-- Note this uses simple/strict left association, so for example:
--  1+2*1*8 is ((1+2)*1)*8 not 1+((2*1)*8) [or 1+(2*(1*8))], and
--  7-(2*2)*8 is (7-(2*2))*8 not 7-((2*2)*8)
--  Does not allow unary minus on the first digit.
-- Uses solve24() from the next task, when it can.
--  (you may want to comment out the last 2 lines/uncomment the if 0, in that file)
--
--include 24_game_solve.exw

--with trace
forward function eval(string equation, sequence unused, integer idx=1)
-- (the above definition is entirely optional, but good coding style)

constant errorcodes = {"digit expected",                    -- 1
                       "')' expected",                      -- 2
                       "digit already used",                -- 3
                       "digit not offered",                 -- 4
                       "operand expected"}                  -- 5

function card(integer idx)  -- (for error handling)
    if idx=1 then return "1st" end if
    if idx=2 then return "2nd" end if
    -- (assumes expression is less than 21 characters)
    return sprintf("%dth",idx)
end function

function errorchar(sequence equation, integer idx)
    if idx>length(equation) then return "" end if
    return sprintf("(%s)",equation[idx])
end function

sequence rset = repeat(0,4)

procedure new_rset()
    for i=1 to length(rset) do
        rset[i] = rand(9)
    end for
end procedure

function get_operand(string equation, integer idx, sequence unused)
integer ch, k,
        error = 1 -- "digit expected"
atom res

    if idx<=length(equation) then
        ch = equation[idx]
        if ch='(' then
            {error,res,unused,idx} = eval(equation,unused,idx+1)
            if error=0
            and idx<=length(equation) then
                ch = equation[idx]
                if ch=')' then
                    return {0,res,unused,idx+1}
                end if
            end if
            if error=0 then
                error = 2   -- "')' expected"
            end if
        elsif ch>='0' and ch<='9' then
            res = ch-'0'
            k = find(res,unused)
            if k!=0 then
                unused[k..k] = {}
                return {0,res,unused,idx+1}
            end if
            if find(res,rset) then
                error = 3  -- "digit already used"
            else
                error = 4  -- "digit not offered"
            end if
        end if
    end if
    return {error,0,unused,idx}
end function

function get_operator(string equation, integer idx)
integer ch, error = 5 -- "operand expected"
    if idx<=length(equation) then
        ch = equation[idx]
        if find(ch,"+-/*") then
            return {0,ch,idx+1}
        end if
    end if
    return {error,0,idx}
end function

function eval(string equation, sequence unused, integer idx=1)
atom lhs, rhs
integer ch, error
    {error,lhs,unused,idx} = get_operand(equation,idx,unused)
    if error=0 then
        while 1 do
            {error,ch,idx} = get_operator(equation,idx)
            if error!=0 then exit end if
            {error,rhs,unused,idx} = get_operand(equation,idx,unused)
            if error!=0 then exit end if
            if    ch='+' then       lhs += rhs
            elsif ch='-' then       lhs -= rhs
            elsif ch='/' then       lhs /= rhs
            elsif ch='*' then       lhs *= rhs
            else ?9/0 -- (should not happen)
            end if
            if idx>length(equation) then
                return {0,lhs,unused,idx}
            end if
            ch = equation[idx]
            if ch=')' then
                return {0,lhs,unused,idx}
            end if
        end while
    end if
    return {error,0,unused,idx}
end function

function strip(string equation)
    for i=length(equation) to 1 by -1 do
        if find(equation[i]," \t\r\n") then
            equation[i..i] = ""
        end if
    end for
    return equation
end function

function strip0(atom a) -- (for error handling)
string res = sprintf("%f",a)
integer ch
    for i=length(res) to 2 by -1 do
        ch = res[i]
        if ch='.' then return res[1..i-1] end if
        if ch!='0' then return res[1..i] end if
    end for
    return res
end function

procedure play()
sequence unused
string equation
integer error,idx
atom res

    new_rset()
    printf(1,"Enter an expression which evaluates to exactly 24\n"&
           "Use all of, and only, the digits %d, %d, %d, and %d\n"&
           "You may only use the operators + - * /\n"&
           "Parentheses and spaces are allowed\n",rset)
    while 1 do
        equation = strip(gets(0))
        if upper(equation)="Q" then exit end if
        if equation="?" then
            puts(1,"\n")
            integer r_solve24 = routine_id("solve24") -- see below
            if r_solve24=-1 then -- (someone copied just this code out?)
                puts(1,"no solve24 routine\n")
            else
                call_proc(r_solve24,{rset})
            end if
        else
            {error,res,unused,idx} = eval(equation, rset)
            if error!=0 then
                printf(1,"  %s on the %s character%s\n",{errorcodes[error],card(idx),errorchar(equation,idx)})
            elsif idx<=length(equation) then
                printf(1,"\neval() returned only having processed %d of %d characters\n",{idx,length(equation)})
            elsif length(unused) then
                printf(1," not all the digits were used\n",error)
            elsif res!=24 then
                printf(1,"\nresult is %s, not 24\n",{strip0(res)})
            else
                puts(1," correct!  Press any key to quit\n")
                if getc(0) then end if
                exit
            end if
        end if
        puts(1,"enter Q to give up and quit\n")
    end while
end procedure

    play()