deal with coin market cap's pagination situation

apps/explorer/lib/explorer/exchange_rates/source.ex

@@ -3,6 +3,7 @@ defmodule Explorer.ExchangeRates.Source do
   Behaviour for fetching exchange rates from external sources.
   """
 
+  alias Explorer.ExchangeRates.Source.CoinMarketCap
   alias Explorer.ExchangeRates.Token
   alias HTTPoison.{Error, Response}
 
@@ -11,6 +12,31 @@ defmodule Explorer.ExchangeRates.Source do
   """
   @spec fetch_exchange_rates(module) :: {:ok, [Token.t()]} | {:error, any}
   def fetch_exchange_rates(source \\ exchange_rates_source()) do
+    if(source == CoinMarketCap) do
+      fetch_exchange_rates_from_paginable_source(source)
+    else
+      fetch_exchange_rates_request(source)
+    end
+  end
+
+  defp fetch_exchange_rates_from_paginable_source(source, page \\ 1) do
+    case HTTPoison.get(source.source_url(page), headers()) do
+      {:ok, %Response{body: body, status_code: 200}} ->
+        cond do
+          body =~ Explorer.coin() -> {:ok, source.format_data(body)}
+          page == source.max_page_number -> {:error, "exchange rates not found for this network"}
+          true -> fetch_exchange_rates_from_paginable_source(source, page + 1)
+        end
+
+      {:ok, %Response{body: body, status_code: status_code}} when status_code in 400..499 ->
+        {:error, decode_json(body)["error"]}
+
+      {:error, %Error{reason: reason}} ->
+        {:error, reason}
+    end
+  end
+
+  defp fetch_exchange_rates_request(source) do
     case HTTPoison.get(source.source_url(), headers()) do
       {:ok, %Response{body: body, status_code: 200}} ->
         {:ok, source.format_data(body)}
@@ -43,6 +69,8 @@ defmodule Explorer.ExchangeRates.Source do
 
   def to_decimal(nil), do: nil
 
+  def to_decimal(%Decimal{} = value), do: value
+
   def to_decimal(value) when is_float(value) do
     Decimal.from_float(value)
   end

apps/explorer/lib/explorer/exchange_rates/source/coin_market_cap.ex

@@ -31,9 +31,15 @@ defmodule Explorer.ExchangeRates.Source.CoinMarketCap do
 
   @impl Source
   def source_url do
-    "#{base_url()}/v1/ticker/?limit=0"
+    source_url(1)
   end
 
+  def source_url(page) do
+    "#{base_url()}/v1/ticker/?start=#{page - 1}00"
+  end
+
+  def max_page_number, do: 10
+
   defp base_url do
     config(:base_url) || "https://api.coinmarketcap.com"
   end