github.com/mattn/go@v0.0.0-20171011075504-07f7db3ea99f/src/go/ast/import.go

// Copyright 2011 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 ast

import (
	"go/token"
	"sort"
	"strconv"
)

// SortImports sorts runs of consecutive import lines in import blocks in f.
// It also removes duplicate imports when it is possible to do so without data loss.
func SortImports(fset *token.FileSet, f *File) {
	for _, d := range f.Decls {
		d, ok := d.(*GenDecl)
		if !ok || d.Tok != token.IMPORT {
			// Not an import declaration, so we're done.
			// Imports are always first.
			break
		}

		if !d.Lparen.IsValid() {
			// Not a block: sorted by default.
			continue
		}

		// Identify and sort runs of specs on successive lines.
		i := 0
		specs := d.Specs[:0]
		for j, s := range d.Specs {
			if j > i && fset.Position(s.Pos()).Line > 1+fset.Position(d.Specs[j-1].End()).Line {
				// j begins a new run. End this one.
				specs = append(specs, sortSpecs(fset, f, d.Specs[i:j])...)
				i = j
			}
		}
		specs = append(specs, sortSpecs(fset, f, d.Specs[i:])...)
		d.Specs = specs

		// Deduping can leave a blank line before the rparen; clean that up.
		if len(d.Specs) > 0 {
			lastSpec := d.Specs[len(d.Specs)-1]
			lastLine := fset.Position(lastSpec.Pos()).Line
			rParenLine := fset.Position(d.Rparen).Line
			for rParenLine > lastLine+1 {
				rParenLine--
				fset.File(d.Rparen).MergeLine(rParenLine)
			}
		}
	}
}

func importPath(s Spec) string {
	t, err := strconv.Unquote(s.(*ImportSpec).Path.Value)
	if err == nil {
		return t
	}
	return ""
}

func importName(s Spec) string {
	n := s.(*ImportSpec).Name
	if n == nil {
		return ""
	}
	return n.Name
}

func importComment(s Spec) string {
	c := s.(*ImportSpec).Comment
	if c == nil {
		return ""
	}
	return c.Text()
}

// collapse indicates whether prev may be removed, leaving only next.
func collapse(prev, next Spec) bool {
	if importPath(next) != importPath(prev) || importName(next) != importName(prev) {
		return false
	}
	return prev.(*ImportSpec).Comment == nil
}

type posSpan struct {
	Start token.Pos
	End   token.Pos
}

func sortSpecs(fset *token.FileSet, f *File, specs []Spec) []Spec {
	// Can't short-circuit here even if specs are already sorted,
	// since they might yet need deduplication.
	// A lone import, however, may be safely ignored.
	if len(specs) <= 1 {
		return specs
	}

	// Record positions for specs.
	pos := make([]posSpan, len(specs))
	for i, s := range specs {
		pos[i] = posSpan{s.Pos(), s.End()}
	}

	// Identify comments in this range.
	// Any comment from pos[0].Start to the final line counts.
	lastLine := fset.Position(pos[len(pos)-1].End).Line
	cstart := len(f.Comments)
	cend := len(f.Comments)
	for i, g := range f.Comments {
		if g.Pos() < pos[0].Start {
			continue
		}
		if i < cstart {
			cstart = i
		}
		if fset.Position(g.End()).Line > lastLine {
			cend = i
			break
		}
	}
	comments := f.Comments[cstart:cend]

	// Assign each comment to the import spec preceding it.
	importComments := map[*ImportSpec][]*CommentGroup{}
	specIndex := 0
	for _, g := range comments {
		for specIndex+1 < len(specs) && pos[specIndex+1].Start <= g.Pos() {
			specIndex++
		}
		s := specs[specIndex].(*ImportSpec)
		importComments[s] = append(importComments[s], g)
	}

	// Sort the import specs by import path.
	// Remove duplicates, when possible without data loss.
	// Reassign the import paths to have the same position sequence.
	// Reassign each comment to abut the end of its spec.
	// Sort the comments by new position.
	sort.Slice(specs, func(i, j int) bool {
		ipath := importPath(specs[i])
		jpath := importPath(specs[j])
		if ipath != jpath {
			return ipath < jpath
		}
		iname := importName(specs[i])
		jname := importName(specs[j])
		if iname != jname {
			return iname < jname
		}
		return importComment(specs[i]) < importComment(specs[j])
	})

	// Dedup. Thanks to our sorting, we can just consider
	// adjacent pairs of imports.
	deduped := specs[:0]
	for i, s := range specs {
		if i == len(specs)-1 || !collapse(s, specs[i+1]) {
			deduped = append(deduped, s)
		} else {
			p := s.Pos()
			fset.File(p).MergeLine(fset.Position(p).Line)
		}
	}
	specs = deduped

	// Fix up comment positions
	for i, s := range specs {
		s := s.(*ImportSpec)
		if s.Name != nil {
			s.Name.NamePos = pos[i].Start
		}
		s.Path.ValuePos = pos[i].Start
		s.EndPos = pos[i].End
		for _, g := range importComments[s] {
			for _, c := range g.List {
				c.Slash = pos[i].End
			}
		}
	}

	sort.Slice(comments, func(i, j int) bool {
		return comments[i].Pos() < comments[j].Pos()
	})

	return specs
}