mirror of
https://github.com/golang/go
synced 2024-11-12 01:10:21 -07:00
sort: simplify semantics of Search.
As discussed earlier. R=gri CC=golang-dev https://golang.org/cl/3025042
This commit is contained in:
parent
81cb189a06
commit
bac478da1c
@ -12,54 +12,54 @@ package sort
|
||||
// sorted. It will often be passed as a closure. For instance, given a slice
|
||||
// of integers, []data, sorted in ascending order, the function
|
||||
//
|
||||
// func(i int) bool { return data[i] <= 23 }
|
||||
// func(i int) bool { return data[i] < 23 }
|
||||
//
|
||||
// can be used to search for the value 23 in data. The relationship expressed
|
||||
// by the function must be "less or equal" if the elements are sorted in ascending
|
||||
// order or "greater or equal" if they are sorted in descending order.
|
||||
// by the function must be "less" if the elements are sorted in ascending
|
||||
// order or "greater" if they are sorted in descending order.
|
||||
// The function f will be called with values of i in the range 0 to n-1.
|
||||
//
|
||||
// For brevity, this discussion assumes ascending sort order. For descending
|
||||
// order, replace <= with >=, and swap 'smaller' with 'larger'.
|
||||
// order, replace < with >, and swap 'smaller' with 'larger'.
|
||||
//
|
||||
// If data[0] <= x and x <= data[n-1], Search returns the index i with:
|
||||
// Search returns the index i with:
|
||||
//
|
||||
// data[i] <= x && x < data[i+1] (0 <= i < n)
|
||||
// data[i-1] < x && x <= data[i]
|
||||
//
|
||||
// where data[n] is assumed to be larger than any x. Thus, i is the index of x
|
||||
// if it is present in the data. It is the responsibility of the caller to
|
||||
// verify the actual presence by testing if data[i] == x.
|
||||
//
|
||||
// If n == 0 or if x is smaller than any element in data (f is always false),
|
||||
// the result is 0. If x is larger than any element in data (f is always true),
|
||||
// the result is n-1.
|
||||
// where data[-1] is assumed to be smaller than any x and data[n] is
|
||||
// assumed to be larger than any x. Thus 0 <= i <= n and i is the first
|
||||
// index of x if x is present in the data. It is the responsibility of
|
||||
// the caller to verify the actual presence by testing if i < n and
|
||||
// data[i] == x.
|
||||
//
|
||||
// To complete the example above, the following code tries to find the element
|
||||
// elem in an integer slice data sorted in ascending order:
|
||||
//
|
||||
// elem := 23
|
||||
// i := sort.Search(len(data), func(i int) bool { return data[i] <= elem })
|
||||
// if len(data) > 0 && data[i] == elem {
|
||||
// i := sort.Search(len(data), func(i int) bool { return data[i] < elem })
|
||||
// if i < len(data) && data[i] == elem {
|
||||
// // elem is present at data[i]
|
||||
// } else {
|
||||
// // elem is not present in data
|
||||
// }
|
||||
//
|
||||
func Search(n int, f func(int) bool) int {
|
||||
// See "A Method of Programming", E.W. Dijkstra,
|
||||
// for arguments on correctness and efficiency.
|
||||
i, j := 0, n
|
||||
for i+1 < j {
|
||||
h := i + (j-i)/2 // avoid overflow when computing h
|
||||
// i < h < j
|
||||
if f(h) {
|
||||
// data[h] <= x
|
||||
i = h
|
||||
// data[h] < x
|
||||
i = h + 1
|
||||
} else {
|
||||
// x < data[h]
|
||||
// x <= data[h]
|
||||
j = h
|
||||
}
|
||||
}
|
||||
// test the final element that the loop did not.
|
||||
if i < j && f(i) {
|
||||
i++
|
||||
}
|
||||
|
||||
return i
|
||||
}
|
||||
|
||||
@ -70,7 +70,7 @@ func Search(n int, f func(int) bool) int {
|
||||
// as specified by Search. The array 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 })
|
||||
return Search(len(a), func(i int) bool { return a[i] < x })
|
||||
}
|
||||
|
||||
|
||||
@ -78,7 +78,7 @@ func SearchInts(a []int, x int) int {
|
||||
// as specified by Search. The array must be sorted in ascending order.
|
||||
//
|
||||
func SearchFloats(a []float, x float) int {
|
||||
return Search(len(a), func(i int) bool { return a[i] <= x })
|
||||
return Search(len(a), func(i int) bool { return a[i] < x })
|
||||
}
|
||||
|
||||
|
||||
@ -86,7 +86,7 @@ func SearchFloats(a []float, x float) int {
|
||||
// as specified by Search. The array 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 })
|
||||
return Search(len(a), func(i int) bool { return a[i] < x })
|
||||
}
|
||||
|
||||
|
||||
|
@ -9,7 +9,7 @@ import "testing"
|
||||
|
||||
func f(a []int, x int) func(int) bool {
|
||||
return func(i int) bool {
|
||||
return a[i] <= x
|
||||
return a[i] < x
|
||||
}
|
||||
}
|
||||
|
||||
@ -23,25 +23,26 @@ var tests = []struct {
|
||||
i int
|
||||
}{
|
||||
{"empty", 0, nil, 0},
|
||||
{"1 1", 1, func(i int) bool { return i <= 1 }, 0},
|
||||
{"1 1", 1, func(i int) bool { return i < 1 }, 1},
|
||||
{"1 false", 1, func(i int) bool { return false }, 0},
|
||||
{"1 true", 1, func(i int) bool { return true }, 0},
|
||||
{"1e9 991", 1e9, func(i int) bool { return i <= 991 }, 991},
|
||||
{"1 true", 1, func(i int) bool { return true }, 1},
|
||||
{"1e9 991", 1e9, func(i int) bool { return i < 991 }, 991},
|
||||
{"1e9 false", 1e9, func(i int) bool { return false }, 0},
|
||||
{"1e9 true", 1e9, func(i int) bool { return true }, 1e9 - 1},
|
||||
{"1e9 true", 1e9, func(i int) bool { return true }, 1e9},
|
||||
{"data -20", len(data), f(data, -20), 0},
|
||||
{"data -10", len(data), f(data, -10), 0},
|
||||
{"data -9", len(data), f(data, -9), 0},
|
||||
{"data -6", len(data), f(data, -6), 0},
|
||||
{"data -9", len(data), f(data, -9), 1},
|
||||
{"data -6", len(data), f(data, -6), 1},
|
||||
{"data -5", len(data), f(data, -5), 1},
|
||||
{"data 3", len(data), f(data, 3), 5},
|
||||
{"data 99", len(data), f(data, 99), 8},
|
||||
{"data 100", len(data), f(data, 100), 11},
|
||||
{"data 101", len(data), f(data, 101), 11},
|
||||
{"data 11", len(data), f(data, 11), 8},
|
||||
{"data 99", len(data), f(data, 99), 9},
|
||||
{"data 100", len(data), f(data, 100), 9},
|
||||
{"data 101", len(data), f(data, 101), 12},
|
||||
{"data 10000", len(data), f(data, 10000), 13},
|
||||
{"data 10001", len(data), f(data, 10001), 13},
|
||||
{"descending a", 7, func(i int) bool { return []int{99, 99, 59, 42, 7, 0, -1, -1}[i] >= 7 }, 4},
|
||||
{"descending 7", 1e9, func(i int) bool { return 1e9-i >= 7 }, 1e9 - 7},
|
||||
{"data 10001", len(data), f(data, 10001), 14},
|
||||
{"descending a", 7, func(i int) bool { return []int{99, 99, 59, 42, 7, 0, -1, -1}[i] > 7 }, 4},
|
||||
{"descending 7", 1e9, func(i int) bool { return 1e9-i > 7 }, 1e9 - 7},
|
||||
}
|
||||
|
||||
|
||||
@ -78,7 +79,7 @@ func TestSearchEfficiency(t *testing.T) {
|
||||
max := log2(n)
|
||||
for x := 0; x < n; x += step {
|
||||
count := 0
|
||||
i := Search(n, func(i int) bool { count++; return i <= x })
|
||||
i := Search(n, func(i int) bool { count++; return i < x })
|
||||
if i != x {
|
||||
t.Errorf("n = %d: expected index %d; got %d", n, x, i)
|
||||
}
|
||||
@ -103,7 +104,7 @@ var wrappertests = []struct {
|
||||
i int
|
||||
}{
|
||||
{"SearchInts", SearchInts(data, 11), 8},
|
||||
{"SearchFloats", SearchFloats(fdata, 2.1), 3},
|
||||
{"SearchFloats", SearchFloats(fdata, 2.1), 4},
|
||||
{"SearchStrings", SearchStrings(sdata, ""), 0},
|
||||
{"IntArray.Search", IntArray(data).Search(0), 2},
|
||||
{"FloatArray.Search", FloatArray(fdata).Search(2.0), 3},
|
||||
|
Loading…
Reference in New Issue
Block a user