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

// 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
    "net/http" // Yes, a web server!
    "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.
    // Qualify it with the package name, fmt.
    fmt.Println("Hello world!")

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

// Functions have parameters in parentheses.
// If there are no parameters, empty parens 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!
}

// Functions can have parameters and (multiple!) return values.
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.
    s := "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 uint32, holds a UTF-8 code point

    f := 3.14195 // float64, an IEEE-754 64-bit floating point number
    c := 3 + 4i  // complex128, represented internally with two float64s

    // Var syntax with an 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
    a3 := [...]int{3, 1, 5} // an array of 3 ints, initialized as shown

    // 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 a3.  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

    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 underbar lets you "use" a variable but discard its value.
    _, _, _, _, _, _, _, _, _ = s2, g, f, u, pi, n, a3, s4, bs
    // Output of course counts as using a variable.
    fmt.Println(s, c, a4, s3, d2, m)

    learnFlowControl() // back in the flow
}

// 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.
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 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() int {
    return 1e6
}

func learnFlowControl() {
    // If statements require brace brackets, and do not require parens.
    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 := 1
    switch x {
    case 0:
    case 1:
        // cases don't "fall through"
    case 2:
        // unreached
    }
    // Like if, for doesn't use parens either.
    for x := 0; x < 3; x++ { // ++ is a statement
        fmt.Println("iteration", x)
    }
    // x == 1 here.

    // For is the only loop statement in Go, but it has alternate forms.
    for { // infinite loop
        break    // just kidding
        continue // unreached
    }
    // 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 > 100 // references x declared above switch statement.
    }
    fmt.Println("xBig:", xBig()) // true (we last assigned 1e6 to x)
    x /= 1e5                     // this makes it == 10
    fmt.Println("xBig:", xBig()) // false now

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

    learnInterfaces() // Good stuff coming up!
}

// 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.
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

    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.
    cc := make(chan chan string)  // a channel of 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.Println("it's a", i)
    case <-cs: // or the value received can be discarded
        fmt.Println("it's a string")
    case <-cc: // 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() {
    // ListenAndServe first parameter is TCP address to listen at.
    // Second parameter is an interface, specifically http.Handler.
    err := http.ListenAndServe(":8080", pair{})
    fmt.Println(err) // don't ignore errors
}

// 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!"))
}