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Source file src/sort/search.go

Documentation: sort

  // Copyright 2010 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // This file implements binary search.
  
  package sort
  
  // 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 := i + (j-i)/2 // 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
  }
  
  // Convenience wrappers for common cases.
  
  // SearchInts searches for x in a sorted slice of ints and returns the index
  // as specified by Search. The return value is the index to insert x if x is
  // not present (it could be len(a)).
  // The slice must be sorted in ascending order.
  //
  func SearchInts(a []int, x int) int {
  	return Search(len(a), func(i int) bool { return a[i] >= x })
  }
  
  // SearchFloat64s searches for x in a sorted slice of float64s and returns the index
  // as specified by Search. The return value is the index to insert x if x is not
  // present (it could be len(a)).
  // The slice must be sorted in ascending order.
  //
  func SearchFloat64s(a []float64, x float64) int {
  	return Search(len(a), func(i int) bool { return a[i] >= x })
  }
  
  // SearchStrings searches for x in a sorted slice of strings and returns the index
  // as specified by Search. The return value is the index to insert x if x is not
  // present (it could be len(a)).
  // The slice must be sorted in ascending order.
  //
  func SearchStrings(a []string, x string) int {
  	return Search(len(a), func(i int) bool { return a[i] >= x })
  }
  
  // Search returns the result of applying SearchInts to the receiver and x.
  func (p IntSlice) Search(x int) int { return SearchInts(p, x) }
  
  // Search returns the result of applying SearchFloat64s to the receiver and x.
  func (p Float64Slice) Search(x float64) int { return SearchFloat64s(p, x) }
  
  // Search returns the result of applying SearchStrings to the receiver and x.
  func (p StringSlice) Search(x string) int { return SearchStrings(p, x) }
  

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