diff --git a/interfaces/01-methods/book.go b/interfaces/01-methods/book.go
new file mode 100644
index 0000000..a001e71
--- /dev/null
+++ b/interfaces/01-methods/book.go
@@ -0,0 +1,36 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type book struct {
+	title string
+	price float64
+}
+
+func (b book) print() {
+	// b is a copy of the original `book` value here.
+	fmt.Printf("%-15s: $%.2f \n", b.title, b.price)
+}
+
+// ----------------------------------------------------------------------------
+// + you can use the same method names among different types.
+// + you don't need to type `printGame`, it's just: `print`.
+//
+// func (b book) printBook() {
+// 	// b is a copy of the original `book` value here.
+// 	fmt.Printf("%-15s: $%.2f \n", b.title, b.price)
+// }
+
+// ----------------------------------------------------------------------------
+// b is a copy of the original `book` value here.
+//
+// func printBook(b book) {
+// 	fmt.Printf("%-15s: $%.2f \n", b.title, b.price)
+// }
diff --git a/interfaces/01-methods/game.go b/interfaces/01-methods/game.go
new file mode 100644
index 0000000..6a8efe6
--- /dev/null
+++ b/interfaces/01-methods/game.go
@@ -0,0 +1,36 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type game struct {
+	title string
+	price float64
+}
+
+func (g game) print() {
+	fmt.Printf("%-15s: $%.2f\n", g.title, g.price)
+}
+
+// PREVIOUS CODE:
+
+// ----------------------------------------------------------------------------
+// you can use same method name among different types.
+// you don't need to type `printGame`, it's just: `print`.
+//
+// func (g game) printGame() {
+// 	fmt.Printf("%-15s: $%.2f\n", g.title, g.price)
+// }
+
+// ----------------------------------------------------------------------------
+// you cannot use the same function name within the same package.
+//
+// func printGame(g game) {
+// 	fmt.Printf("%-15s: $%.2f\n", g.title, g.price)
+// }
diff --git a/interfaces/01-methods/main.go b/interfaces/01-methods/main.go
new file mode 100644
index 0000000..dc3a19b
--- /dev/null
+++ b/interfaces/01-methods/main.go
@@ -0,0 +1,46 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+func main() {
+	mobydick := book{
+		title: "moby dick",
+		price: 10,
+	}
+
+	minecraft := game{
+		title: "minecraft",
+		price: 20,
+	}
+
+	tetris := game{
+		title: "tetris",
+		price: 5,
+	}
+
+	// #4: method expressions
+	// methods are just functions that receive a value of a type.
+	game.print(minecraft) // sends `minecraft` value to `game.print`
+	game.print(tetris)    // sends `tetris` value to `game.print`
+	fmt.Println()
+
+	// #3
+	mobydick.print()  // sends `mobydick` value to `book.print`
+	minecraft.print() // sends `minecraft` value to `game.print`
+	tetris.print()    // sends `tetris` value to `game.print`
+
+	// #2
+	// mobydick.printBook()
+	// minecraft.printGame()
+
+	// #1
+	// printBook(mobydick)
+	// printGame(minecraft)
+}
diff --git a/interfaces/02-receivers/book.go b/interfaces/02-receivers/book.go
new file mode 100644
index 0000000..7b925aa
--- /dev/null
+++ b/interfaces/02-receivers/book.go
@@ -0,0 +1,20 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type book struct {
+	title string
+	price float64
+}
+
+func (b book) print() {
+	// b is a copy of the original `book` value here.
+	fmt.Printf("%-15s: $%.2f \n", b.title, b.price)
+}
diff --git a/interfaces/02-receivers/game.go b/interfaces/02-receivers/game.go
new file mode 100644
index 0000000..dcaf1cf
--- /dev/null
+++ b/interfaces/02-receivers/game.go
@@ -0,0 +1,34 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type game struct {
+	title string
+	price float64
+}
+
+func (g *game) print() {
+	fmt.Printf("%-15s: $%.2f\n", g.title, g.price)
+}
+
+// + discount gets a copy of `*game`.
+// + discount can update the original `game` through the game pointer.
+// + it's better to use the same receiver type: `*game` for all methods.
+func (g *game) discount(ratio float64) {
+	g.price *= (1 - ratio)
+}
+
+// PREVIOUS CODE:
+
+// ----------------------------------------------------------------------------
+// + `g` is a copy: `discount` cannot change the original `g`.
+// func (g game) discount(ratio float64) {
+// 	g.price *= (1 - ratio)
+// }
diff --git a/interfaces/02-receivers/list.go b/interfaces/02-receivers/list.go
new file mode 100644
index 0000000..da097a3
--- /dev/null
+++ b/interfaces/02-receivers/list.go
@@ -0,0 +1,28 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+// + you can attach methods to any concrete type.
+// + rule: you need to declare a new type in the same package.
+type list []game
+
+func (l list) print() {
+	// `list` acts like a `[]game`
+	if len(l) == 0 {
+		fmt.Println("Sorry. Our store is closed. We're waiting for the delivery 🚚.")
+		return
+	}
+
+	fmt.Printf("My Store:\n")
+	fmt.Printf("---------\n")
+	for _, it := range l {
+		it.print()
+	}
+}
diff --git a/interfaces/02-receivers/main.go b/interfaces/02-receivers/main.go
new file mode 100644
index 0000000..deba14e
--- /dev/null
+++ b/interfaces/02-receivers/main.go
@@ -0,0 +1,27 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+func main() {
+	var (
+		// mobydick  = book{title: "moby dick", price: 10}
+		minecraft = game{title: "minecraft", price: 20}
+		tetris    = game{title: "tetris", price: 5}
+	)
+
+	var items []game
+	items = append(items, minecraft, tetris)
+
+	// you can attach methods to a compatible type on the fly:
+	my := list([]game{minecraft, tetris})
+
+	// you can call methods even on a nil value
+	// my = nil
+
+	my.print() // or: list.print(items)
+}
diff --git a/interfaces/03-interfaces/book.go b/interfaces/03-interfaces/book.go
new file mode 100644
index 0000000..7b925aa
--- /dev/null
+++ b/interfaces/03-interfaces/book.go
@@ -0,0 +1,20 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type book struct {
+	title string
+	price float64
+}
+
+func (b book) print() {
+	// b is a copy of the original `book` value here.
+	fmt.Printf("%-15s: $%.2f \n", b.title, b.price)
+}
diff --git a/interfaces/03-interfaces/game.go b/interfaces/03-interfaces/game.go
new file mode 100644
index 0000000..dcaf1cf
--- /dev/null
+++ b/interfaces/03-interfaces/game.go
@@ -0,0 +1,34 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type game struct {
+	title string
+	price float64
+}
+
+func (g *game) print() {
+	fmt.Printf("%-15s: $%.2f\n", g.title, g.price)
+}
+
+// + discount gets a copy of `*game`.
+// + discount can update the original `game` through the game pointer.
+// + it's better to use the same receiver type: `*game` for all methods.
+func (g *game) discount(ratio float64) {
+	g.price *= (1 - ratio)
+}
+
+// PREVIOUS CODE:
+
+// ----------------------------------------------------------------------------
+// + `g` is a copy: `discount` cannot change the original `g`.
+// func (g game) discount(ratio float64) {
+// 	g.price *= (1 - ratio)
+// }
diff --git a/interfaces/03-interfaces/list.go b/interfaces/03-interfaces/list.go
new file mode 100644
index 0000000..1c9c2e9
--- /dev/null
+++ b/interfaces/03-interfaces/list.go
@@ -0,0 +1,33 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type printer interface {
+	print()
+}
+
+type list []printer
+
+func (l list) print() {
+	if len(l) == 0 {
+		fmt.Println("Sorry. Our store is closed. We're waiting for the delivery 🚚.")
+		return
+	}
+
+	fmt.Printf("My Store:\n")
+	fmt.Printf("---------\n")
+	for _, it := range l {
+		it.print()
+
+		// you cannot access to the discount method of the game type.
+		// `it` is a printer not a game.
+		// it.discount(.5)
+	}
+}
diff --git a/interfaces/03-interfaces/main.go b/interfaces/03-interfaces/main.go
new file mode 100644
index 0000000..0e80a9e
--- /dev/null
+++ b/interfaces/03-interfaces/main.go
@@ -0,0 +1,52 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+func main() {
+	var (
+		mobydick  = book{title: "moby dick", price: 10}
+		minecraft = game{title: "minecraft", price: 20}
+		tetris    = game{title: "tetris", price: 5}
+		rubik     = puzzle{title: "rubik's cube", price: 5}
+	)
+
+	var store list
+
+	store = append(store, &minecraft, &tetris, mobydick, rubik)
+
+	tetris.discount(.8)
+	// printer(tetris).discount(.8)
+
+	store.print()
+}
+
+/*
+var my list
+
+// + only the `*game` have methods.
+// + `game` doesn't have any methods.
+// + `item` interface couldn't be satisfied with a `game` value.
+// + it can only be satisfied with a `*game` value.
+// my = my.add(minecraft)
+my = append(my, &minecraft)
+my = append(my, &tetris)
+my = append(my, &mobydick)
+
+// interface value stores a pointer to minecraft
+minecraft.discount(.5)
+my.list()
+
+my.discount(.5)
+my.list()
+*/
+
+// ! cannot add mobydick to the store: it's a book type, not a game type.
+// interfaces to the rescue!
+// mobydick := book{title: "moby dick", price: 10}
+// my.add(&mobydick)
+// ----------------------------------------------------------------
diff --git a/interfaces/03-interfaces/puzzle.go b/interfaces/03-interfaces/puzzle.go
new file mode 100644
index 0000000..09f5156
--- /dev/null
+++ b/interfaces/03-interfaces/puzzle.go
@@ -0,0 +1,19 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type puzzle struct {
+	title string
+	price float64
+}
+
+func (p puzzle) print() {
+	fmt.Printf("%-15s: $%.2f \n", p.title, p.price)
+}
diff --git a/interfaces/04-interfaces/book.go b/interfaces/04-interfaces/book.go
new file mode 100644
index 0000000..4cfccba
--- /dev/null
+++ b/interfaces/04-interfaces/book.go
@@ -0,0 +1,24 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type book struct {
+	title string
+	price float64
+}
+
+func (b book) print() {
+	fmt.Printf("%-15s: $%.2f \n", b.title, b.price)
+}
+
+// TODO: NEW
+func (b book) total() float64 {
+	return b.price
+}
diff --git a/interfaces/04-interfaces/game.go b/interfaces/04-interfaces/game.go
new file mode 100644
index 0000000..7cbe528
--- /dev/null
+++ b/interfaces/04-interfaces/game.go
@@ -0,0 +1,28 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type game struct {
+	title string
+	price float64
+}
+
+func (g *game) print() {
+	fmt.Printf("%-15s: $%.2f\n", g.title, g.price)
+}
+
+func (g *game) discount(ratio float64) {
+	g.price *= (1 - ratio)
+}
+
+// TODO: NEW
+func (g *game) total() float64 {
+	return g.price
+}
diff --git a/interfaces/04-interfaces/list.go b/interfaces/04-interfaces/list.go
new file mode 100644
index 0000000..86a0156
--- /dev/null
+++ b/interfaces/04-interfaces/list.go
@@ -0,0 +1,71 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+// TODO: NEW
+type printer interface {
+	print()
+}
+
+// TODO: NEW
+type pricer interface {
+	total() float64
+}
+
+// TODO: NEW
+// interface embedding
+type item interface {
+	printer
+	pricer
+}
+
+// a slice of item interface values
+type list []item
+
+func (l list) list() {
+	// `list` acts like a `[]game`
+	if len(l) == 0 {
+		fmt.Println("Sorry. Our store is closed. We're waiting for the delivery 🚚.")
+		return
+	}
+
+	fmt.Printf("My Store:\n")
+	fmt.Printf("---------\n")
+	for _, it := range l {
+		it.print()
+	}
+
+	// TODO: NEW
+	fmt.Printf("\nGROSS: $%.2f\n", l.total())
+}
+
+// TODO: NEW
+func (l list) total() (gross float64) {
+	for _, it := range l {
+		gross += it.total()
+	}
+	return gross
+}
+
+// TODO: NEW
+func (l list) discount(ratio float64) {
+	type discounter interface {
+		discount(float64)
+	}
+
+	for _, it := range l {
+		dit, ok := it.(discounter)
+		if !ok {
+			continue
+		}
+
+		dit.discount(ratio)
+	}
+}
diff --git a/interfaces/04-interfaces/main.go b/interfaces/04-interfaces/main.go
new file mode 100644
index 0000000..b3ac142
--- /dev/null
+++ b/interfaces/04-interfaces/main.go
@@ -0,0 +1,35 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+func main() {
+	var (
+		minecraft = game{title: "minecraft", price: 20}
+		tetris    = game{title: "tetris", price: 5}
+		mobydick  = book{title: "moby dick", price: 10}
+	)
+
+	var my list
+	// my.list() // nil receiver is a valid value
+
+	// + only the `*game` have methods.
+	// + `game` doesn't have any methods.
+	// + `item` interface couldn't be satisfied with a `game` value.
+	// + it can only be satisfied with a `*game` value.
+	// my = my.add(minecraft)
+	my = append(my, &minecraft)
+	my = append(my, &tetris)
+	my = append(my, &mobydick)
+
+	// interface value stores a pointer to minecraft
+	minecraft.discount(.5)
+	my.list()
+
+	my.discount(.5)
+	my.list()
+}
diff --git a/interfaces/05-log-parser/01-methods/changes.md b/interfaces/05-log-parser/01-methods/changes.md
new file mode 100644
index 0000000..670639d
--- /dev/null
+++ b/interfaces/05-log-parser/01-methods/changes.md
@@ -0,0 +1,36 @@
+# Problem
+
+## All these functions operate on a *parser value:
++ `main.go`:
+  + `summarize(*p parser)`
++ `parser.go`:
+  + `func parse(p *parser, line string) (r result)`
+  + `func update(p *parser, r result)`
+  + `func err(p *parser) error`
+
+## What is an object in Go?
++ Object = A value of a type : `&parser{}` is an object.
+  + `type parser struct` is the definition (class/blueprint) of the object.
+
+### Methods are behaviors of objects:
++ Parser needs to parse, update (analyse) the parsings, and report an error if any.
++ Parser also needs to summarize the results.
++ So, all these functions belong to a parser object.
+
+# Solution
+
++ Attach the functions to the parser using methods.
++ Methods are the behavior of the parser.
++ Keep the methods that belong to a single type in the same file.
+
+1. Move the `summarize` into `parser.go`.
+2. `parser.go`: Use methods.
+3. `main()`   : Use methods of the `parser`.
+
+# Convince
+
++ `p.` shows the fields and methods belong to the `parser`.
++ `p.` selects a method from the method set of the `parser`.
++ `p.method` sends the `p` as the first parameter to the method.
++ `p` is the receiver: `*pointer`. It's a pointer receiver.
++ Receiver is copied to the method.
\ No newline at end of file
diff --git a/interfaces/05-log-parser/01-methods/log.txt b/interfaces/05-log-parser/01-methods/log.txt
new file mode 100644
index 0000000..940b832
--- /dev/null
+++ b/interfaces/05-log-parser/01-methods/log.txt
@@ -0,0 +1,16 @@
+learngoprogramming.com 10
+learngoprogramming.com 15
+learngoprogramming.com 10
+learngoprogramming.com 20
+learngoprogramming.com 5
+golang.org 40
+golang.org 20
+golang.org 45
+golang.org 15
+blog.golang.org 60
+blog.golang.org 30
+blog.golang.org 20
+blog.golang.org 65
+blog.golang.org 15
+inanc.io 30
+inanc.io 70
\ No newline at end of file
diff --git a/interfaces/05-log-parser/01-methods/log_err_missing.txt b/interfaces/05-log-parser/01-methods/log_err_missing.txt
new file mode 100644
index 0000000..9169c91
--- /dev/null
+++ b/interfaces/05-log-parser/01-methods/log_err_missing.txt
@@ -0,0 +1,16 @@
+learngoprogramming.com 10
+learngoprogramming.com 15
+learngoprogramming.com 10
+learngoprogramming.com 20
+learngoprogramming.com
+golang.org 40
+golang.org 20
+golang.org 45
+golang.org 15
+blog.golang.org 60
+blog.golang.org 30
+blog.golang.org 20
+blog.golang.org 65
+blog.golang.org 15
+inanc.io 30
+inanc.io 70
\ No newline at end of file
diff --git a/interfaces/05-log-parser/01-methods/log_err_negative.txt b/interfaces/05-log-parser/01-methods/log_err_negative.txt
new file mode 100644
index 0000000..feccc0c
--- /dev/null
+++ b/interfaces/05-log-parser/01-methods/log_err_negative.txt
@@ -0,0 +1,16 @@
+learngoprogramming.com 10
+learngoprogramming.com 15
+learngoprogramming.com 10
+learngoprogramming.com 20
+learngoprogramming.com 5
+golang.org 40
+golang.org 20
+golang.org -50
+golang.org 15
+blog.golang.org 60
+blog.golang.org 30
+blog.golang.org 20
+blog.golang.org 65
+blog.golang.org 15
+inanc.io 30
+inanc.io 70
\ No newline at end of file
diff --git a/interfaces/05-log-parser/01-methods/log_err_str.txt b/interfaces/05-log-parser/01-methods/log_err_str.txt
new file mode 100644
index 0000000..59a1fd8
--- /dev/null
+++ b/interfaces/05-log-parser/01-methods/log_err_str.txt
@@ -0,0 +1,16 @@
+learngoprogramming.com 10
+learngoprogramming.com 15
+learngoprogramming.com 10
+learngoprogramming.com 20
+learngoprogramming.com 5
+golang.org FORTY
+golang.org 20
+golang.org 45
+golang.org 15
+blog.golang.org 60
+blog.golang.org 30
+blog.golang.org 20
+blog.golang.org 65
+blog.golang.org 15
+inanc.io 30
+inanc.io 70
\ No newline at end of file
diff --git a/interfaces/05-log-parser/01-methods/main.go b/interfaces/05-log-parser/01-methods/main.go
new file mode 100644
index 0000000..9e6c0ce
--- /dev/null
+++ b/interfaces/05-log-parser/01-methods/main.go
@@ -0,0 +1,39 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+func main() {
+	p := newParser()
+
+	fmt.Printf("%T\n", (*parser).parse)
+	return
+
+	in := bufio.NewScanner(os.Stdin)
+	for in.Scan() {
+		parsed := p.parse(in.Text())
+		p.update(parsed)
+	}
+
+	p.summarize()
+	dumpErrs([]error{in.Err(), p.err()})
+}
+
+// dumpErrs together to simplify multiple error handling
+func dumpErrs(errs []error) {
+	for _, err := range errs {
+		if err != nil {
+			fmt.Println("> Err:", err)
+		}
+	}
+}
diff --git a/interfaces/05-log-parser/01-methods/parser.go b/interfaces/05-log-parser/01-methods/parser.go
new file mode 100644
index 0000000..fb76c07
--- /dev/null
+++ b/interfaces/05-log-parser/01-methods/parser.go
@@ -0,0 +1,100 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"fmt"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+// result stores details about a log line
+type result struct {
+	domain string
+	visits int
+	// add more metrics when needed
+}
+
+// parser keep tracks of the parsing
+type parser struct {
+	sum     map[string]result // metrics per domain
+	domains []string          // unique domain names
+	total   int               // total visits for all domains
+	lines   int               // number of parsed lines (for the error messages)
+	lerr    error             // the last error occurred
+}
+
+// newParser creates and returns a new parser
+func newParser() *parser {
+	return &parser{sum: make(map[string]result)}
+}
+
+// parse result from a log line
+func (p *parser) parse(line string) (r result) {
+	if p.lerr != nil {
+		return
+	}
+
+	p.lines++
+
+	fields := strings.Fields(line)
+	if len(fields) != 2 {
+		p.lerr = fmt.Errorf("wrong input: %v (line #%d)", fields, p.lines)
+		return
+	}
+
+	var err error
+
+	r.domain = fields[0]
+	r.visits, err = strconv.Atoi(fields[1])
+
+	if r.visits < 0 || err != nil {
+		p.lerr = fmt.Errorf("wrong input: %q (line #%d)", fields[1], p.lines)
+	}
+	return
+}
+
+// update the parsing results
+func (p *parser) update(r result) {
+	if p.lerr != nil {
+		return
+	}
+
+	// collect unique domains for easy access to sum map
+	if _, ok := p.sum[r.domain]; !ok {
+		p.domains = append(p.domains, r.domain)
+	}
+
+	// keep track of total visits
+	p.total += r.visits
+
+	// group the log lines by domain
+	p.sum[r.domain] = result{
+		domain: r.domain,
+		visits: r.visits + p.sum[r.domain].visits,
+	}
+}
+
+// summarize the parsing results
+func (p *parser) summarize() {
+	sort.Strings(p.domains)
+
+	fmt.Printf("%-30s %10s\n", "DOMAIN", "VISITS")
+	fmt.Println(strings.Repeat("-", 45))
+
+	for _, domain := range p.domains {
+		fmt.Printf("%-30s %10d\n", domain, p.sum[domain].visits)
+	}
+	fmt.Printf("\n%-30s %10d\n", "TOTAL", p.total)
+}
+
+// err returns the last error encountered
+func (p *parser) err() error {
+	return p.lerr
+}
diff --git a/interfaces/functional/Makefile b/interfaces/05-log-parser/functional/Makefile
similarity index 100%
rename from interfaces/functional/Makefile
rename to interfaces/05-log-parser/functional/Makefile
diff --git a/interfaces/functional/chartwriter.go b/interfaces/05-log-parser/functional/chartwriter.go
similarity index 100%
rename from interfaces/functional/chartwriter.go
rename to interfaces/05-log-parser/functional/chartwriter.go
diff --git a/interfaces/functional/field.go b/interfaces/05-log-parser/functional/field.go
similarity index 100%
rename from interfaces/functional/field.go
rename to interfaces/05-log-parser/functional/field.go
diff --git a/interfaces/functional/filters.go b/interfaces/05-log-parser/functional/filters.go
similarity index 100%
rename from interfaces/functional/filters.go
rename to interfaces/05-log-parser/functional/filters.go
diff --git a/interfaces/functional/groupers.go b/interfaces/05-log-parser/functional/groupers.go
similarity index 100%
rename from interfaces/functional/groupers.go
rename to interfaces/05-log-parser/functional/groupers.go
diff --git a/interfaces/functional/main.go b/interfaces/05-log-parser/functional/main.go
similarity index 100%
rename from interfaces/functional/main.go
rename to interfaces/05-log-parser/functional/main.go
diff --git a/interfaces/functional/pipeline.go b/interfaces/05-log-parser/functional/pipeline.go
similarity index 100%
rename from interfaces/functional/pipeline.go
rename to interfaces/05-log-parser/functional/pipeline.go
diff --git a/interfaces/functional/result.go b/interfaces/05-log-parser/functional/result.go
similarity index 100%
rename from interfaces/functional/result.go
rename to interfaces/05-log-parser/functional/result.go
diff --git a/interfaces/functional/textreader.go b/interfaces/05-log-parser/functional/textreader.go
similarity index 100%
rename from interfaces/functional/textreader.go
rename to interfaces/05-log-parser/functional/textreader.go
diff --git a/interfaces/functional/textwriter.go b/interfaces/05-log-parser/functional/textwriter.go
similarity index 100%
rename from interfaces/functional/textwriter.go
rename to interfaces/05-log-parser/functional/textwriter.go
diff --git a/interfaces/logs/Makefile b/interfaces/05-log-parser/logs/Makefile
similarity index 100%
rename from interfaces/logs/Makefile
rename to interfaces/05-log-parser/logs/Makefile
diff --git a/interfaces/logs/log.json b/interfaces/05-log-parser/logs/log.json
similarity index 100%
rename from interfaces/logs/log.json
rename to interfaces/05-log-parser/logs/log.json
diff --git a/interfaces/logs/log.txt b/interfaces/05-log-parser/logs/log.txt
similarity index 100%
rename from interfaces/logs/log.txt
rename to interfaces/05-log-parser/logs/log.txt
diff --git a/interfaces/logs/log_err_missing.txt b/interfaces/05-log-parser/logs/log_err_missing.txt
similarity index 100%
rename from interfaces/logs/log_err_missing.txt
rename to interfaces/05-log-parser/logs/log_err_missing.txt
diff --git a/interfaces/logs/log_err_negative.txt b/interfaces/05-log-parser/logs/log_err_negative.txt
similarity index 100%
rename from interfaces/logs/log_err_negative.txt
rename to interfaces/05-log-parser/logs/log_err_negative.txt
diff --git a/interfaces/logs/log_err_str.txt b/interfaces/05-log-parser/logs/log_err_str.txt
similarity index 100%
rename from interfaces/logs/log_err_str.txt
rename to interfaces/05-log-parser/logs/log_err_str.txt
diff --git a/interfaces/oop/final/Makefile b/interfaces/05-log-parser/oop/Makefile
similarity index 100%
rename from interfaces/oop/final/Makefile
rename to interfaces/05-log-parser/oop/Makefile
diff --git a/interfaces/oop/final/analysis.go b/interfaces/05-log-parser/oop/analysis.go
similarity index 100%
rename from interfaces/oop/final/analysis.go
rename to interfaces/05-log-parser/oop/analysis.go
diff --git a/interfaces/oop/final/chartsummary.go b/interfaces/05-log-parser/oop/chartsummary.go
similarity index 100%
rename from interfaces/oop/final/chartsummary.go
rename to interfaces/05-log-parser/oop/chartsummary.go
diff --git a/interfaces/oop/final/filters.go b/interfaces/05-log-parser/oop/filters.go
similarity index 100%
rename from interfaces/oop/final/filters.go
rename to interfaces/05-log-parser/oop/filters.go
diff --git a/interfaces/oop/final/groupers.go b/interfaces/05-log-parser/oop/groupers.go
similarity index 100%
rename from interfaces/oop/final/groupers.go
rename to interfaces/05-log-parser/oop/groupers.go
diff --git a/interfaces/oop/final/jsonparser.go b/interfaces/05-log-parser/oop/jsonparser.go
similarity index 100%
rename from interfaces/oop/final/jsonparser.go
rename to interfaces/05-log-parser/oop/jsonparser.go
diff --git a/interfaces/oop/final/main.go b/interfaces/05-log-parser/oop/main.go
similarity index 100%
rename from interfaces/oop/final/main.go
rename to interfaces/05-log-parser/oop/main.go
diff --git a/interfaces/oop/final/readclose.go b/interfaces/05-log-parser/oop/readclose.go
similarity index 100%
rename from interfaces/oop/final/readclose.go
rename to interfaces/05-log-parser/oop/readclose.go
diff --git a/interfaces/oop/final/report.go b/interfaces/05-log-parser/oop/report.go
similarity index 100%
rename from interfaces/oop/final/report.go
rename to interfaces/05-log-parser/oop/report.go
diff --git a/interfaces/oop/final/result.go b/interfaces/05-log-parser/oop/result.go
similarity index 100%
rename from interfaces/oop/final/result.go
rename to interfaces/05-log-parser/oop/result.go
diff --git a/interfaces/oop/final/textparser.go b/interfaces/05-log-parser/oop/textparser.go
similarity index 100%
rename from interfaces/oop/final/textparser.go
rename to interfaces/05-log-parser/oop/textparser.go
diff --git a/interfaces/oop/final/textsummary.go b/interfaces/05-log-parser/oop/textsummary.go
similarity index 100%
rename from interfaces/oop/final/textsummary.go
rename to interfaces/05-log-parser/oop/textsummary.go
diff --git a/interfaces/refactor-notes/refactor-00/changes.md b/interfaces/05-log-parser/refactor-notes/refactor-00/changes.md
similarity index 100%
rename from interfaces/refactor-notes/refactor-00/changes.md
rename to interfaces/05-log-parser/refactor-notes/refactor-00/changes.md
diff --git a/interfaces/refactor-notes/refactor-00/main.go b/interfaces/05-log-parser/refactor-notes/refactor-00/main.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-00/main.go
rename to interfaces/05-log-parser/refactor-notes/refactor-00/main.go
diff --git a/interfaces/refactor-notes/refactor-00/parser.go b/interfaces/05-log-parser/refactor-notes/refactor-00/parser.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-00/parser.go
rename to interfaces/05-log-parser/refactor-notes/refactor-00/parser.go
diff --git a/interfaces/refactor-notes/refactor-00/result.go b/interfaces/05-log-parser/refactor-notes/refactor-00/result.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-00/result.go
rename to interfaces/05-log-parser/refactor-notes/refactor-00/result.go
diff --git a/interfaces/refactor-notes/refactor-00/summarize.go b/interfaces/05-log-parser/refactor-notes/refactor-00/summarize.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-00/summarize.go
rename to interfaces/05-log-parser/refactor-notes/refactor-00/summarize.go
diff --git a/interfaces/refactor-notes/refactor-01/changes.md b/interfaces/05-log-parser/refactor-notes/refactor-01/changes.md
similarity index 100%
rename from interfaces/refactor-notes/refactor-01/changes.md
rename to interfaces/05-log-parser/refactor-notes/refactor-01/changes.md
diff --git a/interfaces/refactor-notes/refactor-01/main.go b/interfaces/05-log-parser/refactor-notes/refactor-01/main.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-01/main.go
rename to interfaces/05-log-parser/refactor-notes/refactor-01/main.go
diff --git a/interfaces/refactor-notes/refactor-01/parser.go b/interfaces/05-log-parser/refactor-notes/refactor-01/parser.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-01/parser.go
rename to interfaces/05-log-parser/refactor-notes/refactor-01/parser.go
diff --git a/interfaces/refactor-notes/refactor-01/result.go b/interfaces/05-log-parser/refactor-notes/refactor-01/result.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-01/result.go
rename to interfaces/05-log-parser/refactor-notes/refactor-01/result.go
diff --git a/interfaces/refactor-notes/refactor-01/summarize.go b/interfaces/05-log-parser/refactor-notes/refactor-01/summarize.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-01/summarize.go
rename to interfaces/05-log-parser/refactor-notes/refactor-01/summarize.go
diff --git a/interfaces/refactor-notes/refactor-02/changes.md b/interfaces/05-log-parser/refactor-notes/refactor-02/changes.md
similarity index 100%
rename from interfaces/refactor-notes/refactor-02/changes.md
rename to interfaces/05-log-parser/refactor-notes/refactor-02/changes.md
diff --git a/interfaces/refactor-notes/refactor-02/main.go b/interfaces/05-log-parser/refactor-notes/refactor-02/main.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-02/main.go
rename to interfaces/05-log-parser/refactor-notes/refactor-02/main.go
diff --git a/interfaces/refactor-notes/refactor-02/parser.go b/interfaces/05-log-parser/refactor-notes/refactor-02/parser.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-02/parser.go
rename to interfaces/05-log-parser/refactor-notes/refactor-02/parser.go
diff --git a/interfaces/refactor-notes/refactor-02/result.go b/interfaces/05-log-parser/refactor-notes/refactor-02/result.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-02/result.go
rename to interfaces/05-log-parser/refactor-notes/refactor-02/result.go
diff --git a/interfaces/refactor-notes/refactor-02/summarize.go b/interfaces/05-log-parser/refactor-notes/refactor-02/summarize.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-02/summarize.go
rename to interfaces/05-log-parser/refactor-notes/refactor-02/summarize.go
diff --git a/interfaces/refactor-notes/refactor-03-deprecated/changes.md b/interfaces/05-log-parser/refactor-notes/refactor-03-deprecated/changes.md
similarity index 100%
rename from interfaces/refactor-notes/refactor-03-deprecated/changes.md
rename to interfaces/05-log-parser/refactor-notes/refactor-03-deprecated/changes.md
diff --git a/interfaces/refactor-notes/refactor-03-deprecated/main.go b/interfaces/05-log-parser/refactor-notes/refactor-03-deprecated/main.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-03-deprecated/main.go
rename to interfaces/05-log-parser/refactor-notes/refactor-03-deprecated/main.go
diff --git a/interfaces/refactor-notes/refactor-03-deprecated/parser.go b/interfaces/05-log-parser/refactor-notes/refactor-03-deprecated/parser.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-03-deprecated/parser.go
rename to interfaces/05-log-parser/refactor-notes/refactor-03-deprecated/parser.go
diff --git a/interfaces/refactor-notes/refactor-03-deprecated/result.go b/interfaces/05-log-parser/refactor-notes/refactor-03-deprecated/result.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-03-deprecated/result.go
rename to interfaces/05-log-parser/refactor-notes/refactor-03-deprecated/result.go
diff --git a/interfaces/refactor-notes/refactor-03-deprecated/scanner.go b/interfaces/05-log-parser/refactor-notes/refactor-03-deprecated/scanner.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-03-deprecated/scanner.go
rename to interfaces/05-log-parser/refactor-notes/refactor-03-deprecated/scanner.go
diff --git a/interfaces/refactor-notes/refactor-03-deprecated/summarize.go b/interfaces/05-log-parser/refactor-notes/refactor-03-deprecated/summarize.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-03-deprecated/summarize.go
rename to interfaces/05-log-parser/refactor-notes/refactor-03-deprecated/summarize.go
diff --git a/interfaces/refactor-notes/refactor-03/changes.md b/interfaces/05-log-parser/refactor-notes/refactor-03/changes.md
similarity index 100%
rename from interfaces/refactor-notes/refactor-03/changes.md
rename to interfaces/05-log-parser/refactor-notes/refactor-03/changes.md
diff --git a/interfaces/refactor-notes/refactor-03/main.go b/interfaces/05-log-parser/refactor-notes/refactor-03/main.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-03/main.go
rename to interfaces/05-log-parser/refactor-notes/refactor-03/main.go
diff --git a/interfaces/refactor-notes/refactor-03/parser.go b/interfaces/05-log-parser/refactor-notes/refactor-03/parser.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-03/parser.go
rename to interfaces/05-log-parser/refactor-notes/refactor-03/parser.go
diff --git a/interfaces/refactor-notes/refactor-03/result.go b/interfaces/05-log-parser/refactor-notes/refactor-03/result.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-03/result.go
rename to interfaces/05-log-parser/refactor-notes/refactor-03/result.go
diff --git a/interfaces/refactor-notes/refactor-03/summarize.go b/interfaces/05-log-parser/refactor-notes/refactor-03/summarize.go
similarity index 100%
rename from interfaces/refactor-notes/refactor-03/summarize.go
rename to interfaces/05-log-parser/refactor-notes/refactor-03/summarize.go
diff --git a/interfaces/logparser-testing/log.txt b/interfaces/05-log-parser/testing/log.txt
similarity index 100%
rename from interfaces/logparser-testing/log.txt
rename to interfaces/05-log-parser/testing/log.txt
diff --git a/interfaces/logparser-testing/log_err_missing.txt b/interfaces/05-log-parser/testing/log_err_missing.txt
similarity index 100%
rename from interfaces/logparser-testing/log_err_missing.txt
rename to interfaces/05-log-parser/testing/log_err_missing.txt
diff --git a/interfaces/logparser-testing/log_err_negative.txt b/interfaces/05-log-parser/testing/log_err_negative.txt
similarity index 100%
rename from interfaces/logparser-testing/log_err_negative.txt
rename to interfaces/05-log-parser/testing/log_err_negative.txt
diff --git a/interfaces/logparser-testing/log_err_str.txt b/interfaces/05-log-parser/testing/log_err_str.txt
similarity index 100%
rename from interfaces/logparser-testing/log_err_str.txt
rename to interfaces/05-log-parser/testing/log_err_str.txt
diff --git a/interfaces/logparser-testing/main.go b/interfaces/05-log-parser/testing/main.go
similarity index 84%
rename from interfaces/logparser-testing/main.go
rename to interfaces/05-log-parser/testing/main.go
index a2372c6..0f5cc83 100644
--- a/interfaces/logparser-testing/main.go
+++ b/interfaces/05-log-parser/testing/main.go
@@ -11,7 +11,7 @@ import (
 	"bufio"
 	"os"
 
-	"github.com/inancgumus/learngo/27-interfaces/logparser-testing/report"
+	"github.com/inancgumus/learngo/interfaces/04-log-parser/testing/report"
 )
 
 func main() {
diff --git a/interfaces/logparser-testing/main_test.go b/interfaces/05-log-parser/testing/main_test.go
similarity index 100%
rename from interfaces/logparser-testing/main_test.go
rename to interfaces/05-log-parser/testing/main_test.go
diff --git a/interfaces/logparser-testing/report/parser.go b/interfaces/05-log-parser/testing/report/parser.go
similarity index 100%
rename from interfaces/logparser-testing/report/parser.go
rename to interfaces/05-log-parser/testing/report/parser.go
diff --git a/interfaces/logparser-testing/report/parser_test.go b/interfaces/05-log-parser/testing/report/parser_test.go
similarity index 100%
rename from interfaces/logparser-testing/report/parser_test.go
rename to interfaces/05-log-parser/testing/report/parser_test.go
diff --git a/interfaces/logparser-testing/report/result.go b/interfaces/05-log-parser/testing/report/result.go
similarity index 100%
rename from interfaces/logparser-testing/report/result.go
rename to interfaces/05-log-parser/testing/report/result.go
diff --git a/interfaces/logparser-testing/report/summary.go b/interfaces/05-log-parser/testing/report/summary.go
similarity index 100%
rename from interfaces/logparser-testing/report/summary.go
rename to interfaces/05-log-parser/testing/report/summary.go
diff --git a/interfaces/logparser-testing/report/summary_test.go b/interfaces/05-log-parser/testing/report/summary_test.go
similarity index 100%
rename from interfaces/logparser-testing/report/summary_test.go
rename to interfaces/05-log-parser/testing/report/summary_test.go
diff --git a/interfaces/logparser-testing/summarize.go b/interfaces/05-log-parser/testing/summarize.go
similarity index 95%
rename from interfaces/logparser-testing/summarize.go
rename to interfaces/05-log-parser/testing/summarize.go
index 1573f4c..a6f5b4c 100644
--- a/interfaces/logparser-testing/summarize.go
+++ b/interfaces/05-log-parser/testing/summarize.go
@@ -13,7 +13,7 @@ import (
 	"os"
 	"strings"
 
-	"github.com/inancgumus/learngo/27-interfaces/logparser-testing/report"
+	"github.com/inancgumus/learngo/interfaces/04-log-parser/testing/report"
 )
 
 // summarize prints the parsing results.