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Source file src/sort/example_multi_test.go

Documentation: sort

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