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// nolint: govet
package main
import (
"encoding/json"
"fmt"
"math"
"os"
"strings"
"github.com/alecthomas/kong"
"github.com/alecthomas/participle/v2"
)
var cli struct {
AST bool `help:"Print AST for expression."`
Set map[string]float64 `short:"s" help:"Set variables."`
Expression []string `arg required help:"Expression to evaluate."`
}
type Operator int
const (
OpMul Operator = iota
OpDiv
OpAdd
OpSub
)
var operatorMap = map[string]Operator{"+": OpAdd, "-": OpSub, "*": OpMul, "/": OpDiv}
func (o *Operator) Capture(s []string) error {
*o = operatorMap[s[0]]
return nil
}
// E --> T {( "+" | "-" ) T}
// T --> F {( "*" | "/" ) F}
// F --> P ["^" F]
// P --> v | "(" E ")" | "-" T
type Value struct {
Number *float64 ` @(Float|Int)`
Variable *string `| @Ident`
Subexpression *Expression `| "(" @@ ")"`
}
type Factor struct {
Base *Value `@@`
Exponent *Value `( "^" @@ )?`
}
type OpFactor struct {
Operator Operator `@("*" | "/")`
Factor *Factor `@@`
}
type Term struct {
Left *Factor `@@`
Right []*OpFactor `@@*`
}
type OpTerm struct {
Operator Operator `@("+" | "-")`
Term *Term `@@`
}
type Expression struct {
Left *Term `@@`
Right []*OpTerm `@@*`
}
// Display
func (o Operator) String() string {
switch o {
case OpMul:
return "*"
case OpDiv:
return "/"
case OpSub:
return "-"
case OpAdd:
return "+"
}
panic("unsupported operator")
}
func (v *Value) String() string {
if v.Number != nil {
return fmt.Sprintf("%g", *v.Number)
}
if v.Variable != nil {
return *v.Variable
}
return "(" + v.Subexpression.String() + ")"
}
func (f *Factor) String() string {
out := f.Base.String()
if f.Exponent != nil {
out += " ^ " + f.Exponent.String()
}
return out
}
func (o *OpFactor) String() string {
return fmt.Sprintf("%s %s", o.Operator, o.Factor)
}
func (t *Term) String() string {
out := []string{t.Left.String()}
for _, r := range t.Right {
out = append(out, r.String())
}
return strings.Join(out, " ")
}
func (o *OpTerm) String() string {
return fmt.Sprintf("%s %s", o.Operator, o.Term)
}
func (e *Expression) String() string {
out := []string{e.Left.String()}
for _, r := range e.Right {
out = append(out, r.String())
}
return strings.Join(out, " ")
}
// Evaluation
func (o Operator) Eval(l, r float64) float64 {
switch o {
case OpMul:
return l * r
case OpDiv:
return l / r
case OpAdd:
return l + r
case OpSub:
return l - r
}
panic("unsupported operator")
}
func (v *Value) Eval(ctx Context) float64 {
switch {
case v.Number != nil:
return *v.Number
case v.Variable != nil:
value, ok := ctx[*v.Variable]
if !ok {
panic("no such variable " + *v.Variable)
}
return value
default:
return v.Subexpression.Eval(ctx)
}
}
func (f *Factor) Eval(ctx Context) float64 {
b := f.Base.Eval(ctx)
if f.Exponent != nil {
return math.Pow(b, f.Exponent.Eval(ctx))
}
return b
}
func (t *Term) Eval(ctx Context) float64 {
n := t.Left.Eval(ctx)
for _, r := range t.Right {
n = r.Operator.Eval(n, r.Factor.Eval(ctx))
}
return n
}
func (e *Expression) Eval(ctx Context) float64 {
l := e.Left.Eval(ctx)
for _, r := range e.Right {
l = r.Operator.Eval(l, r.Term.Eval(ctx))
}
return l
}
type Context map[string]float64
var parser = participle.MustBuild[Expression]()
func main() {
ctx := kong.Parse(&cli,
kong.Description("A basic expression parser and evaluator."),
kong.UsageOnError(),
)
expr, err := parser.ParseString("", strings.Join(cli.Expression, " "))
ctx.FatalIfErrorf(err)
if cli.AST {
json.NewEncoder(os.Stdout).Encode(expr)
} else {
fmt.Println(expr, "=", expr.Eval(cli.Set))
}
}
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