Data commit

Task/Quickselect-algorithm/Jq/quickselect-algorithm-1.jq

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+# Emit the k-th smallest item in the input array,
+# or nothing if k is too small or too large.
+# The smallest corresponds to k==1.
+# The input array may hold arbitrary JSON entities, including null.
+def quickselect(k):
+
+  def partition(pivot):
+    reduce .[] as $x
+      # state: [less, other]
+      ( [ [], [] ];                       # two empty arrays:
+        if    $x  < pivot
+        then .[0] += [$x]                 # add x to less
+        else .[1] += [$x]                 # add x to other
+        end
+      );
+
+  # recursive inner function has arity 0 for efficiency
+  def qs:  # state: [kn, array] where kn counts from 0
+    .[0] as $kn
+     | .[1] as $a
+    | $a[0] as $pivot
+    | ($a[1:] | partition($pivot)) as $p
+    | $p[0] as $left
+    | ($left|length) as $ll
+    | if   $kn == $ll then $pivot
+      elif $kn <  $ll then [$kn, $left] | qs
+      else [$kn - $ll - 1, $p[1] ] | qs
+      end;
+
+  if length < k or k <= 0 then empty else [k-1, .] | qs end;