new function to help print line numbers on the page

apps/block_scout_web/lib/block_scout_web/views/address_contract_view.ex

@@ -14,4 +14,20 @@ defmodule BlockScoutWeb.AddressContractView do
   """
   def format_optimization_text(true), do: gettext("true")
   def format_optimization_text(false), do: gettext("false")
+
+  def contract_lines_with_index(contract_source_code) do
+    contract_lines = String.split(contract_source_code, "\n")
+
+    max_digits =
+      contract_lines
+      |> Enum.count()
+      |> Integer.digits()
+      |> Enum.count()
+
+    contract_lines
+    |> Enum.with_index(1)
+    |> Enum.map(fn {value, line} ->
+      {value, String.pad_leading(to_string(line), max_digits, " ")}
+    end)
+  end
 end

apps/block_scout_web/test/block_scout_web/views/address_contract_view_test.exs

@@ -14,4 +14,59 @@ defmodule BlockScoutWeb.AddressContractViewTest do
       assert AddressContractView.format_optimization_text(false) == "false"
     end
   end
+
+  describe "contract_lines_with_index/1" do
+    test "returns a list of tuples containing two strings each" do
+      code = """
+      pragma solidity >=0.4.22 <0.6.0;
+
+      struct Proposal {
+        uint voteCount;
+      }
+
+      address chairperson;
+      mapping(address => Voter) voters;
+      Proposal[] proposals;
+
+      constructor(uint8 _numProposals) public {
+        chairperson = msg.sender;
+        voters[chairperson].weight = 1;
+        proposals.length = _numProposals;
+      }
+      """
+
+      result = AddressContractView.contract_lines_with_index(code)
+
+      assert result == [
+               {"pragma solidity >=0.4.22 <0.6.0;", " 1"},
+               {"", " 2"},
+               {"struct Proposal {", " 3"},
+               {"  uint voteCount;", " 4"},
+               {"}", " 5"},
+               {"", " 6"},
+               {"address chairperson;", " 7"},
+               {"mapping(address => Voter) voters;", " 8"},
+               {"Proposal[] proposals;", " 9"},
+               {"", "10"},
+               {"constructor(uint8 _numProposals) public {", "11"},
+               {"  chairperson = msg.sender;", "12"},
+               {"  voters[chairperson].weight = 1;", "13"},
+               {"  proposals.length = _numProposals;", "14"},
+               {"}", "15"},
+               {"", "16"}
+             ]
+    end
+
+    test "returns a list of tuples and the second element always has n chars with x lines" do
+      chars = 3
+      lines = 100
+      result = AddressContractView.contract_lines_with_index(Enum.join(1..lines, "\n"))
+      assert Enum.all?(result, fn {_, number} -> String.length(number) == chars end)
+    end
+
+    test "returns a list of tuples and the first element is just a line from the original string" do
+      result = AddressContractView.contract_lines_with_index("a\nb\nc\nd\ne")
+      assert Enum.map(result, fn {line, _number} -> line end) == ["a", "b", "c", "d", "e"]
+    end
+  end
 end