good good study, day day up!

vendor/gopl.io/ch3/basename1/main.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 72.
+
+// Basename1 reads file names from stdin and prints the base name of each one.
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+func main() {
+	input := bufio.NewScanner(os.Stdin)
+	for input.Scan() {
+		fmt.Println(basename(input.Text()))
+	}
+	// NOTE: ignoring potential errors from input.Err()
+}
+
+//!+
+// basename removes directory components and a .suffix.
+// e.g., a => a, a.go => a, a/b/c.go => c, a/b.c.go => b.c
+func basename(s string) string {
+	// Discard last '/' and everything before.
+	for i := len(s) - 1; i >= 0; i-- {
+		if s[i] == '/' {
+			s = s[i+1:]
+			break
+		}
+	}
+	// Preserve everything before last '.'.
+	for i := len(s) - 1; i >= 0; i-- {
+		if s[i] == '.' {
+			s = s[:i]
+			break
+		}
+	}
+	return s
+}
+
+//!-

vendor/gopl.io/ch3/basename2/main.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 72.
+
+// Basename2 reads file names from stdin and prints the base name of each one.
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
+
+func main() {
+	input := bufio.NewScanner(os.Stdin)
+	for input.Scan() {
+		fmt.Println(basename(input.Text()))
+	}
+	// NOTE: ignoring potential errors from input.Err()
+}
+
+// basename removes directory components and a trailing .suffix.
+// e.g., a => a, a.go => a, a/b/c.go => c, a/b.c.go => b.c
+//!+
+func basename(s string) string {
+	slash := strings.LastIndex(s, "/") // -1 if "/" not found
+	s = s[slash+1:]
+	if dot := strings.LastIndex(s, "."); dot >= 0 {
+		s = s[:dot]
+	}
+	return s
+}
+
+//!-

vendor/gopl.io/ch3/comma/main.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 73.
+
+// Comma prints its argument numbers with a comma at each power of 1000.
+//
+// Example:
+// 	$ go build gopl.io/ch3/comma
+//	$ ./comma 1 12 123 1234 1234567890
+// 	1
+// 	12
+// 	123
+// 	1,234
+// 	1,234,567,890
+//
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	for i := 1; i < len(os.Args); i++ {
+		fmt.Printf("  %s\n", comma(os.Args[i]))
+	}
+}
+
+//!+
+// comma inserts commas in a non-negative decimal integer string.
+func comma(s string) string {
+	n := len(s)
+	if n <= 3 {
+		return s
+	}
+	return comma(s[:n-3]) + "," + s[n-3:]
+}
+
+//!-

vendor/gopl.io/ch3/mandelbrot/main.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 61.
+//!+
+
+// Mandelbrot emits a PNG image of the Mandelbrot fractal.
+package main
+
+import (
+	"image"
+	"image/color"
+	"image/png"
+	"math/cmplx"
+	"os"
+)
+
+func main() {
+	const (
+		xmin, ymin, xmax, ymax = -2, -2, +2, +2
+		width, height          = 1024, 1024
+	)
+
+	img := image.NewRGBA(image.Rect(0, 0, width, height))
+	for py := 0; py < height; py++ {
+		y := float64(py)/height*(ymax-ymin) + ymin
+		for px := 0; px < width; px++ {
+			x := float64(px)/width*(xmax-xmin) + xmin
+			z := complex(x, y)
+			// Image point (px, py) represents complex value z.
+			img.Set(px, py, mandelbrot(z))
+		}
+	}
+	png.Encode(os.Stdout, img) // NOTE: ignoring errors
+}
+
+func mandelbrot(z complex128) color.Color {
+	const iterations = 200
+	const contrast = 15
+
+	var v complex128
+	for n := uint8(0); n < iterations; n++ {
+		v = v*v + z
+		if cmplx.Abs(v) > 2 {
+			return color.Gray{255 - contrast*n}
+		}
+	}
+	return color.Black
+}
+
+//!-
+
+// Some other interesting functions:
+
+func acos(z complex128) color.Color {
+	v := cmplx.Acos(z)
+	blue := uint8(real(v)*128) + 127
+	red := uint8(imag(v)*128) + 127
+	return color.YCbCr{192, blue, red}
+}
+
+func sqrt(z complex128) color.Color {
+	v := cmplx.Sqrt(z)
+	blue := uint8(real(v)*128) + 127
+	red := uint8(imag(v)*128) + 127
+	return color.YCbCr{128, blue, red}
+}
+
+// f(x) = x^4 - 1
+//
+// z' = z - f(z)/f'(z)
+//    = z - (z^4 - 1) / (4 * z^3)
+//    = z - (z - 1/z^3) / 4
+func newton(z complex128) color.Color {
+	const iterations = 37
+	const contrast = 7
+	for i := uint8(0); i < iterations; i++ {
+		z -= (z - 1/(z*z*z)) / 4
+		if cmplx.Abs(z*z*z*z-1) < 1e-6 {
+			return color.Gray{255 - contrast*i}
+		}
+	}
+	return color.Black
+}

vendor/gopl.io/ch3/netflag/netflag.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 77.
+
+// Netflag demonstrates an integer type used as a bit field.
+package main
+
+import (
+	"fmt"
+	. "net"
+)
+
+//!+
+func IsUp(v Flags) bool     { return v&FlagUp == FlagUp }
+func TurnDown(v *Flags)     { *v &^= FlagUp }
+func SetBroadcast(v *Flags) { *v |= FlagBroadcast }
+func IsCast(v Flags) bool   { return v&(FlagBroadcast|FlagMulticast) != 0 }
+
+func main() {
+	var v Flags = FlagMulticast | FlagUp
+	fmt.Printf("%b %t\n", v, IsUp(v)) // "10001 true"
+	TurnDown(&v)
+	fmt.Printf("%b %t\n", v, IsUp(v)) // "10000 false"
+	SetBroadcast(&v)
+	fmt.Printf("%b %t\n", v, IsUp(v))   // "10010 false"
+	fmt.Printf("%b %t\n", v, IsCast(v)) // "10010 true"
+}
+
+//!-

vendor/gopl.io/ch3/printints/main.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 74.
+
+// Printints demonstrates the use of bytes.Buffer to format a string.
+package main
+
+import (
+	"bytes"
+	"fmt"
+)
+
+//!+
+// intsToString is like fmt.Sprint(values) but adds commas.
+func intsToString(values []int) string {
+	var buf bytes.Buffer
+	buf.WriteByte('[')
+	for i, v := range values {
+		if i > 0 {
+			buf.WriteString(", ")
+		}
+		fmt.Fprintf(&buf, "%d", v)
+	}
+	buf.WriteByte(']')
+	return buf.String()
+}
+
+func main() {
+	fmt.Println(intsToString([]int{1, 2, 3})) // "[1, 2, 3]"
+}
+
+//!-

vendor/gopl.io/ch3/surface/main.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 58.
+//!+
+
+// Surface computes an SVG rendering of a 3-D surface function.
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+const (
+	width, height = 600, 320            // canvas size in pixels
+	cells         = 100                 // number of grid cells
+	xyrange       = 30.0                // axis ranges (-xyrange..+xyrange)
+	xyscale       = width / 2 / xyrange // pixels per x or y unit
+	zscale        = height * 0.4        // pixels per z unit
+	angle         = math.Pi / 6         // angle of x, y axes (=30°)
+)
+
+var sin30, cos30 = math.Sin(angle), math.Cos(angle) // sin(30°), cos(30°)
+
+func main() {
+	fmt.Printf("<svg xmlns='http://www.w3.org/2000/svg' "+
+		"style='stroke: grey; fill: white; stroke-width: 0.7' "+
+		"width='%d' height='%d'>", width, height)
+	for i := 0; i < cells; i++ {
+		for j := 0; j < cells; j++ {
+			ax, ay := corner(i+1, j)
+			bx, by := corner(i, j)
+			cx, cy := corner(i, j+1)
+			dx, dy := corner(i+1, j+1)
+			fmt.Printf("<polygon points='%g,%g %g,%g %g,%g %g,%g'/>\n",
+				ax, ay, bx, by, cx, cy, dx, dy)
+		}
+	}
+	fmt.Println("</svg>")
+}
+
+func corner(i, j int) (float64, float64) {
+	// Find point (x,y) at corner of cell (i,j).
+	x := xyrange * (float64(i)/cells - 0.5)
+	y := xyrange * (float64(j)/cells - 0.5)
+
+	// Compute surface height z.
+	z := f(x, y)
+
+	// Project (x,y,z) isometrically onto 2-D SVG canvas (sx,sy).
+	sx := width/2 + (x-y)*cos30*xyscale
+	sy := height/2 + (x+y)*sin30*xyscale - z*zscale
+	return sx, sy
+}
+
+func f(x, y float64) float64 {
+	r := math.Hypot(x, y) // distance from (0,0)
+	return math.Sin(r) / r
+}
+
+//!-