From 878db39080607ba476ba8d8f547ad28259efb6a9 Mon Sep 17 00:00:00 2001
From: Sibi Prabakaran <sibi@psibi.in>
Date: Fri, 5 Jun 2020 16:55:52 +0530
Subject: Optimize date plugin

We avoid calling getTimeZone for each of the time the date has to be
updated. Instead, it's computed once at the start and re-used for each
invocation.

Looking at the implementation of 'getTimeZone', we can see that it's
very expensive:
https://www.stackage.org/haddock/lts-15.15/time-1.9.3/src/Data-Time-LocalTime-Internal-TimeZone.html#getTimeZone

It calls a C FFI each time to get the time
zone (getTimeZoneCTime). This is something which we can avoid and the
MR implements that.

I have been using my xmobar with this patch and the result has been
quite good. My xmobar CPU usage has used to hit 3~7%
intermittently. With this MR, It hits only 0.7% intermittently which
is nice. :-)
---
 changelog.md               |  3 +++
 src/Xmobar/Plugins/Date.hs | 14 +++++++++++---
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/changelog.md b/changelog.md
index e77af85..5a02fc4 100644
--- a/changelog.md
+++ b/changelog.md
@@ -5,6 +5,9 @@ _New features_
   - New plugin `HandleReader` for reading data from a Haskell `Handle`. This is
     useful if you are running xmobar from within a Haskell program.
   - Build with ghc 8.10 allowed.
+  - Optimize date plugin by avoiding calling getTimeZone for each of
+    the time the date has to be updated. Instead, it's computed once
+    at the start and re-used for each invocation.
 
 ## Version 0.33 (February, 2020)
 
diff --git a/src/Xmobar/Plugins/Date.hs b/src/Xmobar/Plugins/Date.hs
index 1cb0596..83ca1c9 100644
--- a/src/Xmobar/Plugins/Date.hs
+++ b/src/Xmobar/Plugins/Date.hs
@@ -31,8 +31,16 @@ data Date = Date String String Int
 
 instance Exec Date where
     alias (Date _ a _) = a
-    run   (Date f _ _) = date f
     rate  (Date _ _ r) = r
+    start (Date f _ r) cb = do
+      t <- getCurrentTime
+      zone <- getTimeZone t
+      go zone
+     where
+      go zone = doEveryTenthSeconds r $ date zone f >>= cb
 
-date :: String -> IO String
-date format = fmap (formatTime defaultTimeLocale format) getZonedTime
+date :: TimeZone -> String -> IO String
+date timezone format = do
+  time <- getCurrentTime
+  let zonedTime = utcToZonedTime timezone time
+  pure $ formatTime defaultTimeLocale format zonedTime
-- 
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