Commit a4a82241 authored by Robert Griesemer's avatar Robert Griesemer

use a bootstrap array to avoid allocation for short vectors

R=r
https://golang.org/cl/165078
parent 8c22dd24
......@@ -9,12 +9,28 @@ package vector
// Vector is the container itself.
// The zero value for Vector is an empty vector ready to use.
type Vector struct {
a []interface{};
a []interface{};
bootstrap [8]interface{};
}
func (p *Vector) realloc(length, capacity int) (b []interface{}) {
if length <= cap(p.bootstrap) && capacity <= cap(p.bootstrap) {
// don't allocate; use pre-allocated bootstrap array
b = p.bootstrap[0:length]
} else {
b = make([]interface{}, length, capacity)
}
copy(b, p.a);
p.a = b;
return;
}
// Insert n elements at position i.
func expand(a []interface{}, i, n int) []interface{} {
func (p *Vector) expand(i, n int) {
a := p.a;
// make sure we have enough space
len0 := len(a);
len1 := len0 + n;
......@@ -24,21 +40,20 @@ func expand(a []interface{}, i, n int) []interface{} {
} else {
// not enough space - double capacity
capb := cap(a) * 2;
if capb <= len1 {
if capb < len1 {
// still not enough - use required length
capb = len1
}
// capb > len1
b := make([]interface{}, len1, capb);
copy(b, a);
a = b;
// capb >= len1
a = p.realloc(len1, capb);
}
// make a hole
for j := len0 - 1; j >= i; j-- {
a[j+n] = a[j]
}
return a;
p.a = a;
}
......@@ -46,15 +61,14 @@ func expand(a []interface{}, i, n int) []interface{} {
// If the new length is shorter than the current length, Resize discards
// trailing elements. If the new length is longer than the current length,
// Resize adds nil elements. The capacity parameter is ignored unless the
// new length or capacity is longer that the current capacity.
// new length or capacity is longer that the current capacity. The resized
// vector's capacity may be larger than the requested capacity.
func (p *Vector) Resize(length, capacity int) *Vector {
a := p.a;
if length > cap(a) || capacity > cap(a) {
// not enough space or larger capacity requested explicitly
b := make([]interface{}, length, capacity);
copy(b, a);
a = b;
a = p.realloc(length, capacity)
} else if length < len(a) {
// clear trailing elements
for i := range a[length:] {
......@@ -101,7 +115,7 @@ func (p *Vector) Data() []interface{} {
// Insert inserts into the vector an element of value x before
// the current element at index i.
func (p *Vector) Insert(i int, x interface{}) {
p.a = expand(p.a, i, 1);
p.expand(i, 1);
p.a[i] = x;
}
......@@ -121,7 +135,7 @@ func (p *Vector) Delete(i int) {
// InsertVector inserts into the vector the contents of the Vector
// x such that the 0th element of x appears at index i after insertion.
func (p *Vector) InsertVector(i int, x *Vector) {
p.a = expand(p.a, i, len(x.a));
p.expand(i, len(x.a));
copy(p.a[i:i+len(x.a)], x.a);
}
......
......@@ -27,8 +27,8 @@ func checkSize(t *testing.T, v VectorInterface, len, cap int) {
if v.Len() != len {
t.Errorf("expected len = %d; found %d", len, v.Len())
}
if v.Cap() != cap {
t.Errorf("expected cap = %d; found %d", cap, v.Cap())
if v.Cap() < cap {
t.Errorf("expected cap >= %d; found %d", cap, v.Cap())
}
}
......
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