src/cmd/cover/cover.go - go - Git at Google

 // 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 main

 import (
 	"bytes"
 	"flag"
 	"fmt"
 	"go/ast"
 	"go/parser"
 	"go/token"
 	"io"
 	"log"
 	"os"
 	"sort"

 	"cmd/internal/edit"
 	"cmd/internal/objabi"
 )

 const usageMessage = "" +
 	`Usage of 'go tool cover':
 Given a coverage profile produced by 'go test':
 	go test -coverprofile=c.out

 Open a web browser displaying annotated source code:
 	go tool cover -html=c.out

 Write out an HTML file instead of launching a web browser:
 	go tool cover -html=c.out -o coverage.html

 Display coverage percentages to stdout for each function:
 	go tool cover -func=c.out

 Finally, to generate modified source code with coverage annotations
 (what go test -cover does):
 	go tool cover -mode=set -var=CoverageVariableName program.go
 `

 func usage() {
 	fmt.Fprintln(os.Stderr, usageMessage)
 	fmt.Fprintln(os.Stderr, "Flags:")
 	flag.PrintDefaults()
 	fmt.Fprintln(os.Stderr, "\n  Only one of -html, -func, or -mode may be set.")
 	os.Exit(2)
 }

 var (
 	mode    = flag.String("mode", "", "coverage mode: set, count, atomic")
 	varVar  = flag.String("var", "GoCover", "name of coverage variable to generate")
 	output  = flag.String("o", "", "file for output; default: stdout")
 	htmlOut = flag.String("html", "", "generate HTML representation of coverage profile")
 	funcOut = flag.String("func", "", "output coverage profile information for each function")
 )

 var profile string // The profile to read; the value of -html or -func

 var counterStmt func(*File, string) string

 const (
 	atomicPackagePath = "sync/atomic"
 	atomicPackageName = "_cover_atomic_"
 )

 func main() {
 	objabi.AddVersionFlag()
 	flag.Usage = usage
 	flag.Parse()

 	// Usage information when no arguments.
 	if flag.NFlag() == 0 && flag.NArg() == 0 {
 		flag.Usage()
 	}

 	err := parseFlags()
 	if err != nil {
 		fmt.Fprintln(os.Stderr, err)
 		fmt.Fprintln(os.Stderr, `For usage information, run "go tool cover -help"`)
 		os.Exit(2)
 	}

 	// Generate coverage-annotated source.
 	if *mode != "" {
 		annotate(flag.Arg(0))
 		return
 	}

 	// Output HTML or function coverage information.
 	if *htmlOut != "" {
 		err = htmlOutput(profile, *output)
 	} else {
 		err = funcOutput(profile, *output)
 	}

 	if err != nil {
 		fmt.Fprintf(os.Stderr, "cover: %v\n", err)
 		os.Exit(2)
 	}
 }

 // parseFlags sets the profile and counterStmt globals and performs validations.
 func parseFlags() error {
 	profile = *htmlOut
 	if *funcOut != "" {
 		if profile != "" {
 			return fmt.Errorf("too many options")
 		}
 		profile = *funcOut
 	}

 	// Must either display a profile or rewrite Go source.
 	if (profile == "") == (*mode == "") {
 		return fmt.Errorf("too many options")
 	}

 	if *varVar != "" && !token.IsIdentifier(*varVar) {
 		return fmt.Errorf("-var: %q is not a valid identifier", *varVar)
 	}

 	if *mode != "" {
 		switch *mode {
 		case "set":
 			counterStmt = setCounterStmt
 		case "count":
 			counterStmt = incCounterStmt
 		case "atomic":
 			counterStmt = atomicCounterStmt
 		default:
 			return fmt.Errorf("unknown -mode %v", *mode)
 		}

 		if flag.NArg() == 0 {
 			return fmt.Errorf("missing source file")
 		} else if flag.NArg() == 1 {
 			return nil
 		}
 	} else if flag.NArg() == 0 {
 		return nil
 	}
 	return fmt.Errorf("too many arguments")
 }

 // Block represents the information about a basic block to be recorded in the analysis.
 // Note: Our definition of basic block is based on control structures; we don't break
 // apart && and ||. We could but it doesn't seem important enough to bother.
 type Block struct {
 	startByte token.Pos
 	endByte   token.Pos
 	numStmt   int
 }

 // File is a wrapper for the state of a file used in the parser.
 // The basic parse tree walker is a method of this type.
 type File struct {
 	fset    *token.FileSet
 	name    string // Name of file.
 	astFile *ast.File
 	blocks  []Block
 	content []byte
 	edit    *edit.Buffer
 }

 // findText finds text in the original source, starting at pos.
 // It correctly skips over comments and assumes it need not
 // handle quoted strings.
 // It returns a byte offset within f.src.
 func (f *File) findText(pos token.Pos, text string) int {
 	b := []byte(text)
 	start := f.offset(pos)
 	i := start
 	s := f.content
 	for i < len(s) {
 		if bytes.HasPrefix(s[i:], b) {
 			return i
 		}
 		if i+2 <= len(s) && s[i] == '/' && s[i+1] == '/' {
 			for i < len(s) && s[i] != '\n' {
 				i++
 			}
 			continue
 		}
 		if i+2 <= len(s) && s[i] == '/' && s[i+1] == '*' {
 			for i += 2; ; i++ {
 				if i+2 > len(s) {
 					return 0
 				}
 				if s[i] == '*' && s[i+1] == '/' {
 					i += 2
 					break
 				}
 			}
 			continue
 		}
 		i++
 	}
 	return -1
 }

 // Visit implements the ast.Visitor interface.
 func (f *File) Visit(node ast.Node) ast.Visitor {
 	switch n := node.(type) {
 	case *ast.BlockStmt:
 		// If it's a switch or select, the body is a list of case clauses; don't tag the block itself.
 		if len(n.List) > 0 {
 			switch n.List[0].(type) {
 			case *ast.CaseClause: // switch
 				for _, n := range n.List {
 					clause := n.(*ast.CaseClause)
 					f.addCounters(clause.Colon+1, clause.Colon+1, clause.End(), clause.Body, false)
 				}
 				return f
 			case *ast.CommClause: // select
 				for _, n := range n.List {
 					clause := n.(*ast.CommClause)
 					f.addCounters(clause.Colon+1, clause.Colon+1, clause.End(), clause.Body, false)
 				}
 				return f
 			}
 		}
 		f.addCounters(n.Lbrace, n.Lbrace+1, n.Rbrace+1, n.List, true) // +1 to step past closing brace.
 	case *ast.IfStmt:
 		if n.Init != nil {
 			ast.Walk(f, n.Init)
 		}
 		ast.Walk(f, n.Cond)
 		ast.Walk(f, n.Body)
 		if n.Else == nil {
 			return nil
 		}
 		// The elses are special, because if we have
 		//	if x {
 		//	} else if y {
 		//	}
 		// we want to cover the "if y". To do this, we need a place to drop the counter,
 		// so we add a hidden block:
 		//	if x {
 		//	} else {
 		//		if y {
 		//		}
 		//	}
 		elseOffset := f.findText(n.Body.End(), "else")
 		if elseOffset < 0 {
 			panic("lost else")
 		}
 		f.edit.Insert(elseOffset+4, "{")
 		f.edit.Insert(f.offset(n.Else.End()), "}")

 		// We just created a block, now walk it.
 		// Adjust the position of the new block to start after
 		// the "else". That will cause it to follow the "{"
 		// we inserted above.
 		pos := f.fset.File(n.Body.End()).Pos(elseOffset + 4)
 		switch stmt := n.Else.(type) {
 		case *ast.IfStmt:
 			block := &ast.BlockStmt{
 				Lbrace: pos,
 				List:   []ast.Stmt{stmt},
 				Rbrace: stmt.End(),
 			}
 			n.Else = block
 		case *ast.BlockStmt:
 			stmt.Lbrace = pos
 		default:
 			panic("unexpected node type in if")
 		}
 		ast.Walk(f, n.Else)
 		return nil
 	case *ast.SelectStmt:
 		// Don't annotate an empty select - creates a syntax error.
 		if n.Body == nil || len(n.Body.List) == 0 {
 			return nil
 		}
 	case *ast.SwitchStmt:
 		// Don't annotate an empty switch - creates a syntax error.
 		if n.Body == nil || len(n.Body.List) == 0 {
 			if n.Init != nil {
 				ast.Walk(f, n.Init)
 			}
 			if n.Tag != nil {
 				ast.Walk(f, n.Tag)
 			}
 			return nil
 		}
 	case *ast.TypeSwitchStmt:
 		// Don't annotate an empty type switch - creates a syntax error.
 		if n.Body == nil || len(n.Body.List) == 0 {
 			if n.Init != nil {
 				ast.Walk(f, n.Init)
 			}
 			ast.Walk(f, n.Assign)
 			return nil
 		}
 	case *ast.FuncDecl:
 		// Don't annotate functions with blank names - they cannot be executed.
 		if n.Name.Name == "_" {
 			return nil
 		}
 	}
 	return f
 }

 func annotate(name string) {
 	fset := token.NewFileSet()
 	content, err := os.ReadFile(name)
 	if err != nil {
 		log.Fatalf("cover: %s: %s", name, err)
 	}
 	parsedFile, err := parser.ParseFile(fset, name, content, parser.ParseComments)
 	if err != nil {
 		log.Fatalf("cover: %s: %s", name, err)
 	}

 	file := &File{
 		fset:    fset,
 		name:    name,
 		content: content,
 		edit:    edit.NewBuffer(content),
 		astFile: parsedFile,
 	}
 	if *mode == "atomic" {
 		// Add import of sync/atomic immediately after package clause.
 		// We do this even if there is an existing import, because the
 		// existing import may be shadowed at any given place we want
 		// to refer to it, and our name (_cover_atomic_) is less likely to
 		// be shadowed.
 		file.edit.Insert(file.offset(file.astFile.Name.End()),
 			fmt.Sprintf("; import %s %q", atomicPackageName, atomicPackagePath))
 	}

 	ast.Walk(file, file.astFile)
 	newContent := file.edit.Bytes()

 	fd := os.Stdout
 	if *output != "" {
 		var err error
 		fd, err = os.Create(*output)
 		if err != nil {
 			log.Fatalf("cover: %s", err)
 		}
 	}

 	fmt.Fprintf(fd, "//line %s:1\n", name)
 	fd.Write(newContent)

 	// After printing the source tree, add some declarations for the counters etc.
 	// We could do this by adding to the tree, but it's easier just to print the text.
 	file.addVariables(fd)
 }

 // setCounterStmt returns the expression: __count[23] = 1.
 func setCounterStmt(f *File, counter string) string {
 	return fmt.Sprintf("%s = 1", counter)
 }

 // incCounterStmt returns the expression: __count[23]++.
 func incCounterStmt(f *File, counter string) string {
 	return fmt.Sprintf("%s++", counter)
 }

 // atomicCounterStmt returns the expression: atomic.AddUint32(&__count[23], 1)
 func atomicCounterStmt(f *File, counter string) string {
 	return fmt.Sprintf("%s.AddUint32(&%s, 1)", atomicPackageName, counter)
 }

 // newCounter creates a new counter expression of the appropriate form.
 func (f *File) newCounter(start, end token.Pos, numStmt int) string {
 	stmt := counterStmt(f, fmt.Sprintf("%s.Count[%d]", *varVar, len(f.blocks)))
 	f.blocks = append(f.blocks, Block{start, end, numStmt})
 	return stmt
 }

 // addCounters takes a list of statements and adds counters to the beginning of
 // each basic block at the top level of that list. For instance, given
 //
 //	S1
 //	if cond {
 //		S2
 // 	}
 //	S3
 //
 // counters will be added before S1 and before S3. The block containing S2
 // will be visited in a separate call.
 // TODO: Nested simple blocks get unnecessary (but correct) counters
 func (f *File) addCounters(pos, insertPos, blockEnd token.Pos, list []ast.Stmt, extendToClosingBrace bool) {
 	// Special case: make sure we add a counter to an empty block. Can't do this below
 	// or we will add a counter to an empty statement list after, say, a return statement.
 	if len(list) == 0 {
 		f.edit.Insert(f.offset(insertPos), f.newCounter(insertPos, blockEnd, 0)+";")
 		return
 	}
 	// Make a copy of the list, as we may mutate it and should leave the
 	// existing list intact.
 	list = append([]ast.Stmt(nil), list...)
 	// We have a block (statement list), but it may have several basic blocks due to the
 	// appearance of statements that affect the flow of control.
 	for {
 		// Find first statement that affects flow of control (break, continue, if, etc.).
 		// It will be the last statement of this basic block.
 		var last int
 		end := blockEnd
 		for last = 0; last < len(list); last++ {
 			stmt := list[last]
 			end = f.statementBoundary(stmt)
 			if f.endsBasicSourceBlock(stmt) {
 				// If it is a labeled statement, we need to place a counter between
 				// the label and its statement because it may be the target of a goto
 				// and thus start a basic block. That is, given
 				//	foo: stmt
 				// we need to create
 				//	foo: ; stmt
 				// and mark the label as a block-terminating statement.
 				// The result will then be
 				//	foo: COUNTER[n]++; stmt
 				// However, we can't do this if the labeled statement is already
 				// a control statement, such as a labeled for.
 				if label, isLabel := stmt.(*ast.LabeledStmt); isLabel && !f.isControl(label.Stmt) {
 					newLabel := *label
 					newLabel.Stmt = &ast.EmptyStmt{
 						Semicolon: label.Stmt.Pos(),
 						Implicit:  true,
 					}
 					end = label.Pos() // Previous block ends before the label.
 					list[last] = &newLabel
 					// Open a gap and drop in the old statement, now without a label.
 					list = append(list, nil)
 					copy(list[last+1:], list[last:])
 					list[last+1] = label.Stmt
 				}
 				last++
 				extendToClosingBrace = false // Block is broken up now.
 				break
 			}
 		}
 		if extendToClosingBrace {
 			end = blockEnd
 		}
 		if pos != end { // Can have no source to cover if e.g. blocks abut.
 			f.edit.Insert(f.offset(insertPos), f.newCounter(pos, end, last)+";")
 		}
 		list = list[last:]
 		if len(list) == 0 {
 			break
 		}
 		pos = list[0].Pos()
 		insertPos = pos
 	}
 }

 // hasFuncLiteral reports the existence and position of the first func literal
 // in the node, if any. If a func literal appears, it usually marks the termination
 // of a basic block because the function body is itself a block.
 // Therefore we draw a line at the start of the body of the first function literal we find.
 // TODO: what if there's more than one? Probably doesn't matter much.
 func hasFuncLiteral(n ast.Node) (bool, token.Pos) {
 	if n == nil {
 		return false, 0
 	}
 	var literal funcLitFinder
 	ast.Walk(&literal, n)
 	return literal.found(), token.Pos(literal)
 }

 // statementBoundary finds the location in s that terminates the current basic
 // block in the source.
 func (f *File) statementBoundary(s ast.Stmt) token.Pos {
 	// Control flow statements are easy.
 	switch s := s.(type) {
 	case *ast.BlockStmt:
 		// Treat blocks like basic blocks to avoid overlapping counters.
 		return s.Lbrace
 	case *ast.IfStmt:
 		found, pos := hasFuncLiteral(s.Init)
 		if found {
 			return pos
 		}
 		found, pos = hasFuncLiteral(s.Cond)
 		if found {
 			return pos
 		}
 		return s.Body.Lbrace
 	case *ast.ForStmt:
 		found, pos := hasFuncLiteral(s.Init)
 		if found {
 			return pos
 		}
 		found, pos = hasFuncLiteral(s.Cond)
 		if found {
 			return pos
 		}
 		found, pos = hasFuncLiteral(s.Post)
 		if found {
 			return pos
 		}
 		return s.Body.Lbrace
 	case *ast.LabeledStmt:
 		return f.statementBoundary(s.Stmt)
 	case *ast.RangeStmt:
 		found, pos := hasFuncLiteral(s.X)
 		if found {
 			return pos
 		}
 		return s.Body.Lbrace
 	case *ast.SwitchStmt:
 		found, pos := hasFuncLiteral(s.Init)
 		if found {
 			return pos
 		}
 		found, pos = hasFuncLiteral(s.Tag)
 		if found {
 			return pos
 		}
 		return s.Body.Lbrace
 	case *ast.SelectStmt:
 		return s.Body.Lbrace
 	case *ast.TypeSwitchStmt:
 		found, pos := hasFuncLiteral(s.Init)
 		if found {
 			return pos
 		}
 		return s.Body.Lbrace
 	}
 	// If not a control flow statement, it is a declaration, expression, call, etc. and it may have a function literal.
 	// If it does, that's tricky because we want to exclude the body of the function from this block.
 	// Draw a line at the start of the body of the first function literal we find.
 	// TODO: what if there's more than one? Probably doesn't matter much.
 	found, pos := hasFuncLiteral(s)
 	if found {
 		return pos
 	}
 	return s.End()
 }

 // endsBasicSourceBlock reports whether s changes the flow of control: break, if, etc.,
 // or if it's just problematic, for instance contains a function literal, which will complicate
 // accounting due to the block-within-an expression.
 func (f *File) endsBasicSourceBlock(s ast.Stmt) bool {
 	switch s := s.(type) {
 	case *ast.BlockStmt:
 		// Treat blocks like basic blocks to avoid overlapping counters.
 		return true
 	case *ast.BranchStmt:
 		return true
 	case *ast.ForStmt:
 		return true
 	case *ast.IfStmt:
 		return true
 	case *ast.LabeledStmt:
 		return true // A goto may branch here, starting a new basic block.
 	case *ast.RangeStmt:
 		return true
 	case *ast.SwitchStmt:
 		return true
 	case *ast.SelectStmt:
 		return true
 	case *ast.TypeSwitchStmt:
 		return true
 	case *ast.ExprStmt:
 		// Calls to panic change the flow.
 		// We really should verify that "panic" is the predefined function,
 		// but without type checking we can't and the likelihood of it being
 		// an actual problem is vanishingly small.
 		if call, ok := s.X.(*ast.CallExpr); ok {
 			if ident, ok := call.Fun.(*ast.Ident); ok && ident.Name == "panic" && len(call.Args) == 1 {
 				return true
 			}
 		}
 	}
 	found, _ := hasFuncLiteral(s)
 	return found
 }

 // isControl reports whether s is a control statement that, if labeled, cannot be
 // separated from its label.
 func (f *File) isControl(s ast.Stmt) bool {
 	switch s.(type) {
 	case *ast.ForStmt, *ast.RangeStmt, *ast.SwitchStmt, *ast.SelectStmt, *ast.TypeSwitchStmt:
 		return true
 	}
 	return false
 }

 // funcLitFinder implements the ast.Visitor pattern to find the location of any
 // function literal in a subtree.
 type funcLitFinder token.Pos

 func (f *funcLitFinder) Visit(node ast.Node) (w ast.Visitor) {
 	if f.found() {
 		return nil // Prune search.
 	}
 	switch n := node.(type) {
 	case *ast.FuncLit:
 		*f = funcLitFinder(n.Body.Lbrace)
 		return nil // Prune search.
 	}
 	return f
 }

 func (f *funcLitFinder) found() bool {
 	return token.Pos(*f) != token.NoPos
 }

 // Sort interface for []block1; used for self-check in addVariables.

 type block1 struct {
 	Block
 	index int
 }

 type blockSlice []block1

 func (b blockSlice) Len() int           { return len(b) }
 func (b blockSlice) Less(i, j int) bool { return b[i].startByte < b[j].startByte }
 func (b blockSlice) Swap(i, j int)      { b[i], b[j] = b[j], b[i] }

 // offset translates a token position into a 0-indexed byte offset.
 func (f *File) offset(pos token.Pos) int {
 	return f.fset.Position(pos).Offset
 }

 // addVariables adds to the end of the file the declarations to set up the counter and position variables.
 func (f *File) addVariables(w io.Writer) {
 	// Self-check: Verify that the instrumented basic blocks are disjoint.
 	t := make([]block1, len(f.blocks))
 	for i := range f.blocks {
 		t[i].Block = f.blocks[i]
 		t[i].index = i
 	}
 	sort.Sort(blockSlice(t))
 	for i := 1; i < len(t); i++ {
 		if t[i-1].endByte > t[i].startByte {
 			fmt.Fprintf(os.Stderr, "cover: internal error: block %d overlaps block %d\n", t[i-1].index, t[i].index)
 			// Note: error message is in byte positions, not token positions.
 			fmt.Fprintf(os.Stderr, "\t%s:#%d,#%d %s:#%d,#%d\n",
 				f.name, f.offset(t[i-1].startByte), f.offset(t[i-1].endByte),
 				f.name, f.offset(t[i].startByte), f.offset(t[i].endByte))
 		}
 	}

 	// Declare the coverage struct as a package-level variable.
 	fmt.Fprintf(w, "\nvar %s = struct {\n", *varVar)
 	fmt.Fprintf(w, "\tCount     [%d]uint32\n", len(f.blocks))
 	fmt.Fprintf(w, "\tPos       [3 * %d]uint32\n", len(f.blocks))
 	fmt.Fprintf(w, "\tNumStmt   [%d]uint16\n", len(f.blocks))
 	fmt.Fprintf(w, "} {\n")

 	// Initialize the position array field.
 	fmt.Fprintf(w, "\tPos: [3 * %d]uint32{\n", len(f.blocks))

 	// A nice long list of positions. Each position is encoded as follows to reduce size:
 	// - 32-bit starting line number
 	// - 32-bit ending line number
 	// - (16 bit ending column number << 16) | (16-bit starting column number).
 	for i, block := range f.blocks {
 		start := f.fset.Position(block.startByte)
 		end := f.fset.Position(block.endByte)

 		start, end = dedup(start, end)

 		fmt.Fprintf(w, "\t\t%d, %d, %#x, // [%d]\n", start.Line, end.Line, (end.Column&0xFFFF)<<16|(start.Column&0xFFFF), i)
 	}

 	// Close the position array.
 	fmt.Fprintf(w, "\t},\n")

 	// Initialize the position array field.
 	fmt.Fprintf(w, "\tNumStmt: [%d]uint16{\n", len(f.blocks))

 	// A nice long list of statements-per-block, so we can give a conventional
 	// valuation of "percent covered". To save space, it's a 16-bit number, so we
 	// clamp it if it overflows - won't matter in practice.
 	for i, block := range f.blocks {
 		n := block.numStmt
 		if n > 1<<16-1 {
 			n = 1<<16 - 1
 		}
 		fmt.Fprintf(w, "\t\t%d, // %d\n", n, i)
 	}

 	// Close the statements-per-block array.
 	fmt.Fprintf(w, "\t},\n")

 	// Close the struct initialization.
 	fmt.Fprintf(w, "}\n")

 	// Emit a reference to the atomic package to avoid
 	// import and not used error when there's no code in a file.
 	if *mode == "atomic" {
 		fmt.Fprintf(w, "var _ = %s.LoadUint32\n", atomicPackageName)
 	}
 }

 // It is possible for positions to repeat when there is a line
 // directive that does not specify column information and the input
 // has not been passed through gofmt.
 // See issues #27530 and #30746.
 // Tests are TestHtmlUnformatted and TestLineDup.
 // We use a map to avoid duplicates.

 // pos2 is a pair of token.Position values, used as a map key type.
 type pos2 struct {
 	p1, p2 token.Position
 }

 // seenPos2 tracks whether we have seen a token.Position pair.
 var seenPos2 = make(map[pos2]bool)

 // dedup takes a token.Position pair and returns a pair that does not
 // duplicate any existing pair. The returned pair will have the Offset
 // fields cleared.
 func dedup(p1, p2 token.Position) (r1, r2 token.Position) {
 	key := pos2{
 		p1: p1,
 		p2: p2,
 	}

 	// We want to ignore the Offset fields in the map,
 	// since cover uses only file/line/column.
 	key.p1.Offset = 0
 	key.p2.Offset = 0

 	for seenPos2[key] {
 		key.p2.Column++
 	}
 	seenPos2[key] = true

 	return key.p1, key.p2
 }
	// 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 main

	import (
	"bytes"
	"flag"
	"fmt"
	"go/ast"
	"go/parser"
	"go/token"
	"io"
	"log"
	"os"
	"sort"

	"cmd/internal/edit"
	"cmd/internal/objabi"
	)

	const usageMessage = "" +
	`Usage of 'go tool cover':
	Given a coverage profile produced by 'go test':
	go test -coverprofile=c.out

	Open a web browser displaying annotated source code:
	go tool cover -html=c.out

	Write out an HTML file instead of launching a web browser:
	go tool cover -html=c.out -o coverage.html

	Display coverage percentages to stdout for each function:
	go tool cover -func=c.out

	Finally, to generate modified source code with coverage annotations
	(what go test -cover does):
	go tool cover -mode=set -var=CoverageVariableName program.go
	`

	func usage() {
	fmt.Fprintln(os.Stderr, usageMessage)
	fmt.Fprintln(os.Stderr, "Flags:")
	flag.PrintDefaults()
	fmt.Fprintln(os.Stderr, "\n Only one of -html, -func, or -mode may be set.")
	os.Exit(2)
	}

	var (
	mode = flag.String("mode", "", "coverage mode: set, count, atomic")
	varVar = flag.String("var", "GoCover", "name of coverage variable to generate")
	output = flag.String("o", "", "file for output; default: stdout")
	htmlOut = flag.String("html", "", "generate HTML representation of coverage profile")
	funcOut = flag.String("func", "", "output coverage profile information for each function")
	)

	var profile string // The profile to read; the value of -html or -func

	var counterStmt func(*File, string) string

	const (
	atomicPackagePath = "sync/atomic"
	atomicPackageName = "_cover_atomic_"
	)

	func main() {
	objabi.AddVersionFlag()
	flag.Usage = usage
	flag.Parse()

	// Usage information when no arguments.
	if flag.NFlag() == 0 && flag.NArg() == 0 {
	flag.Usage()
	}

	err := parseFlags()
	if err != nil {
	fmt.Fprintln(os.Stderr, err)
	fmt.Fprintln(os.Stderr, `For usage information, run "go tool cover -help"`)
	os.Exit(2)
	}

	// Generate coverage-annotated source.
	if *mode != "" {
	annotate(flag.Arg(0))
	return
	}

	// Output HTML or function coverage information.
	if *htmlOut != "" {
	err = htmlOutput(profile, *output)
	} else {
	err = funcOutput(profile, *output)
	}

	if err != nil {
	fmt.Fprintf(os.Stderr, "cover: %v\n", err)
	os.Exit(2)
	}
	}

	// parseFlags sets the profile and counterStmt globals and performs validations.
	func parseFlags() error {
	profile = *htmlOut
	if *funcOut != "" {
	if profile != "" {
	return fmt.Errorf("too many options")
	}
	profile = *funcOut
	}

	// Must either display a profile or rewrite Go source.
	if (profile == "") == (*mode == "") {
	return fmt.Errorf("too many options")
	}

	if varVar != "" && !token.IsIdentifier(varVar) {
	return fmt.Errorf("-var: %q is not a valid identifier", *varVar)
	}

	if *mode != "" {
	switch *mode {
	case "set":
	counterStmt = setCounterStmt
	case "count":
	counterStmt = incCounterStmt
	case "atomic":
	counterStmt = atomicCounterStmt
	default:
	return fmt.Errorf("unknown -mode %v", *mode)
	}

	if flag.NArg() == 0 {
	return fmt.Errorf("missing source file")
	} else if flag.NArg() == 1 {
	return nil
	}
	} else if flag.NArg() == 0 {
	return nil
	}
	return fmt.Errorf("too many arguments")
	}

	// Block represents the information about a basic block to be recorded in the analysis.
	// Note: Our definition of basic block is based on control structures; we don't break
	// apart && and \|\|. We could but it doesn't seem important enough to bother.
	type Block struct {
	startByte token.Pos
	endByte token.Pos
	numStmt int
	}

	// File is a wrapper for the state of a file used in the parser.
	// The basic parse tree walker is a method of this type.
	type File struct {
	fset *token.FileSet
	name string // Name of file.
	astFile *ast.File
	blocks []Block
	content []byte
	edit *edit.Buffer
	}

	// findText finds text in the original source, starting at pos.
	// It correctly skips over comments and assumes it need not
	// handle quoted strings.
	// It returns a byte offset within f.src.
	func (f *File) findText(pos token.Pos, text string) int {
	b := []byte(text)
	start := f.offset(pos)
	i := start
	s := f.content
	for i < len(s) {
	if bytes.HasPrefix(s[i:], b) {
	return i
	}
	if i+2 <= len(s) && s[i] == '/' && s[i+1] == '/' {
	for i < len(s) && s[i] != '\n' {
	i++
	}
	continue
	}
	if i+2 <= len(s) && s[i] == '/' && s[i+1] == '*' {
	for i += 2; ; i++ {
	if i+2 > len(s) {
	return 0
	}
	if s[i] == '*' && s[i+1] == '/' {
	i += 2
	break
	}
	}
	continue
	}
	i++
	}
	return -1
	}

	// Visit implements the ast.Visitor interface.
	func (f *File) Visit(node ast.Node) ast.Visitor {
	switch n := node.(type) {
	case *ast.BlockStmt:
	// If it's a switch or select, the body is a list of case clauses; don't tag the block itself.
	if len(n.List) > 0 {
	switch n.List[0].(type) {
	case *ast.CaseClause: // switch
	for _, n := range n.List {
	clause := n.(*ast.CaseClause)
	f.addCounters(clause.Colon+1, clause.Colon+1, clause.End(), clause.Body, false)
	}
	return f
	case *ast.CommClause: // select
	for _, n := range n.List {
	clause := n.(*ast.CommClause)
	f.addCounters(clause.Colon+1, clause.Colon+1, clause.End(), clause.Body, false)
	}
	return f
	}
	}
	f.addCounters(n.Lbrace, n.Lbrace+1, n.Rbrace+1, n.List, true) // +1 to step past closing brace.
	case *ast.IfStmt:
	if n.Init != nil {
	ast.Walk(f, n.Init)
	}
	ast.Walk(f, n.Cond)
	ast.Walk(f, n.Body)
	if n.Else == nil {
	return nil
	}
	// The elses are special, because if we have
	// if x {
	// } else if y {
	// }
	// we want to cover the "if y". To do this, we need a place to drop the counter,
	// so we add a hidden block:
	// if x {
	// } else {
	// if y {
	// }
	// }
	elseOffset := f.findText(n.Body.End(), "else")
	if elseOffset < 0 {
	panic("lost else")
	}
	f.edit.Insert(elseOffset+4, "{")
	f.edit.Insert(f.offset(n.Else.End()), "}")

	// We just created a block, now walk it.
	// Adjust the position of the new block to start after
	// the "else". That will cause it to follow the "{"
	// we inserted above.
	pos := f.fset.File(n.Body.End()).Pos(elseOffset + 4)
	switch stmt := n.Else.(type) {
	case *ast.IfStmt:
	block := &ast.BlockStmt{
	Lbrace: pos,
	List: []ast.Stmt{stmt},
	Rbrace: stmt.End(),
	}
	n.Else = block
	case *ast.BlockStmt:
	stmt.Lbrace = pos
	default:
	panic("unexpected node type in if")
	}
	ast.Walk(f, n.Else)
	return nil
	case *ast.SelectStmt:
	// Don't annotate an empty select - creates a syntax error.
	if n.Body == nil \|\| len(n.Body.List) == 0 {
	return nil
	}
	case *ast.SwitchStmt:
	// Don't annotate an empty switch - creates a syntax error.
	if n.Body == nil \|\| len(n.Body.List) == 0 {
	if n.Init != nil {
	ast.Walk(f, n.Init)
	}
	if n.Tag != nil {
	ast.Walk(f, n.Tag)
	}
	return nil
	}
	case *ast.TypeSwitchStmt:
	// Don't annotate an empty type switch - creates a syntax error.
	if n.Body == nil \|\| len(n.Body.List) == 0 {
	if n.Init != nil {
	ast.Walk(f, n.Init)
	}
	ast.Walk(f, n.Assign)
	return nil
	}
	case *ast.FuncDecl:
	// Don't annotate functions with blank names - they cannot be executed.
	if n.Name.Name == "_" {
	return nil
	}
	}
	return f
	}

	func annotate(name string) {
	fset := token.NewFileSet()
	content, err := os.ReadFile(name)
	if err != nil {
	log.Fatalf("cover: %s: %s", name, err)
	}
	parsedFile, err := parser.ParseFile(fset, name, content, parser.ParseComments)
	if err != nil {
	log.Fatalf("cover: %s: %s", name, err)
	}

	file := &File{
	fset: fset,
	name: name,
	content: content,
	edit: edit.NewBuffer(content),
	astFile: parsedFile,
	}
	if *mode == "atomic" {
	// Add import of sync/atomic immediately after package clause.
	// We do this even if there is an existing import, because the
	// existing import may be shadowed at any given place we want
	// to refer to it, and our name (_cover_atomic_) is less likely to
	// be shadowed.
	file.edit.Insert(file.offset(file.astFile.Name.End()),
	fmt.Sprintf("; import %s %q", atomicPackageName, atomicPackagePath))
	}

	ast.Walk(file, file.astFile)
	newContent := file.edit.Bytes()

	fd := os.Stdout
	if *output != "" {
	var err error
	fd, err = os.Create(*output)
	if err != nil {
	log.Fatalf("cover: %s", err)
	}
	}

	fmt.Fprintf(fd, "//line %s:1\n", name)
	fd.Write(newContent)

	// After printing the source tree, add some declarations for the counters etc.
	// We could do this by adding to the tree, but it's easier just to print the text.
	file.addVariables(fd)
	}

	// setCounterStmt returns the expression: __count[23] = 1.
	func setCounterStmt(f *File, counter string) string {
	return fmt.Sprintf("%s = 1", counter)
	}

	// incCounterStmt returns the expression: __count[23]++.
	func incCounterStmt(f *File, counter string) string {
	return fmt.Sprintf("%s++", counter)
	}

	// atomicCounterStmt returns the expression: atomic.AddUint32(&__count[23], 1)
	func atomicCounterStmt(f *File, counter string) string {
	return fmt.Sprintf("%s.AddUint32(&%s, 1)", atomicPackageName, counter)
	}

	// newCounter creates a new counter expression of the appropriate form.
	func (f *File) newCounter(start, end token.Pos, numStmt int) string {
	stmt := counterStmt(f, fmt.Sprintf("%s.Count[%d]", *varVar, len(f.blocks)))
	f.blocks = append(f.blocks, Block{start, end, numStmt})
	return stmt
	}

	// addCounters takes a list of statements and adds counters to the beginning of
	// each basic block at the top level of that list. For instance, given
	//
	// S1
	// if cond {
	// S2
	// }
	// S3
	//
	// counters will be added before S1 and before S3. The block containing S2
	// will be visited in a separate call.
	// TODO: Nested simple blocks get unnecessary (but correct) counters
	func (f *File) addCounters(pos, insertPos, blockEnd token.Pos, list []ast.Stmt, extendToClosingBrace bool) {
	// Special case: make sure we add a counter to an empty block. Can't do this below
	// or we will add a counter to an empty statement list after, say, a return statement.
	if len(list) == 0 {
	f.edit.Insert(f.offset(insertPos), f.newCounter(insertPos, blockEnd, 0)+";")
	return
	}
	// Make a copy of the list, as we may mutate it and should leave the
	// existing list intact.
	list = append([]ast.Stmt(nil), list...)
	// We have a block (statement list), but it may have several basic blocks due to the
	// appearance of statements that affect the flow of control.
	for {
	// Find first statement that affects flow of control (break, continue, if, etc.).
	// It will be the last statement of this basic block.
	var last int
	end := blockEnd
	for last = 0; last < len(list); last++ {
	stmt := list[last]
	end = f.statementBoundary(stmt)
	if f.endsBasicSourceBlock(stmt) {
	// If it is a labeled statement, we need to place a counter between
	// the label and its statement because it may be the target of a goto
	// and thus start a basic block. That is, given
	// foo: stmt
	// we need to create
	// foo: ; stmt
	// and mark the label as a block-terminating statement.
	// The result will then be
	// foo: COUNTER[n]++; stmt
	// However, we can't do this if the labeled statement is already
	// a control statement, such as a labeled for.
	if label, isLabel := stmt.(*ast.LabeledStmt); isLabel && !f.isControl(label.Stmt) {
	newLabel := *label
	newLabel.Stmt = &ast.EmptyStmt{
	Semicolon: label.Stmt.Pos(),
	Implicit: true,
	}
	end = label.Pos() // Previous block ends before the label.
	list[last] = &newLabel
	// Open a gap and drop in the old statement, now without a label.
	list = append(list, nil)
	copy(list[last+1:], list[last:])
	list[last+1] = label.Stmt
	}
	last++
	extendToClosingBrace = false // Block is broken up now.
	break
	}
	}
	if extendToClosingBrace {
	end = blockEnd
	}
	if pos != end { // Can have no source to cover if e.g. blocks abut.
	f.edit.Insert(f.offset(insertPos), f.newCounter(pos, end, last)+";")
	}
	list = list[last:]
	if len(list) == 0 {
	break
	}
	pos = list[0].Pos()
	insertPos = pos
	}
	}

	// hasFuncLiteral reports the existence and position of the first func literal
	// in the node, if any. If a func literal appears, it usually marks the termination
	// of a basic block because the function body is itself a block.
	// Therefore we draw a line at the start of the body of the first function literal we find.
	// TODO: what if there's more than one? Probably doesn't matter much.
	func hasFuncLiteral(n ast.Node) (bool, token.Pos) {
	if n == nil {
	return false, 0
	}
	var literal funcLitFinder
	ast.Walk(&literal, n)
	return literal.found(), token.Pos(literal)
	}

	// statementBoundary finds the location in s that terminates the current basic
	// block in the source.
	func (f *File) statementBoundary(s ast.Stmt) token.Pos {
	// Control flow statements are easy.
	switch s := s.(type) {
	case *ast.BlockStmt:
	// Treat blocks like basic blocks to avoid overlapping counters.
	return s.Lbrace
	case *ast.IfStmt:
	found, pos := hasFuncLiteral(s.Init)
	if found {
	return pos
	}
	found, pos = hasFuncLiteral(s.Cond)
	if found {
	return pos
	}
	return s.Body.Lbrace
	case *ast.ForStmt:
	found, pos := hasFuncLiteral(s.Init)
	if found {
	return pos
	}
	found, pos = hasFuncLiteral(s.Cond)
	if found {
	return pos
	}
	found, pos = hasFuncLiteral(s.Post)
	if found {
	return pos
	}
	return s.Body.Lbrace
	case *ast.LabeledStmt:
	return f.statementBoundary(s.Stmt)
	case *ast.RangeStmt:
	found, pos := hasFuncLiteral(s.X)
	if found {
	return pos
	}
	return s.Body.Lbrace
	case *ast.SwitchStmt:
	found, pos := hasFuncLiteral(s.Init)
	if found {
	return pos
	}
	found, pos = hasFuncLiteral(s.Tag)
	if found {
	return pos
	}
	return s.Body.Lbrace
	case *ast.SelectStmt:
	return s.Body.Lbrace
	case *ast.TypeSwitchStmt:
	found, pos := hasFuncLiteral(s.Init)
	if found {
	return pos
	}
	return s.Body.Lbrace
	}
	// If not a control flow statement, it is a declaration, expression, call, etc. and it may have a function literal.
	// If it does, that's tricky because we want to exclude the body of the function from this block.
	// Draw a line at the start of the body of the first function literal we find.
	// TODO: what if there's more than one? Probably doesn't matter much.
	found, pos := hasFuncLiteral(s)
	if found {
	return pos
	}
	return s.End()
	}

	// endsBasicSourceBlock reports whether s changes the flow of control: break, if, etc.,
	// or if it's just problematic, for instance contains a function literal, which will complicate
	// accounting due to the block-within-an expression.
	func (f *File) endsBasicSourceBlock(s ast.Stmt) bool {
	switch s := s.(type) {
	case *ast.BlockStmt:
	// Treat blocks like basic blocks to avoid overlapping counters.
	return true
	case *ast.BranchStmt:
	return true
	case *ast.ForStmt:
	return true
	case *ast.IfStmt:
	return true
	case *ast.LabeledStmt:
	return true // A goto may branch here, starting a new basic block.
	case *ast.RangeStmt:
	return true
	case *ast.SwitchStmt:
	return true
	case *ast.SelectStmt:
	return true
	case *ast.TypeSwitchStmt:
	return true
	case *ast.ExprStmt:
	// Calls to panic change the flow.
	// We really should verify that "panic" is the predefined function,
	// but without type checking we can't and the likelihood of it being
	// an actual problem is vanishingly small.
	if call, ok := s.X.(*ast.CallExpr); ok {
	if ident, ok := call.Fun.(*ast.Ident); ok && ident.Name == "panic" && len(call.Args) == 1 {
	return true
	}
	}
	}
	found, _ := hasFuncLiteral(s)
	return found
	}

	// isControl reports whether s is a control statement that, if labeled, cannot be
	// separated from its label.
	func (f *File) isControl(s ast.Stmt) bool {
	switch s.(type) {
	case ast.ForStmt, ast.RangeStmt, ast.SwitchStmt, ast.SelectStmt, *ast.TypeSwitchStmt:
	return true
	}
	return false
	}

	// funcLitFinder implements the ast.Visitor pattern to find the location of any
	// function literal in a subtree.
	type funcLitFinder token.Pos

	func (f *funcLitFinder) Visit(node ast.Node) (w ast.Visitor) {
	if f.found() {
	return nil // Prune search.
	}
	switch n := node.(type) {
	case *ast.FuncLit:
	*f = funcLitFinder(n.Body.Lbrace)
	return nil // Prune search.
	}
	return f
	}

	func (f *funcLitFinder) found() bool {
	return token.Pos(*f) != token.NoPos
	}

	// Sort interface for []block1; used for self-check in addVariables.

	type block1 struct {
	Block
	index int
	}

	type blockSlice []block1

	func (b blockSlice) Len() int { return len(b) }
	func (b blockSlice) Less(i, j int) bool { return b[i].startByte < b[j].startByte }
	func (b blockSlice) Swap(i, j int) { b[i], b[j] = b[j], b[i] }

	// offset translates a token position into a 0-indexed byte offset.
	func (f *File) offset(pos token.Pos) int {
	return f.fset.Position(pos).Offset
	}

	// addVariables adds to the end of the file the declarations to set up the counter and position variables.
	func (f *File) addVariables(w io.Writer) {
	// Self-check: Verify that the instrumented basic blocks are disjoint.
	t := make([]block1, len(f.blocks))
	for i := range f.blocks {
	t[i].Block = f.blocks[i]
	t[i].index = i
	}
	sort.Sort(blockSlice(t))
	for i := 1; i < len(t); i++ {
	if t[i-1].endByte > t[i].startByte {
	fmt.Fprintf(os.Stderr, "cover: internal error: block %d overlaps block %d\n", t[i-1].index, t[i].index)
	// Note: error message is in byte positions, not token positions.
	fmt.Fprintf(os.Stderr, "\t%s:#%d,#%d %s:#%d,#%d\n",
	f.name, f.offset(t[i-1].startByte), f.offset(t[i-1].endByte),
	f.name, f.offset(t[i].startByte), f.offset(t[i].endByte))
	}
	}

	// Declare the coverage struct as a package-level variable.
	fmt.Fprintf(w, "\nvar %s = struct {\n", *varVar)
	fmt.Fprintf(w, "\tCount [%d]uint32\n", len(f.blocks))
	fmt.Fprintf(w, "\tPos [3 * %d]uint32\n", len(f.blocks))
	fmt.Fprintf(w, "\tNumStmt [%d]uint16\n", len(f.blocks))
	fmt.Fprintf(w, "} {\n")

	// Initialize the position array field.
	fmt.Fprintf(w, "\tPos: [3 * %d]uint32{\n", len(f.blocks))

	// A nice long list of positions. Each position is encoded as follows to reduce size:
	// - 32-bit starting line number
	// - 32-bit ending line number
	// - (16 bit ending column number << 16) \| (16-bit starting column number).
	for i, block := range f.blocks {
	start := f.fset.Position(block.startByte)
	end := f.fset.Position(block.endByte)

	start, end = dedup(start, end)

	fmt.Fprintf(w, "\t\t%d, %d, %#x, // [%d]\n", start.Line, end.Line, (end.Column&0xFFFF)<<16\|(start.Column&0xFFFF), i)
	}

	// Close the position array.
	fmt.Fprintf(w, "\t},\n")

	// Initialize the position array field.
	fmt.Fprintf(w, "\tNumStmt: [%d]uint16{\n", len(f.blocks))

	// A nice long list of statements-per-block, so we can give a conventional
	// valuation of "percent covered". To save space, it's a 16-bit number, so we
	// clamp it if it overflows - won't matter in practice.
	for i, block := range f.blocks {
	n := block.numStmt
	if n > 1<<16-1 {
	n = 1<<16 - 1
	}
	fmt.Fprintf(w, "\t\t%d, // %d\n", n, i)
	}

	// Close the statements-per-block array.
	fmt.Fprintf(w, "\t},\n")

	// Close the struct initialization.
	fmt.Fprintf(w, "}\n")

	// Emit a reference to the atomic package to avoid
	// import and not used error when there's no code in a file.
	if *mode == "atomic" {
	fmt.Fprintf(w, "var _ = %s.LoadUint32\n", atomicPackageName)
	}
	}

	// It is possible for positions to repeat when there is a line
	// directive that does not specify column information and the input
	// has not been passed through gofmt.
	// See issues #27530 and #30746.
	// Tests are TestHtmlUnformatted and TestLineDup.
	// We use a map to avoid duplicates.

	// pos2 is a pair of token.Position values, used as a map key type.
	type pos2 struct {
	p1, p2 token.Position
	}

	// seenPos2 tracks whether we have seen a token.Position pair.
	var seenPos2 = make(map[pos2]bool)

	// dedup takes a token.Position pair and returns a pair that does not
	// duplicate any existing pair. The returned pair will have the Offset
	// fields cleared.
	func dedup(p1, p2 token.Position) (r1, r2 token.Position) {
	key := pos2{
	p1: p1,
	p2: p2,
	}

	// We want to ignore the Offset fields in the map,
	// since cover uses only file/line/column.
	key.p1.Offset = 0
	key.p2.Offset = 0

	for seenPos2[key] {
	key.p2.Column++
	}
	seenPos2[key] = true

	return key.p1, key.p2
	}