From cef342d57c89dcb962545432bd0bf0edaccc8708 Mon Sep 17 00:00:00 2001
From: frosch <frosch@openttd.org>
Date: Wed, 30 Oct 2013 10:01:49 +0000
Subject: [PATCH] (svn r25931) -Fix [FS#5786-ish]: [NoGo] Preserve the relative
 town growth progress when changing the town growth rate.

---
 src/town_cmd.cpp | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/town_cmd.cpp b/src/town_cmd.cpp
index d431242c3e..38894fe766 100644
--- a/src/town_cmd.cpp
+++ b/src/town_cmd.cpp
@@ -2527,7 +2527,20 @@ CommandCost CmdTownGrowthRate(TileIndex tile, DoCommandFlag flags, uint32 p1, ui
 	if (t == NULL) return CMD_ERROR;
 
 	if (flags & DC_EXEC) {
-		t->growth_rate = (p2 == 0) ? 0 : p2 | TOWN_GROW_RATE_CUSTOM;
+		if (p2 == 0) {
+			/* Clear TOWN_GROW_RATE_CUSTOM, UpdateTownGrowRate will determine a proper value */
+			t->growth_rate = 0;
+		} else {
+			uint old_rate = t->growth_rate & ~TOWN_GROW_RATE_CUSTOM;
+			if (t->grow_counter >= old_rate) {
+				/* This also catches old_rate == 0 */
+				t->grow_counter = p2;
+			} else {
+				/* Scale grow_counter, so half finished houses stay half finished */
+				t->grow_counter = t->grow_counter * p2 / old_rate;
+			}
+			t->growth_rate = p2 | TOWN_GROW_RATE_CUSTOM;
+		}
 		UpdateTownGrowRate(t);
 		InvalidateWindowData(WC_TOWN_VIEW, p1);
 	}