Resolve "Best way to get number of hit DOMs"

Closes #46 (closed)

km3io/tools.py

 #!/usr/bin/env python3
+import numba as nb
+import numpy as np
 import uproot
+
 # 110 MB based on the size of the largest basket found so far in km3net
 BASKET_CACHE_SIZE = 110 * 1024**2
 BASKET_CACHE = uproot.cache.ThreadSafeArrayCache(BASKET_CACHE_SIZE)
@@ -40,3 +43,45 @@ def to_num(value):
         except (ValueError, TypeError):
             pass
     return value
+
+
+@nb.jit(nopython=True)
+def unique(array, dtype=np.int64):
+    """Return the unique elements of an array with a given dtype.
+
+    The performance is better for pre-sorted input arrays.
+
+    """
+    n = len(array)
+    out = np.empty(n, dtype)
+    last = array[0]
+    entry_idx = 0
+    out[entry_idx] = last
+    for i in range(1, n):
+        current = array[i]
+        if current == last: # shortcut for sorted arrays
+            continue
+        already_present = False
+        for j in range(entry_idx + 1):
+            if current == out[j]:
+                already_present = True
+                break
+        if not already_present:
+            entry_idx += 1
+            out[entry_idx] = current
+        last = current
+    return out[:entry_idx+1]
+
+
+@nb.jit(nopython=True)
+def uniquecount(array, dtype=np.int64):
+    """Count the number of unique elements in a jagged Awkward1 array."""
+    n = len(array)
+    out = np.empty(n, dtype)
+    for i in range(n):
+        sub_array = array[i]
+        if len(sub_array) == 0:
+            out[i] = 0
+        else:
+            out[i] = len(unique(sub_array))
+    return out