Deterministic tx cost computation in CI (#1712)

Follow up to #1711 .

~A draft as it will only work once that one is merged.~

.github/workflows/ci-nix.yaml

@@ -401,8 +401,10 @@ jobs:
 
     - name: "Compute costs on both old and new"
       run: |
-        nix run ./new/#tx-cost >new.md
-        nix run ./old/#tx-cost >old.md
+        SEED=$RANDOM
+        echo "Using random seed $SEED"
+        nix run ./new/#tx-cost -- --seed $SEED >new.md
+        nix run ./old/#tx-cost -- --seed $SEED >old.md
 
     - name: "Compute the difference markdown"
       run: |

.github/workflows/cost-differences/diff.py

@@ -24,7 +24,7 @@ def parties(df):
 def utxo(df):
   return df.set_index("UTxO")
 
-def compare_to_md(f, old, new):
+def compare(f, old, new):
   # New should be better, so we compare to that.
   df = f(new) - f(old)
 
@@ -34,6 +34,9 @@ def compare_to_md(f, old, new):
   # Round everything to 2 decimals
   df = df.round(2)
 
+  return df
+
+def to_markdown(df):
   # Add colour
   def update_colour(x):
     if x == 0:
@@ -51,30 +54,37 @@ def update_colour(x):
 
   return df.to_markdown()
 
-print("Transaction cost differences")
 
-# First is the script size
+# We ignore the first table, namely the metadata, that's why the base/branch
+# index starts at 1.
+
+diffs = [
+  # First is the script size
+    (headers[0], compare( script_size, base[1], branch[1]))
+  # Then Init,
+  , (headers[1], compare( parties,     base[2], branch[2]))
+  # Then Commit is different; it doesn't have a "Parties" column
+  , (headers[2], compare( utxo,        base[3], branch[3]))
+  ]
 
-print(f"## {headers[0]}")
-print("")
-print( compare_to_md( script_size, base[1], branch[1]) )
+# Then the remaining are all the same.
+for i in range(4, 9 + 1):
+  diffs.append(( headers[i - 1]
+               , compare( parties, base[i], branch[i] )
+               ))
 
-# Then Init,
-print("")
-print(f"## {headers[1]}")
-print("")
-print( compare_to_md(parties, base[2], branch[2]) )
+# Check that ther was _some_ difference, at least.
+some_change = any( df.to_numpy().sum() != 0 for _, df in diffs )
 
-# Then Commit is different; it doesn't have a "Parties" column
+print("# Transaction cost differences")
 
-print("")
-print(f"## {headers[2]}")
-print("")
-print( compare_to_md(utxo, base[3], branch[3]) )
+if not (some_change):
+  print("No cost or size differences found")
+  exit(0)
 
-# The remaining are all the same as Init.
-for i in range(4, 9 + 1):
+for (header, df) in diffs:
   print("")
-  print(f"## {headers[i - 1]}")
+  print(f"## {header}")
   print("")
-  print( compare_to_md(parties, base[i], branch[i]) )
+  diff = to_markdown(df)
+  print(diff)