174
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175 3d6 max 15 - always returns 15 or lower
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176
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177 I decided to put min and max as having the same precedence as + and -,
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178 because if they had higher precedence, then:
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179
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180 3d6+2 min 10
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181
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182 would be equivalent to 3d6+10. (It would take the max of 2 and 10, then
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183 add that to 3d6). One problem that does arise here is with multiplication
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184 and division: [1d6 min 5 * 2] will be equivalent to [1d6 min 10], since
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185 multiplication has higher precedence. We may just want to warn people
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186 that min and max can be screwy unless you parenthesize, unless someone can
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187 think of a better way to handle them.
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188
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189
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190 high-op ::= * | / | mod
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191
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192 The / is integer division, of course, since we're doing integer math.
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193
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194
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195 number ::= <digit>+ | -<digit>+
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196
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197 Positive and negative numbers are allowed. This means that, syntactically,
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198 [-2d-4] is legal. Do we want to modify the BNF to disallow this, or handle
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199 it on a semantic level?
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200
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201
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202 name ::= <letter>[<letter>|<digit>]*
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203
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204 We may want to expand to allow underscores and dashes in user-created names.
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205
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206
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207
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208 letter ::= A-Z | a-z
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209
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210 digit ::= 0-9
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211
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212 relation ::= < | > | <= | >= | => | =< | = | ==
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213
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214
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215
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216 Well, that's the BNF. Again, at the end is a copy without all the running
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217 commentary.
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218
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219
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220 Thoughts on Implementation:
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221
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222 First, I think a sort of "dice library" of common functions needs to be
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223 created. This would include rolling a set of dice, getting the highest
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224 of a group of dice, growing and shrinking dice from a set based on
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225 conditions, and so on. These functions should be available for use by
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226 custom-written dice types.
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227
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228 Next, that library should be used as a tool in implementing a dice-string
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229 interpreter. That will require creating a parser for the dice-string
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230 'language'. This could be either a custom-written parser, or possibly
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231 one created with some of the Python parser generators. A custom-written
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232 parser may take longer to do and be a bit more finicky to maintain, but
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233 it would remove a dependency from the code.
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234
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235 User-created flags and dice types could be supported in two ways:
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236
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237 - First, by allowing users to specify strings in the dice language
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238 that the flags/expressions would expand to -- basically, allowing
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239 dice macros.
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240
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241 - Second, by adding hooks for python modules to be associated with
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242 user-created dice or flag types. This is likely to be the more
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243 complicated of the two solutions, but it would also be more
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244 flexible.
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245
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246 Personally, I think both are desirable -- the first, so that
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247 non-programming users can create simple die and flag types. The
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248 second, because by design, there are some things this dice system
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249 just won't do.
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250
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251
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252 Further work needed:
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253
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254 - specs for the "dice library"
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255
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256 - specs on an interface for python modules meant to be dice and
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257 flag types.
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258
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259 ----------------------------------------------------------------
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260
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261 start ::= <expression> |
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262 <comparison>
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263
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264 comparison ::= <expression> <condition>
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265
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266 expression ::= <term> | <term> <low-op> <factor>
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267
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268 term ::= <factor> | <factor> <high-op> <unit>
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269
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270 factor ::= <atom> | <dice_set>
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271
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272 atom ::= <number> | ( <expression> )
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273
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274 dice_set ::= <dice> <dice_set>, <dice> | <expr> of <dice> | <dice> each |
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275 lastroll
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276
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277 dice ::= <atom>d<atom> | <atom>d<name> | <dice> <flag>
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278
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279 flag ::= reroll <condition> |
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280 reroll <slice> |
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281 grow <condition> |
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282 shrink <condition> |
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283 drop <condition> |
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284 drop <slice> |
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285 take <condition> |
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286 take <slice> |
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287 <slice> |
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288 <name> <condition> |
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289 <name> <slice> |
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290 <name>
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291
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292 slice ::= highest | lowest | highest <number> | lowest <number>
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293
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294 condition ::= <relation> <unit>
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295
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296 low-op ::= + | -
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297
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298 high-op ::= * | / | mod| max | min
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299
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300 number ::= <digit>+ | -<digit>+
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301
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302 name ::= <letter>[<letter>|<digit>]*
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303
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304 letter ::= A-Z | a-z
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305
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306 digit ::= 0-9
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307
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308 relation ::= < | > | <= | >= | => | =< | = | ==
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309
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