doc: add Slices: usage and internals article

Originally published on the Go blog on 5 Jan 2011:
http://blog.golang.org/2011/01/go-slices-usage-and-internals.html

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5516046

doc/Makefile

@@ -11,6 +11,7 @@ GOFILES=\
 HTML=\
 	articles/defer_panic_recover.html\
 	articles/error_handling.html\
+	articles/slices_usage_and_internals.html\
 	effective_go.html\
 	go1.html\
 	go_tutorial.html\

doc/progs/run

@@ -61,6 +61,7 @@ for i in \
 	$effective_go \
 	$error_handling \
 	$go_tutorial \
+	slices.go \
 	go1.go \
 ; do
 	$GC $i

doc/progs/slices.go

+package main
+
+import (
+	"io/ioutil"
+	"regexp"
+)
+
+func AppendByte(slice []byte, data ...byte) []byte {
+	m := len(slice)
+	n := m + len(data)
+	if n > cap(slice) { // if necessary, reallocate
+		// allocate double what's needed, for future growth.
+		newSlice := make([]byte, (n+1)*2)
+		copy(newSlice, slice)
+		slice = newSlice
+	}
+	slice = slice[0:n]
+	copy(slice[m:n], data)
+	return slice
+}
+
+// STOP OMIT
+
+// Filter returns a new slice holding only
+// the elements of s that satisfy f()
+func Filter(s []int, fn func(int) bool) []int {
+	var p []int // == nil
+	for _, i := range s {
+		if fn(i) {
+			p = append(p, i)
+		}
+	}
+	return p
+}
+
+// STOP OMIT
+
+var digitRegexp = regexp.MustCompile("[0-9]+")
+
+func FindDigits(filename string) []byte {
+	b, _ := ioutil.ReadFile(filename)
+	return digitRegexp.Find(b)
+}
+
+// STOP OMIT
+
+func CopyDigits(filename string) []byte {
+	b, _ := ioutil.ReadFile(filename)
+	b = digitRegexp.Find(b)
+	c := make([]byte, len(b))
+	copy(c, b)
+	return c
+}
+
+// STOP OMIT