(in reader\n) print


/token_delim (;,"` \n{}\(\)[]) def
/token_number (0123456789-) def

% read_number: read a single number from string/idx
% string idx -> read_number -> number string new_idx
/read_number {
    %(in read_number\n) print
    /idx exch def
    /str exch def
    /start idx def
    /cnt 0 def
    { % loop
        idx str length ge { exit } if % EOF, break loop
        /ch str idx get def  % current character
        ch 48 ge ch 57 le and 45 ch eq or { %if number
            /cnt cnt 1 add def
        }{ % else
            exit
        } ifelse
        /idx idx 1 add def % increment idx
    } loop

    str start cnt getinterval cvi % the matched number
    str idx % return: number string new_idx
} def


% read_symbol: read a single symbol from string/idx
% string idx -> read_symbol -> name string new_idx
/read_symbol {
    %(in read_symbol\n) print
    /idx exch def
    /str exch def
    /start idx def
    /cnt 0 def
    { % loop
        idx str length ge { exit } if % EOF, break loop
        /ch str idx 1 getinterval def
        token_delim ch search { % if token delimeter
            pop pop pop exit
        }{ % else not a delim
            pop
            /cnt cnt 1 add def
        } ifelse
        /idx idx 1 add def % increment idx
    } loop

    str start cnt getinterval cvn % the matched symbol
    str idx % return: symbol string new_idx
} def


% read_string: read a single string from string/idx
% string idx -> read_string -> new_string string new_idx
/read_string {
    %(in read_string\n) print
    /idx exch 1 add def
    /str exch def
    /start idx def
    /cnt 0 def
    { % loop
        idx str length ge { %if EOF
            (unexpected EOF reading string)
            throw
        } if
        /ch str idx get def  % current character
        /idx idx 1 add def
        ch 34 eq { exit } if % '"' is end of string
        /cnt cnt 1 add def
    } loop
    str start cnt getinterval % the matched string
    str idx % return: new_string string new_idx
} def


% read_atom: read a single atom from string/idx
% string idx -> read_atom -> int string new_idx
/read_atom {
    %(in read_atom\n) print
    /idx exch def
    /str exch def
    str length idx le { % ifelse
        exit % EOF
    }{
        /ch str idx get def  % current character
        %ch 48 ge ch 57 le and 45 ch eq or { %if number
        ch 48 ge ch 57 le and { %if number
            str idx read_number
        }{ ch 34 eq { %elseif double-quote
            str idx read_string
        }{
            str idx read_symbol
            /idx exch def pop
            dup /nil eq { %if nil
                pop null str idx
            }{ dup /true eq { %elseif true
                pop true str idx
            }{ dup /false eq { %elseif false
                pop false str idx
            }{ %else
                str idx % return the original symbol/name
            } ifelse } ifelse } ifelse
        } ifelse } ifelse
    }ifelse

    % return: atom string new_idx
} def

% read_list: read a single list from string/idx
% string idx -> read_list -> list string new_idx
/read_list {
    %(in read_list\n) print
    /idx exch 1 add def
    /str exch def
    [
    { % loop
        str idx read_spaces /idx exch def pop
        str length idx le { %if EOF
            (unexpected EOF reading list)
            throw
        } if
        /ch str idx get def  % current character
        ch 41 eq { exit } if % ')' is end of list
        str idx read_form /idx exch def pop
    } loop
    ]
    str idx 1 add
} def

% read_spaces: advance idx to the first non-whitespace
% string idx -> read_form -> string new_idx
/read_spaces {
    %(in read_spaces\n) print
    /idx exch def
    /str exch def
    { % loop
        str length idx le { exit } if % EOF, break loop
        /ch str idx get def  % current character
        % if not whitespace then exit
        ch 32 ne ch 10 ne ch 44 ne and and { exit } if
        /idx idx 1 add def % increment idx
    } loop

    str idx % return: string new_idx
} def

% read_form: read the next form from string start at idx
% string idx -> read_form -> ast string new_idx
/read_form {
    %(in read_form\n) print
    read_spaces
    /idx exch def
    /str exch def

    idx str length ge { exit } if % EOF, break loop
    /ch str idx get def  % current character
    ch 40 eq { %if (
        str idx read_list
    }{ ch 91 eq { %elseif [
        (unexpected '[') throw
    }{ ch 93 eq { %elseif ]
        (unexpected ']') throw
    }{ % else
        str idx read_atom
    } ifelse } ifelse } ifelse

    %(stack vvv\n) print
    %pstack
    %(stack ^^^\n) print

    % return: ast string new_idx
} def

% string -> read_str -> ast
/read_str {
    %(in read_str\n) print
    0  % current index into the string
    read_form

    pop pop  % drop the string, idx. return: ast
} def