go-parsekit/example_basiccalculator2_test.go

// Let's write the hello world of parsers: a calculator that can interpret
// calculations that looks like:
//
//     " -10 + (10.8+ (3 *-20-3*(8 +-4.12)) + 10)/5  "
//
// More formally, a calculation is defined as:
//
//     calculation : expr EOF
//     expr        : term ((ADD|SUB) term)*
//     term        : factor ((MUL|DIV) factor)*
//     space       : (SPACE|TAB)*
//     factor      : space (FLOAT | LPAREN expr RPAREN) space
package parsekit_test

import (
	"fmt"
	"math"
	"strconv"

	"git.makaay.nl/mauricem/go-parsekit"
)

func Example_basicCalculator2() {
	for _, c := range []struct {
		input    string
		expected float64
	}{
		{"1", 1},
		{"(123.10)", 123.10},
		{"1 + 2 + 3 + 4 + 5", 15},
		{"1 * 2 * 3 * 4 * 5 * 0.6", 72},
		{"(3.05+2)*(4.3+5.12)", 47.571},
		{"8.10 + 999/233", 12.387554},
		{" -10 + (10.8+ (3 *-20-3*(8 +-4.12)) + 10)/5  ", -20.168},
		{"", 0},
		{"(", 0},
		{"10+20-", 0},
		{"10+20-(4*10))", 0},
		{"10+20-((4*10) + 17", 0},
	} {
		output, err := Compute(c.input)
		output = math.Round(output*1000000) / 1000000 // to make the expectation comparisons usable
		if err != nil {
			fmt.Printf("Input: %q, got error: %s\n", c.input, err)
		} else {
			fmt.Printf("Input: %q, got outcome: %f, correct = %t\n", c.input, output, output == c.expected)
		}
	}
	// Output:
	// Input: "1", got outcome: 1.000000, correct = true
	// Input: "(123.10)", got outcome: 123.100000, correct = true
	// Input: "1 + 2 + 3 + 4 + 5", got outcome: 15.000000, correct = true
	// Input: "1 * 2 * 3 * 4 * 5 * 0.6", got outcome: 72.000000, correct = true
	// Input: "(3.05+2)*(4.3+5.12)", got outcome: 47.571000, correct = true
	// Input: "8.10 + 999/233", got outcome: 12.387554, correct = true
	// Input: " -10 + (10.8+ (3 *-20-3*(8 +-4.12)) + 10)/5  ", got outcome: -20.168000, correct = true
	// Input: "", got error: unexpected end of file
	// Input: "(", got error: unexpected end of file
	// Input: "10+20-", got error: unexpected end of file
	// Input: "10+20-(4*10))", got error: unexpected character ')' (expected end of file)
	// Input: "10+20-((4*10) + 17", got error: unexpected end of file (expected ')')
}

// ---------------------------------------------------------------------------
// Implementation of the parser
// ---------------------------------------------------------------------------

// calculator implements a recursive descent parser that is responsible for parsing
// the input computation string according to the grammar.
// It offloads the actual computation to a separate interpreter.
type calculator struct {
	interpreter interpreter
	result      float64
}

// Compute takes a calculation string as input and returns the interpreted result
// value for the calculation. An error can be returned as well, in case the
// computation fails for some reason.
func Compute(input string) (float64, *parsekit.Error) {
	c := &calculator{}
	parser := parsekit.NewParser(c.computation)
	err := parser.Execute(input)
	return c.result, err
}

func (c *calculator) computation(p *parsekit.ParseAPI) {
	p.Handle(c.expr)
	p.ExpectEndOfFile()

	c.result = c.interpreter.result
}

// expr : term ((ADD|SUB) term)*
func (c *calculator) expr(p *parsekit.ParseAPI) {
	c.interpreter.push()

	var pc, a = parsekit.C, parsekit.A
	p.Handle(c.term)
	for p.On(pc.Any(a.Add, a.Subtract)).Skip() {
		c.interpreter.pushOperator(p.LastMatch)
		p.Handle(c.term)
		c.interpreter.eval()
	}

	c.interpreter.pop()
}

// term : factor ((MUL|DIV) factor)*
func (c *calculator) term(p *parsekit.ParseAPI) {
	c.interpreter.push()

	var pc, a = parsekit.C, parsekit.A
	p.Handle(c.factor)
	for p.On(pc.Any(a.Multiply, a.Divide)).Skip() {
		c.interpreter.pushOperator(p.LastMatch)
		p.Handle(c.factor)
		c.interpreter.eval()
	}

	c.interpreter.pop()
}

// factor : space (FLOAT | LPAREN expr RPAREN) space
func (c *calculator) factor(p *parsekit.ParseAPI) {
	var pc, a = parsekit.C, parsekit.A
	p.On(a.Whitespace).Skip()
	switch {
	case p.On(pc.Signed(a.Float)).Accept():
		floatStr := p.BufLiteral()
		p.BufClear()
		value, err := strconv.ParseFloat(floatStr, 64)
		if err != nil {
			p.EmitError("invalid number %s: %s", floatStr, err)
		} else {
			c.interpreter.pushValue(value)
		}
	case p.On(a.LeftParen).Skip():
		p.Handle(c.expr)
		if !p.On(a.RightParen).Skip() {
			p.Expects("')'")
			p.UnexpectedInput()
		}
	default:
		p.UnexpectedInput()
	}
	p.On(a.Whitespace).Skip()
}

// ---------------------------------------------------------------------------
// The computational interpreter, used by the calculator.
// ---------------------------------------------------------------------------

type stackFrame struct {
	a  float64
	b  float64
	op func(a, b float64) float64
}

type interpreter struct {
	stack  []*stackFrame
	top    *stackFrame
	result float64
}

func (i *interpreter) push() *stackFrame {
	f := &stackFrame{}
	i.stack = append(i.stack, f)
	i.top = f
	i.pushOperator("VAL")
	return f
}

func (i *interpreter) pop() float64 {
	value := i.eval()
	i.stack = i.stack[0 : len(i.stack)-1]
	if len(i.stack) > 0 {
		i.top = i.stack[len(i.stack)-1]
		i.pushValue(value)
	} else {
		i.result = i.top.b
		i.top = nil
	}
	return value
}

func (i *interpreter) pushValue(value float64) {
	i.top.a, i.top.b = i.top.b, value
}

func (i *interpreter) pushOperator(op string) {
	switch op {
	case "VAL":
		i.top.op = func(a, b float64) float64 { return b }
	case "+":
		i.top.op = func(a, b float64) float64 { return a + b }
	case "-":
		i.top.op = func(a, b float64) float64 { return a - b }
	case "*":
		i.top.op = func(a, b float64) float64 { return a * b }
	case "/":
		i.top.op = func(a, b float64) float64 { return a / b }
	default:
		panic(fmt.Sprintf("Unhandled op name: %s", op))
	}
}

func (i *interpreter) eval() float64 {
	value := i.top.op(i.top.a, i.top.b)
	i.pushValue(value)
	i.pushOperator("VAL")
	return value
}