CLN: separate out groupby algorithms to separate lib

Author: jreback

pandas/_libs/algos.pxd

@@ -0,0 +1,39 @@
+from util cimport numeric
+from numpy cimport float64_t, double_t
+
+cpdef numeric kth_smallest(numeric[:] a, Py_ssize_t k)
+
+cdef inline Py_ssize_t swap(numeric *a, numeric *b) nogil except -1:
+    cdef numeric t
+
+    # cython doesn't allow pointer dereference so use array syntax
+    t = a[0]
+    a[0] = b[0]
+    b[0] = t
+    return 0
+
+
+cdef inline kth_smallest_c(float64_t* a, Py_ssize_t k, Py_ssize_t n):
+    cdef:
+        Py_ssize_t i, j, l, m
+        double_t x, t
+
+    l = 0
+    m = n -1
+    while (l<m):
+        x = a[k]
+        i = l
+        j = m
+
+        while 1:
+            while a[i] < x: i += 1
+            while x < a[j]: j -= 1
+            if i <= j:
+                swap(&a[i], &a[j])
+                i += 1; j -= 1
+
+            if i > j: break
+
+        if j < k: l = i
+        if k < i: m = j
+    return a[k]