// Single line comment
/* Multi-
 line comment */

 /* A build tag is a line comment starting with // +build
  and can be executed by go build -tags="foo bar" command.
  Build tags are placed before the package clause near or at the top of the file
  followed by a blank line or other line comments. */
// +build prod, dev, test

// A package clause starts every source file.
// Main is a special name declaring an executable rather than a library.
package main

// Import declaration declares library packages referenced in this file.
import (
    "fmt"       // A package in the Go standard library.
    "io/ioutil" // Implements some I/O utility functions.
    m "math"    // Math library with local alias m.
    "net/http"  // Yes, a web server!
    "os"        // OS functions like working with the file system
    "strconv"   // String conversions.
)

// A function definition. Main is special. It is the entry point for the
// executable program. Love it or hate it, Go uses brace brackets.
func main() {
    // Println outputs a line to stdout.
    // It comes from the package fmt.
    fmt.Println("Hello world!")

    // Call another function within this package.
    beyondHello()
}

// Functions have parameters in parentheses.
// If there are no parameters, empty parentheses are still required.
func beyondHello() {
    var x int // Variable declaration. Variables must be declared before use.
    x = 3     // Variable assignment.
    // "Short" declarations use := to infer the type, declare, and assign.
    y := 4
    sum, prod := learnMultiple(x, y)        // Function returns two values.
    fmt.Println("sum:", sum, "prod:", prod) // Simple output.
    learnTypes()                            // < y minutes, learn more!
}

/* <- multiline comment
Functions can have parameters and (multiple!) return values.
Here `x`, `y` are the arguments and `sum`, `prod` is the signature (what's returned).
Note that `x` and `sum` receive the type `int`.
*/
func learnMultiple(x, y int) (sum, prod int) {
    return x + y, x * y // Return two values.
}

// Some built-in types and literals.
func learnTypes() {
    // Short declaration usually gives you what you want.
    str := "Learn Go!" // string type.

    s2 := `A "raw" string literal
can include line breaks.` // Same string type.

    // Non-ASCII literal. Go source is UTF-8.
    g := 'Σ' // rune type, an alias for int32, holds a unicode code point.

    f := 3.14195 // float64, an IEEE-754 64-bit floating point number.
    c := 3 + 4i  // complex128, represented internally with two float64's.

    // var syntax with initializers.
    var u uint = 7 // Unsigned, but implementation dependent size as with int.
    var pi float32 = 22. / 7

    // Conversion syntax with a short declaration.
    n := byte('\n') // byte is an alias for uint8.

    // Arrays have size fixed at compile time.
    var a4 [4]int           // An array of 4 ints, initialized to all 0.
    a5 := [...]int{3, 1, 5, 10, 100} // An array initialized with a fixed size of five
    // elements, with values 3, 1, 5, 10, and 100.

    // Arrays have value semantics.
    a4_cpy := a4            // a4_cpy is a copy of a4, two separate instances.
    a4_cpy[0] = 25          // Only a4_cpy is changed, a4 stays the same.
    fmt.Println(a4_cpy[0] == a4[0]) // false

    // Slices have dynamic size. Arrays and slices each have advantages
    // but use cases for slices are much more common.
    s3 := []int{4, 5, 9}    // Compare to a5. No ellipsis here.
    s4 := make([]int, 4)    // Allocates slice of 4 ints, initialized to all 0.
    var d2 [][]float64      // Declaration only, nothing allocated here.
    bs := []byte("a slice") // Type conversion syntax.

    // Slices (as well as maps and channels) have reference semantics.
    s3_cpy := s3            // Both variables point to the same instance.
    s3_cpy[0] = 0           // Which means both are updated.
    fmt.Println(s3_cpy[0] == s3[0]) // true 

    // Because they are dynamic, slices can be appended to on-demand.
    // To append elements to a slice, the built-in append() function is used.
    // First argument is a slice to which we are appending. Commonly,
    // the array variable is updated in place, as in example below.
    s := []int{1, 2, 3}     // Result is a slice of length 3.
    s = append(s, 4, 5, 6)  // Added 3 elements. Slice now has length of 6.
    fmt.Println(s) // Updated slice is now [1 2 3 4 5 6]

    // To append another slice, instead of list of atomic elements we can
    // pass a reference to a slice or a slice literal like this, with a
    // trailing ellipsis, meaning take a slice and unpack its elements,
    // appending them to slice s.
    s = append(s, []int{7, 8, 9}...) // Second argument is a slice literal.
    fmt.Println(s)  // Updated slice is now [1 2 3 4 5 6 7 8 9]

    p, q := learnMemory() // Declares p, q to be type pointer to int.
    fmt.Println(*p, *q)   // * follows a pointer. This prints two ints.

    // Maps are a dynamically growable associative array type, like the
    // hash or dictionary types of some other languages.
    m := map[string]int{"three": 3, "four": 4}
    m["one"] = 1

    // Unused variables are an error in Go.
    // The underscore lets you "use" a variable but discard its value.
    _, _, _, _, _, _, _, _, _, _ = str, s2, g, f, u, pi, n, a5, s4, bs
    // Usually you use it to ignore one of the return values of a function
    // For example, in a quick and dirty script you might ignore the
    // error value returned from os.Create, and expect that the file
    // will always be created.
    file, _ := os.Create("output.txt")
    fmt.Fprint(file, "This is how you write to a file, by the way")
    file.Close()

    // Output of course counts as using a variable.
    fmt.Println(s, c, a4, s3, d2, m)

    learnFlowControl() // Back in the flow.
}

// It is possible, unlike in many other languages for functions in go
// to have named return values.
// Assigning a name to the type being returned in the function declaration line
// allows us to easily return from multiple points in a function as well as to
// only use the return keyword, without anything further.
func learnNamedReturns(x, y int) (z int) {
    z = x * y
    return // z is implicit here, because we named it earlier.
}

// Go is fully garbage collected. It has pointers but no pointer arithmetic.
// You can make a mistake with a nil pointer, but not by incrementing a pointer.
// Unlike in C/Cpp taking and returning an address of a local varible is also safe. 
func learnMemory() (p, q *int) {
    // Named return values p and q have type pointer to int.
    p = new(int) // Built-in function new allocates memory.
    // The allocated int slice is initialized to 0, p is no longer nil.
    s := make([]int, 20) // Allocate 20 ints as a single block of memory.
    s[3] = 7             // Assign one of them.
    r := -2              // Declare another local variable.
    return &s[3], &r     // & takes the address of an object.
}

func expensiveComputation() float64 {
    return m.Exp(10)
}

func learnFlowControl() {
    // If statements require brace brackets, and do not require parentheses.
    if true {
        fmt.Println("told ya")
    }
    // Formatting is standardized by the command line command "go fmt".
    if false {
        // Pout.
    } else {
        // Gloat.
    }
    // Use switch in preference to chained if statements.
    x := 42.0
    switch x {
    case 0:
    case 1, 2: // Can have multiple matches on one case
    case 42:
        // Cases don't "fall through".
        /*
        There is a `fallthrough` keyword however, see:
          https://github.com/golang/go/wiki/Switch#fall-through
        */
    case 43:
        // Unreached.
    default:
        // Default case is optional.
    }

    // Type switch allows switching on the type of something instead of value
    var data interface{}
    data = ""
    switch c := data.(type) {
    case string:
        fmt.Println(c, "is a string")
    case int64:
        fmt.Printf("%d is an int64\n", c)
    default:
        // all other cases
    }

    // Like if, for doesn't use parens either.
    // Variables declared in for and if are local to their scope.
    for x := 0; x < 3; x++ { // ++ is a statement.
        fmt.Println("iteration", x)
    }
    // x == 42 here.

    // For is the only loop statement in Go, but it has alternate forms.
    for { // Infinite loop.
        break    // Just kidding.
        continue // Unreached.
    }

    // You can use range to iterate over an array, a slice, a string, a map, or a channel.
    // range returns one (channel) or two values (array, slice, string and map).
    for key, value := range map[string]int{"one": 1, "two": 2, "three": 3} {
        // for each pair in the map, print key and value
        fmt.Printf("key=%s, value=%d\n", key, value)
    }
    // If you only need the value, use the underscore as the key
    for _, name := range []string{"Bob", "Bill", "Joe"} {
        fmt.Printf("Hello, %s\n", name)
    }

    // As with for, := in an if statement means to declare and assign
    // y first, then test y > x.
    if y := expensiveComputation(); y > x {
        x = y
    }
    // Function literals are closures.
    xBig := func() bool {
        return x > 10000 // References x declared above switch statement.
    }
    x = 99999
    fmt.Println("xBig:", xBig()) // true
    x = 1.3e3                    // This makes x == 1300
    fmt.Println("xBig:", xBig()) // false now.

    // What's more is function literals may be defined and called inline,
    // acting as an argument to function, as long as:
    // a) function literal is called immediately (),
    // b) result type matches expected type of argument.
    fmt.Println("Add + double two numbers: ",
        func(a, b int) int {
            return (a + b) * 2
        }(10, 2)) // Called with args 10 and 2
    // => Add + double two numbers: 24

    // When you need it, you'll love it.
    goto love
love:

    learnFunctionFactory() // func returning func is fun(3)(3)
    learnDefer()      // A quick detour to an important keyword.
    learnInterfaces() // Good stuff coming up!
}

func learnFunctionFactory() {
    // Next two are equivalent, with second being more practical
    fmt.Println(sentenceFactory("summer")("A beautiful", "day!"))

    d := sentenceFactory("summer")
    fmt.Println(d("A beautiful", "day!"))
    fmt.Println(d("A lazy", "afternoon!"))
}

// Decorators are common in other languages. Same can be done in Go
// with function literals that accept arguments.
func sentenceFactory(mystring string) func(before, after string) string {
    return func(before, after string) string {
        return fmt.Sprintf("%s %s %s", before, mystring, after) // new string
    }
}

func learnDefer() (ok bool) {
    // A defer statement pushes a function call onto a list. The list of saved
    // calls is executed AFTER the surrounding function returns.
    defer fmt.Println("deferred statements execute in reverse (LIFO) order.")
    defer fmt.Println("\nThis line is being printed first because")
    // Defer is commonly used to close a file, so the function closing the
    // file stays close to the function opening the file.
    return true
}

// Define Stringer as an interface type with one method, String.
type Stringer interface {
    String() string
}

// Define pair as a struct with two fields, ints named x and y.
type pair struct {
    x, y int
}

// Define a method on type pair. Pair now implements Stringer because Pair has defined all the methods in the interface.
func (p pair) String() string { // p is called the "receiver"
    // Sprintf is another public function in package fmt.
    // Dot syntax references fields of p.
    return fmt.Sprintf("(%d, %d)", p.x, p.y)
}

func learnInterfaces() {
    // Brace syntax is a "struct literal". It evaluates to an initialized
    // struct. The := syntax declares and initializes p to this struct.
    p := pair{3, 4}
    fmt.Println(p.String()) // Call String method of p, of type pair.
    var i Stringer          // Declare i of interface type Stringer.
    i = p                   // Valid because pair implements Stringer
    // Call String method of i, of type Stringer. Output same as above.
    fmt.Println(i.String())

    // Functions in the fmt package call the String method to ask an object
    // for a printable representation of itself.
    fmt.Println(p) // Output same as above. Println calls String method.
    fmt.Println(i) // Output same as above.

    learnVariadicParams("great", "learning", "here!")
}

// Functions can have variadic parameters.
func learnVariadicParams(myStrings ...interface{}) {
    // Iterate each value of the variadic.
    // The underbar here is ignoring the index argument of the array.
    for _, param := range myStrings {
        fmt.Println("param:", param)
    }

    // Pass variadic value as a variadic parameter.
    fmt.Println("params:", fmt.Sprintln(myStrings...))

    learnErrorHandling()
}

func learnErrorHandling() {
    // ", ok" idiom used to tell if something worked or not.
    m := map[int]string{3: "three", 4: "four"}
    if x, ok := m[1]; !ok { // ok will be false because 1 is not in the map.
        fmt.Println("no one there")
    } else {
        fmt.Print(x) // x would be the value, if it were in the map.
    }
    // An error value communicates not just "ok" but more about the problem.
    if _, err := strconv.Atoi("non-int"); err != nil { // _ discards value
        // prints 'strconv.ParseInt: parsing "non-int": invalid syntax'
        fmt.Println(err)
    }
    // We'll revisit interfaces a little later. Meanwhile,
    learnConcurrency()
}

// c is a channel, a concurrency-safe communication object.
func inc(i int, c chan int) {
    c <- i + 1 // <- is the "send" operator when a channel appears on the left.
}

// We'll use inc to increment some numbers concurrently.
func learnConcurrency() {
    // Same make function used earlier to make a slice. Make allocates and
    // initializes slices, maps, and channels.
    c := make(chan int)
    // Start three concurrent goroutines. Numbers will be incremented
    // concurrently, perhaps in parallel if the machine is capable and
    // properly configured. All three send to the same channel.
    go inc(0, c) // go is a statement that starts a new goroutine.
    go inc(10, c)
    go inc(-805, c)
    // Read three results from the channel and print them out.
    // There is no telling in what order the results will arrive!
    fmt.Println(<-c, <-c, <-c) // channel on right, <- is "receive" operator.

    cs := make(chan string)       // Another channel, this one handles strings.
    ccs := make(chan chan string) // A channel of string channels.
    go func() { c <- 84 }()       // Start a new goroutine just to send a value.
    go func() { cs <- "wordy" }() // Again, for cs this time.
    // Select has syntax like a switch statement but each case involves
    // a channel operation. It selects a case at random out of the cases
    // that are ready to communicate.
    select {
    case i := <-c: // The value received can be assigned to a variable,
        fmt.Printf("it's a %T", i)
    case <-cs: // or the value received can be discarded.
        fmt.Println("it's a string")
    case <-ccs: // Empty channel, not ready for communication.
        fmt.Println("didn't happen.")
    }
    // At this point a value was taken from either c or cs. One of the two
    // goroutines started above has completed, the other will remain blocked.

    learnWebProgramming() // Go does it. You want to do it too.
}

// A single function from package http starts a web server.
func learnWebProgramming() {

    // First parameter of ListenAndServe is TCP address to listen to.
    // Second parameter is an interface, specifically http.Handler.
    go func() {
        err := http.ListenAndServe(":8080", pair{})
        fmt.Println(err) // don't ignore errors
    }()

    requestServer()
}

// Make pair an http.Handler by implementing its only method, ServeHTTP.
func (p pair) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    // Serve data with a method of http.ResponseWriter.
    w.Write([]byte("You learned Go in Y minutes!"))
}

func requestServer() {
    resp, err := http.Get("http://localhost:8080")
    fmt.Println(err)
    defer resp.Body.Close()
    body, err := ioutil.ReadAll(resp.Body)
    fmt.Printf("\nWebserver said: `%s`", string(body))
}

$ go run hello-world.go
hello world

$ go build hello-world.go
$ ls
hello-world    hello-world.go

$ ./hello-world
hello world