sort.Search()

sort.Search() 提交于遥远的2010年11月11日,提交者是Go三位创始人之一的Robert Griesemer[1], 随Go第一个正式版本一起发布

从这个意义上说,是标准库元老级的函数了~

sort.Search()[2] 用于在排序的切片或数组中查找元素

// Search uses binary search to find and return the smallest index i
// in [0, n) at which f(i) is true, assuming that on the range [0, n),
// f(i) == true implies f(i+1) == true. That is, Search requires that
// f is false for some (possibly empty) prefix of the input range [0, n)
// and then true for the (possibly empty) remainder; Search returns
// the first true index. If there is no such index, Search returns n.
// (Note that the "not found" return value is not -1 as in, for instance,
// strings.Index.)
// Search calls f(i) only for i in the range [0, n).
//
// A common use of Search is to find the index i for a value x in
// a sorted, indexable data structure such as an array or slice.
// In this case, the argument f, typically a closure, captures the value
// to be searched for, and how the data structure is indexed and
// ordered.
//
// For instance, given a slice data sorted in ascending order,
// the call Search(len(data), func(i int) bool { return data[i] >= 23 })
// returns the smallest index i such that data[i] >= 23. If the caller
// wants to find whether 23 is in the slice, it must test data[i] == 23
// separately.
//
// Searching data sorted in descending order would use the <=
// operator instead of the >= operator.
//
// To complete the example above, the following code tries to find the value
// x in an integer slice data sorted in ascending order:
//
// x := 23
// i := sort.Search(len(data), func(i int) bool { return data[i] >= x })
// if i < len(data) && data[i] == x {
//  // x is present at data[i]
// } else {
//  // x is not present in data,
//  // but i is the index where it would be inserted.
// }
//
// As a more whimsical example, this program guesses your number:
//
// func GuessingGame() {
//  var s string
//  fmt.Printf("Pick an integer from 0 to 100.\n")
//  answer := sort.Search(100, func(i int) bool {
//   fmt.Printf("Is your number <= %d? ", i)
//   fmt.Scanf("%s", &s)
//   return s != "" && s[0] == 'y'
//  })
//  fmt.Printf("Your number is %d.\n", answer)
// }
func Search(n int, f func(int) bool) int {
 // Define f(-1) == false and f(n) == true.
 // Invariant: f(i-1) == false, f(j) == true.
 i, j := 0, n
 for i < j {
  h := int(uint(i+j) >> 1) // avoid overflow when computing h
  // i ≤ h < j
  if !f(h) {
   i = h + 1 // preserves f(i-1) == false
  } else {
   j = h // preserves f(j) == true
  }
 }
 // i == j, f(i-1) == false, and f(j) (= f(i)) == true  =>  answer is i.
 return i
}