Go Golang : Merge Sort Stack Overflow - merge

Question

http://play.golang.org/p/rRccL6YHtQ
I just implement the same code as in CLRS
Pseudocode from CLRS
Merge-Sort(A, p, r)
if p < r
q = [(q+r)/2]
Merge-Sort(A, p, q)
Merge-Sort(A, q+1, r)
Merge(A, p, q, r)
Merge(A, p, q, r)
n_1 = q - p + 1
n_2 = r - q
let L[1 .. n_1 + 1] and R[1 .. n_2 + 1] be new arrays
for i = 1 to n_1
L[i] = A[p+i-1]
for j = 1 to n_2
R[j] = A[q+j]
L[n_1 + 1] = INFINITE
R[n_2 + 1] = INFINITE
i = 1
j = 1
for k = p to r
if L[i] <= R[j]
A[k] = L[i]
i = i + 1
else A[k] = R[j]
j = j + 1
But I am getting the stack overflow in the merge sort.
[9 -13 4 -2 3 1 -10 21 12]
runtime: goroutine stack exceeds 250000000-byte limit
fatal error: stack overflow
runtime stack:
runtime.throw(0x1b4980, 0x20280)
How do I make this work?
func MergeSort(slice []int, first, last int) {
if len(slice) < 2 {
return
}
if first < last {
mid := len(slice) / 2
MergeSort(slice, first, mid)
MergeSort(slice, mid+1, last)
Merge(slice, first, mid, last)
}
}
thanks a lot!

Answer 1

mid := len(slice) / 2
That's not where the middle should go. The middle is supposed to be halfway between first and last, which define the region of the slice you're sorting, not halfway through the slice. Alternatively, you can slice the slice to make new slices and drop first and last.

Answer 2

You don't seem to be re-slicing the slice you're merge-sorting, so there is no termination of the recursion.
You could either re-slice the slice before recursion, or you can do something like length := last - first (this may have an off-by-one, depending if "last" is the last index or one past the last index) and then go from there.