89 lines
2.4 KiB
Forth
89 lines
2.4 KiB
Forth
A standard RETRO image only supports decimal base numbers. This
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shows a way to add support for multiple bases.
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I begin by creating a variable for the current base and words to
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set this to specific values quickly.
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~~~
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#10 'Base var-n
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:decimal #10 !Base ;
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:binary #2 !Base ;
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:octal #8 !Base ;
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:hex #16 !Base ;
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~~~
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Next, a string constant with the symbols for each digit. Note that
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I'm only going to support uppercase for hexadecimal.
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~~~
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'0123456789ABCDEF 'DIGITS s:const
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~~~
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So conversion to a numeric value is pretty easy. The basic idea
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here is:
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- set a variable to hold the numeric value to zero (as a stating point)
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- check to see if the first character is - for negative, set a modifier
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- for each character in the string:
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- convert to a numeric value (in this case, it's the index in the
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DIGITS string)
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- Multiply the last value of the number accumulator by the base
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- Add the converted value
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- Store the result in the number accumulator
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- Multiply the final number by the modifier
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~~~
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{{
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'Number var
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'Mod var
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:convert (c-) &DIGITS swap s:index/char @Number @Base * + !Number ;
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:check-sign (s-s) dup fetch $- eq? [ #-1 !Mod n:inc ] [ #1 !Mod ] choose ;
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---reveal---
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:s:to-number-with-base (s-n)
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#0 !Number check-sign [ convert ] s:for-each @Number @Mod * ;
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}}
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~~~
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Going the other way, back to a string, follows a similar process.
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- Take a value
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- Repeat:
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- Divide by `Base`
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- Convert result to character and append to a buffer
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- If remainder is not zero, repeat
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- If number is negative, append the '-' symbol to the buffer
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- Reverse the buffer contents to return a string in the correct order
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~~~
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{{
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'String d:create #12 allot
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:check-sign (n-) n:negative? [ $- buffer:add ] if ;
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:n->digit (n-c) &DIGITS + fetch ;
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:convert (n-) [ @Base /mod swap n->digit buffer:add dup n:zero? ] until drop ;
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---reveal---
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:n:to-string-with-base (n-s)
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[ &String buffer:set dup n:abs convert check-sign ] buffer:preserve
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&String s:reverse ;
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}}
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~~~
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The `n:to-string-with-base` returns a representation of binary numbers, but
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not the actual bitwise representation. The next word takes care of this.
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~~~
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{{
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'Selector var
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:bit (f-c) [ $0 ] [ $1 ] choose ;
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---reveal---
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:n:binary-rep (n-s)
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dup n:negative? [ n:inc &n:odd? ] [ &n:even? ] choose !Selector
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[ s:empty buffer:set
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#32 [ dup @Selector call bit buffer:add #2 / ] times drop
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buffer:start s:reverse
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] buffer:preserve ;
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}}
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~~~
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