// This program takes the name of a whitelist file (a sequence of integers) as argument and filters any
// entry that is on the whitelist from standard input, leaving only integers that are not on the whitelist
// on standard output. It uses the binary search algorithm, implemented in the method Rank(),
// to accomplish the task efficiently.
func main() {
if len(os.Args) <= 1 {
fmt.Printf("usage: %s tinyW.txt < tinyT.txt\n", filepath.Base(os.Args[0]))
return
}
whitelist, err := in.ReadAllIntsFromFile(os.Args[1])
if err != nil {
log.Fatal(err)
}
sort.Ints(whitelist)
stdin := stdin.NewStdin()
for !stdin.IsEmpty() {
// Read key, print if not in whitelist
key, err := stdin.ReadInt()
if err != nil {
log.Fatal(err)
}
if bsearch.Rank(key, whitelist) == -1 {
fmt.Println(key)
}
}
}
// Count counts doubles that sum to 0 (ignoring integer overflow).
// This implementation takes linearithmic time
// Returns the number of doubles (i, j) with i < j such that a[i] + a[j] == 0.
// The improved algorithm is based on the fact that an entry a[i] is one of a pair that sums to
// 0 if and only if the value -a[i] is in the array (and a[i] is not zero). To solve the problem,
// we sort the array (to enable binary search) and then, for every entry a[i] in the array, do a
// binary search for -a[i] with Rank() in bsearch package. If the result is an index j with j > i,
// we increment the count.
func Count(a []int) int {
sort.Ints(a)
n := len(a)
cnt := 0
for i := 0; i < n; i++ {
if bsearch.Rank(-a[i], a) > i {
cnt++
}
}
return cnt
}
// Count returns the number of triples (i, j, k) with i < j < k such that a[i] + a[j] + a[k] == 0.
// This implementation ignores integer overflow.
// This implementation uses sorting and binary search and takes time proportional to N^2 log N,
// where N is the number of integers.
func Count(a []int) int {
sort.Ints(a)
n := len(a)
if containsDuplicates(a) {
panic("array contains duplicate integers")
}
cnt := 0
for i := 0; i < n; i++ {
for j := i + 1; j < n; j++ {
if bsearch.Rank(-(a[i]+a[j]), a) > j {
cnt++
}
}
}
return cnt
}
