go.tools/cmd/cover: get cover counters on if conditions

This requires a little more tree rewriting to put a block around the if of an "else if".
More tests too.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/11042045

cmd/cover/cover.go

@@ -146,6 +146,38 @@ func (f *File) Visit(node ast.Node) ast.Visitor {
 			}
 		}
 		n.List = f.addCounters(n.Lbrace, n.Rbrace+1, n.List, true) // +1 to step past closing brace.
+	case *ast.IfStmt:
+		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 {
+		//		}
+		//	}
+		const backupToElse = token.Pos(len("else ")) // The AST doesn't remember the else location. We can make an accurate guess.
+		switch stmt := n.Else.(type) {
+		case *ast.IfStmt:
+			block := &ast.BlockStmt{
+				Lbrace: stmt.If - backupToElse, // So the covered part looks like it starts at the "else".
+				List:   []ast.Stmt{stmt},
+				Rbrace: stmt.End(),
+			}
+			n.Else = block
+		case *ast.BlockStmt:
+			stmt.Lbrace -= backupToElse // So the block looks like it starts at the "else".
+		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 {
@@ -507,7 +539,7 @@ func (f *File) addVariables(w io.Writer) {
 	for i, block := range f.blocks {
 		start := f.fset.Position(block.startByte)
 		end := f.fset.Position(block.endByte)
-		fmt.Fprintf(w, "\t\t%d, %d, %#x, // %d\n", start.Line, end.Line, (end.Column&0xFFFF)<<16|(start.Column&0xFFFF), i)
+		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.

cmd/cover/testdata/main.go

@@ -33,32 +33,61 @@ func check(line, count uint32) {
 	counters[b] = true
 }
 
+// checkVal is a version of check that returns its extra argument,
+// so it can be used in conditionals.
+func checkVal(line, count uint32, val int) int {
+	b := block{
+		count,
+		line,
+	}
+	counters[b] = true
+	return val
+}
+
+var PASS = true
+
 // verify checks the expected counts against the actual. It runs after the test has completed.
 func verify() {
-	ok := true
 	for b := range counters {
-		got := count(b.line)
+		got, index := count(b.line)
 		if b.count == anything && got != 0 {
 			got = anything
 		}
 		if got != b.count {
-			fmt.Fprintf(os.Stderr, "test_go:%d expected count %d got %d\n", b.line, b.count, got)
-			ok = false
+			fmt.Fprintf(os.Stderr, "test_go:%d expected count %d got %d [counter %d]\n", b.line, b.count, got, index)
+			PASS = false
 		}
 	}
-	if !ok {
+	if !PASS {
 		fmt.Fprintf(os.Stderr, "FAIL\n")
 		os.Exit(2)
 	}
 }
 
-func count(line uint32) uint32 {
-	// Linear search is fine.
+// count returns the count and index for the counter at the specified line.
+func count(line uint32) (uint32, int) {
+	// Linear search is fine. Choose perfect fit over approximate.
+	// We can have a closing brace for a range on the same line as a condition for an "else if"
+	// and we don't want that brace to steal the count for the condition on the "if".
+	// Therefore we test for a perfect (lo==line && hi==line) match, but if we can't
+	// find that we take the first imperfect match.
+	index := -1
+	indexLo := uint32(1e9)
 	for i := range coverTest.Count {
 		lo, hi := coverTest.Pos[3*i], coverTest.Pos[3*i+1]
-		if lo <= line && line <= hi {
-			return coverTest.Count[i]
+		if lo == line && line == hi {
+			return coverTest.Count[i], i
+		}
+		// Choose the earliest match (the counters are in unpredictable order).
+		if lo <= line && line <= hi && indexLo > lo {
+			index = i
+			indexLo = lo
 		}
 	}
-	return 0
+	if index == -1 {
+		fmt.Fprintln(os.Stderr, "cover_test: no counter for line", line)
+		PASS = false
+		return 0, 0
+	}
+	return coverTest.Count[index], index
 }

cmd/cover/testdata/test.go

@@ -15,8 +15,10 @@ const anything = 1e9 // Just some unlikely value that means "we got here, don't
 func testAll() {
 	testSimple()
 	testBlockRun()
+	testIf()
 	testFor()
 	testSwitch()
+	testTypeSwitch()
 	testSelect1()
 	testSelect2()
 }
@@ -25,6 +27,48 @@ func testSimple() {
 	check(LINE, 1)
 }
 
+func testIf() {
+	if true {
+		check(LINE, 1)
+	} else {
+		check(LINE, 0)
+	}
+	if false {
+		check(LINE, 0)
+	} else {
+		check(LINE, 1)
+	}
+	for i := 0; i < 3; i++ {
+		if checkVal(LINE, 3, i) <= 2 {
+			check(LINE, 3)
+		}
+		if checkVal(LINE, 3, i) <= 1 {
+			check(LINE, 2)
+		}
+		if checkVal(LINE, 3, i) <= 0 {
+			check(LINE, 1)
+		}
+	}
+	for i := 0; i < 3; i++ {
+		if checkVal(LINE, 3, i) <= 1 {
+			check(LINE, 2)
+		} else {
+			check(LINE, 1)
+		}
+	}
+	for i := 0; i < 3; i++ {
+		if checkVal(LINE, 3, i) <= 0 {
+			check(LINE, 1)
+		} else if checkVal(LINE, 2, i) <= 1 {
+			check(LINE, 1)
+		} else if checkVal(LINE, 1, i) <= 2 {
+			check(LINE, 1)
+		} else if checkVal(LINE, 0, i) <= 3 {
+			check(LINE, 0)
+		}
+	}
+}
+
 func testFor() {
 	for i := 0; i < 10; i++ {
 		check(LINE, 10)
@@ -64,6 +108,24 @@ func testSwitch() {
 	}
 }
 
+func testTypeSwitch() {
+	var x = []interface{}{1, 2.0, "hi"}
+	for _, v := range x {
+		switch v.(type) {
+		case int:
+			check(LINE, 1)
+		case float64:
+			check(LINE, 1)
+		case string:
+			check(LINE, 1)
+		case complex128:
+			check(LINE, 0)
+		default:
+			check(LINE, 0)
+		}
+	}
+}
+
 func testSelect1() {
 	c := make(chan int)
 	go func() {