debug/dwarf: factor finding unit containing entry offset

This factors out the code for finding which unit contains an offset in
the "info" section.  The new code also replaces linear search with a
binary search.  The line table reader will also need this
functionality.

Change-Id: I2076e4fc6719b6f06fd2796cbbc7548ec1876cb3
Reviewed-on: https://go-review.googlesource.com/6733Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/debug/dwarf/entry.go

@@ -308,18 +308,13 @@ func (r *Reader) Seek(off Offset) {
 		return
 	}
 
-	// TODO(rsc): binary search (maybe a new package)
-	var i int
-	var u *unit
-	for i = range d.unit {
-		u = &d.unit[i]
-		if u.off <= off && off < u.off+Offset(len(u.data)) {
+	i := d.offsetToUnit(off)
+	if i == -1 {
+		r.err = errors.New("offset out of range")
+	}
+	u := &d.unit[i]
 	r.unit = i
 	r.b = makeBuf(r.d, u, "info", off, u.data[off-u.off:])
-			return
-		}
-	}
-	r.err = errors.New("offset out of range")
 }
 
 // maybeNextUnit advances to the next unit if this one is finished.

src/debug/dwarf/unit.go

@@ -4,7 +4,10 @@
 
 package dwarf
 
-import "strconv"
+import (
+	"sort"
+	"strconv"
+)
 
 // DWARF debug info is split into a sequence of compilation units.
 // Each unit has its own abbreviation table and address size.
@@ -88,3 +91,20 @@ func (d *Data) parseUnits() ([]unit, error) {
 	}
 	return units, nil
 }
+
+// offsetToUnit returns the index of the unit containing offset off.
+// It returns -1 if no unit contains this offset.
+func (d *Data) offsetToUnit(off Offset) int {
+	// Find the unit after off
+	next := sort.Search(len(d.unit), func(i int) bool {
+		return d.unit[i].off > off
+	})
+	if next == 0 {
+		return -1
+	}
+	u := &d.unit[next-1]
+	if u.off <= off && off < u.off+Offset(len(u.data)) {
+		return next - 1
+	}
+	return -1
+}