%{
import std.stdio;
double[string] variables;
enum Errors { undefinedVariables = 1, divideByZero = 2, syntaxError = 3 };
uint errors;

void report_errors()
{
    auto report = "Errors:";
    if (errors & Errors.undefinedVariables) {
        report ~= " \"Undefined Variables\"";
    }
    if (errors & Errors.divideByZero) {
        report ~= " \"Divide by Zero\"";
    }
    if (errors & Errors.syntaxError) {
        report ~= " \"Syntax Errors\"";
    }
    stderr.writeln(report);
}
%}

%field  double value
%field  string id

%token          EOL     (\n)
%token          PLUS    "+"
%token          MINUS   "-"
%token          TIMES   "*"
%token          DIVIDE  "/"
%token          ASSIGN  "="
%token  <value> NUMBER  ([0-9]+(\.[0-9]+){0,1})
%token  <id>    ID      ([a-zA-Z]+)
%token          LPR     "("
%token          RPR     ")"

%skip   ([\t\r ]+)

%right  UMINUS
%left   TIMES DIVIDE
%left   PLUS MINUS
%left   EOL

%%
line: setup expr ?(errors > 0?) !{report_errors();!}
    | setup expr !{writeln($2.value);!}
    | setup ID "=" expr ?(errors == 0?) !{variables[$2.id] = $4.value;!}
    | setup ID "=" expr !{report_errors();!}
    | line EOL line
    | line EOL
    .

setup: !{errors = 0;!}.

expr: expr "+" expr ?($1.value == 0?) !{$$.value = $3.value;!}
    | expr "+" expr ?($3.value == 0?) !{$$.value = $1.value;!}
    | expr "+" expr !{$$.value = $1.value + $3.value;!}
    | expr "-" expr ?($1.value == 0?) !{$$.value = -$3.value;!}
    | expr "-" expr ?($3.value == 0?) !{$$.value = $1.value;!}
    | expr "-" expr !{$$.value = $1.value - $3.value;!}
    | expr "*" expr ?($1.value == 0 || $3.value == 0?) !{$$.value = -$3.value;!}
    | expr "*" expr ?($1.value == 1?) !{$$.value = $3.value;!}
    | expr "*" expr ?($3.value == 1?) !{$$.value = $1.value;!}
    | expr "*" expr !{$$.value = $1.value * $3.value;!}
    | expr "/" expr ?($3.value == 1?) !{$$.value = $1.value;!}
    | expr "/" expr ?($3.value == 0?) !{errors |= Errors.divideByZero;!}
    | expr "/" expr ?($1.value == 0?) !{$$.value = 0;!}
    | expr "/" expr !{$$.value = $1.value / $3.value;!}
    | "(" expr ")" !{$$.value = $2.value;!}
    | "-" expr %prec UMINUS !{$$.value = $2.value;!}
    | NUMBER !{$$.value = $1.value;!}
    | ID ?($1.id in variables?) !{$$.value = variables[$1.id];!}
    | ID !{errors |= Errors.undefinedVariables; $$.value = 0;!}
    | %error !{errors |= Errors.syntaxError;!}
    .