Source: import.go in package go/ast


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)

Not an import declaration, so we're done. Imports are always first.

			break
		}

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 {

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 := lineAt(fset, lastSpec.Pos())
			rParenLine := lineAt(fset, d.Rparen)
			for rParenLine > lastLine+1 {
				rParenLine--
				fset.File(d.Rparen).MergeLine(rParenLine)
			}
		}
	}
}

func lineAt(fset *token.FileSet, pos token.Pos) int {
	return fset.PositionFor(pos, false).Line
}

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
}

type cgPos struct {
	left bool // true if comment is to the left of the spec, false otherwise.
	cg   *CommentGroup
}

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.

	begSpecs := pos[0].Start
	endSpecs := pos[len(pos)-1].End
	beg := fset.File(begSpecs).LineStart(lineAt(fset, begSpecs))
	endLine := lineAt(fset, endSpecs)
	endFile := fset.File(endSpecs)
	var end token.Pos
	if endLine == endFile.LineCount() {
		end = endSpecs
	} else {
		end = endFile.LineStart(endLine + 1) // beginning of next line
	}
	first := len(f.Comments)
	last := -1
	for i, g := range f.Comments {
		if g.End() >= end {
			break

g.End() < end

comment is within the range [beg, end[ of import declarations

			if i < first {
				first = i
			}
			if i > last {
				last = i
			}
		}
	}

	var comments []*CommentGroup
	if last >= 0 {
		comments = f.Comments[first : last+1]
	}

Assign each comment to the import spec on the same line.

	importComments := map[*ImportSpec][]cgPos{}
	specIndex := 0
	for _, g := range comments {
		for specIndex+1 < len(specs) && pos[specIndex+1].Start <= g.Pos() {
			specIndex++
		}

A block comment can appear before the first import spec.

		if specIndex == 0 && pos[specIndex].Start > g.Pos() {
			left = true
		} else if specIndex+1 < len(specs) && // Or it can appear on the left of an import spec.
			lineAt(fset, pos[specIndex].Start)+1 == lineAt(fset, g.Pos()) {
			specIndex++
			left = true
		}
		s := specs[specIndex].(*ImportSpec)
		importComments[s] = append(importComments[s], cgPos{left: left, cg: 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 the spec on the same line. 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(lineAt(fset, p))
		}
	}
	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.cg.List {
				if g.left {
					c.Slash = pos[i].Start - 1

An import spec can have both block comment and a line comment to its right. In that case, both of them will have the same pos. But while formatting the AST, the line comment gets moved to after the block comment.

					c.Slash = pos[i].End
				}
			}
		}
	}

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

	return specs