From 3622895f09d7c6f6a3d608031c790750f995327b Mon Sep 17 00:00:00 2001
From: Ben Whittaker <ben@benw.ca>
Date: Tue, 28 Sep 2021 22:16:34 -0400
Subject: [PATCH] Layout: avoid floating point numbers

The previous approach involved calculating the width of each element
independently, and deriving the `x` coordinate of each element by
summing the previous widths. However, if the widths are rounded, summing
them leads to accumulated rounding errors.

This change effectively takes the alternate approach of calculating the
`x` coordinate of each element independently, and then deriving the
widths from the differences between adjacent values of `x`.

As a bonus, this change corrects a couple issues in the
`padding_with_fill` test.
---
 modules/Layout.js                        |  44 +++++++++++++----------
 tests/Layout/tests/filly_issue820.bmp    | Bin 15562 -> 15562 bytes
 tests/Layout/tests/padding_with_fill.bmp | Bin 15562 -> 15562 bytes
 3 files changed, 26 insertions(+), 18 deletions(-)

diff --git a/modules/Layout.js b/modules/Layout.js
index 9f3a805be..53447c503 100644
--- a/modules/Layout.js
+++ b/modules/Layout.js
@@ -239,29 +239,37 @@ Layout.prototype.layout = function (l) {
   // exw,exh = extra width/height available
   switch (l.type) {
     case "h": {
-      var x = l.x + (0|l.pad);
+      var acc_w = l.x + (0|l.pad);
+      var accfillx = 0;
       var fillx = l.c && l.c.reduce((a,l)=>a+(0|l.fillx),0);
-      if (!fillx) { x += (l.w-l._w)/2; }
+      if (!fillx) { acc_w += (l.w-l._w)>>1; }
+      var x = acc_w;
       l.c.forEach(c => {
-        c.w = c._w + ((0|c.fillx)*(l.w-l._w)/(fillx||1));
-        c.h = c.filly ? l.h - (l.pad<<1) : c._h;
-        c.x = x;
-        c.y = l.y + (0|l.pad) + (1+(0|c.valign))*(l.h-(l.pad<<1)-c.h)/2;
-        x += c.w;
+        c.x = 0|x;
+        acc_w += c._w;
+        accfillx += 0|c.fillx;
+        x = acc_w + Math.floor(accfillx*(l.w-l._w)/(fillx||1));
+        c.w = 0|(x - c.x);
+        c.h = 0|(c.filly ? l.h - (l.pad<<1) : c._h);
+        c.y = 0|(l.y + (0|l.pad) + ((1+(0|c.valign))*(l.h-(l.pad<<1)-c.h)>>1));
         if (c.c) this.layout(c);
       });
       break;
     }
     case "v": {
-      var y = l.y + (0|l.pad);;
+      var acc_h = l.y + (0|l.pad);
+      var accfilly = 0;
       var filly = l.c && l.c.reduce((a,l)=>a+(0|l.filly),0);
-      if (!filly) { y += (l.h-l._h)/2 }
+      if (!filly) { acc_h += (l.h-l._h)>>1; }
+      var y = acc_h;
       l.c.forEach(c => {
-        c.w = c.fillx ? l.w - (l.pad<<1) : c._w;
-        c.h = c._h + ((0|c.filly)*(l.h-l._h)/(filly||1));
-        c.y = y;
-        c.x = l.x + (0|l.pad) + (1+(0|c.halign))*(l.w-(l.pad<<1)-c.w)/2;
-        y += c.h;
+        c.y = 0|y;
+        acc_h += c._h;
+        accfilly += 0|c.filly;
+        y = acc_h + Math.floor(accfilly*(l.h-l._h)/(filly||1));
+        c.h = 0|(y - c.y);
+        c.w = 0|(c.fillx ? l.w - (l.pad<<1) : c._w);
+        c.x = 0|(l.x + (0|l.pad) + ((1+(0|c.halign))*(l.w-(l.pad<<1)-c.w)>>1));
         if (c.c) this.layout(c);
       });
       break;
@@ -288,8 +296,8 @@ Layout.prototype.update = function() {
     if (l.r&1) { // rotation
       var t = l._w;l._w=l._h;l._h=t;
     }
-    l._w = Math.max(l._w + (l.pad<<1), 0|l.width);
-    l._h = Math.max(l._h + (l.pad<<1), 0|l.height);
+    l._w = 0|Math.max(l._w + (l.pad<<1), 0|l.width);
+    l._h = 0|Math.max(l._h + (l.pad<<1), 0|l.height);
   }
   var cb = {
     "txt" : function(l) {
@@ -349,8 +357,8 @@ Layout.prototype.update = function() {
   } else {
     l.w = l._w;
     l.h = l._h;
-    l.x = (w-l.w)/2;
-    l.y = y+(h-l.h)/2;
+    l.x = (w-l.w)>>1;
+    l.y = y+((h-l.h)>>1);
   }
   // layout children
   this.layout(l);
diff --git a/tests/Layout/tests/filly_issue820.bmp b/tests/Layout/tests/filly_issue820.bmp
index 886ba97d8967b1211ae7eb43c8f5bc5cf7cc55a7..adfe9164ef3a4f2d98a312d7f88857b26662606e 100644
GIT binary patch
delta 179
zcmX?Ad8%^5ZjH$o#T+IVD6vgmsUbAEK$K;2si-ZGW}O_ViNR<5KRH1Ns2U;*;{o;V
z(h_9(&%nSxIZy_u86p5R%o!vJW?vAo1JYtZZ3l%NK`e+CMp@U%2L$+0I6yUtLg7$d
PA(I0&f;I=}y7B`6mx()H

delta 197
zcmX?Ad8%^5ZjH$WN-UF2By4~*+vJrRLO_~zvZ@%6FUmSOQWJyE%Kw3ZfqyciEL;V$
z2+)jOT7pdcA0{6Zwg(#t<RXlONCWvSldp)`0j&^Y0UF0YIZy`70qIK=3WHJ*DMne>
c$p-}Z!Frac`vcjmjQo=yN;q#0&<*AX0H-B9ng9R*

diff --git a/tests/Layout/tests/padding_with_fill.bmp b/tests/Layout/tests/padding_with_fill.bmp
index 15e05b0d980d41eb76f4bc70456126994e45bf63..2c785bbc03cb5da5dad6ba34c80debce9ae9b1a3 100644
GIT binary patch
delta 55
zcmV-70LcH!dCGaPjRdph1Xl!;Tmo3LJPE85vp^lQ1GAeTYXXzNAy~5pBjN(HKqjOE
Nvy3Tj1G7>scn^Lw6+Qp}

delta 67
zcmV-J0KEUodCGaPjRcb*0yC4q1SqqR1XKjGJPE85vydDM1hexX69ThdBjN&+&?7IC
ZZzh7XnI=mEvpgym1CziZShG?scn@!C7`y-g