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* docs: Add tutorial on bitflags * add bit about max flags, also fix horrible admonition font size * accidentally a word * better terminology * include contra's docs, better distinction between value and variable (hopefully), better example without a leading 0 * fuck * this should stay higher up * fix operator
115 lines
3.3 KiB
Markdown
115 lines
3.3 KiB
Markdown
# Advanced Bitwise Operations
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Expanding on information covered in [Bitflags](./bitflags.md), this document
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further explores how other bitwise operators can be used. This document uses the
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prefix `0b` to denote a binary number, to distinguish them from numbers in base
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ten. For example, the base ten number 5 is `0b101`, and the base ten number
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1,110 is `0b10001010110`.
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## Or Operator
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The Or operator can be used merge bitflags, even if they contain the same flag.
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```
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var/a = 5 // 0b101
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var/b = 6 // 0b110
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var/c = a | b // 0b111 = 7
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```
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The Or operator can be used in conjunction with each other to reduce line count.
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This example from the previous bitflags document can be simplified.
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```dm
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var/player_abilities = WALK
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player_abilities |= SING
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player_abilities |= READ
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```
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Instead of assigning twice, we can simplify.
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```dm
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var/player_abilities = DANCE
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player_abilities |= (SING|READ)
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```
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Note that these are now wrapped in parentheses, primarily to make them easier to
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read that they are a pair of OR'd bitflags.
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## And Operator
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The And operator can be used to check if 2 variables contain the same bitflag.
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```
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var/a = 5 // 0b101
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var/b = 6 // 0b110
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var/c = a & b // 0b100 = 4
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```
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The only bit that stayed true was the bit that was in both inputs. This is
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different to the Or operator, where it only had to be in one input.
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In the previous section, we turned a flag off using this example:
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```dm
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player_abilities &= ~SING
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```
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This done by using the & (And) operator and a negation of the bitflag. Negating
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this bitflag replace all the 1s with 0s and all the 0s with 1s.
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```
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#define DANCE (1 << 4) // 0b10000 = 16
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to_chat(world, DANCE) // 0b10000 = 16, note that all the leading zeros have been hidden here.
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to_chat(world, ~DANCE) // 0b111111111111111111101111 = 16777199
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```
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When the `&= ~DANCE` operation is performed, it takes negated version, where the
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flag is only zero and applies the and operation
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```
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var/player_abilities = (DANCE|SING|SWIM)
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player_abilities &= ~DANCE // remove the dance flag
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to_chat(world, player_abilities) // now just contains (SING | SWIM)
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```
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## Exclusive Or Operator.
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The exclusive or operator `^` acts as a "toggle", and doesn't require knowledge
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of what is inside the flags, as seen in the example below.
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```
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var/player_abilities = (DANCE|SING|SWIM)
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player_abilities ^= DANCE // remove the dance flag
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to_chat(world, player_abilities) // (SING | SWIM)
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player_abilities ^= DANCE // enable the dance flag
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to_chat(world, player_abilities) // (DANCE | SING | SWIM)
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```
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When used between two variables, this can be used to check the differences
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between them.
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```
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var/a = 3 // 0b010
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var/b = 5 // 0b101
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var/c = a ^ b // 0b110 = 6
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```
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## Bitwise Shift Right
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Finally, in conjunction to `<<` bitwise left shift, there is a bitwise right
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shift, used with `>>`. These shifts can also be used with numbers other than
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one, as shown in the example below.
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```dm
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var/a = 5 // 0b101 = 5
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var/b = (a<<3) // 0b101000 = 40
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var/c = (b>>1) // 0b10100 = 20
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```
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However, using these shifts are not common outside of defining bitflags, bitwise
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right shift is barely used.
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Not to be confused, these operators (`<<` and `>>`) are also used as "output"
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and "input" operators for mobs, savefiles, etc. These are only rarely used,
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users should be using helper procs like to_chat() instead.
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