diff --git a/.github/workflows/check_code_format.yml b/.github/workflows/check_code_format.yml
new file mode 100644
index 00000000..44dea45f
--- /dev/null
+++ b/.github/workflows/check_code_format.yml
@@ -0,0 +1,35 @@
+---
+name: check_code_format
+
+# yamllint disable-line rule:truthy
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - main
+  pull_request:
+
+jobs:
+  check_format_code:
+    name: check format code
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+
+      - uses: jiro4989/setup-nim-action@v1
+
+      - name: Format code
+        run: |
+          git clean -f -x -d
+          nim prettyfy
+
+      - name: Fail if needs reformatting
+        run: |
+          if [[ $(git status --porcelain) ]]; then
+             echo "please reformat/prettyfy these files:"
+             git status --porcelain=v1
+             exit 1
+          fi
+...
diff --git a/config.nims b/config.nims
index 4cec23c5..5e37d79a 100644
--- a/config.nims
+++ b/config.nims
@@ -1,5 +1,5 @@
 # Nim compilation defaults for the whole repository
---gc: arc
+#--gc: arc
 if defined(release) or defined(danger):
   --opt: speed
   --passC: "-flto"
diff --git a/dynamic_programming/catalan_numbers.nim b/dynamic_programming/catalan_numbers.nim
index 32d53a59..8536cfd5 100644
--- a/dynamic_programming/catalan_numbers.nim
+++ b/dynamic_programming/catalan_numbers.nim
@@ -68,7 +68,7 @@ func catalanNumbersCompileTime(index: Natural): Positive =
     const catalanTable = createCatalanTable(12)
   when sizeof(int) == 4:
     const catalanTable = createCatalanTable(20)
-  when sizeof(int) == 8: 
+  when sizeof(int) == 8:
     const catalanTable = createCatalanTable(36)
   catalanTable[index-1]
 
@@ -89,7 +89,7 @@ when isMainModule:
     263747951750360, 1002242216651368, 3814986502092304, 14544636039226909,
     55534064877048198, 212336130412243110, 812944042149730764,
     3116285494907301262]
-  
+
   static:
     for i in 0 ..< 36:
       doAssert (CatalanNumbersList[i] == createCatalanTable(36)[i])
@@ -100,7 +100,7 @@ when isMainModule:
       for index in 0 .. limit:
         let catalanNumber = catalanNumbersRecursive(index)
         check catalanNumber == CatalanNumbersList[index]
-    
+
     test "The thirty-one first Catalan numbers recursively, second formula":
       let limit = LowerLimit
       for index in 0 .. limit:
@@ -109,32 +109,32 @@ when isMainModule:
 
     test "The sixteen first Catalan numbers iteratively":
       check catalanNumbers(16) == expectedResult
-    
+
     test "We can compute up to the thirty-seventh Catalan number iteratively":
       let limit = UpperLimit
       let catalanNumbersSeq = catalanNumbers(limit)
       for index in 0 .. limit:
         check catalanNumbersSeq[index] == CatalanNumbersList[index]
-    
+
     test "The thirty-seventh first Catalan numbers with iterator":
       let limit = UpperLimit
       var index = 0
       for catalanNumber in catalanNumbersIt(limit):
         check catalanNumber == CatalanNumbersList[index]
         inc index
-      
+
     test "Test the catalan number function with binomials":
       let limit = LowerLimit
       for index in 0 .. limit:
         check catalanNumbers2(index) == CatalanNumbersList[index]
-    
+
     test "The Catalan Numbers binomial formula overflows at 31":
       doAssertRaises(OverflowDefect):
         discard catalanNumbers2(LowerLimit + 1)
-    
+
     test "Uses compile-time table":
       check catalanNumbersCompileTime(UpperLimit) == Positive(3116285494907301262)
-    
+
     test "The compile-time table overflows at 37":
       doAssertRaises(OverflowDefect):
         discard catalanNumbersCompileTime(UpperLimit + 1)
diff --git a/dynamic_programming/viterbi.nim b/dynamic_programming/viterbi.nim
index 958d38bd..c7a2cdfe 100644
--- a/dynamic_programming/viterbi.nim
+++ b/dynamic_programming/viterbi.nim
@@ -12,20 +12,20 @@ func viterbi*[S, O](hmm: HiddenMarkovModel[S], observations: seq[O]): seq[S] =
   var
     probabilities = newSeq[seq[float]](len(observations))
     backpointers = newSeq[seq[int]](len(observations))
-  
+
   # Initialization
   for i in 0 ..< len(observations):
     probabilities[i] = newSeq[float](len(hmm.states))
     backpointers[i] = newSeq[int](len(hmm.states))
-  
+
   for state in 0 ..< len(hmm.states):
     probabilities[0][state] = hmm.startProbability[state] *
      hmm.emissionProbability[state][observations[0].ord]
     backpointers[0][state] = 0
-  
+
   # Forward Pass - Derive the probabilities
   for nObs in 1 ..< len(observations):
-    var 
+    var
       obs = observations[nObs].ord
     for state in 0 ..< len(hmm.states):
       # Compute the argmax for probability of the current state
@@ -34,7 +34,7 @@ func viterbi*[S, O](hmm: HiddenMarkovModel[S], observations: seq[O]): seq[S] =
         max_prob_state = 0
       for prior_state in 0 ..< len(hmm.states):
         var
-          prob = probabilities[nObs - 1][prior_state] * 
+          prob = probabilities[nObs - 1][prior_state] *
           hmm.transitionProbability[prior_state][state] *
           hmm.emissionProbability[state][obs]
         if prob > max_prob:
@@ -43,7 +43,7 @@ func viterbi*[S, O](hmm: HiddenMarkovModel[S], observations: seq[O]): seq[S] =
       # Update probabilities and backpointers
       probabilities[nObs][state] = max_prob
       backpointers[nObs][state] = max_prob_state
-  
+
   # Final observation
   var
     max_prob = -1.0
@@ -52,10 +52,10 @@ func viterbi*[S, O](hmm: HiddenMarkovModel[S], observations: seq[O]): seq[S] =
     if probabilities[len(observations) - 1][state] > max_prob:
       max_prob = probabilities[len(observations) - 1][state]
       max_prob_state = state
-  
+
   result = newSeq[S](len(observations))
   result[^1] = hmm.states[max_prob_state]
-  
+
   # Backward Pass - Derive the states from the probabilities
   for i in 1 ..< len(observations):
     result[^(i+1)] = hmm.states[backpointers[^i][max_prob_state]]
@@ -71,7 +71,7 @@ when isMainModule:
           Healthy, Fever
         Observations = enum
           Normal, Cold, Dizzy
-      var 
+      var
         hmm = HiddenMarkovModel[States]()
         observations = @[Normal, Cold, Dizzy]
       hmm.states = @[Healthy, Fever]