132 lines
4.0 KiB
Go
132 lines
4.0 KiB
Go
// Copyright 2013 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package sort_test
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import (
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"fmt"
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"sort"
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)
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// A Change is a record of source code changes, recording user, language, and delta size.
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type Change struct {
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user string
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language string
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lines int
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}
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type lessFunc func(p1, p2 *Change) bool
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// multiSorter implements the Sort interface, sorting the changes within.
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type multiSorter struct {
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changes []Change
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less []lessFunc
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}
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// Sort sorts the argument slice according to the less functions passed to OrderedBy.
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func (ms *multiSorter) Sort(changes []Change) {
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ms.changes = changes
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sort.Sort(ms)
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}
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// OrderedBy returns a Sorter that sorts using the less functions, in order.
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// Call its Sort method to sort the data.
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func OrderedBy(less ...lessFunc) *multiSorter {
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return &multiSorter{
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less: less,
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}
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}
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// Len is part of sort.Interface.
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func (ms *multiSorter) Len() int {
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return len(ms.changes)
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}
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// Swap is part of sort.Interface.
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func (ms *multiSorter) Swap(i, j int) {
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ms.changes[i], ms.changes[j] = ms.changes[j], ms.changes[i]
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}
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// Less is part of sort.Interface. It is implemented by looping along the
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// less functions until it finds a comparison that is either Less or
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// !Less. Note that it can call the less functions twice per call. We
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// could change the functions to return -1, 0, 1 and reduce the
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// number of calls for greater efficiency: an exercise for the reader.
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func (ms *multiSorter) Less(i, j int) bool {
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p, q := &ms.changes[i], &ms.changes[j]
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// Try all but the last comparison.
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var k int
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for k = 0; k < len(ms.less)-1; k++ {
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less := ms.less[k]
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switch {
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case less(p, q):
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// p < q, so we have a decision.
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return true
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case less(q, p):
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// p > q, so we have a decision.
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return false
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}
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// p == q; try the next comparison.
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}
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// All comparisons to here said "equal", so just return whatever
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// the final comparison reports.
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return ms.less[k](p, q)
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}
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var changes = []Change{
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{"gri", "Go", 100},
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{"ken", "C", 150},
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{"glenda", "Go", 200},
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{"rsc", "Go", 200},
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{"r", "Go", 100},
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{"ken", "Go", 200},
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{"dmr", "C", 100},
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{"r", "C", 150},
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{"gri", "Smalltalk", 80},
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}
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// ExampleMultiKeys demonstrates a technique for sorting a struct type using different
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// sets of multiple fields in the comparison. We chain together "Less" functions, each of
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// which compares a single field.
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func Example_sortMultiKeys() {
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// Closures that order the Change structure.
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user := func(c1, c2 *Change) bool {
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return c1.user < c2.user
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}
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language := func(c1, c2 *Change) bool {
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return c1.language < c2.language
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}
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increasingLines := func(c1, c2 *Change) bool {
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return c1.lines < c2.lines
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}
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decreasingLines := func(c1, c2 *Change) bool {
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return c1.lines > c2.lines // Note: > orders downwards.
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}
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// Simple use: Sort by user.
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OrderedBy(user).Sort(changes)
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fmt.Println("By user:", changes)
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// More examples.
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OrderedBy(user, increasingLines).Sort(changes)
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fmt.Println("By user,<lines:", changes)
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OrderedBy(user, decreasingLines).Sort(changes)
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fmt.Println("By user,>lines:", changes)
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OrderedBy(language, increasingLines).Sort(changes)
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fmt.Println("By language,<lines:", changes)
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OrderedBy(language, increasingLines, user).Sort(changes)
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fmt.Println("By language,<lines,user:", changes)
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// Output:
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// By user: [{dmr C 100} {glenda Go 200} {gri Go 100} {gri Smalltalk 80} {ken C 150} {ken Go 200} {r Go 100} {r C 150} {rsc Go 200}]
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// By user,<lines: [{dmr C 100} {glenda Go 200} {gri Smalltalk 80} {gri Go 100} {ken C 150} {ken Go 200} {r Go 100} {r C 150} {rsc Go 200}]
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// By user,>lines: [{dmr C 100} {glenda Go 200} {gri Go 100} {gri Smalltalk 80} {ken Go 200} {ken C 150} {r C 150} {r Go 100} {rsc Go 200}]
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// By language,<lines: [{dmr C 100} {ken C 150} {r C 150} {r Go 100} {gri Go 100} {ken Go 200} {glenda Go 200} {rsc Go 200} {gri Smalltalk 80}]
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// By language,<lines,user: [{dmr C 100} {ken C 150} {r C 150} {gri Go 100} {r Go 100} {glenda Go 200} {ken Go 200} {rsc Go 200} {gri Smalltalk 80}]
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}
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