Add backtracking parser for arithmetic example

examples/arithmetic/backtrackingparser/parser.go

@@ -0,0 +1,88 @@
+package backtrackingparser
+
+import (
+	"fmt"
+	"regexp"
+
+	"github.com/shivamMg/rd"
+	. "github.com/shivamMg/rd/examples/arithmetic/tokens"
+)
+
+const Grammar = `
+	Expr   = Term "+" Expr | Term "-" Expr | Term
+	Term   = Factor "*" Term | Factor "/" Term | Factor
+	Factor = "(" Expr ")" | "-" Factor | Number
+`
+
+var (
+	numberRegex = regexp.MustCompile(`^(\d*\.\d+|\d+)$`)
+	b           *rd.Builder
+)
+
+func Expr() (ok bool) {
+	b.Enter("Expr")
+	defer b.Exit(&ok)
+
+	if Term() && b.Match(Plus) && Expr() {
+		return true
+	}
+	b.Reset()
+	if Term() && b.Match(Minus) && Expr() {
+		return true
+	}
+	b.Reset()
+	return Term()
+}
+
+func Term() (ok bool) {
+	b.Enter("Term")
+	defer b.Exit(&ok)
+
+	if Factor() && b.Match(Star) && Term() {
+		return true
+	}
+	b.Reset()
+	if Factor() && b.Match(Slash) && Term() {
+		return true
+	}
+	b.Reset()
+	return Factor()
+}
+
+func Factor() (ok bool) {
+	b.Enter("Factor")
+	defer b.Exit(&ok)
+
+	if b.Match(OpenParen) && Expr() && b.Match(CloseParen) {
+		return true
+	}
+	b.Reset()
+	if b.Match(Minus) && Factor() {
+		return true
+	}
+	return Number()
+}
+
+func Number() (ok bool) {
+	b.Enter("Number")
+	defer b.Exit(&ok)
+
+	token, ok := b.Next()
+	if !ok {
+		return false
+	}
+	if numberRegex.MatchString(fmt.Sprint(token)) {
+		b.Add(token)
+		return true
+	}
+	b.Reset()
+	return false
+}
+
+func Parse(tokens []rd.Token) (parseTree *rd.Tree, debugTree string, err error) {
+	b = rd.NewBuilder(tokens)
+	if ok := Expr(); !ok {
+		return nil, b.DebugTree().Sprint(), b.Err()
+	}
+	return b.Tree(), b.DebugTree().Sprint(), nil
+}

examples/arithmetic/main.go

@@ -6,17 +6,9 @@ import (
 	"strings"
 
 	"github.com/shivamMg/rd"
+	"github.com/shivamMg/rd/examples/arithmetic/parser"
 )
 
-// Grammar without recursion. Left-factored. Needs single lookahead. Suitable for R.D. parsing.
-const Grammar = `
-	Expr   = Term Expr'
-	Expr'  = "+" Expr | "-" Expr | ε
-	Term   = Factor Term'
-	Term'  = "*" Term | "/" Term | ε
-	Factor = "(" Expr ")" | "-" Factor | Number
-`
-
 func main() {
 	if len(os.Args) < 2 {
 		fmt.Println("invalid arguments. pass arithmetic expression as argument")
@@ -32,12 +24,12 @@ func main() {
 	printTokens(tokens)
 
 	fmt.Println("Grammar in EBNF:")
-	fmt.Println(Grammar)
+	fmt.Println(parser.Grammar)
 
-	parseTree, debugTree, err := Parse(tokens)
+	parseTree, debugTree, err := parser.Parse(tokens)
 	if err != nil {
 		fmt.Println("Parsing failed.", err)
-		fmt.Println("Debug Tree:\n", debugTree)
+		fmt.Print("Debug Tree:\n", debugTree, "\n")
 		os.Exit(1)
 	}
 	fmt.Print("Parse Tree:\n\n")

examples/arithmetic/main_test.go

@@ -4,17 +4,17 @@ import (
 	"testing"
 
 	"github.com/shivamMg/rd"
+	"github.com/shivamMg/rd/examples/arithmetic/parser"
 )
 
 func TestArithmeticExpressionsGrammar(t *testing.T) {
 	tokens := []rd.Token{"2.8", "+", "(", "3", "-", ".733", ")", "/", "23"}
-	b = rd.NewBuilder(tokens)
-	ok := Expr()
-	if !ok {
+	parseTree, debugTree, err := parser.Parse(tokens)
+	if err != nil {
 		t.Error("parsing failed")
 	}
 
-	debugTree := `Expr(true)
+	expectedDebugTree := `Expr(true)
 ├─ Term(true)
 │  ├─ Factor(true)
 │  │  ├─ 2.8 ≠ (
@@ -69,12 +69,11 @@ func TestArithmeticExpressionsGrammar(t *testing.T) {
          ├─ <no tokens left> ≠ +
          └─ <no tokens left> ≠ -
 `
-	got := b.DebugTree().Sprint()
-	if got != debugTree {
-		t.Errorf("invalid debug tree. expected: %s\ngot: %s\n", debugTree, got)
+	if debugTree != expectedDebugTree {
+		t.Errorf("invalid debug tree. expected: %s\ngot: %s\n", expectedDebugTree, debugTree)
 	}
 
-	parseTree := `Expr
+	expectedParseTree := `Expr
 ├─ Term
 │  ├─ Factor
 │  │  └─ Number
@@ -117,8 +116,8 @@ func TestArithmeticExpressionsGrammar(t *testing.T) {
       └─ Expr'
          └─ ε
 `
-	got = b.Tree().Sprint()
-	if got != parseTree {
-		t.Errorf("invalid parse tree. want: %s\ngot: %s\n", parseTree, got)
+	got := parseTree.Sprint()
+	if got != expectedParseTree {
+		t.Errorf("invalid parse tree. want: %s\ngot: %s\n", expectedParseTree, got)
 	}
 }

examples/arithmetic/parser.go → examples/arithmetic/parser/parser.go

@@ -1,12 +1,21 @@
-package main
+package parser
 
 import (
 	"fmt"
 	"regexp"
 
 	"github.com/shivamMg/rd"
+	. "github.com/shivamMg/rd/examples/arithmetic/tokens"
 )
 
+const Grammar = `
+	Expr   = Term Expr'
+	Expr'  = "+" Expr | "-" Expr | ε
+	Term   = Factor Term'
+	Term'  = "*" Term | "/" Term | ε
+	Factor = "(" Expr ")" | "-" Factor | Number
+`
+
 var (
 	numberRegex = regexp.MustCompile(`^(\d*\.\d+|\d+)$`)
 	b           *rd.Builder
@@ -79,7 +88,6 @@ func Number() (ok bool) {
 		b.Add(token)
 		return true
 	}
-	b.Reset()
 	return false
 }
 

examples/arithmetic/tokens.go → examples/arithmetic/tokens/tokens.go

@@ -1,4 +1,4 @@
-package main
+package tokens
 
 const (
 	Plus       = "+"

examples/pl0/main.go

@@ -10,38 +10,6 @@ import (
 	"github.com/shivamMg/rd/examples/pl0/parser"
 )
 
-// Grammar is PL/0's grammar in EBNF. Copied from https://en.wikipedia.org/wiki/PL/0#Grammar
-const Grammar = `
-	program = block "." .
-
-	block =
-		["const" ident "=" number {"," ident "=" number} ";"]
-		["var" ident {"," ident} ";"]
-		{"procedure" ident ";" block ";"} statement .
-
-	statement =
-		ident ":=" expression
-		| "!" expression
-		| "?" ident
-		| "call" ident
-		| "begin" statement {";" statement } "end"
-		| "if" condition "then" statement
-		| "while" condition "do" statement .
-
-	condition =
-		"odd" expression
-		| expression ("="|"#"|"<"|"<="|">"|">=") expression .
-
-	expression = ["+"|"-"] term {("+"|"-") term} .
-
-	term = factor {("*"|"/") factor} .
-
-	factor =
-		ident
-		| number
-		| "(" expression ")" .
-`
-
 func main() {
 	if len(os.Args) != 2 {
 		fmt.Println("invalid arguments. pass PL/0 program file as an argument")
@@ -60,7 +28,7 @@ func main() {
 	}
 	fmt.Println("Tokens:", tokens)
 
-	fmt.Println("\nGrammar:", Grammar)
+	fmt.Println("\nGrammar:", parser.Grammar)
 
 	parseTree, debugTree, err := parser.Parse(tokens)
 	if err != nil {

examples/pl0/parser/parser.go

@@ -8,6 +8,38 @@ import (
 	. "github.com/shivamMg/rd/examples/pl0/tokens"
 )
 
+// Grammar is PL/0's grammar in EBNF. Copied from https://en.wikipedia.org/wiki/PL/0#Grammar
+const Grammar = `
+	program = block "." .
+
+	block =
+		["const" ident "=" number {"," ident "=" number} ";"]
+		["var" ident {"," ident} ";"]
+		{"procedure" ident ";" block ";"} statement .
+
+	statement =
+		ident ":=" expression
+		| "!" expression
+		| "?" ident
+		| "call" ident
+		| "begin" statement {";" statement } "end"
+		| "if" condition "then" statement
+		| "while" condition "do" statement .
+
+	condition =
+		"odd" expression
+		| expression ("="|"#"|"<"|"<="|">"|">=") expression .
+
+	expression = ["+"|"-"] term {("+"|"-") term} .
+
+	term = factor {("*"|"/") factor} .
+
+	factor =
+		ident
+		| number
+		| "(" expression ")" .
+`
+
 func Parse(tokens []rd.Token) (parseTree *rd.Tree, debugTree string, err error) {
 	b := rd.NewBuilder(tokens)
 	if ok := Program(b); !ok {