September 2017 Update

Task/ABC-Problem/Jq/abc-problem-1.jq

@@ -0,0 +1,29 @@
+# when_index(cond;ary) returns the index of the first element in ary
+# that satisfies cond; it uses a helper function that takes advantage
+# of tail-recursion optimization in recent versions of jq.
+def index_when(cond; ary):
+  # state variable: counter
+  def when: if . >= (ary | length) then null
+            elif ary[.] | cond then .
+            else (.+1) | when
+            end;
+  0 | when;
+
+# Attempt to match a single letter with a block;
+# return null if no match, else the remaining blocks
+def match_letter(letter):
+  . as $ary | index_when( index(letter); $ary ) as $ix
+  | if $ix == null then null
+    else del( .[$ix] )
+    end;
+
+# Usage: string | abc(blocks)
+def abc(blocks):
+  if length == 0 then true
+  else
+    .[0:1] as $letter
+    | (blocks | match_letter( $letter )) as $blks
+    | if $blks == null then false
+      else .[1:] | abc($blks)
+      end
+  end;