go.tools/container/intsets: fix compile error on 32-bit platforms.

Also, fix typo.

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/99310043

container/intsets/sparse.go

@@ -18,9 +18,9 @@ package intsets
 // - Add InsertAll(...int), RemoveAll(...int)
 // - Add 'bool changed' results for {Intersection,Difference}With too.
 //
-// TODO(adonovan): implement Dense, a sparse bit vector with a similar
-// API.  The space usage would be proportional to Max(), not Len(),
-// and the implementation would be based upon big.Int.
+// TODO(adonovan): implement Dense, a dense bit vector with a similar API.
+// The space usage would be proportional to Max(), not Len(), and the
+// implementation would be based upon big.Int.
 
 import (
 	"bytes"
@@ -131,7 +131,7 @@ func (b *block) empty() bool {
 func (b *block) len() int {
 	var l int
 	for _, w := range b.bits {
-		l += int(popcount(w))
+		l += popcount(w)
 	}
 	return l
 }
@@ -144,7 +144,7 @@ func (b *block) max() int {
 	// Decrement bi by number of high zeros in last.bits.
 	for i := len(b.bits) - 1; i >= 0; i-- {
 		if w := b.bits[i]; w != 0 {
-			return bi - int(nlz(w)) - 1
+			return bi - nlz(w) - 1
 		}
 		bi -= bitsPerWord
 	}

container/intsets/util.go

@@ -19,8 +19,8 @@ func init() {
 }
 
 // popcount returns the population count (number of set bits) of x.
-func popcount(x word) word {
-	return word(a[byte(x>>(0*8))] +
+func popcount(x word) int {
+	return int(a[byte(x>>(0*8))] +
 		a[byte(x>>(1*8))] +
 		a[byte(x>>(2*8))] +
 		a[byte(x>>(3*8))] +
@@ -32,7 +32,7 @@ func popcount(x word) word {
 
 // nlz returns the number of leading zeros of x.
 // From Hacker's Delight, fig 5.11.
-func nlz(x word) word {
+func nlz(x word) int {
 	x |= (x >> 1)
 	x |= (x >> 2)
 	x |= (x >> 4)

container/intsets/util_test.go

@@ -7,8 +7,17 @@ package intsets
 import "testing"
 
 func TestNLZ(t *testing.T) {
-	if x := nlz(0x0000801000000000); x != 16 {
-		t.Errorf("bad %d", x)
+	// Test the platform-specific edge case.
+	// NB: v must be a var (not const) so that the word() conversion is dynamic.
+	// Otherwise the compiler will report an error.
+	v := uint64(0x0000801000000000)
+	n := nlz(word(v))
+	want := 32 // (on 32-bit)
+	if bitsPerWord == 64 {
+		want = 16
+	}
+	if n != want {
+		t.Errorf("%d-bit nlz(%d) = %d, want %d", bitsPerWord, v, n, want)
 	}
 }