strconv: slightly simplified roundShortest; better comments

Change-Id: If886f15468680f7e1c589873066b4391eb9784b5
Reviewed-on: https://go-review.googlesource.com/14856Reviewed-by: Alan Donovan <adonovan@google.com>

src/strconv/ftoa.go

@@ -286,25 +286,23 @@ func roundShortest(d *decimal, mant uint64, exp int, flt *floatInfo) {
 	// Now we can figure out the minimum number of digits required.
 	// Walk along until d has distinguished itself from upper and lower.
 	for i := 0; i < d.nd; i++ {
-		var l, m, u byte // lower, middle, upper digits
+		l := byte('0') // lower digit
 		if i < lower.nd {
 			l = lower.d[i]
-		} else {
-			l = '0'
 		}
-		m = d.d[i]
+		m := d.d[i]    // middle digit
+		u := byte('0') // upper digit
 		if i < upper.nd {
 			u = upper.d[i]
-		} else {
-			u = '0'
 		}
 
 		// Okay to round down (truncate) if lower has a different digit
-		// or if lower is inclusive and is exactly the result of rounding down.
-		okdown := l != m || (inclusive && l == m && i+1 == lower.nd)
+		// or if lower is inclusive and is exactly the result of rounding
+		// down (i.e., and we have reached the final digit of lower).
+		okdown := l != m || inclusive && i+1 == lower.nd
 
-		// Okay to round up if upper has a different digit and
-		// either upper is inclusive or upper is bigger than the result of rounding up.
+		// Okay to round up if upper has a different digit and either upper
+		// is inclusive or upper is bigger than the result of rounding up.
 		okup := m != u && (inclusive || m+1 < u || i+1 < upper.nd)
 
 		// If it's okay to do either, then round to the nearest one.

src/strconv/ftoa_test.go

@@ -18,7 +18,7 @@ type ftoaTest struct {
 	s    string
 }
 
-func fdiv(a, b float64) float64 { return a / b } // keep compiler in the dark
+func fdiv(a, b float64) float64 { return a / b }
 
 const (
 	below1e23 = 99999999999999974834176
@@ -94,8 +94,8 @@ var ftoatests = []ftoaTest{
 	{above1e23, 'f', -1, "100000000000000010000000"},
 	{above1e23, 'g', -1, "1.0000000000000001e+23"},
 
-	{fdiv(5e-304, 1e20), 'g', -1, "5e-324"},
-	{fdiv(-5e-304, 1e20), 'g', -1, "-5e-324"},
+	{fdiv(5e-304, 1e20), 'g', -1, "5e-324"},   // avoid constant arithmetic
+	{fdiv(-5e-304, 1e20), 'g', -1, "-5e-324"}, // avoid constant arithmetic
 
 	{32, 'g', -1, "32"},
 	{32, 'g', 0, "3e+01"},