good good study, day day up!

vendor/gopl.io/ch6/intset/intset.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 165.
+
+// Package intset provides a set of integers based on a bit vector.
+package intset
+
+import (
+	"bytes"
+	"fmt"
+)
+
+//!+intset
+
+// An IntSet is a set of small non-negative integers.
+// Its zero value represents the empty set.
+type IntSet struct {
+	words []uint64
+}
+
+// Has reports whether the set contains the non-negative value x.
+func (s *IntSet) Has(x int) bool {
+	word, bit := x/64, uint(x%64)
+	return word < len(s.words) && s.words[word]&(1<<bit) != 0
+}
+
+// Add adds the non-negative value x to the set.
+func (s *IntSet) Add(x int) {
+	word, bit := x/64, uint(x%64)
+	for word >= len(s.words) {
+		s.words = append(s.words, 0)
+	}
+	s.words[word] |= 1 << bit
+}
+
+// UnionWith sets s to the union of s and t.
+func (s *IntSet) UnionWith(t *IntSet) {
+	for i, tword := range t.words {
+		if i < len(s.words) {
+			s.words[i] |= tword
+		} else {
+			s.words = append(s.words, tword)
+		}
+	}
+}
+
+//!-intset
+
+//!+string
+
+// String returns the set as a string of the form "{1 2 3}".
+func (s *IntSet) String() string {
+	var buf bytes.Buffer
+	buf.WriteByte('{')
+	for i, word := range s.words {
+		if word == 0 {
+			continue
+		}
+		for j := 0; j < 64; j++ {
+			if word&(1<<uint(j)) != 0 {
+				if buf.Len() > len("{") {
+					buf.WriteByte(' ')
+				}
+				fmt.Fprintf(&buf, "%d", 64*i+j)
+			}
+		}
+	}
+	buf.WriteByte('}')
+	return buf.String()
+}
+
+//!-string