std/slices: add Grow

Author: mertcandav

std/slices/slices.jule

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD 3-Clause
 // license that can be found in the LICENSE file.
 
+use "std/runtime"
+
 // Reports whether slices are the same length and contains same elements.
 // The nil slices considered as zero-length slices.
 // The floating-point NaNs are not considered equal.
@@ -75,4 +77,20 @@ fn Reverse[S: ~[]E, E](mut s: S) {
 	for i < j; i, j = i+1, j-1 {
 		s[i], s[j] = s[j], s[i]
 	}
+}
+
+// Increases the slice's capacity, if necessary, to guarantee space for
+// another n elements. After Grow(n), at least n elements can be appended
+// to the slice without another allocation. If n is negative or too large to
+// allocate the memory, Grow panics.
+fn Grow[S: ~[]E, E](mut s: S, mut n: int): S {
+	if n < 0 {
+		panic("cannot be negative")
+	}
+	n -= cap(s) - len(s)
+	if n > 0 {
+		// Use runtime implementation to grow properly like `append` calls.
+		runtime::prememappend(&s, n)
+	}
+	ret s
 }

std/slices/slices_test.jule

@@ -96,4 +96,13 @@ fn testReverse(t: &testing::T) {
 	rs3 := [2, 1]
 	Reverse(s3)
 	t.Assert(Equal(s3, rs3), "s3 != rs3")
+}
+
+#test
+fn testGrow(t: &testing::T) {
+	mut buf := make([]byte, 128)
+	appendTest := append(buf, make([]byte, 1024)...)
+	buf = Grow(buf, 1024)
+	t.Assert(cap(buf) == cap(appendTest), "cap(buf) != cap(appendtest)")
+	t.Assert(cap(buf) >= 128+1024, "cap(buf) < 128+1024")
 }