Infix to Pre- or Postfix in GNU Smalltalk

infix-prefix.st

Object subclass: Conversion [
    isOperator: c [
        (c isAlphaNumeric) ifFalse: [ ^true ].
        (c isAlphaNumeric) ifTrue: [ ^false ].
    ]
    
    getPriority: char [
        (char = $-) ifTrue: [ ^1 ].
        (char = $+) ifTrue: [ ^1 ].
        (char = $/) ifTrue: [ ^2 ].
        (char = $*) ifTrue: [ ^2 ].
        (char = $^) ifTrue: [ ^3 ].
        ^0.
    ]
    
    infixToPostfix: infixString [
        |infix l char_stack output|
        infix := ('(', infixString,')') asArray.
        l := infix size.
        char_stack := #().
        output := ''.
        
        1 to: l do: [:i |
            ((infix at: i) isAlphaNumeric) ifTrue: [
                output := output, ((infix at: i) asString).
            ] ; ifFalse: [
            ((infix at: i) = $() ifTrue: [
                char_stack := #($)), char_stack.
            ] ; ifFalse: [
            ((infix at: i) = $)) ifTrue: [
                [(char_stack at: 1) = $(] whileFalse: [
                    output := output, (char_stack at: 1) asString.
                    char_stack := char_stack allButFirst.
                ].
                char_stack := char_stack allButFirst.
            ] ; ifFalse: [
            (self isOperator: (char_stack at: 1)) ifTrue: [
                [(self getPriority: (infix at: i)) <= 
                 (self getPriority: (char_stack at: 1))] whileTrue: [
                    output := output, (char_stack at: 1) asString.
                    char_stack := char_stack allButFirst.
                ].
                char_stack := ((infix at: i) asString) asArray, char_stack.
            ]]]].
        ].
        ^output.
    ]
    
    infixToPrefix: infixStr [
        |infix l prefix|
        infix := infixStr asArray.
        l := infix size.
        prefix := ''.
        infix := infix reverse.
        
        1 to: l do: [:i |
            ((infix at: i) = $() ifTrue: [
                infix at: i put: $).
            ] ; ifFalse: [
            ((infix at: i) = $)) ifTrue: [
                infix at: i put: $(.
            ]].
        ].
        
        prefix := self infixToPostfix: (infix asString).
        prefix := prefix reverse.
        ^prefix.
    ]
]

Eval [
    |s out|
    s := '(a-b/c)*(a/k-l)'.
    out := (Conversion new) infixToPostfix: s.
    out printNl.
]